The Circulatory System

ICSE Class 10 Biology • Chapter 06

1. Composition of Blood

Blood: A fluid connective tissue that acts as a transport medium.
Average Volume: 5-6 Litres.

Component	Details	Function
Plasma (55%)	Non-living matrix. 90% water.	Transports nutrients, hormones, CO2, urea. Contains Fibrinogen (for clotting).
RBC (Erythrocytes)	Biconcave, No Nucleus (to increase surface area for O2), Life: 120 days.	Carries Oxygen (as Oxyhaemoglobin) and CO2 (as Carbaminohaemoglobin).
WBC (Leukocytes)	Nucleated, Irregular shape (Amoeboid movement).	Defense: Phagocytosis (Neutrophils) & Antibody production (Lymphocytes).
Platelets (Thrombocytes)	Small fragments, Non-nucleated.	Initiates Clotting (Coagulation) at injury site.

Mechanism of Blood Clotting

Injured Tissue/Platelets release Thrombokinase (enzyme).
Thrombokinase converts inactive Prothrombin into active Thrombin (Requires $Ca^{++}$).
Thrombin converts soluble Fibrinogen into insoluble Fibrin threads.
Fibrin creates a mesh that traps RBCs to form a Clot.

Practice Q1: Reasoning

BOARD QUESTION Why are RBCs biconcave and non-nucleated?

Ans:
1. Biconcave: Increases surface area for oxygen absorption.
2. Non-nucleated: Provides more space to accommodate Haemoglobin.

2. Blood Groups (ABO System)

Universal Donor: O (Rh -ve) [No Antigens].
Universal Recipient: AB (Rh +ve) [No Antibodies].

3. The Human Heart

Enclosed in a double-walled membrane called Pericardium. Pericardial Fluid reduces friction.

Double Circulation

Blood passes through the heart twice in one complete circuit.

Pulmonary Circulation: Right Ventricle $\rightarrow$ Pulmonary Artery $\rightarrow$ Lungs (Oxygenation) $\rightarrow$ Pulmonary Vein $\rightarrow$ Left Auricle.
Systemic Circulation: Left Ventricle $\rightarrow$ Aorta $\rightarrow$ Body Parts (Deoxygenation) $\rightarrow$ Vena Cava $\rightarrow$ Right Auricle.

Hepatic Portal System

Veins starting from the Stomach and Intestine (Gut) do not go directly to the heart. Instead, they unite to form the Hepatic Portal Vein which enters the Liver.
Significance: Liver regulates blood sugar (Glycogen) and detoxifies toxins before blood goes to the heart.

Practice Q2: Circulation

CONCEPT CHECK Name the blood vessel that carries oxygenated blood to the heart muscles.

Ans: Coronary Artery.

4. Cardiac Cycle & Sounds

Systole: Contraction Phase.
Diastole: Relaxation Phase.
LUBB: First sound. Closure of AV Valves (Tricuspid/Bicuspid) at start of Ventricular Systole.
DUP: Second sound. Closure of Semilunar Valves at start of Ventricular Diastole.

Cardiac Output: Volume of blood pumped by heart per minute. $\approx 5$ Litres.

5. Blood Vessels

Feature	Artery	Vein
Direction	Away from Heart.	Towards Heart.
Wall	Thick, Elastic.	Thin, Less muscular.
Lumen	Narrow.	Wide.
Valves	Absent.	Present (Prevent backflow).
Blood Flow	Jerky, High Pressure.	Smooth, Low Pressure.

Exam Practice Questions (PYQ Trends)

Year: 2018

BOARD QUESTION Give the biological term for: The phase of cardiac cycle in which the auricles contract.

Ans: Atrial Systole.

Differentiation

THINKING Differentiate between Serum and Plasma.

Ans: Plasma is the liquid part of blood containing clotting factors (fibrinogen). Serum is Plasma minus clotting factors (fibrinogen).