Body Fluids and Circulation

Blood is a fluid connective tissue consisting of plasma (55%) and formed elements (45%). It transports respiratory gases, nutrients, hormones, and metabolic wastes.

Plasma is a straw-coloured fluid containing water (~92%), proteins (fibrinogen, globulins, albumins), glucose, amino acids, lipids, and inorganic salts.

Formed elements:

• RBCs (Erythrocytes) — 5 million/mm³; biconcave; no nucleus in mature mammalian RBCs; contain haemoglobin; life span ~120 days; produced in red bone marrow (haematopoiesis).
• WBCs (Leucocytes) — 6,000–8,000/mm³; nucleated; defend against pathogens.
  — Granulocytes: Neutrophils (most common, phagocytic), Eosinophils (anti-allergic), Basophils (release histamine).
  — Agranulocytes: Lymphocytes (produce antibodies), Monocytes (become macrophages).
• Platelets (Thrombocytes) — 150,000–400,000/mm³; cell fragments; essential for clotting.

ABO blood groups — based on presence or absence of antigens A and B on RBC surface:

Rh factor — Rh antigen present in ~80% of humans (Rh⁺). Rh incompatibility during pregnancy can cause erythroblastosis foetalis.

Blood coagulation: Injury → platelets aggregate → release thromboplastin → prothrombin → thrombin → fibrinogen → fibrin (clot).

The human heart is a myogenic (self-generating) muscular organ located in the thoracic cavity between the lungs. It has four chambers: two atria (thin-walled, receiving) and two ventricles (thick-walled, pumping).

Valves:

• Tricuspid valve — between right atrium and right ventricle (3 flaps).
• Bicuspid (mitral) valve — between left atrium and left ventricle (2 flaps).
• Semilunar valves — at the opening of aorta and pulmonary artery; prevent backflow.

Nodal tissue (conduction system):

• SA node (sinoatrial node) — in right atrium wall; pacemaker of the heart; initiates impulse.
• AV node (atrioventricular node) — at atrioventricular junction.
• Bundle of His and Purkinje fibres — transmit impulse to ventricular walls.

The cardiac cycle is the sequence of events in one heartbeat (~0.8 seconds at 72 beats/min):

• Atrial systole — 0.1 s; atria contract, blood enters ventricles.
• Ventricular systole — 0.3 s; ventricles contract, blood ejected into aorta and pulmonary artery.
• Joint diastole — 0.4 s; all chambers relax; heart fills passively.

Cardiac output = Stroke volume × Heart rate = ~70 mL × 72 beats/min = ~5 L/min.

ECG (Electrocardiograph):

• P wave — atrial depolarisation (SA node fires, atria contract).
• QRS complex — ventricular depolarisation (ventricles contract).
• T wave — ventricular repolarisation (ventricles relax).

Double circulation means blood passes through the heart twice per complete circuit — once through the pulmonary circuit and once through the systemic circuit. This ensures oxygenated and deoxygenated blood never mix.

• Pulmonary circulation — right heart → lungs → left heart (blood gets oxygenated).
• Systemic circulation — left heart → body → right heart (blood delivers O₂ to tissues).

Lymphatic system — returns excess tissue fluid to blood via lymph capillaries and lymph vessels. Lymph nodes filter lymph and are sites of immune response.

Disorders:

• Hypertension — blood pressure consistently above 120/80 mmHg; damages arteries.
• Coronary artery disease (CAD) — narrowing of coronary arteries due to atherosclerosis.
• Angina pectoris — chest pain due to insufficient blood supply to heart muscle.
• Heart failure (Cardiac failure) — heart unable to pump sufficient blood; should not be confused with cardiac arrest.