Class 11 Biology NCERT Notes- Chapter 15: Body Fluids and Circulation

Detailed Study Notes – Chapter 15: Body Fluids and Circulation (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. Introduction to Body Fluids and Circulation

All living cells require a continuous supply of nutrients, oxygen (O₂), and other essential substances. Simultaneously, waste and harmful byproducts must be removed to maintain healthy tissue function. To facilitate this transport, organisms have evolved various mechanisms.

• Simple Organisms: Sponges and coelenterates circulate water from their surroundings through their body cavities for substance exchange.
• Complex Organisms: Higher organisms, including humans, use specialised internal fluids. The most common is blood, a special connective tissue. Another fluid, lymph, also aids in transporting certain substances.

2. Blood: Composition and Function

Blood is a connective tissue composed of a fluid matrix (plasma) and formed elements.

2.1 Plasma

Plasma is a straw-colored, viscous fluid that constitutes approximately 55% of the blood’s volume.

• Composition:
  • Water: 90-92%
  • Proteins: 6-8%, including:
    • Fibrinogen: Essential for blood clotting or coagulation.
    • Globulins: Primarily involved in the body’s defence mechanisms.
    • Albumins: Help maintain osmotic balance.
  • Minerals: Small amounts of Na⁺, Ca⁺⁺, Mg⁺⁺, HCO₃⁻, Cl⁻, etc.
  • Other Substances: Glucose, amino acids, and lipids are present as they are transported throughout the body.
• Serum: Plasma from which the clotting factors have been removed.

2.2 Formed Elements

Constituting about 45% of blood, the formed elements include erythrocytes (RBCs), leucocytes (WBCs), and platelets.

• Erythrocytes (Red Blood Cells – RBCs):
  • Abundance: The most numerous blood cells, with 5 to 5.5 million per mm³ in a healthy adult male.
  • Formation: Produced in the red bone marrow in adults.
  • Structure: Biconcave in shape and lack a nucleus in most mammals.
  • Function: Contain haemoglobin, a red, iron-containing complex protein (12-16 gms per 100 ml of blood) that is crucial for transporting respiratory gases.
  • Lifespan: Average lifespan is 120 days, after which they are destroyed in the spleen, often called the “graveyard of RBCs.”
• Leucocytes (White Blood Cells – WBCs):
  • Characteristics: Colourless (due to lack of haemoglobin), nucleated, and relatively few in number (6,000-8,000 per mm³). They are generally short-lived.
  • Categories:
    1. Granulocytes: Neutrophils, Eosinophils, Basophils.
    2. Agranulocytes: Lymphocytes, Monocytes.
  • Types and Functions:
    • Neutrophils (60-65%): The most abundant WBCs. They are phagocytic, destroying foreign organisms.
    • Basophils (0.5-1%): The least abundant. They secrete histamine, serotonin, and heparin and are involved in inflammatory reactions.
    • Eosinophils (2-3%): Resist infections and are associated with allergic reactions.
    • Monocytes (6-8%): Phagocytic cells that destroy foreign organisms.
    • Lymphocytes (20-25%): Include ‘B’ and ‘T’ forms, both responsible for the body’s immune responses.
• Platelets (Thrombocytes):
  • Origin: Cell fragments produced from special bone marrow cells called megakaryocytes.
  • Count: Normal blood contains 1,500,000 to 3,500,000 platelets per mm³.
  • Function: Release substances involved in the coagulation or clotting of blood. A low platelet count can lead to clotting disorders and excessive blood loss.

3. Blood Groups

Human blood is classified into different groups based on specific antigens on the surface of RBCs. The two most important groupings are ABO and Rh.

3.1 ABO Grouping

This system is based on the presence or absence of two surface antigens, A and B, on RBCs, and two natural antibodies, anti-A and anti-B, in the plasma. Mismatched blood transfusions can cause severe clumping (destruction of RBCs).

Blood Group	Antigens on RBCs	Antibodies in Plasma	Can Receive Blood From (Donor’s Group)
A	A	anti-B	A, O
B	B	anti-A	B, O
AB	A, B	nil	AB, A, B, O
O	nil	anti-A, anti-B	O

• Universal Donors: Individuals with blood group O can donate to all other groups.
• Universal Recipients: Individuals with blood group AB can receive blood from all other groups.

3.2 Rh Grouping

• Rh Antigen: An antigen, similar to one found in Rhesus monkeys, present on the RBC surface of about 80% of humans, who are termed Rh positive (Rh+ve). Those lacking the antigen are Rh negative (Rh-ve).
• Rh Incompatibility: An Rh-ve person exposed to Rh+ve blood will produce anti-Rh antibodies. This is critical during blood transfusions and pregnancy.
• Erythroblastosis Foetalis: A condition that can occur when an Rh-ve mother carries an Rh+ve foetus. During the first delivery, maternal blood may be exposed to the foetus’s Rh+ve blood, causing the mother to develop anti-Rh antibodies. In subsequent pregnancies, these antibodies can cross the placenta and destroy the RBCs of an Rh+ve foetus, leading to severe anaemia, jaundice, or death. This can be prevented by administering anti-Rh antibodies to the mother immediately after the first delivery.

4. Coagulation of Blood

Blood clotting is a vital mechanism to prevent excessive blood loss from an injury. A clot, or coagulum, is a dark reddish-brown scum formed from a network of threads called fibrins, which trap dead and damaged formed elements.

The clotting process is a cascade of enzymic reactions:

1. Injury stimulates platelets and tissues to release factors that activate the coagulation mechanism.
2. An enzyme complex, thrombokinase, is formed.
3. Thrombokinase converts the inactive plasma substance prothrombin into the active enzyme thrombin.
4. Thrombin then converts inactive fibrinogen in the plasma into active fibrin.
5. Calcium ions (Ca⁺⁺) play a very important role in this cascade.

5. Lymph (Tissue Fluid)

• Formation: As blood flows through capillaries, water and small soluble substances filter out into the spaces between tissue cells, forming interstitial fluid or tissue fluid. Larger proteins and most formed elements remain in the blood vessels.
• Function: It facilitates the exchange of nutrients, gases, and waste between the blood and cells.
• Lymphatic System: A network of vessels that collects this fluid and drains it back into the major veins. The fluid within this system is called lymph.
• Composition: Lymph is a colourless fluid containing specialised lymphocytes responsible for immune responses. Its mineral distribution is similar to plasma. It is also an important carrier for nutrients and hormones, and it absorbs fats through lacteals in the intestinal villi.

6. Circulatory Pathways

• Open Circulatory System: Found in arthropods and molluscs. Blood is pumped by the heart through large vessels into open spaces or body cavities called sinuses.
• Closed Circulatory System: Found in annelids and chordates. Blood is always circulated through a closed network of blood vessels. This system allows for more precise regulation of blood flow.

Vertebrate Heart Evolution:

• Fishes: 2-chambered heart (one atrium, one ventricle). Pumps deoxygenated blood to the gills for oxygenation. This is single circulation.
• Amphibians & Reptiles (except crocodiles): 3-chambered heart (two atria, one ventricle). The single ventricle pumps out mixed oxygenated and deoxygenated blood, known as incomplete double circulation.
• Crocodiles, Birds & Mammals: 4-chambered heart (two atria, two ventricles). Oxygenated and deoxygenated blood are kept separate. This system, with two distinct pathways, is called double circulation.

7. Human Circulatory System

This system consists of the heart, a network of blood vessels, and blood.

7.1 The Heart

• Location & Structure: A muscular, mesodermally derived organ the size of a clenched fist, located in the thoracic cavity between the lungs. It is protected by a double-walled bag called the pericardium. It has four chambers: two upper atria and two lower, larger ventricles.
• Septa:
  • Inter-atrial septum: Separates the right and left atria.
  • Inter-ventricular septum: A thick wall separating the right and left ventricles.
• Valves (Ensure one-way blood flow):
  • Tricuspid Valve: Between the right atrium and right ventricle.
  • Bicuspid (Mitral) Valve: Between the left atrium and left ventricle.
  • Semilunar Valves: Guard the openings of the ventricles into the pulmonary artery and the aorta.

7.2 Nodal Tissue and Conduction System

The heart is made of specialised cardiac muscle that is autoexcitable (can generate action potentials without external stimuli).

• Sino-atrial Node (SAN): A patch of nodal tissue in the upper right corner of the right atrium. It generates 70-75 action potentials per minute, initiating the heartbeat. It is called the pacemaker.
• Atrio-ventricular Node (AVN): Located in the lower-left corner of the right atrium.
• AV Bundle (Bundle of His) and Purkinje Fibres: The AV bundle continues from the AVN, divides into right and left branches, and spreads throughout the ventricular musculature via Purkinje fibres, conducting the action potential to the ventricles.

7.3 Cardiac Cycle

This is the sequential repetition of events in the heart, consisting of contraction (systole) and relaxation (diastole). With a heart rate of 72 beats/min, one cardiac cycle lasts 0.8 seconds.

1. Joint Diastole: All four chambers are relaxed. Blood flows from the vena cava and pulmonary veins into the ventricles.
2. Atrial Systole: The SAN generates an impulse, causing both atria to contract. This pushes about 30% more blood into the ventricles.
3. Ventricular Systole: The impulse travels to the ventricles, causing them to contract. The tricuspid and bicuspid valves close (producing the “lub” sound). Pressure builds, forcing the semilunar valves open, and blood is ejected into the pulmonary artery and aorta.
4. Ventricular Diastole: The ventricles relax, and pressure falls. The semilunar valves close to prevent backflow (producing the “dub” sound). The atria, which were filling with blood, now push the tricuspid and bicuspid valves open, and the cycle repeats.

• Stroke Volume: The volume of blood pumped by each ventricle per beat (approx. 70 mL).
• Cardiac Output: The volume of blood pumped by each ventricle per minute. It is calculated as Stroke Volume × Heart Rate (approx. 5 Litres/min).

7.4 Electrocardiogram (ECG)

An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.

• P-wave: Represents the electrical excitation (depolarisation) of the atria, leading to atrial contraction.
• QRS Complex: Represents the depolarisation of the ventricles, which initiates ventricular contraction (systole).
• T-wave: Represents the return of the ventricles to their normal state (repolarisation). The end of the T-wave marks the end of systole.

The ECG is of great clinical significance, as deviations from the normal shape can indicate disease or abnormalities.

8. Double Circulation

Humans have two distinct circulatory pathways:

1. Pulmonary Circulation: The path of deoxygenated blood from the right ventricle to the lungs via the pulmonary artery, and the return of oxygenated blood from the lungs to the left atrium via the pulmonary veins.
2. Systemic Circulation: The path of oxygenated blood from the left ventricle to all body tissues via the aorta and its branches, and the return of deoxygenated blood from the tissues to the right atrium via the vena cava.

• Hepatic Portal System: A unique vascular connection where the hepatic portal vein carries blood from the intestine to the liver before it enters systemic circulation.
• Coronary System: A dedicated system of vessels that supplies blood to and from the heart muscle itself.

9. Regulation of Cardiac Activity

• Intrinsic Regulation: The heart is myogenic, meaning its normal activities are auto-regulated by its specialised nodal tissue.
• Extrinsic Regulation:
  • Neural Control: A centre in the medulla oblongata moderates heart function via the Autonomic Nervous System (ANS).
    • Sympathetic nerves increase heart rate and contraction strength.
    • Parasympathetic nerves decrease heart rate.
  • Hormonal Control: Adrenal medullary hormones can increase cardiac output.

10. Disorders of the Circulatory System

• High Blood Pressure (Hypertension): Blood pressure that is consistently higher than the normal 120/80 mmHg. A reading of 140/90 mmHg or higher indicates hypertension, which can lead to heart disease and damage vital organs like the brain and kidneys.
• Coronary Artery Disease (CAD): Also known as atherosclerosis, it is caused by the deposition of calcium, fat, cholesterol, and fibrous tissues in the coronary arteries, narrowing their lumen and affecting blood supply to the heart muscle.
• Angina (Angina Pectoris): A symptom of acute chest pain that occurs when the heart muscle does not receive enough oxygen. It is common in middle-aged and elderly individuals.
• Heart Failure: A condition where the heart is not pumping blood effectively enough to meet the body’s needs. It is distinct from cardiac arrest (heart stops beating) and a heart attack (heart muscle damage). Congestion of the lungs is a primary symptom.

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Q&A

Short-Answer Questions (Answer in 2-3 sentences)

1. What are the three major types of proteins found in blood plasma and their primary functions?
2. Describe the structure and primary function of erythrocytes (RBCs).
3. Why is the spleen called the “graveyard of RBCs”?
4. What are the two main categories of leucocytes, and which cell types fall under each?
5. What is the role of neutrophils and monocytes?
6. Explain the function of basophils in the body’s inflammatory response.
7. What are platelets, and why are they essential for the body?
8. Explain the basis of the ABO blood grouping system.
9. Why is a person with blood group O considered a “universal donor”?
10. What is the Rh antigen, and how does it determine if a person is Rh positive or negative?
11. Briefly explain the condition known as erythroblastosis foetalis.
12. Describe the final steps of blood coagulation, involving fibrinogen and fibrin.
13. What role do calcium ions play in blood clotting?
14. What is lymph, and how is it formed from tissue fluid?
15. Differentiate between an open and a closed circulatory system.
16. Describe the key difference between single circulation in fishes and incomplete double circulation in amphibians.
17. What is the function of the pericardium?
18. Name the four valves of the human heart and state their locations.
19. Why is the sino-atrial node (SAN) called the pacemaker of the heart?
20. Define stroke volume and cardiac output.
21. What events in the cardiac cycle are associated with the “lub” and “dub” heart sounds?
22. What does the P-wave on an ECG represent?
23. What electrical event in the heart is represented by the QRS complex?
24. Differentiate between pulmonary and systemic circulation.
25. What is Coronary Artery Disease (CAD)?

Multiple-Choice Questions (MCQs)

1. Which component makes up 55% of the blood’s volume? a) Formed Elements b) Plasma c) Erythrocytes d) Leucocytes
2. Which plasma protein is primarily involved in blood clotting? a) Albumin b) Globulin c) Fibrinogen d) Serotonin
3. What is the average lifespan of a Red Blood Cell (RBC)? a) 60 days b) 90 days c) 120 days d) 150 days
4. Which are the most abundant leucocytes in the blood? a) Lymphocytes b) Neutrophils c) Basophils d) Eosinophils
5. Which cells are responsible for the body’s immune responses? a) Eosinophils b) Platelets c) Monocytes d) Lymphocytes
6. A person with blood group AB is a universal recipient because they have: a) Both anti-A and anti-B antibodies in their plasma. b) No antigens on their RBCs. c) No antibodies in their plasma. d) Only antigen A on their RBCs.
7. The conversion of prothrombin to thrombin is catalysed by: a) Fibrin b) Thrombokinase c) Calcium ions d) Heparin
8. In which organisms is an open circulatory system present? a) Annelids and Chordates b) Arthropods and Molluscs c) Fishes and Amphibians d) Birds and Mammals
9. Which animal group possesses a 3-chambered heart? a) Birds b) Fishes c) Amphibians d) Mammals
10. The opening between the right atrium and the right ventricle is guarded by the: a) Mitral valve b) Semilunar valve c) Bicuspid valve d) Tricuspid valve
11. The rhythmic contractile activity of the heart is initiated and maintained by the: a) Atrio-ventricular node (AVN) b) Purkinje fibres c) Sino-atrial node (SAN) d) Bundle of His
12. The duration of a single cardiac cycle is approximately: a) 0.5 seconds b) 0.8 seconds c) 1.0 second d) 1.2 seconds
13. What is the average cardiac output of a healthy individual? a) 2 litres/min b) 3 litres/min c) 5 litres/min d) 7 litres/min
14. The second heart sound (“dub”) is associated with the closure of which valves? a) Tricuspid and Bicuspid valves b) Semilunar valves c) Tricuspid valve only d) Bicuspid valve only
15. On an ECG, the repolarisation of the ventricles is represented by the: a) P-wave b) QRS complex c) T-wave d) Q-wave
16. The hepatic portal vein carries blood from the ______ to the ______. a) Liver, Intestine b) Kidney, Liver c) Intestine, Liver d) Lungs, Heart
17. A heart that is auto-regulated by specialised nodal tissue is called: a) Neurogenic b) Myogenic c) Pacemaker d) Systolic
18. A repeated blood pressure reading of 140/90 mmHg or higher indicates: a) Angina b) Heart Failure c) Atherosclerosis d) Hypertension
19. The symptom of acute chest pain due to insufficient oxygen reaching the heart muscle is called: a) Angina Pectoris b) Cardiac Arrest c) Heart Attack d) Hypertension
20. What is plasma without the clotting factors called? a) Lymph b) Tissue Fluid c) Serum d) Blood

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Answer Keys

Answer Key: Short-Answer Questions

1. The three major plasma proteins are fibrinogen, globulins, and albumins. Fibrinogen is essential for blood coagulation, globulins are involved in defence mechanisms, and albumins help maintain the body’s osmotic balance.
2. Erythrocytes are biconcave discs that lack a nucleus in most mammals. Their primary function is to transport respiratory gases, which is accomplished by the iron-containing protein haemoglobin within them.
3. The spleen is called the “graveyard of RBCs” because after their average lifespan of 120 days, old and worn-out red blood cells are removed from circulation and destroyed in the spleen.
4. The two main categories are granulocytes and agranulocytes. Granulocytes include neutrophils, eosinophils, and basophils, while agranulocytes include lymphocytes and monocytes.
5. Neutrophils and monocytes are both phagocytic cells. Their role is to engulf and destroy foreign organisms, such as bacteria, that enter the body, thus playing a key part in the immune defence.
6. Basophils are involved in inflammatory reactions. They secrete substances like histamine, serotonin, and heparin, which mediate inflammation and allergic responses.
7. Platelets, or thrombocytes, are cell fragments produced in the bone marrow. They are essential for blood coagulation, as they release various substances that initiate and participate in the clotting cascade to prevent excessive blood loss.
8. The ABO system is based on the presence or absence of two antigens, A and B, on the surface of RBCs. The plasma contains corresponding natural antibodies (anti-A or anti-B) against the antigens that are absent on the RBCs.
9. A person with blood group O is a “universal donor” because their RBCs have neither A nor B antigens on their surface. Therefore, their blood can be transfused into individuals of any ABO blood group without causing an immune reaction, leading to clumping.
10. The Rh antigen is another antigen found on the surface of RBCs. Individuals who have this antigen are called Rh positive (Rh+ve), while those who lack it are called Rh negative (Rh-ve).
11. Erythroblastosis foetalis is a condition where an Rh-negative mother carries an Rh-positive foetus. In subsequent pregnancies, maternal anti-Rh antibodies can cross the placenta and destroy the foetal RBCs, causing severe anaemia and jaundice in the baby.
12. The final step of coagulation involves the enzyme thrombin converting the soluble plasma protein fibrinogen into insoluble threads of fibrin. This fibrin network then traps blood cells and platelets to form a stable clot.
13. Calcium ions play a very important role as a cofactor in multiple steps of the clotting cascade. They are required for the series of linked enzymatic reactions that lead to the formation of the enzyme complex thrombokinase.
14. Lymph is the fluid present in the lymphatic system. It is formed when interstitial fluid (tissue fluid), which leaks out from blood capillaries into the spaces between cells, is collected by the network of lymphatic vessels.
15. In an open circulatory system, blood is pumped into open body cavities called sinuses, directly bathing the tissues. In a closed circulatory system, blood is confined within a network of vessels and does not leave them.
16. Fishes have single circulation where the 2-chambered heart pumps deoxygenated blood to the gills and then to the body. Amphibians have a 3-chambered heart where oxygenated and deoxygenated blood mix in a single ventricle, a system known as incomplete double circulation.
17. The pericardium is a double-walled membranous bag that encloses the heart. It contains pericardial fluid, which protects the heart from shock and reduces friction during its pumping action.
18. The four valves are the tricuspid valve (between right atrium and ventricle), the bicuspid/mitral valve (between left atrium and ventricle), and two semilunar valves (at the exit of the right ventricle into the pulmonary artery and the left ventricle into the aorta).
19. The SAN is called the pacemaker because it has the ability to generate the maximum number of action potentials (70-75 per minute) without external stimuli. This intrinsic rhythm initiates and maintains the rhythmic contractile activity of the entire heart.
20. Stroke volume is the volume of blood pumped out by each ventricle during one cardiac cycle, which is approximately 70 mL. Cardiac output is the total volume pumped by each ventricle per minute and is the product of stroke volume and heart rate (approx. 5 litres/min).
21. The first heart sound, “lub,” is produced by the closure of the tricuspid and bicuspid valves at the beginning of ventricular systole. The second heart sound, “dub,” is produced by the closure of the semilunar valves at the end of ventricular systole.
22. The P-wave on an ECG represents the electrical excitation, or depolarisation, of the atria. This electrical event leads to the contraction of both atria (atrial systole).
23. The QRS complex represents the depolarisation of the ventricles. This electrical activity initiates ventricular contraction, or ventricular systole.
24. Pulmonary circulation involves pumping deoxygenated blood from the right ventricle to the lungs for oxygenation and its return to the left atrium. Systemic circulation involves pumping oxygenated blood from the left ventricle to the rest of the body and returning deoxygenated blood to the right atrium.
25. Coronary Artery Disease (CAD), or atherosclerosis, is a condition that affects the blood vessels supplying the heart muscle. It is caused by the buildup of deposits like calcium, fat, and cholesterol, which narrows the arteries and restricts blood flow.

Answer Key: Multiple-Choice Questions

1. b) Plasma
2. c) Fibrinogen
3. c) 120 days
4. b) Neutrophils
5. d) Lymphocytes
6. c) No antibodies in their plasma.
7. b) Thrombokinase
8. b) Arthropods and Molluscs
9. c) Amphibians
10. d) Tricuspid valve
11. c) Sino-atrial node (SAN)
12. b) 0.8 seconds
13. c) 5 litres/min
14. b) Semilunar valves
15. c) T-wave
16. c) Intestine, Liver
17. b) Myogenic
18. d) Hypertension
19. a) Angina Pectoris
20. c) Serum

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Essay Questions with Answers

1. Describe the composition and functions of the formed elements in human blood.

• Answer: The formed elements constitute about 45% of human blood and include erythrocytes, leucocytes, and platelets.
  • Erythrocytes (RBCs) are the most abundant cells, numbering 5-5.5 million per mm³. They are biconcave, anucleated cells formed in the red bone marrow. Their primary function is the transport of respiratory gases, facilitated by the iron-containing protein haemoglobin, which gives blood its red colour. They have a lifespan of about 120 days.
  • Leucocytes (WBCs) are colourless, nucleated cells involved in the body’s defence mechanisms. They are less numerous (6000-8000 per mm³) and are categorised into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Neutrophils and monocytes are phagocytic, engulfing foreign invaders. Eosinophils combat infections and are involved in allergic reactions. Basophils secrete histamine and other chemicals for inflammatory responses. Lymphocytes (B and T cells) are crucial for specific immune responses.
  • Platelets (Thrombocytes) are cell fragments from megakaryocytes in the bone marrow, with a normal count of 1.5 to 3.5 million per mm³. They play a vital role in blood coagulation by releasing factors that initiate the clotting cascade at the site of an injury, preventing excessive blood loss.

2. Explain the entire process of blood coagulation, starting from an injury.

• Answer: Blood coagulation, or clotting, is a complex mechanism to prevent excessive blood loss following an injury or trauma. The process is a cascade of enzymatic reactions involving multiple clotting factors present in the plasma in an inactive form.
  1. Initiation: When an injury occurs, platelets in the blood are stimulated to release certain factors. The damaged tissues at the site of injury also release factors that can initiate coagulation.
  2. Formation of Thrombokinase: These factors trigger a series of linked enzymic reactions (a cascade process) that lead to the formation of an enzyme complex called thrombokinase. Calcium ions (Ca⁺⁺) are essential for this stage.
  3. Conversion of Prothrombin to Thrombin: The thrombokinase complex then acts as an enzyme to convert an inactive plasma protein, prothrombin, into its active form, thrombin.
  4. Formation of Fibrin: Thrombin, in turn, acts as an enzyme to convert another inactive plasma protein, soluble fibrinogen, into insoluble threads of fibrin.
  5. Clot Formation: These fibrin threads form a network at the site of the injury. This network traps dead and damaged formed elements of the blood (RBCs, WBCs, platelets), forming a dark reddish-brown structure called a clot or coagulum, which seals the wound and stops the bleeding.

3. What is meant by “double circulation”? Describe the pulmonary and systemic circuits in humans and explain the significance of this system.

• Answer: Double circulation is a system found in birds and mammals where blood passes through the heart twice for each complete circuit of the body. This involves two separate circulatory pathways: the pulmonary circulation and the systemic circulation.
  • Pulmonary Circulation: This circuit carries deoxygenated blood away from the heart to the lungs and returns oxygenated blood back to the heart. The pathway starts when the right ventricle pumps deoxygenated blood into the pulmonary artery, which carries it to the lungs. In the lungs, the blood releases carbon dioxide and picks up oxygen. The now oxygenated blood is carried back to the left atrium of the heart by the pulmonary veins.
  • Systemic Circulation: This circuit carries oxygenated blood from the heart to the rest of the body and returns deoxygenated blood to the heart. The pathway begins when the left ventricle pumps oxygenated blood into the aorta. The aorta branches into a network of arteries, arterioles, and capillaries that supply nutrients and oxygen to all body tissues. Deoxygenated blood, containing waste products, is then collected by venules and veins, which merge into the vena cava, emptying it back into the right atrium.
  • Significance: The primary significance of double circulation is the complete separation of oxygenated and deoxygenated blood. This ensures that the body’s tissues receive a highly efficient supply of oxygen, which is necessary to support the high metabolic rate of warm-blooded animals like mammals and birds. This separation prevents mixing and maintains a high-pressure gradient in the systemic circuit, allowing for rapid delivery of blood to distant body parts.

4. Detail the sequence of events that occur during one cardiac cycle, including the roles of the heart chambers, valves, and the associated heart sounds.

• Answer: A cardiac cycle is the sequence of events that occurs from the beginning of one heartbeat to the beginning of the next, lasting about 0.8 seconds. It consists of the systole (contraction) and diastole (relaxation) of the atria and ventricles.
  1. Joint Diastole: To begin, all four chambers are in a relaxed state. The tricuspid and bicuspid valves are open, allowing blood to flow passively from the vena cava and pulmonary veins into the right and left ventricles, respectively. The semilunar valves are closed.
  2. Atrial Systole: The sino-atrial node (SAN) generates an action potential, causing both atria to contract simultaneously. This pushes the remaining blood (about 30% more) into the ventricles.
  3. Ventricular Systole: The action potential is conducted to the ventricles via the AVN and AV bundle, causing them to contract. The ventricular pressure rises sharply, forcing the tricuspid and bicuspid valves to shut. This closure prevents backflow into the atria and produces the first heart sound, “lub”.
  4. Ejection: As ventricular pressure continues to increase and exceeds the pressure in the pulmonary artery and aorta, the semilunar valves are forced open. Blood is then ejected from the right ventricle into the pulmonary artery and from the left ventricle into the aorta.
  5. Ventricular Diastole: Following ejection, the ventricles relax, and ventricular pressure falls. When the pressure drops below that in the aorta and pulmonary artery, blood attempts to flow back, causing the semilunar valves to snap shut. This closure produces the second heart sound, “dub”. As the ventricles continue to relax and their pressure falls below atrial pressure, the tricuspid and bicuspid valves open, allowing the ventricles to start filling again, thus beginning a new cycle.

5. Describe the structure and function of the heart’s conduction system. Why is this system crucial for coordinated cardiac function?

• Answer: The heart’s conduction system consists of specialised cardiac musculature, called nodal tissue, which is autoexcitable and responsible for generating and transmitting the electrical impulses that cause the heart to beat. The key components are the Sino-atrial Node (SAN), Atrio-ventricular Node (AVN), Atrio-ventricular Bundle (AV Bundle or Bundle of His), and Purkinje fibres.
  • Sino-atrial Node (SAN): Located in the upper right corner of the right atrium, the SAN is the heart’s natural pacemaker. It has the highest rate of intrinsic depolarisation, generating 70-75 action potentials per minute, which initiates the normal heartbeat.
  • Atrio-ventricular Node (AVN): Located in the lower-left corner of the right atrium, the AVN receives the impulse from the SAN. It slightly delays the impulse before passing it on, which allows the atria to finish contracting before the ventricles start.
  • AV Bundle (Bundle of His) and Purkinje Fibres: From the AVN, the impulse travels down the AV bundle, which runs through the inter-ventricular septum. The bundle then divides into right and left branches that spread out into a network of fine Purkinje fibres throughout the ventricular walls. These fibres rapidly conduct the impulse to the entire ventricular musculature.
  • Importance of Coordination: This system is crucial because it ensures the heart contracts in a coordinated and efficient manner. The impulse originating in the SAN first causes both atria to contract together. The delay at the AVN ensures the ventricles are full of blood before they are signaled to contract. The rapid conduction through the Purkinje fibres ensures that both ventricles contract simultaneously from the bottom up, effectively ejecting blood into the aorta and pulmonary artery. Without this precise, sequential timing, the heart’s pumping action would be inefficient, and blood circulation would be compromised.

6. Explain the concept of ABO and Rh blood group incompatibility, providing specific examples of potential problems during transfusion and pregnancy.

• Answer: Blood group incompatibility occurs when the immune system of a recipient reacts to antigens present on the RBCs of donated blood, or in the case of pregnancy, on the RBCs of a foetus. The two most clinically significant systems are ABO and Rh.
  • ABO Incompatibility: This system is based on A and B antigens on RBCs and corresponding anti-A and anti-B antibodies in the plasma. A problem arises if, for example, a person with blood group A (who has anti-B antibodies) receives blood from a donor with group B. The recipient’s anti-B antibodies would attack the donor’s B-antigen-bearing RBCs, causing them to clump together (agglutinate) and be destroyed (haemolysis). This can lead to blocked blood vessels, kidney failure, and death. This is why cross-matching is essential before any transfusion.
  • Rh Incompatibility: This is based on the presence (Rh+ve) or absence (Rh-ve) of the Rh antigen. An Rh-ve person does not naturally have anti-Rh antibodies but will produce them if exposed to Rh+ve blood.
    • Transfusion: If an Rh-ve person receives Rh+ve blood, they will become sensitised and develop anti-Rh antibodies. A subsequent transfusion with Rh+ve blood would then trigger a severe immune reaction.
    • Pregnancy: A major issue is erythroblastosis foetalis. If an Rh-ve mother conceives an Rh+ve child, she can become sensitised to the Rh antigen during the first childbirth when foetal blood enters her circulation. In subsequent pregnancies with an Rh+ve foetus, her anti-Rh antibodies can cross the placenta and attack the foetal RBCs, leading to life-threatening anaemia and jaundice for the newborn. This is preventable by administering anti-Rh antibodies to the mother after the first delivery.

7. Compare and contrast the composition and functions of blood and lymph.

• Answer: Blood and lymph are both crucial body fluids involved in transport, but they differ significantly in composition and primary roles.
  • Composition:
    • Blood is a complex fluid connective tissue composed of plasma (55%) and formed elements (45%). Plasma is mostly water but rich in proteins (albumin, globulin, fibrinogen), glucose, minerals, and hormones. The formed elements include red blood cells (RBCs), white blood cells (WBCs), and platelets. The presence of haemoglobin in RBCs makes blood red.
    • Lymph is essentially filtered blood plasma. It is a colourless fluid formed when interstitial fluid is collected by lymphatic vessels. Its composition is similar to blood plasma but with a much lower concentration of large proteins, as they mostly remain within the blood capillaries. Lymph lacks RBCs and platelets but contains a high concentration of specialised white blood cells, particularly lymphocytes.
  • Functions:
    • Blood’s primary functions include the transport of respiratory gases (O₂ and CO₂) via RBCs, distribution of nutrients and hormones, regulation of body temperature and pH, and defence against pathogens via WBCs. It is also responsible for clotting to prevent blood loss, thanks to platelets and fibrinogen.
    • Lymph’s primary functions are related to immunity and fluid balance. The lymphocytes within the lymph are central to the body’s immune responses. The lymphatic system collects excess tissue fluid and returns it to the bloodstream, maintaining fluid balance and preventing edema. Lymph also plays a specialised role in transporting absorbed fats from the intestine to the blood via lacteals.

8. What is an Electrocardiogram (ECG)? Draw and label a standard ECG and explain what each wave or complex represents in terms of the heart’s electrical activity.

• Answer: An Electrocardiogram (ECG) is a non-invasive diagnostic tool that produces a graphical representation of the electrical activity of the heart during a cardiac cycle. By placing electrodes on the skin, it records the electrical impulses generated by the heart’s nodal tissue as they spread through the cardiac muscle. (A student would typically draw the ECG diagram here.) A standard ECG trace consists of several distinct waves labelled P, Q, R, S, and T.
  • P-wave: This is the first small, upward wave. It represents the electrical excitation, or depolarisation of the atria. This electrical event triggers the contraction of both atria (atrial systole).
  • QRS Complex: This is the largest and most prominent part of the ECG. It represents the depolarisation of the ventricles. The contraction of the ventricles (ventricular systole) begins shortly after the Q wave. The large size of this complex is due to the greater muscle mass of the ventricles compared to the atria.
  • T-wave: This is a broader, upward wave that follows the QRS complex. It represents the repolarisation of the ventricles, which is the return of the ventricular muscle to a resting state. The end of the T-wave marks the end of ventricular systole. The regular shape and timing of these waves are consistent in healthy individuals. Any significant deviation can indicate a possible abnormality or cardiac disease, making the ECG a tool of great clinical importance for assessing heart function and health.

9. Describe four major disorders of the human circulatory system, explaining their causes and effects on the body.

• Answer: The human circulatory system is susceptible to several disorders that can significantly impact health. Four major ones are:
  1. High Blood Pressure (Hypertension): This is a condition where the pressure of blood against the artery walls is consistently elevated. Normal blood pressure is around 120/80 mmHg; hypertension is diagnosed with repeated readings of 140/90 mmHg or higher. It is often caused by lifestyle factors like diet and stress, but can also have genetic or other medical causes. High blood pressure forces the heart to work harder and can damage arteries, leading to serious complications like heart disease, heart failure, stroke, and kidney disease.
  2. Coronary Artery Disease (CAD): Also known as atherosclerosis, CAD affects the arteries that supply blood to the heart muscle. It is caused by the buildup of plaque—deposits of calcium, fat, cholesterol, and fibrous tissues—on the inner walls of the coronary arteries. This plaque narrows the lumen of the arteries, restricting blood flow to the heart. This can lead to chest pain (angina) and, if a plaque ruptures and forms a clot, can cause a heart attack by completely blocking blood flow.
  3. Angina (Angina Pectoris): This is not a disease itself, but a symptom of an underlying heart problem, usually CAD. It is characterised by acute chest pain, pressure, or discomfort that occurs when the heart muscle doesn’t get as much oxygen-rich blood as it needs. The pain is typically triggered by physical exertion or emotional stress and subsides with rest. It is a warning sign that the blood flow through the coronary arteries is compromised.
  4. Heart Failure: This is a chronic condition where the heart muscle is unable to pump blood effectively enough to meet the body’s needs for blood and oxygen. It is often caused by conditions that have damaged or weakened the heart, such as CAD or hypertension. It is not the same as a heart attack or cardiac arrest. A common symptom is congestion of the lungs (leading to shortness of breath), which is why it is sometimes called congestive heart failure. Other effects include fatigue, weakness, and swelling in the legs and abdomen.

10. Explain how cardiac activity is regulated, discussing both the intrinsic mechanism and the roles of the nervous and endocrine systems.

• Answer: Cardiac activity is regulated through a combination of intrinsic control and extrinsic modulation by the nervous and endocrine systems.
  • Intrinsic Regulation: The heart is described as myogenic, meaning its beat originates within the heart muscle itself, independent of external nerves. This is due to the specialised nodal tissue, particularly the sino-atrial node (SAN) or pacemaker. The SAN spontaneously generates electrical impulses at a regular rate (70-75 times per minute), which then spread through the heart’s conduction system to cause a coordinated contraction. This intrinsic system maintains the basic rhythm of the heart.
  • Extrinsic Regulation (Nervous System): While the heart can beat on its own, its rate and force can be modified to meet the body’s changing needs. A special neural centre in the medulla oblongata of the brainstem regulates cardiac function through the Autonomic Nervous System (ANS).
    • Sympathetic Nerves: These nerves can increase the rate of heartbeat, strengthen ventricular contractions, and thereby increase cardiac output. This is the “fight-or-flight” response, preparing the body for action.
    • Parasympathetic Nerves: These nerves (primarily the vagus nerve) have the opposite effect. They decrease the rate of heartbeat and the speed of impulse conduction, thus reducing cardiac output. This is the “rest-and-digest” response.
  • Extrinsic Regulation (Endocrine System): Hormones can also influence cardiac activity. Adrenal medullary hormones, such as adrenaline (epinephrine) and noradrenaline (norepinephrine), are released during stress or excitement. These hormones act similarly to the sympathetic nervous system, increasing heart rate and contractility, which in turn increases cardiac output.

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Comprehensive Glossary

Term	Definition
Agranulocytes	A category of white blood cells (leucocytes) that includes lymphocytes and monocytes.
Albumins	A major protein in blood plasma that helps in maintaining osmotic balance.
Angina Pectoris	A symptom of acute chest pain that appears when not enough oxygen is reaching the heart muscle.
Antibody	A major protein in blood plasma primarily involved in the defence mechanisms of the body.
Antigen	Chemicals (often on the surface of cells like RBCs) that can induce an immune response.
Aorta	The largest artery in the body, which carries oxygenated blood from the left ventricle to the systemic circulation.
Atherosclerosis	A condition, often referred to as Coronary Artery Disease (CAD), where deposits of calcium, fat, and other substances build up in the arteries, narrowing them.
Atrium (pl. Atria)	The two relatively small upper chambers of the heart that receive blood.
Atrio-ventricular Node (AVN)	A mass of nodal tissue in the lower left corner of the right atrium that conducts the cardiac impulse from the atria to the ventricles.
Autoexcitable	The ability of a tissue, like the cardiac nodal musculature, to generate action potentials without any external stimuli.
Basophils	A type of granulocyte (WBC) that secretes histamine, serotonin, and heparin and is involved in inflammatory reactions.
Bicuspid Valve	Also called the mitral valve; a valve with two cusps that guards the opening between the left atrium and the left ventricle.
Blood	A special connective tissue consisting of a fluid matrix (plasma) and formed elements, used to transport materials in higher organisms.
Blood Groups	Classifications of blood (e.g., ABO, Rh) based on the presence or absence of specific antigens on the surface of red blood cells.
Bundle of His	Also called the Atrio-ventricular (AV) bundle; a bundle of nodal fibres that transmits the cardiac impulse from the AVN to the ventricles.
Cardiac Arrest	A condition when the heart stops beating.
Cardiac Cycle	The sequential event in the heart which is cyclically repeated, consisting of the systole and diastole of both atria and ventricles.
Cardiac Output	The volume of blood pumped out by each ventricle per minute; calculated as stroke volume multiplied by heart rate (approx. 5 litres/min).
Closed Circulatory System	A system in which blood pumped by the heart is always circulated through a closed network of blood vessels (e.g., in chordates).
Coagulation	The process of blood clotting, which is a mechanism to prevent excessive loss of blood from the body.
Diastole	The phase of the cardiac cycle when a heart chamber relaxes and fills with blood.
Double Circulation	A circulatory system with two distinct pathways: pulmonary and systemic circulation, ensuring complete separation of oxygenated and deoxygenated blood.
Electrocardiogram (ECG)	A graphical representation of the electrical activity of the heart during a cardiac cycle.
Eosinophils	A type of granulocyte (WBC) that resists infections and is associated with allergic reactions.
Erythroblastosis Foetalis	A condition where a pregnant Rh-negative mother’s antibodies destroy the red blood cells of her Rh-positive foetus.
Erythrocytes	Red blood cells (RBCs); the most abundant cells in blood, responsible for transporting respiratory gases.
Fibrin	A network of insoluble protein threads formed from fibrinogen during blood clotting, which traps formed elements to create a clot.
Fibrinogen	A major protein in blood plasma that is converted into fibrin during blood coagulation.
Formed Elements	Erythrocytes, leucocytes, and platelets, which collectively constitute about 45% of the blood.
Globulins	White blood cells (WBCs); colourless, nucleated cells involved in the body’s defense mechanisms.
Granulocytes	A category of white blood cells (leucocytes) that includes neutrophils, eosinophils, and basophils.
Haemoglobin	A red-colored, iron-containing complex protein in red blood cells that transports oxygen and carbon dioxide.
Heart Attack	A condition where the heart muscle is suddenly damaged by an inadequate blood supply.
Heart Failure	A state where the heart is not pumping blood effectively enough to meet the needs of the body.
Hepatic Portal System	A unique vascular connection where the hepatic portal vein carries blood from the intestine to the liver.
Hypertension	High blood pressure; a condition where blood pressure is consistently higher than the normal 120/80 mmHg.
Leucocytes	A term for a heart whose activities are auto-regulated by specialised muscles (nodal tissue) within the heart itself.
Lymph	A colourless fluid present in the lymphatic system, containing specialised lymphocytes, that is responsible for immune responses and fluid balance.
Lymphocytes	A type of agranulocyte (WBC) of two major types, B and T forms, responsible for the immune responses of the body.
Megakaryocytes	Special cells in the bone marrow from which platelets are produced as fragments.
Monocytes	A type of agranulocyte (WBC) that is phagocytic and destroys foreign organisms.
Myogenic	A term for a heart whose activities are auto-regulated by specialized muscles (nodal tissue) within the heart itself.
Neutrophils	The most abundant type of granulocyte (WBC), which is phagocytic and destroys foreign organisms.
Open Circulatory System	A system where blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses (e.g., in arthropods).
Pacemaker	A name given to the sino-atrial node (SAN) because it initiates and maintains the rhythmic contractile activity of the heart.
Pericardium	A double-walled membranous bag that encloses the heart and contains pericardial fluid.
Plasma	The straw-colored, viscous fluid matrix of blood, constituting about 55% of its volume.
Platelets	Also called thrombocytes; cell fragments involved in the coagulation or clotting of blood.
Pulmonary Circulation	The circulatory pathway that carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.
Purkinje Fibres	Minute nodal fibres distributed throughout the ventricular musculature that conduct the cardiac impulse.
QRS Complex	A component of an ECG that represents the depolarisation of the ventricles, initiating their contraction.
P-wave	A component of an ECG that represents the electrical excitation (depolarisation) of the atria.
Rh Factor	An antigen found on the surface of RBCs in the majority of humans.
Semilunar Valves	Valves that guard the openings of the right and left ventricles into the pulmonary artery and the aorta, respectively.
Serum	Plasma from which the clotting factors have been removed.
Sino-atrial Node (SAN)	A patch of nodal tissue in the right atrium that functions as the heart’s pacemaker.
Spleen	An organ where old red blood cells are destroyed; also known as the “graveyard of RBCs.”
Stroke Volume	The volume of blood pumped out by each ventricle during a cardiac cycle (approx. 70 mL).
Systemic Circulation	The circulatory pathway that carries oxygenated blood from the left ventricle to all body tissues and returns deoxygenated blood to the right atrium.
Systole	The phase of the cardiac cycle when a heart chamber contracts and pumps blood.
Thrombin	An enzyme that converts fibrinogen into fibrin during blood clotting.
Thrombocytes	Another name for platelets.
Thrombokinase	An enzyme complex required for the conversion of prothrombin to thrombin.
Tricuspid Valve	A valve with three muscular flaps that guards the opening between the right atrium and the right ventricle.
T-wave	A component of an ECG that represents the repolarisation of the ventricles.
Universal Donor	An individual with blood group ‘O’, whose blood can be donated to persons with any other blood group.
Universal Recipient	An individual with blood group ‘AB’, who can accept blood from persons with any other blood group.
Ventricle	The two larger, lower chambers of the heart that pump blood out to the lungs and the rest of the body.