Body Fluids and Circulation

Key Concepts

Composition of Blood

Blood is a special connective tissue consisting of a fluid matrix (plasma) and formed elements.

• Plasma (55%): A straw-colored viscous fluid. 90-92% water, 6-8% proteins (Fibrinogen for clotting, Globulins for defense, Albumins for osmotic balance). Plasma without clotting factors is called Serum.
• Formed Elements (45%):
  • Erythrocytes (RBCs): Biconcave, lack nucleus (in most mammals), contain haemoglobin for gas transport. Life span: 120 days.
  • Leucocytes (WBCs): Nucleated, part of the immune system. Includes Granulocytes (Neutrophils, Eosinophils, Basophils) and Agranulocytes (Lymphocytes, Monocytes).
  • Platelets (Thrombocytes): Cell fragments from megakaryocytes involved in blood coagulation.

Blood Groups and Coagulation

• ABO Grouping: Based on A and B antigens on RBCs. Group O is the universal donor; AB is the universal recipient.
• Rh Grouping: Based on the Rh antigen. Erythroblastosis foetalis occurs when an Rh-ve mother carries an Rh+ve foetus.
• Coagulation (Clotting): A "cascade" process. Prothrombin is converted to Thrombin (by thrombokinase), which converts Fibrinogen to Fibrin threads that trap dead cells to form a clot. Calcium ions are essential.

Human Circulatory System

• Heart Anatomy: A 4-chambered muscular organ tilted to the left. Protected by the pericardium.
  • Valves: Tricuspid (right side), Bicuspid/Mitral (left side), and Semilunar valves prevent backward flow.
  • Nodal Tissue: The Sino-atrial node (SAN) is the pacemaker as it generates the maximum action potentials (70-75/min).
• Cardiac Cycle: Includes Atrial systole, Ventricular systole, and Joint diastole. Duration: 0.8 seconds.
• Cardiac Output: Stroke volume (70 mL) $\times$ Heart rate ($\approx 72$ bpm) $\approx 5$ Litres/min.
• Heart Sounds: 'Lub' (closure of tricuspid/bicuspid valves) and 'Dub' (closure of semilunar valves).

Electrocardiogram (ECG)

A graphical representation of the heart's electrical activity.

• P-wave: Depolarisation (excitation) of atria.
• QRS complex: Depolarisation of ventricles (marks beginning of systole).
• T-wave: Repolarisation (return to normal) of ventricles.

Double Circulation

Humans possess two separate circulatory pathways:

1. Pulmonary Circulation: Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary veins $\rightarrow$ Left atrium.
2. Systemic Circulation: Left ventricle $\rightarrow$ Aorta $\rightarrow$ Body tissues $\rightarrow$ Vena cava $\rightarrow$ Right atrium.

• Hepatic Portal System: A unique vascular connection between the digestive tract and the liver.

Regulation and Disorders

• Regulation: The heart is myogenic (auto-regulated). The medulla oblongata can moderate activity via the Autonomic Nervous System (ANS).
• Hypertension: Blood pressure higher than normal (e.g., 140/90).
• Coronary Artery Disease (CAD): Atherosclerosis; narrowing of arteries due to deposits.
• Angina Pectoris: Acute chest pain due to inadequate oxygen reaching the heart.
• Heart Failure: Heart not pumping blood effectively; often involves lung congestion.

Activities

• Measure your pulse rate and notice its rhythm.
• Learn about the "universal donor" and "universal recipient" concepts through a blood donation drive.
• Draw a diagram of the human heart and trace the path of blood during double circulation.