Body Fluids and Circulation

The body needs a continuous supply of oxygen, nutrients, and removal of waste products. This is achieved by the circulatory system, which includes blood, lymph, blood vessels, and the heart.

Blood

Blood is a connective tissue comprising plasma and formed elements.

Plasma (55% of blood volume): Straw-coloured fluid; 90-92% water; contains plasma proteins (albumin, globulins, fibrinogen), glucose, amino acids, lipids, hormones, ions, and waste products.
Fibrinogen → fibrin (in blood clotting)
Serum = plasma without clotting factors (fibrinogen removed after clotting)

Formed Elements (45% of blood volume):

Red Blood Cells (Erythrocytes, RBCs):
Count: ~5 million/mm³ in males; ~4.5 million/mm³ in females
Biconcave disc shape; no nucleus (in mammals); rich in haemoglobin
Lifespan: ~120 days; destroyed in spleen ("graveyard of RBCs")
Produced in red bone marrow (erythropoiesis)

White Blood Cells (Leucocytes, WBCs):
Count: 6000-8000/mm³ (much fewer than RBCs)
Granulocytes: neutrophils (most common; phagocytosis), eosinophils (anti-allergic; anti-parasitic), basophils (contain histamine and heparin; involved in inflammation)
Agranulocytes: lymphocytes (B cells — antibodies, T cells — cell-mediated immunity), monocytes (become macrophages; phagocytosis)

Platelets (Thrombocytes):
Count: 1.5-3.5 lakh/mm³
Tiny fragments of megakaryocytes; no nucleus
Essential for blood clotting (haemostasis)

Blood Groups

ABO system: Based on antigens (agglutinogens) on RBC surface and antibodies (agglutinins) in plasma.
Blood group A: antigen A on RBC; antibody b (anti-B) in plasma
Blood group B: antigen B on RBC; antibody a (anti-A) in plasma
Blood group AB: antigens A and B; no antibodies; universal recipient
Blood group O: no antigens; antibodies a and b; universal donor

Rh factor: Another antigen on RBC. Rh+ individuals have it; Rh- do not.
Rh incompatibility: if Rh- mother carries Rh+ foetus — mother may develop anti-Rh antibodies (after first delivery or miscarriage). In subsequent pregnancies, these antibodies cross placenta → Erythroblastosis foetalis (haemolytic disease of newborn). Prevented by giving Rh- mother anti-Rh antibodies (RhoGAM) after first delivery.

Blood Coagulation (Clotting)

1.Blood clotting involves a cascade of reactions:
2.Injury → release of thromboplastin (clotting factor III) from damaged tissues
3.Thromboplastin + Ca2+ + clotting factors → converts prothrombin (inactive) → thrombin (active enzyme)
4.Thrombin converts fibrinogen (soluble) → fibrin (insoluble network/mesh)
5.Fibrin + trapped blood cells → clot (thrombus)

Vitamin K is essential for synthesis of several clotting factors (II, VII, IX, X).

Lymph

Fluid that leaks from blood capillaries into tissue spaces is called interstitial fluid. When this fluid enters lymph capillaries, it becomes lymph.
Colourless; similar composition to plasma but less protein
Contains lymphocytes (immunity)
Lymph vessels drain into lymph nodes (filter lymph) → thoracic duct and right lymphatic duct → return to blood (subclavian veins)
Functions: return protein and fluid to blood; fat absorption from intestine (chylomicrons through lacteals); immunity

Human Heart

The heart is a four-chambered muscular pump:
Right side (low-pressure): receives deoxygenated blood, pumps to lungs (pulmonary circulation)
Left side (high-pressure): receives oxygenated blood from lungs, pumps to body (systemic circulation)

Chambers: Right atrium (RA), Right ventricle (RV), Left atrium (LA), Left ventricle (LV)
Valves:
Tricuspid valve: between RA and RV (3 cusps)
Bicuspid/Mitral valve: between LA and LV (2 cusps)
Semilunar valves: at the base of pulmonary artery (pulmonary semilunar) and aorta (aortic semilunar)

Nodal Tissue (Conduction system):
SA node (Sinoatrial node): in right atrium wall; pacemaker — generates electrical impulse at 70-75 beats/min
AV node (Atrioventricular node): at atrioventricular junction; slight delay allows atria to contract before ventricles
Bundle of His: carries impulse to ventricles
Purkinje fibres: distribute impulse through ventricular muscle

Cardiac Cycle

One complete heartbeat = cardiac cycle ≈ 0.8 seconds at 75 beats/min.
Atrial systole (0.1 s): atria contract; blood pushed into ventricles
Ventricular systole (0.3 s): ventricles contract; blood pumped to lungs and body; AV valves close (lubb — S1 sound); semilunar valves open
Joint diastole (0.4 s): all chambers relax; semilunar valves close (dubb — S2 sound); AV valves open; filling begins

Cardiac Output (CO) = Stroke Volume x Heart Rate = 70 mL x 75 beats/min = ~5250 mL/min ≈ 5 L/min

ECG (Electrocardiogram): Records electrical activity of heart.
P wave: atrial depolarisation (atrial systole)
QRS complex: ventricular depolarisation (ventricular systole)
T wave: ventricular repolarisation (ventricular diastole)

Disorders

Hypertension: Sustained high blood pressure (>140/90 mmHg); risk factor for stroke, heart attack
Coronary Artery Disease (CAD): Narrowing of coronary arteries by atherosclerosis (plaque); causes angina pectoris (chest pain on exertion) or myocardial infarction (heart attack)
Heart failure: Heart cannot pump enough blood to meet body's needs

Common mistakes

The SA node, NOT the AV node, is the pacemaker.
Universal donor (O group) has NO antigens, but does have BOTH antibodies — not truly safe for all, but used in emergencies.
Heart sounds: "Lubb" (S1) = closure of AV valves at start of ventricular systole; "Dubb" (S2) = closure of semilunar valves at end of ventricular systole.
Lymph is NOT blood — it lacks RBCs and has less protein than plasma.

Summary

Blood consists of plasma and formed elements (RBCs, WBCs, platelets). The heart is a four-chambered pump regulated by the SA node. The cardiac cycle involves coordinated contraction and relaxation producing heart sounds and electrical waves seen on ECG. Blood groups (ABO and Rh) and clotting mechanism are critical for transfusions and surgery.