EduDias: ICSE 8 Biology Circulatory System Basic

1.0 The Circulatory System: Blood

The circulatory system is the transport network of the human body. It consists of Blood, Blood Vessels, and the Heart. Blood is a fluid connective tissue that circulates throughout the body, carrying oxygen, nutrients, and waste products.

Composition

Blood: Composed of a straw-colored liquid called Plasma (55%) and cellular structures called Formed Elements (45%).

Components of Blood

🔴 RBCs (Erythrocytes): Circular, biconcave, and lack a nucleus (to accommodate more Haemoglobin). They transport oxygen.
⚪ WBCs (Leukocytes): Irregularly shaped cells that act as the "soldiers" of the body by fighting infections.
🧬 Platelets (Thrombocytes): Tiny cell fragments that help in Blood Clotting at the site of an injury.

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Fig 6.1: Components of Human Blood

RBCs vs. WBCs

Feature	RBCs	WBCs
Shape	Biconcave disc.	Irregular/Amoeboid.
Nucleus	Absent (in mature RBCs).	Present.
Function	Oxygen transport.	Defense mechanism.

💡 Did You Know?

Haemoglobin has a very high affinity for carbon monoxide. This is why inhaling smoke is so dangerous—it prevents oxygen from binding to your blood!

⚠️ Important Term:

The process by which WBCs squeeze through the walls of capillaries to fight germs is called Diapedesis. This is a common one-word answer question in ICSE exams.

2.0 Blood Vessels: The Transport Highways

Blood travels through the body in a closed network of tubes called Blood Vessels. Depending on their structure and function, they are classified into three types.

Comparison of Blood Vessels

Feature	Arteries	Veins	Capillaries
Function	Carry blood away from the heart.	Carry blood towards the heart.	Exchange of materials between blood and cells.
Wall Thickness	Thick and elastic.	Thin and less elastic.	Extremely thin (one-cell thick).
Valves	Absent.	Present (prevent backflow).	Absent.
Blood Flow	Flows with high pressure/jerks.	Flows smoothly with low pressure.	Flows very slowly.

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Fig 6.2: Structural Differences between Artery, Vein, and Capillary

Did You Know about the Exceptions?

🩺 Pulmonary Artery: The only artery that carries deoxygenated blood (from heart to lungs).
🫁 Pulmonary Vein: The only vein that carries oxygenated blood (from lungs to heart).

⚠️ Exam Tip:

In reasoning questions, if asked why Arteries have thick walls, the answer is: To withstand the high pressure of blood pumped out by the heart.

3.0 The Human Heart: Structure and Function

The heart is a muscular organ about the size of a closed fist, located in the chest cavity. Its primary job is to act as a double pump to circulate blood throughout the body.

Structure

The heart is enclosed in a double-walled membranous sac called the Pericardium, which protects the heart from shocks and friction.

The Four Chambers

To prevent the mixing of oxygenated and deoxygenated blood, the human heart is divided into four chambers:

⏫ Right & Left Atria (Auricles): The upper, thin-walled receiving chambers.
⏬ Right & Left Ventricles: The lower, thick-walled pumping chambers.
🚧 Septum: The muscular wall that separates the right and left sides of the heart.

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Fig 6.3: Internal Structure of the Human Heart

Valves of the Heart

Valves ensure that blood flows in only one direction:

Tricuspid Valve: Between Right Atrium and Right Ventricle.
Bicuspid (Mitral) Valve: Between Left Atrium and Left Ventricle.
Semi-lunar Valves: Located at the base of the Pulmonary Artery and Aorta.

⚠️ Exam Tip:

In diagrams, always remember that the Left Ventricle has the thickest walls. Why? Because it has to pump blood to the entire body, whereas the right ventricle only pumps blood to the nearby lungs.

4.0 Double Circulation and Heartbeat

In humans, blood passes through the heart twice for each complete circuit of the body. This is known as Double Circulation.

The Two Circuits

Pulmonary Circulation: The flow of blood between the Heart and Lungs. Deoxygenated blood goes to the lungs to get oxygen and returns as oxygenated blood.
Systemic Circulation: The flow of blood between the Heart and the rest of the Body. Oxygenated blood is pumped to all organs and returns as deoxygenated blood.

Mechanism

Cardiac Cycle: One complete contraction (Systole) and relaxation (Diastole) of the heart chambers. This makes up one heartbeat.

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Fig 6.4: Schematic representation of Double Circulation

Heart Sounds: LUB-DUB

The sounds we hear through a stethoscope are caused by the closing of heart valves:

LUB (1st Sound): Low-pitched and long. Caused by the closure of Bicuspid and Tricuspid valves.
DUB (2nd Sound): High-pitched and short. Caused by the closure of Semi-lunar valves.

⚠️ Important Note:

Pulse Rate is exactly equal to the Heart Rate. An average adult heart beats 72 to 80 times per minute under normal resting conditions.