VARDAAN LEARNING INSTITUTE | POWERED BY VARDAAN COMET
Circulatory System
ICSE Class 8 Biology • Chapter 6 (Detailed Master Notes)
Chapter Overview
A city survives because it has an organized transport network of roads and delivery trucks bringing supplies to every house. In the human body, this transport network is the Circulatory System. It continuously pumps blood to deliver oxygen and nutrients to every single cell, and removes toxic wastes.
6.1 Blood: The Liquid Tissue
Blood is a vital fluid connective tissue. It consists of a liquid part and a solid cellular part.
1. Plasma (The Liquid Part)
Plasma is a pale-yellow liquid that forms 55% of the blood. It consists mostly of water ($90\%$) and dissolved proteins, digested food, hormones, and waste materials.
2. Blood Cells (The Solid Part)
Three main types of cells float freely inside the blood plasma.
| Cell Type | Scientific Name | Function |
|---|---|---|
| Red Blood Cells (RBCs) | Erythrocytes | They look like tiny discs. They contain a red iron pigment called Haemoglobin, which binds to oxygen in the lungs and carries it to all body cells. |
| White Blood Cells (WBCs) | Leucocytes | They act as the soldiers of the body. They protect the body against infectious diseases by completely destroying harmful bacteria and viruses. |
| Blood Platelets | Thrombocytes | They play a central role in blood clotting. If you get a cut, platelets quickly gather at the site and form a plug to stop the bleeding. |
6.2 Blood Vessels: The Transport Pipes
Blood flows through a completely closed network of tubes.
- Arteries: Thick-walled, elastic vessels that carry oxygen-rich (pure) blood strictly away from the heart to the entire body. (Exception: Pulmonary Artery). The blood flows under very high pressure.
- Veins: Thin-walled vessels that carry carbon dioxide-rich (impure) blood strictly back to the heart. They contain special one-way valves to absolutely prevent the blood from flowing backwards.
- Capillaries: Extremely narrow, microscopic tubes that connect arteries to veins deep inside the organs. Because their walls are remarkably thin, the actual exchange of gases and nutrients happens here.
AI Image Prompt: A medical illustration showing the connection of blood vessels. On one side, a thick red Artery branches out into very fine, microscopic purple Capillary networks embedded in tissue. The capillaries then converge and merge together to form a wide blue Vein on the other side.
6.3 The Human Heart
The heart is an incredible muscular pumping organ located in the center of the chest cavity, slightly tilted to the left. It works continuously without ever taking a rest.
Structure of the Heart
The human heart is securely divided into exactly four chambers mapping to four distinct rooms.
- Upper Chambers: Right Atrium and Left Atrium. They act as receiving rooms. They have thin walls.
- Lower Chambers: Right Ventricle and Left Ventricle. They are the main pumping rooms. They have very thick muscular walls because they must forcefully push blood out of the heart.
A thick muscular central wall (septum) completely separates the right side of the heart from the left side, preventing the pure oxygenated blood from mixing with the impure deoxygenated blood.
6.4 The Double Circulation Pathway
Blood physically passes through the human heart twice to complete one full cycle.
- Pulmonary Circulation: The Right Ventricle pumps dark, impure blood specifically to the lungs to pick up fresh oxygen. The bright red, pure blood returns to the Left Atrium.
- Systemic Circulation: The Left Ventricle pumps that pure blood completely out to the entire body. After the body cells use the oxygen, the impure blood returns strictly to the Right Atrium.
Q1. Why do veins have specialized one-way valves inside them, while arteries do not?
Answer: Blood flowing inside arteries is actively pushed by the powerful pumping action of the thick heart muscles; thus, it flows continuously forward under enormous high pressure. In contrast, blood flowing inside veins is under very low pressure and must climb upwards against gravity from the legs back to the chest. The valves strictly prevent gravity from pulling the blood back down.
Q2. State the central function of the red pigment Haemoglobin.
Answer: Haemoglobin has a strong chemical affinity for oxygen. As blood passes through the lungs, haemoglobin quickly binds with fresh oxygen to distinctly form Oxyhaemoglobin, safely transporting it to oxygen-starved cells deep inside the body tissues.