CBSE Class 11 Biology Chapter 18 Notes: Body Fluids and Circulation

Chapter Introduction

Body Fluids and Circulation is Chapter 18 in the Human Physiology unit of NCERT Biology for Class 11. All living cells require a constant supply of nutrients and oxygen and removal of metabolic wastes. In complex organisms, specialised body fluids — blood and lymph — perform this vital transport function through the circulatory system.

This chapter covers the composition of blood and its formed elements, the ABO and Rh blood group systems, the mechanism of blood coagulation, the lymphatic system, the structure of the human heart, the cardiac cycle and heart sounds, the electrocardiogram (ECG), the concept of double circulation, the hepatic portal system, and common circulatory disorders. Mastering these topics is essential for both the CBSE board exam and NEET.

Key Definitions

1. Plasma — The fluid (liquid) part of blood constituting about 55% of blood volume; contains water, proteins (albumin, globulin, fibrinogen), and dissolved substances such as glucose, amino acids, urea, and ions.

2. Serum — Plasma from which clotting factors (especially fibrinogen) have been removed; it is the fluid that remains after blood has clotted.

3. Erythrocytes (RBCs) — Red blood cells; biconcave, enucleate (in mammals), contain haemoglobin, and are responsible for oxygen and carbon dioxide transport.

4. Leucocytes (WBCs) — White blood cells; nucleated and colourless, involved in immune defence. Classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

5. Thrombocytes (Platelets) — Cell fragments derived from megakaryocytes; essential for blood clotting and release of thromboplastin at injury sites.

6. Coagulation — The process of blood clot formation that prevents excessive blood loss after injury; involves a cascade of reactions converting fibrinogen to fibrin.

7. Cardiac cycle — One complete heartbeat comprising atrial systole, ventricular systole, and joint diastole; lasts approximately 0.8 seconds at 72 beats per minute.

8. Stroke volume — The volume of blood pumped out by a ventricle during each contraction; approximately 70 mL in a healthy adult at rest.

9. Cardiac output — The volume of blood pumped by each ventricle per minute; calculated as stroke volume multiplied by heart rate (approximately 5 L/min).

10. Double circulation — A circulatory system in which blood passes through the heart twice during one complete cycle — once through pulmonary circulation and once through systemic circulation.

11. Hepatic portal system — A portal circulation in which the hepatic portal vein carries blood from the gastrointestinal tract and spleen to the liver before it enters the systemic circulation.

Detailed Concepts

Blood: Plasma

Blood is a specialised connective tissue consisting of a fluid matrix called plasma and suspended formed elements.

Plasma constitutes about 55% of total blood volume:

• Water: 90–92% of plasma — acts as solvent and transport medium
• Proteins: 6–8% of plasma
• Albumin — maintains osmotic pressure; most abundant plasma protein
• Globulin — involved in immune responses (antibodies are gamma globulins)
• Fibrinogen — essential for blood coagulation; converted to fibrin during clotting
• Other dissolved substances: glucose, amino acids, urea, lipids, enzymes, hormones, vitamins, and ions (Na+, K+, Ca2+, Cl−, HCO3−)

Plasma without fibrinogen is called serum. This distinction is important: serum is used in blood grouping and diagnostic tests because it contains antibodies but no clotting factors.

Formed Elements

Formed elements make up about 45% of blood volume and include erythrocytes, leucocytes, and platelets.

Key points about formed elements:

• RBCs are the most abundant cells in blood. Their biconcave shape increases surface area for gas exchange and allows flexible passage through narrow capillaries.
• RBCs are produced in the red bone marrow and destroyed in the spleen (the "graveyard of RBCs").
• WBCs are classified into granulocytes (neutrophils, eosinophils, basophils — contain granules in cytoplasm) and agranulocytes (lymphocytes, monocytes — lack visible granules).
• Platelets are not true cells; they are fragments of large cells called megakaryocytes.

Blood Groups

The ABO blood group system was discovered by Karl Landsteiner. It is based on the presence or absence of two surface antigens (A and B) on RBCs.

Important principles:

• Antigens on RBCs determine the blood group; antibodies in plasma determine compatibility.
• If incompatible blood is transfused, antibodies in the recipient's plasma will agglutinate (clump) the donor's RBCs — this is a transfusion reaction, which can be fatal.
• Blood group AB has no antibodies, so it can receive blood from any group — hence universal recipient.
• Blood group O has no antigens, so it can donate to any group — hence universal donor. However, group O individuals can only receive from group O because they have both anti-a and anti-b antibodies.

Rh factor:

• Another important blood group system based on the Rh (Rhesus) D antigen.
• Rh+ individuals have the D antigen on their RBCs; Rh− individuals do not.
• Rh antibodies are not naturally present — they are produced only after exposure to Rh+ blood.
• Erythroblastosis foetalis: If an Rh− mother carries an Rh+ foetus, during the first delivery some foetal Rh+ RBCs enter the mother's circulation, causing her to produce anti-Rh antibodies. In a subsequent Rh+ pregnancy, these antibodies cross the placenta and destroy foetal RBCs, causing haemolytic disease of the newborn. This is why Rh factor testing is critical during pregnancy.

Blood Coagulation

Blood coagulation (clotting) is a defence mechanism that prevents excessive blood loss after injury. It involves a cascade of enzymatic reactions:

1. Injury occurs → platelets accumulate at the site and release thromboplastin (thrombokinase). 2. Thromboplastin, along with calcium ions (Ca2+), converts inactive prothrombin into active thrombin. 3. Thrombin acts on soluble fibrinogen and converts it into insoluble fibrin threads. 4. Fibrin threads form a mesh that traps RBCs, WBCs, and platelets → forming the clot. 5. The clot retracts, pulling the edges of the wound together and sealing it.

Summary of the coagulation cascade:

Vitamin K is essential for the synthesis of prothrombin and several clotting factors in the liver. Deficiency of vitamin K impairs clotting.

Anticoagulants prevent clotting:

• Heparin — natural anticoagulant released by basophils and liver; prevents thrombin formation
• Sodium citrate — binds Ca2+, removing it from the clotting cascade
• EDTA — chelates calcium ions
• Warfarin — inhibits vitamin K-dependent synthesis of clotting factors

Lymph

Lymph is a colourless fluid that circulates through the lymphatic system. It is similar to blood plasma but differs in important ways:

• Lymph contains no RBCs, no platelets, and fewer proteins (large plasma proteins like fibrinogen are absent)
• It contains WBCs, mainly lymphocytes
• Lymph is formed when interstitial fluid (tissue fluid) enters lymph capillaries

Functions of lymph:

1. Returns interstitial fluid and proteins that have leaked from blood capillaries back to the circulatory system — this maintains blood volume and pressure. 2. Transports digested fats from the small intestine — fat is absorbed into lacteals (lymph capillaries in the villi) and transported through the lymphatic system before entering the bloodstream. 3. Provides immune defence — lymph nodes filter lymph and contain lymphocytes that destroy pathogens.

The lymphatic system also includes lymph nodes, the spleen, the thymus, and tonsils — all important for immunity.

Circulatory Pathways

Animals have two main types of circulatory systems:

In open circulation, blood (haemolymph) flows through body cavities and directly bathes tissues. In closed circulation, blood flows entirely within vessels, and exchange with tissues occurs across capillary walls by diffusion.

Humans have a closed circulatory system with a four-chambered heart that supports double circulation.

Human Heart

The human heart is a muscular organ roughly the size of a fist, located in the thoracic cavity between the lungs, slightly left of the midline.

Structure:

• 4 chambers: 2 atria (upper, receiving chambers) + 2 ventricles (lower, pumping chambers)
• Wall thickness: Left ventricle has the thickest wall (pumps blood to the entire body against high resistance); right ventricle is thinner (pumps only to the lungs, which are nearby); atria have the thinnest walls (they only push blood into ventricles)
• Septum: Inter-atrial and inter-ventricular septa separate oxygenated and deoxygenated blood

Valves:

Other features:

• Pericardium: Double-walled fibrous sac surrounding the heart; contains pericardial fluid that reduces friction during heartbeat
• Coronary arteries: Branch from the aorta and supply blood to the heart muscle itself
• Nodal tissue: Sino-atrial (SA) node — the pacemaker of the heart, initiates each cardiac cycle; Atrio-ventricular (AV) node — relays signals to ventricles via the bundle of His and Purkinje fibres

Cardiac Cycle

The cardiac cycle is the sequence of events during one complete heartbeat. At a normal heart rate of 72 beats per minute, one cardiac cycle takes approximately 0.8 seconds.

Key calculations:

• Stroke volume = volume of blood pumped per beat ≈ 70 mL
• Cardiac output = Stroke volume × Heart rate = 70 mL × 72 bpm = 5040 mL/min ≈ 5 L/min
• During vigorous exercise, cardiac output can increase to 20–25 L/min

Heart sounds:

• S1 ("lub"): Caused by closure of AV valves (bicuspid and tricuspid) at the start of ventricular systole
• S2 ("dub"): Caused by closure of semilunar valves (aortic and pulmonary) at the start of ventricular diastole
• A third heart sound (S3) is sometimes heard in young adults and is usually benign

ECG (Electrocardiogram)

An ECG is a graphical recording of the electrical activity of the heart obtained by placing electrodes on the body surface. Each heartbeat produces a characteristic wave pattern.

Important intervals:

• PR interval: Time from the beginning of atrial depolarisation to the beginning of ventricular depolarisation; normally about 0.12–0.16 seconds. Prolonged PR interval may indicate AV block.
• ST segment: The period between ventricular depolarisation and repolarisation; should be at baseline. Elevation or depression of the ST segment indicates myocardial injury or ischaemia.

Clinical significance of ECG abnormalities:

• Enlarged P wave → atrial enlargement (e.g., in mitral stenosis)
• Widened QRS complex → ventricular conduction delay (e.g., bundle branch block)
• Inverted T wave → myocardial ischaemia
• Elevated ST segment → acute myocardial infarction
• Irregular R-R intervals → arrhythmia

Double Circulation

Double circulation means that blood passes through the heart twice during one complete circuit of the body. It consists of two loops:

Hepatic Portal System

The hepatic portal system is a unique circulation in which blood from the gastrointestinal tract passes through the liver before entering the systemic circulation.

Pathway:

Functions and significance:

1. Nutrient processing: The liver receives blood rich in absorbed nutrients (glucose, amino acids, lipids) from the intestines. It converts excess glucose to glycogen (glycogenesis), synthesises plasma proteins, and processes amino acids. 2. Detoxification: The liver detoxifies harmful substances absorbed from the gut (alcohol, drugs, ammonia → urea) before they reach the systemic circulation. 3. Storage: The liver stores glycogen, vitamins (A, D, B12), and iron. 4. Defence: Kupffer cells in the liver phagocytose bacteria and debris from gut blood.

This portal system ensures that blood from the digestive tract is "screened" by the liver — a critical protective and metabolic function.

Disorders of the Circulatory System

Important Diagrams

The following diagrams are essential for the CBSE exam. Practise drawing and labelling them:

1. Human heart (longitudinal section) — Show all 4 chambers (right and left atria, right and left ventricles), 4 valves (bicuspid, tricuspid, pulmonary semilunar, aortic semilunar), major blood vessels (superior and inferior vena cava, pulmonary artery, pulmonary veins, aorta), and the septa.

2. Cardiac cycle flow chart — Show the three phases (atrial systole → ventricular systole → joint diastole) with events, valve positions, and heart sounds at each stage.

3. ECG tracing — Draw the P wave, QRS complex, and T wave with correct relative sizes and positions. Label each wave and indicate what it represents.

4. Double circulation diagram — Draw two loops: the pulmonary loop (right ventricle → lungs → left atrium) and the systemic loop (left ventricle → body → right atrium). Use colour coding (blue for deoxygenated, red for oxygenated).

5. Blood coagulation cascade — Show the stepwise pathway: injury → thromboplastin release → prothrombin → thrombin → fibrinogen → fibrin → clot formation.

Common Mistakes

1. Confusing serum with plasma — Serum is plasma WITHOUT fibrinogen. Plasma still has all clotting factors; serum does not. If asked for the difference, always mention fibrinogen.

2. Saying blood group O is universal recipient — O is the universal donor (no antigens on RBCs). AB is the universal recipient (no antibodies in plasma). Do not mix these up.

3. Confusing bicuspid and tricuspid valves — Bicuspid (mitral) valve is on the left side (between left atrium and left ventricle). Tricuspid valve is on the right side (between right atrium and right ventricle). Remember: "tri" = right (both have the letter "r").

4. Forgetting that joint diastole is the longest phase — Joint diastole occupies 0.4 seconds, which is the longest single phase of the cardiac cycle (0.4 > 0.3 > 0.1). Many students incorrectly write ventricular systole as the longest.

5. Writing that the left ventricle has the thinnest wall — The left ventricle has the thickest wall because it must pump blood to the entire body against high resistance. The atria have the thinnest walls.

6. Confusing pulmonary artery and pulmonary vein blood content — The pulmonary artery carries deoxygenated blood (away from heart to lungs); the pulmonary vein carries oxygenated blood (from lungs to heart). These are exceptions to the general rule that arteries carry oxygenated blood.

7. Forgetting vitamin K in the coagulation cascade — Vitamin K is essential for the synthesis of prothrombin and several clotting factors in the liver. Without it, clotting is impaired. Always mention it when writing about coagulation.

Exam Tips

1. Draw the human heart diagram — This is a frequently asked 5-mark question. Label all 4 chambers, 4 valves, and the major blood vessels (vena cava, pulmonary artery, pulmonary veins, aorta). Show the direction of blood flow with arrows.

2. ABO blood groups — Always mention both antigens on RBCs AND antibodies in plasma for each blood group. A complete answer includes the donation and receiving compatibility.

3. Cardiac cycle — Write the total time (0.8 s) and break it down clearly: atrial systole (0.1 s) + ventricular systole (0.3 s) + joint diastole (0.4 s). Mention heart sounds and which valves produce them.

4. ECG — Use the mnemonic: P (P-wave = P-atrial depolarisation), QRS (ventricular depolarisation), T (ventricular repolarisation). Remember that atrial repolarisation is hidden in the QRS complex.

5. Double circulation — Explain BOTH loops with complete start and end points. Use the format: start chamber → vessel → organ → vessel → end chamber.

6. Coagulation — Memorise the key sequence: thromboplastin → prothrombin → thrombin → fibrinogen → fibrin. Also mention the role of Ca2+ and vitamin K.

7. Rh factor — Explain erythroblastosis foetalis as a second-pregnancy problem. In the first pregnancy, the mother is sensitised; in the second, her anti-Rh antibodies attack foetal RBCs. This nuance is crucial for full marks.

Quick Revision

• Blood: Plasma (55%) + Formed elements (45%) — RBCs, WBCs, platelets
• Plasma proteins: Albumin (osmotic pressure), Globulin (immunity), Fibrinogen (clotting)
• ABO blood groups: A (A antigen, anti-b), B (B antigen, anti-a), AB (both antigens, no antibodies — universal recipient), O (no antigens, both antibodies — universal donor)
• Rh factor: Rh+ (D antigen present), Rh− (D antigen absent); erythroblastosis foetalis occurs in second pregnancy of Rh− mother with Rh+ foetus
• Coagulation cascade: Thromboplastin → Prothrombin → Thrombin → Fibrinogen → Fibrin; Ca2+ and vitamin K are essential
• Lymph: Returns interstitial fluid to blood; transports fats via lacteals; provides immune defence
• Heart: 4 chambers, 4 valves (bicuspid left, tricuspid right, 2 semilunar); left ventricle has thickest wall
• Cardiac cycle: 0.8 s total — atrial systole 0.1 s, ventricular systole 0.3 s, joint diastole 0.4 s
• Cardiac output: ~5 L/min (stroke volume 70 mL × heart rate 72 bpm)
• ECG: P wave (atrial depolarisation), QRS complex (ventricular depolarisation), T wave (ventricular repolarisation)
• Double circulation: Pulmonary loop (right ventricle → lungs → left atrium) + Systemic loop (left ventricle → body → right atrium)
• Hepatic portal system: Blood from GI tract → liver (via portal vein) → systemic circulation; enables nutrient processing and detoxification
• Disorders: Hypertension (>140/90 mm Hg), CAD (atherosclerosis), angina, heart failure, myocardial infarction