Revision: Class 11 >> Body Fluids and Circulation NEET (UG) Body Fluids and Circulation

A closed blood circulatory system is one in which blood flows entirely within blood vessels, as seen in humans and other vertebrates.

An open blood circulatory system is found in animals like insects, where blood flows freely through body cavities without confined blood vessels.

Blood is a fluid that circulates in the heart, arteries, capillaries, and veins of vertebrate animals. It transports nutrients and oxygen throughout the body and removes waste.

The plasma from which the protein fibrinogen has been removed is called serum. 

Phagocytosis is a process in which most WBCs, particularly the neutrophils, engulf particle-like solid substances, especially bacteria.

The solid mass left behind after clotting, formed by fibrin and trapped blood cells, is called clot or thrombus.

Diapedesis is the movement of white blood cells from lymph capillary walls to neighbouring tissues, often for immunological defence.

The enzyme released by injured tissues and disintegrating platelets that initiates clotting is called thrombokinase (also known as Thromboplastin or Factor X or Stuart factor).

A person with blood group AB is called a universal recipient because they can receive blood from all major blood groups (A, B, AB, and O) without risk of incompatibility.

Rhesus factor is a hereditary protein present in red blood cells. If present, the individual is Rh-positive; if absent, they are Rh-negative.

Blood transfusion is the process of introducing blood from a healthy donor into the bloodstream of a patient, typically through a vein, often during surgery or after heavy blood loss.

A Blood Recipient is a person who receives blood during a transfusion, requiring compatibility with the donor's blood group.

Antigens are specific proteins present on the surface of red blood cells that determine an individual’s blood group (e.g., Antigen A or Antigen B).

Antibodies are proteins present in blood plasma that react against specific antigens not found on the individual's own red blood cells, playing a key role in blood group compatibility.

Lymph is the fluid formed from tissue fluid that enters lymph vessels. It helps in returning excess fluid to the blood and plays a role in immunity.

1. The heart's cyclical repetition of successive events, known as the cardiac cycle, is made up of the contraction and relaxation of the ventricles and atria during diastole.
2. A cardiac cycle lasts about 0.8 seconds. The trial systole (0.1 second), ventricular systole (0.3 second), and full cardiac diastole (0.4 second) are the phases of the heart cycle. 

1. Cardiac output is the volume of blood that the heart pumps out in a minute. It is computed by multiplying heart rate (the number of beats per minute) by stroke volume, which is the amount of blood pumped by each ventricle per minute.
2. A healthy person's heart beats 72 times per minute, pumping out roughly 70 millilitres of blood with each beat. As a result, the average cardiac output is five litres, or 5000 mL.

Double circulation is the process in which blood passes through the heart twice during one complete cycle—once for pulmonary circulation (to and from the lungs) and once for systemic circulation (to and from the body).

• Blood is a fluid connective tissue made of plasma (55%) and formed elements (RBCs, WBCs, platelets).
• Plasma contains water, proteins, electrolytes, hormones, and waste products.
• RBCs transport oxygen and carbon dioxide, WBCs help in immunity, and platelets help in blood clotting.
• Blood is slightly alkaline (pH ~7.4) and maintains proper physiological balance (homeostasis).
• Blood helps in the transport of nutrients, wastes, and hormones, and also regulates body temperature.

• Plasma is a straw-coloured, slightly alkaline fluid and forms about 55% of blood.
• It contains 90–92% water and 6–8% proteins.
• Major proteins are: fibrinogen (clotting), globulins (defence), and albumin (osmotic balance).
• Plasma also contains electrolytes (Na⁺, Ca²⁺, Mg²⁺, Cl⁻, HCO₃⁻) and nutrients like glucose, amino acids, and lipids.
• Plasma without clotting factors is called serum.

• Platelets are oval-shaped, enucleated cell fragments found in mammals only. Normal count = 2.5–4.5 lakh/mm³. Lifespan = 3–5 days, destroyed mainly in the spleen.
• At injury site, platelets disintegrate → release thrombokinase (thromboplastin/Factor X) → initiates clotting cascade. Vitamin K is essential for the synthesis of prothrombin in the liver.
• Thrombokinase + Ca²⁺ ions → converts inactive prothrombin → active thrombin.
• Thrombin converts soluble fibrinogen → insoluble fibrin threads → mesh traps blood cells → forms a clot sealing the wound.
• Clot contracts → squeezes out serum → leaves behind a solid mass called thrombus. Low platelet count = Thrombocytopenia (excessive bleeding).

• WBCs are colourless, nucleated cells lacking haemoglobin. Normal count = 5000–11000/mm³. Produced in red bone marrow, lymph nodes, liver and spleen. Short-lived (~2 weeks).
• Two types — Granulocytes (Neutrophils, Eosinophils, Basophils) and Agranulocytes (Lymphocytes, Monocytes). Most WBCs are amoeboid and enter tissues via diapedesis.
• Neutrophils (70%) and Monocytes are phagocytic — engulf germs and damaged cells. Eosinophils initiate an allergic response. Basophils release histamine, heparin and serotonin.
• Lymphocytes (25–30%) provide immunity — B-lymphocytes (antibody-mediated) and T-lymphocytes (cell-mediated). Form the basis of immunity and vaccination.
• WBC count increases during infection, stress or leukaemia (leukocytosis) and decreases in viral illness or bone marrow disorders (leukopenia).

• Platelets are oval-shaped, enucleated cell fragments found in mammals only. Normal count = 2.5–4.5 lakh/mm³. Lifespan = 3–5 days, destroyed mainly in the spleen.
• At injury site, platelets disintegrate → release thrombokinase (thromboplastin/Factor X) → initiates clotting cascade. Vitamin K is essential for the synthesis of prothrombin in the liver.
• Thrombokinase + Ca²⁺ ions → converts inactive prothrombin → active thrombin.
• Thrombin converts soluble fibrinogen → insoluble fibrin threads → mesh traps blood cells → forms a clot sealing the wound.
• Clot contracts → squeezes out serum → leaves behind a solid mass called thrombus. Low platelet count = Thrombocytopenia (excessive bleeding).

• ABO System: Introduced by Karl Landsteiner (1900). Based on the presence/absence of antigen A and B on RBCs, four groups: A, B, AB, O.
• Universal Donor & Acceptor: AB = universal acceptor (both antigens, no antibodies); O = universal donor (no antigens, both antibodies).
• Rh Factor: Discovered by Landsteiner & Wiener (1940). 80–85% people are Rh⁺; the rest are Rh⁻.
• HDN (Erythroblastosis Foetalis): When a Rh⁻ mother carries a Rh⁺ foetus, she produces anti-Rh antibodies (after the first delivery), which attack subsequent Rh⁺ foetuses.
• Prevention: Rh⁻ mother is injected with anti-D antibodies during all pregnancies with Rh⁺ foetus to prevent HDN.

• When a blood vessel is injured, platelets disintegrate, and injured tissue releases thromboplastin, which initiates the clotting process.
• Thromboplastin converts inactive prothrombin → thrombin in the presence of Ca²⁺ ions (via formation of prothrombinase/thrombokinase).
• Thrombin then converts soluble fibrinogen → insoluble fibrin threads.
• Fibrin threads trap formed elements (RBCs, WBCs, platelets) to form a dark reddish-brown clot (scum), sealing the wound.

• Lymphatic system consists of lymph, lymphatic vessels, organs and tissues. The word "lymph" means clear water.
• Lymph is similar to blood but lacks RBCs, platelets and some proteins. It contains mainly WBCs (lymphocytes), 94% water and 6% solids (proteins & fats).
• Lymph is formed from tissue fluid and flows through lymph vessels by contraction of nearby muscles. It drains into veins via thoracic duct and right lymphatic duct.
• Lymph supplies nutrients and O₂ to deep tissues where blood cannot reach, and drains excess tissue fluid, waste and proteins back into the bloodstream.
• Lymph absorbs fats from the intestine and defends the body by removing bacteria and housing immune cells (lymphocytes).

• Two types of circulatory pathways - Open (arthropods & molluscs, blood into body cavities, less regulated) and Closed (annelids & chordates, a closed network of vessels, more precise).
• All vertebrates have a muscular chambered heart - Fish = 2-chambered, Amphibians & Reptiles (except crocodiles) = 3-chambered, Crocodiles, Birds & Mammals = 4-chambered.
• In fish, the heart pumps deoxygenated blood, which is oxygenated by the gills.
• Amphibians and reptiles have incomplete double circulation - oxygenated and deoxygenated blood mix in a single ventricle.
• Birds and mammals have complete double circulation - separate pathways for oxygenated and deoxygenated blood.
• Open system = simple, less energy-intensive; Closed system = more efficient, precise regulation.

• Arteries carry oxygenated blood from the heart to tissues, while veins bring deoxygenated blood back; capillaries connect them.
• Exception: Pulmonary artery carries deoxygenated blood, and pulmonary vein carries oxygenated blood.
• The heart is fist-sized, located in the thorax between the lungs, and protected by the pericardium.
• The heart has four chambers separated by septa, with valves (tricuspid, bicuspid/mitral, semilunar) ensuring one-way blood flow.
• Heart sounds: “Lub” is due to closure of the tricuspid and bicuspid valves, and “Dub” is due to closure of the semilunar valves.
• The SA node (pacemaker) generates impulses (70–75/min) and controls the heartbeat through the AV node and the bundle of His.

• The cardiac cycle starts with joint diastole, where all chambers are relaxed, and blood flows into the ventricles through open AV valves.
• The SA node generates an impulse, causing atrial systole, pushing extra blood into the ventricles.
• The impulse passes through the AV node and bundle of His, causing ventricular systole.
• During ventricular systole, tricuspid and bicuspid valves close (lub sound) and semilunar valves open to pump blood into the arteries.
• In ventricular diastole, ventricles relax, semilunar valves close (dub sound), and AV valves reopen for filling.
• Each cycle pumps about 70 mL of blood (stroke volume), and cardiac output is about 5 L/min (stroke volume × heart rate).

• ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. The machine used to record it is called an electrocardiograph.
• P-wave - Represents depolarisation of atria, causing contraction of both atria.
• QRS Complex - Represents depolarisation of the ventricles, initiating ventricular contraction. Contraction begins immediately after Q → marks the start of systole.
• T-wave - Represents repolarisation of ventricles. End of T-wave = end of systole.
• Key intervals - P-Q interval = 0.16 sec, Q-T interval = represents full ventricular activity, RR interval = time between two heartbeats.
• Important distinction - Electrocardiograph = the instrument, Electrocardiogram = the recording/graph produced.

• The heart is myogenic - its activities are intrinsically regulated by nodal tissue (SAN and AVN), meaning it generates its own impulse.
• A neural centre in the medulla oblongata moderates cardiac function through the ANS.
• Sympathetic nerves (ANS) increase heart rate, ventricular contraction strength and cardiac output.
• Parasympathetic nerves (ANS) decrease heart rate, slow conduction speed and reduce cardiac output. (AIPMT 2007)
• Adrenal medullary hormones also increase cardiac output.

• Hypertension (high BP) is when blood pressure is above 120/80 mm Hg; ≥140/90 mm Hg is serious and can damage the heart, brain, and kidneys.
• Coronary Artery Disease (CAD) or atherosclerosis occurs due to fat, cholesterol, and calcium deposits, narrowing the arteries supplying the heart.
• Angina (angina pectoris) is chest pain caused by reduced oxygen supply to the heart muscles due to restricted blood flow.
• Heart failure is when the heart cannot pump enough blood to meet the body's needs; it is different from a heart attack and cardiac arrest.
• Coronary thrombosis is the formation of a blood clot in the coronary arteries, often in the left anterior descending artery.
• These disorders mainly affect blood flow and oxygen supply, leading to serious cardiovascular problems.