Body Fluids and Circulation
Welcome to HSLC Guru! In this lesson we cover ASSEB Class 11 Biology Chapter 18 — Body Fluids and Circulation. You will learn about the composition of blood and lymph, ABO and Rh blood grouping, the mechanism of coagulation, the structure and working of the human heart, the cardiac cycle, ECG, double circulation, regulation of cardiac activity and major circulatory disorders. The chapter is presented with summary paragraphs, exam-oriented question answers, MCQs, fill in the blanks, true or false statements and a glossary table — all aligned strictly with the ASSEB syllabus.
Summary
Body fluids transport nutrients, gases, hormones and wastes between cells. The two main circulating fluids in humans are blood and lymph. Blood is a special connective tissue made of fluid plasma (about 55%) and formed elements (about 45%). Plasma contains water (90–92%), proteins (fibrinogen, globulins, albumins), minerals, glucose, urea and hormones. Formed elements include erythrocytes (RBCs) which contain haemoglobin and carry oxygen, leucocytes (WBCs — neutrophils, eosinophils, basophils, lymphocytes, monocytes) which fight infection, and thrombocytes (platelets) which help in clotting. Lymph is a colourless fluid formed from the interstitial fluid; it lacks RBCs but contains lymphocytes and drains back into the blood through lymphatic vessels.
Blood is grouped on the basis of surface antigens. The ABO system recognises four groups — A, B, AB and O — based on the presence of antigens A and B on RBCs and corresponding antibodies in plasma. The Rh factor is another antigen; people having it are Rh⁺ and those without it are Rh⁻. An Rh⁻ mother carrying an Rh⁺ foetus may develop anti-Rh antibodies leading to erythroblastosis foetalis. Coagulation of blood is a cascade in which injured tissue and platelets release thromboplastin which, with Ca²⁺, converts prothrombin to thrombin; thrombin then converts soluble fibrinogen into insoluble fibrin threads that form the clot. Circulatory pathways are of two types — open circulation (as in arthropods, where blood flows through sinuses) and closed circulation (as in annelids and vertebrates, where blood flows through definite vessels) — the latter being more efficient.
The human heart is a four-chambered muscular pump enclosed in a pericardial sac. It has two atria and two ventricles separated by septa. Valves prevent backflow — the tricuspid valve guards the right atrio-ventricular opening, the bicuspid (mitral) valve guards the left, and semilunar valves guard the openings of the pulmonary artery and the aorta. The conducting system consists of the SA node (pacemaker) in the right atrium, the AV node, the bundle of His and Purkinje fibres. The cardiac cycle consists of atrial systole, ventricular systole and joint diastole, taking about 0.8 seconds at 72 beats per minute. Cardiac output = stroke volume × heart rate (≈ 70 mL × 72 = 5 L/min). The ECG records electrical activity of the heart with P, QRS and T waves representing atrial depolarisation, ventricular depolarisation and ventricular repolarisation respectively.
Humans show double circulation — pulmonary circulation between heart and lungs and systemic circulation between heart and body — so oxygenated and deoxygenated blood do not mix. Cardiac activity is regulated by the medulla oblongata through the autonomic nervous system: the sympathetic nerves increase heart rate while the parasympathetic (vagus) decreases it; adrenaline also stimulates the heart. Major disorders include hypertension (BP higher than 140/90 mm Hg), coronary artery disease (CAD) due to narrowing of coronary vessels by plaques (atherosclerosis), angina pectoris (chest pain due to inadequate oxygen supply to the myocardium), heart failure and myocardial infarction or heart attack caused by sudden blockage of a coronary artery.
1-Mark Questions
Q1. What is plasma?
Answer: Plasma is the straw-coloured fluid component of blood that constitutes about 55% of its volume.
Q2. Name the pacemaker of the human heart.
Answer: The Sino-Atrial node (SA node) located in the right atrium is the pacemaker of the human heart.
Q3. Which protein helps in oxygen transport in RBCs?
Answer: Haemoglobin helps in transport of oxygen in red blood cells.
Q4. Name the universal blood donor and universal recipient.
Answer: Group O is the universal donor and group AB is the universal recipient.
Q5. What is the normal blood pressure of a healthy adult?
Answer: Approximately 120/80 mm Hg (systolic/diastolic).
Q6. What does the QRS complex of an ECG represent?
Answer: The QRS complex represents the depolarisation of the ventricles.
Q7. Define cardiac output.
Answer: Cardiac output is the volume of blood pumped by each ventricle per minute, normally about 5 litres in a healthy adult at rest.
Q8. Name the valve between the left atrium and left ventricle.
Answer: The bicuspid valve, also known as the mitral valve.
Q9. What is lymph?
Answer: Lymph is a colourless tissue fluid containing lymphocytes that drains back to the blood through lymphatic vessels.
Q10. Name the largest WBC in human blood.
Answer: Monocyte is the largest WBC in human blood.
2–3 Mark Questions
Q1. Differentiate between open and closed circulatory systems.
Answer: In an open circulatory system blood is pumped from the heart into open spaces called sinuses where it bathes the tissues directly (e.g. arthropods, molluscs). In a closed circulatory system blood flows through a continuous network of arteries, veins and capillaries (e.g. annelids and vertebrates), allowing precise and faster regulation of flow and blood pressure.
Q2. Write a short note on the formed elements of blood.
Answer: Formed elements are the cellular components of blood — Erythrocytes (RBCs) are biconcave, anucleated cells (5 million/mm³) with haemoglobin for O₂ transport. Leucocytes (WBCs, 6000–8000/mm³) are nucleated and protect the body against infection — neutrophils, eosinophils, basophils (granulocytes), lymphocytes and monocytes (agranulocytes). Platelets (thrombocytes, 1.5–3.5 lakh/mm³) release factors needed for blood clotting.
Q3. Explain ABO blood grouping.
Answer: Karl Landsteiner classified blood into four groups — A, B, AB and O — based on the presence of antigens A and B on RBC surface and natural antibodies in plasma. Group A has antigen A and antibody b; group B has antigen B and antibody a; group AB has both antigens and no antibody (universal recipient); group O has no antigen and both antibodies (universal donor).
Q4. What is erythroblastosis foetalis?
Answer: When an Rh⁻ mother carries an Rh⁺ foetus, during her second pregnancy maternal anti-Rh antibodies cross the placenta and destroy foetal RBCs causing severe anaemia and jaundice in the foetus or new-born — a condition called erythroblastosis foetalis.
Q5. Briefly describe the cardiac cycle.
Answer: The cardiac cycle is the sequence of events occurring in one heart beat. It consists of atrial systole (0.1 s) when atria contract and pump blood into ventricles, ventricular systole (0.3 s) when ventricles contract pushing blood into the aorta and pulmonary artery, and joint diastole (0.4 s) when the whole heart relaxes. One complete cycle takes about 0.8 seconds.
Q6. What is double circulation? Why is it important?
Answer: Double circulation means blood passes through the heart twice in a complete circuit. The pulmonary circulation carries deoxygenated blood from heart to lungs and oxygenated blood back. The systemic circulation supplies oxygenated blood from heart to body and returns deoxygenated blood. It prevents mixing of oxygenated and deoxygenated blood, ensuring efficient oxygen supply to tissues.
5–7 Mark Questions
Q1. Describe the structure of the human heart with reference to its chambers, valves and conducting system.
Answer: The human heart is a hollow, muscular, four-chambered organ situated in the thoracic cavity between the lungs and enclosed by a double-walled pericardium with pericardial fluid that protects it from shock. The wall has three layers — outer epicardium, middle thick muscular myocardium and inner endocardium. The heart has two upper thin-walled atria separated by an interatrial septum and two lower thick-walled ventricles separated by an interventricular septum. The right atrium receives deoxygenated blood through the superior and inferior vena cavae, while the left atrium receives oxygenated blood through pulmonary veins. Valves ensure unidirectional flow — the tricuspid valve guards the right atrio-ventricular opening, the bicuspid (mitral) valve guards the left atrio-ventricular opening, and semilunar valves are present at the base of the pulmonary artery and aorta. The conducting system consists of the SA node (pacemaker, situated in the right atrium near the opening of superior vena cava), the AV node lying in the lower right corner of the right atrium, the bundle of His arising from the AV node and dividing into right and left bundle branches in the interventricular septum, and Purkinje fibres which spread throughout the ventricular musculature transmitting impulses for ventricular contraction.
Q2. Describe the mechanism of blood coagulation.
Answer: Blood coagulation is the process by which liquid blood forms a solid clot at the site of injury to prevent excessive blood loss. When a blood vessel is injured, platelets aggregate and release thromboplastin (also produced by injured tissue). In the presence of calcium ions (Ca²⁺) and other clotting factors, thromboplastin converts the inactive plasma protein prothrombin into active thrombin. Thrombin then catalyses the conversion of soluble fibrinogen present in plasma into insoluble fibrin threads. These fibrin threads form a network or mesh which traps blood cells and platelets, producing a clot. The clot seals the wound and prevents further blood loss while allowing the underlying tissue to heal. Vitamin K is essential for synthesis of prothrombin in the liver. Defects in clotting cause haemophilia, a hereditary bleeding disorder.
Q3. Explain the events of the cardiac cycle and give the components of an ECG.
Answer: The cardiac cycle is the sequence of mechanical and electrical events that occur during one complete heartbeat. It begins with joint diastole when all four chambers are relaxed and blood flows passively from atria into ventricles through open AV valves. The SA node generates an action potential that depolarises the atria producing atrial systole (0.1 s) which pushes additional blood into the ventricles. The impulse reaches the AV node, then travels through the bundle of His and Purkinje fibres causing ventricular systole (0.3 s); the AV valves close (first heart sound ‘lubb’) and ventricles pump blood into the aorta and pulmonary artery through semilunar valves. The semilunar valves then close (second heart sound ‘dubb’) as ventricles relax during ventricular diastole. The whole cycle lasts about 0.8 seconds at 72 beats/min. An electrocardiogram (ECG) records the electrical activity of the heart and shows three main waves — the P wave indicates atrial depolarisation, the QRS complex indicates ventricular depolarisation, and the T wave indicates ventricular repolarisation. Any deviation in the shape, size or interval of these waves helps diagnose cardiac abnormalities.
Q4. Discuss major disorders of the circulatory system.
Answer: Several disorders affect the human circulatory system. Hypertension (high blood pressure) is a sustained rise of arterial pressure above 140/90 mm Hg; it strains the heart, damages vessels, and increases risk of stroke and kidney failure. Atherosclerosis is the deposition of cholesterol, calcium and fibrous tissue in the inner wall of arteries forming plaques; it narrows the lumen and reduces elasticity. Coronary Artery Disease (CAD), often called atherosclerosis of coronary vessels, restricts oxygen and nutrient supply to cardiac muscle. Angina pectoris is acute chest pain experienced when oxygen supply to the myocardium is inadequate, especially during exertion or stress. Heart attack (myocardial infarction) occurs when a coronary artery is suddenly blocked by a clot or plaque, causing death of cardiac muscle in that region. Heart failure means the heart is unable to pump enough blood to meet body needs and results in pulmonary congestion. Smoking, obesity, lack of exercise, high-fat diet, diabetes and stress are major risk factors; healthy lifestyle, balanced diet and regular exercise prevent these disorders.
Q5. Describe the regulation of cardiac activity.
Answer: Although the heart beat is myogenic (initiated by SA node), its rate and force are regulated by the nervous and endocrine systems to suit the body’s demands. The cardiac centres in the medulla oblongata of the brain coordinate this regulation through the autonomic nervous system. The sympathetic nerves release noradrenaline at the SA node and ventricular muscle, increasing heart rate (tachycardia) and force of contraction during exercise, fear or anger. The parasympathetic nerves, mainly the vagus nerve, release acetylcholine at the SA node decreasing heart rate (bradycardia) during rest. Hormonal regulation involves adrenaline and noradrenaline from the adrenal medulla which stimulate the heart, and thyroxine which increases metabolic rate and thereby heart rate. Stretch receptors (baroreceptors) in the aorta and carotid sinus monitor blood pressure and reflexively adjust heart activity. Thus, cardiac output is precisely matched to body needs.
Multiple Choice Questions (MCQs)
Q1. The pacemaker of the heart is —
(a) AV node (b) SA node (c) Bundle of His (d) Purkinje fibres
Answer: (b) SA node
Q2. Universal blood donor is —
(a) A (b) B (c) AB (d) O
Answer: (d) O
Q3. Normal heart rate of an adult is —
(a) 50 (b) 72 (c) 90 (d) 120 beats/min
Answer: (b) 72 beats/min
Q4. Bicuspid valve lies between —
(a) Right atrium and ventricle (b) Left atrium and ventricle (c) Aorta and ventricle (d) Pulmonary artery and ventricle
Answer: (b) Left atrium and ventricle
Q5. The QRS complex of ECG represents —
(a) Atrial depolarisation (b) Ventricular depolarisation (c) Atrial repolarisation (d) Ventricular repolarisation
Answer: (b) Ventricular depolarisation
Q6. Which of the following is not a granulocyte?
(a) Neutrophil (b) Eosinophil (c) Basophil (d) Lymphocyte
Answer: (d) Lymphocyte
Q7. Soluble plasma protein involved in clotting is —
(a) Albumin (b) Fibrinogen (c) Globulin (d) Haemoglobin
Answer: (b) Fibrinogen
Q8. Cardiac output is —
(a) Stroke volume × heart rate (b) Heart rate − stroke volume (c) Stroke volume / heart rate (d) None
Answer: (a) Stroke volume × heart rate
Q9. The deposition of cholesterol on artery walls is called —
(a) Angina (b) Atherosclerosis (c) Hypertension (d) Anaemia
Answer: (b) Atherosclerosis
Q10. Lymph differs from blood in lacking —
(a) WBC (b) Plasma (c) RBC (d) Platelets only
Answer: (c) RBC
Fill in the Blanks
Q1. The pigment that carries oxygen in RBC is __________.
Answer: haemoglobin
Q2. The pacemaker of the heart is __________.
Answer: SA node
Q3. Insoluble protein that forms the clot is __________.
Answer: fibrin
Q4. The heart sound ‘lubb’ is produced by closure of __________ valves.
Answer: atrio-ventricular (tricuspid and bicuspid)
Q5. Blood pressure higher than 140/90 mm Hg is called __________.
Answer: hypertension
True or False
Q1. Lymph contains RBCs.
Answer: False
Q2. Blood group AB is the universal recipient.
Answer: True
Q3. The right ventricle pumps oxygenated blood to the body.
Answer: False
Q4. Vagus nerve decreases the heart rate.
Answer: True
Q5. The T wave of ECG indicates atrial depolarisation.
Answer: False
Glossary
| Term | Meaning |
|---|---|
| Plasma | Fluid part of blood (about 55%) carrying proteins, salts and nutrients. |
| Erythrocyte | Red blood cell containing haemoglobin for O₂ transport. |
| Leucocyte | White blood cell that defends the body against infection. |
| Thrombocyte | Platelet that helps in blood clotting. |
| Lymph | Colourless tissue fluid containing lymphocytes; lacks RBCs. |
| Antigen | Surface molecule on RBCs that determines blood group. |
| Rh factor | An antigen first found in rhesus monkey; basis of Rh⁺/Rh⁻ grouping. |
| Coagulation | Process of conversion of liquid blood into a clot. |
| Fibrin | Insoluble protein threads that form the blood clot. |
| SA node | Sino-Atrial node — pacemaker of the heart. |
| AV node | Atrio-Ventricular node that delays and relays impulses to ventricles. |
| Bundle of His | Specialised fibres in interventricular septum conducting impulses. |
| Purkinje fibres | Fibres spreading throughout ventricles to trigger contraction. |
| Cardiac cycle | Sequence of events in one heartbeat (~0.8 sec). |
| Cardiac output | Volume of blood pumped per minute by each ventricle (~5 L). |
| ECG | Electrocardiogram — recording of electrical activity of the heart. |
| Double circulation | Two-circuit blood flow — pulmonary and systemic. |
| Hypertension | Persistent high blood pressure (>140/90 mm Hg). |
| Atherosclerosis | Hardening of arteries due to plaque deposition. |
| Angina pectoris | Chest pain due to insufficient O₂ supply to heart muscle. |
| Heart attack | Sudden death of cardiac muscle due to blocked coronary artery. |