Body Fluids and Circulation

NCERT Line-by-Line Breakdown for NEET 2026

Unit 5: Human Physiology

Simple organisms like sponges circulate water from their surroundings. However, complex humans need special fluids to transport nutrients, oxygen, and waste. In this chapter, we master Blood, Lymph, the Human Heart, and the Cardiac Cycle.

1. Blood: The River of Life

A specialized connective tissue consisting of a fluid matrix, plasma, and formed elements.

A. Plasma (55%)

Straw-colored, viscous fluid. 90-92% Water, 6-8% Proteins.

Fibrinogen: Coagulation (Clotting).
Globulins: Defense (Antibodies).
Albumins: Osmotic balance.

B. Formed Elements (45%)

Cell	Scientific Name	Count (/mm³)	Key Feature
RBC	Erythrocytes	5 – 5.5 Million	Biconcave, No nucleus, Hb (Transport O₂).
WBC	Leucocytes	6,000 – 8,000	Nucleated, Defense. (Granulocytes & Agranulocytes).
Platelets	Thrombocytes	1.5 – 3.5 Lakhs	Cell fragments (Megakaryocytes). Clotting.

🩸 Blood Grouping (NEET Favorite):

ABO Grouping: Based on Antigens A and B on RBCs. O- is Universal Donor; AB+ is Universal Recipient.
Rh Grouping: Rh antigen (Rhesus). Erythroblastosis foetalis: Occurs if Mother is Rh-ve and Fetus is Rh+ve. (Treatment: Anti-Rh antibodies).

2. Lymph (Tissue Fluid)

As blood passes through capillaries, some water and small water-soluble substances move out into spaces between cells. This is Interstitial Fluid or Lymph.

Colorless fluid containing specialized Lymphocytes (Immune response).
Carrier for nutrients, hormones, etc.
Fats are absorbed through lymph in Lacteals (intestinal villi).

3. Human Circulatory System

A. The Heart

Mesodermally derived organ situated in the thoracic cavity. Protected by double-walled Pericardium.

[Image of human heart internal structure]

Valves (Prevent Backflow)
• Tricuspid: Between Right Atrium & Right Ventricle.
• Bicuspid (Mitral): Between Left Atrium & Left Ventricle.
• Semilunar: At opening of Pulmonary Artery & Aorta.

Nodal Tissue (Auto-excitable)
• SAN (Sino-atrial Node): The Pacemaker (Right upper corner of RA). Generates 70-75 APs/min.
• AVN (Atrio-ventricular Node): Left lower corner of RA.

B. Double Circulation

Pulmonary: RV → Pulmonary Artery → Lungs → Pulmonary Vein → LA.
Systemic: LV → Aorta → Body Tissues → Vena Cava → RA.

Hepatic Portal System: Unique vascular connection between Digestive tract and Liver.

4. Cardiac Cycle & ECG

A. Cardiac Cycle

Sequential event in the heart which is cyclically repeated. Duration: 0.8 seconds.

Joint Diastole: All 4 chambers relaxed. AV valves open.
Atrial Systole: SA node fires. Atria contract (Increases flow by 30%).
Ventricular Systole: Ventricles contract. AV valves close (“LUB” sound). Semilunar valves open.
Ventricular Diastole: Ventricles relax. Semilunar valves close (“DUB” sound).

Cardiac Output = Stroke Vol (70 mL) × Heart Rate (72) ≈ 5000 mL (5L) per minute.

B. Electrocardiograph (ECG)

Graphical representation of the electrical activity of the heart.

[Image of normal ECG wave P QRS T]

Wave	Significance
P-Wave	Depolarisation of Atria (Contraction).
QRS Complex	Depolarisation of Ventricles (Contraction begins). Counting QRS = Heart Rate.
T-Wave	Repolarisation of Ventricles (Return to normal). End of T-wave = End of Systole.

5. Disorders of Circulatory System

Hypertension (High BP)
BP higher than 140/90 mmHg (Normal is 120/80). Affects brain, kidney.

CAD (Atherosclerosis)
Deposition of Calcium, Fat, Cholesterol in arteries. Lumen becomes narrow.

Angina Pectoris
Acute chest pain due to insufficient oxygen reaching heart muscle.

Heart Failure
Heart is not pumping blood effectively enough. (Congestive Heart Failure).
Note: Different from Cardiac Arrest (Heart stops) or Heart Attack (Muscle death).

📝 Rapid Fire MCQs

Q1. The second heart sound ‘DUB’ is associated with the closure of:

A) Tricuspid valve
B) Semilunar valves
C) Bicuspid valve
D) Tricuspid and Bicuspid valves

Click to check Answer

Answer: B) Semilunar valves (End of ventricular systole).

Q2. Which blood group is the universal donor?

A) AB+
B) O+
C) AB-
D) O-