Every cell needs a supply of oxygen and nutrients and a way to remove waste. In humans, blood carries many of these substances through blood vessels, while lymph helps return tissue fluid to the circulation and transports certain materials.
Table of Contents
Composition of Blood
Blood is a fluid connective tissue made of plasma and formed elements.
Plasma
Plasma is the liquid part of blood. It is mostly water and contains dissolved nutrients, salts, hormones, waste substances and proteins.
| Plasma protein | Main function |
|---|---|
| Fibrinogen | Helps in blood clotting |
| Globulins | Include proteins involved in defence |
| Albumins | Help maintain osmotic balance |
Formed Elements
- Red blood cells (RBCs): Contain haemoglobin and transport respiratory gases. Mature human RBCs lack a nucleus.
- White blood cells (WBCs): Take part in defence and immune responses.
- Platelets: Cell fragments that help initiate blood clotting.
Blood Groups
The ABO blood group depends on whether A and B antigens are present on the surface of RBCs.
| Blood group | Antigen on RBCs | Antibody in plasma |
|---|---|---|
| A | A | Anti-B |
| B | B | Anti-A |
| AB | A and B | Neither anti-A nor anti-B |
| O | Neither A nor B | Anti-A and anti-B |
The Rh group depends mainly on the presence or absence of the Rh(D) antigen. A person with this antigen is described as Rh positive; a person without it is Rh negative.
Blood must be properly tested and matched before transfusion. ABO and Rh labels alone do not replace compatibility testing.
How Does Blood Clot?
A damaged blood vessel triggers a series of reactions that help prevent excessive blood loss. Platelets and clotting factors participate in this process. The soluble protein fibrinogen is converted into fibrin, whose thread-like network helps form a clot. Calcium ions also play an important role.
What Is Lymph?
Some fluid leaves blood capillaries and surrounds the body’s cells as tissue fluid. Much of it returns to the blood; the lymphatic system collects the remaining fluid. Once it enters lymphatic vessels, it is called lymph.
Lymph contains immune cells called lymphocytes. It also helps transport absorbed fats from the intestine and returns fluid to the bloodstream.
Human Circulatory System
The human circulatory system contains the heart, blood and a closed network of blood vessels. The heart has four chambers:
- Right atrium and right ventricle
- Left atrium and left ventricle
Valves keep blood flowing in one direction. The tricuspid valve lies between the right atrium and right ventricle; the mitral valve lies between the left atrium and left ventricle. Semilunar valves guard the openings into the pulmonary artery and aorta.
Path of Blood Through the Heart
Follow the route in two parts:
To the lungs: Body → Venae cavae → Right atrium → Right ventricle → Pulmonary artery → Lungs
To the body: Lungs → Pulmonary veins → Left atrium → Left ventricle → Aorta → Body
Notice the exceptions to a common shortcut: the pulmonary artery carries deoxygenated blood, while the pulmonary veins carry oxygenated blood.
Double Circulation
In double circulation, blood passes through the heart twice during one complete trip around the body:
- Pulmonary circulation: Right ventricle → Lungs → Left atrium.
- Systemic circulation: Left ventricle → Body tissues → Right atrium.
This arrangement keeps oxygenated blood returning from the lungs separate from deoxygenated blood returning from the body.
Cardiac Cycle and Cardiac Output
The cardiac cycle is one repeated sequence of heart contraction and relaxation.
- Systole means contraction.
- Diastole means relaxation.
The atria contract to help fill the ventricles. The ventricles then contract to pump blood into the pulmonary artery and aorta. Heart valves open and close as pressures change, preventing backflow.
Stroke volume is the blood pumped by one ventricle in one beat. Cardiac output is the blood pumped by one ventricle in one minute:
Cardiac output = Stroke volume × Heart rate
The familiar first and second heart sounds are associated mainly with closure of the atrioventricular and semilunar valves, respectively.
How Is Heartbeat Controlled?
Specialised heart tissue can generate electrical impulses. The sinoatrial node (SA node) starts the usual heartbeat and is called the heart’s pacemaker. The impulse spreads across the atria, reaches the atrioventricular node (AV node) and then travels through conducting fibres to the ventricles.
The nervous system and hormones can modify heart rate and the strength of contraction to meet the body’s needs.
What Does an ECG Show?
An electrocardiogram (ECG) records the heart’s electrical activity.
| ECG feature | Represents |
|---|---|
| P wave | Atrial depolarisation |
| QRS complex | Ventricular depolarisation |
| T wave | Ventricular repolarisation |
An ECG records electrical changes associated with the cardiac cycle; it does not directly measure the amount of blood pumped.
Circulatory Disorders
- Hypertension: Persistently elevated blood pressure.
- Coronary artery disease: Narrowing of arteries supplying the heart muscle.
- Angina: Chest discomfort associated with insufficient oxygen supply to the heart muscle.
- Heart failure: The heart cannot pump effectively enough to meet the body’s needs.
Quick Revision
- Plasma: Liquid part of blood.
- RBCs: Mainly transport respiratory gases.
- WBCs: Participate in defence.
- Platelets: Help with clotting.
- Lymph: Helps return tissue fluid and transport absorbed fats.
- SA node: Starts the usual heartbeat.
- Pulmonary circulation: Heart to lungs and back.
- Systemic circulation: Heart to body tissues and back.
Check Your Understanding
- Name the three formed elements of blood.
- Which plasma protein is important in clotting?
- How many chambers does the human heart have?
- Why is human circulation called double circulation?
- What does the QRS complex represent on an ECG?
Answers: (1) RBCs, WBCs and platelets. (2) Fibrinogen. (3) Four. (4) Blood passes through the heart twice in one complete circuit. (5) Ventricular depolarisation.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What are the two major circulating body fluids discussed here?
Blood and lymph are the major fluids discussed in this chapter.
What is plasma?
Plasma is the liquid part of blood containing water, proteins, nutrients and dissolved substances.
What are the formed elements of blood?
Red blood cells, white blood cells and platelets.
What is the main function of red blood cells?
They transport oxygen using haemoglobin.
What is the main function of white blood cells?
They help defend the body against infection and other threats.
What do platelets do?
They help initiate blood clot formation after injury.
What are the ABO blood groups based on?
They depend on the presence or absence of A and B antigens on red blood cells.
What is the Rh factor?
It refers mainly to the presence or absence of the D antigen on red blood cells.
How many chambers does a human heart have?
The heart has four chambers: two atria and two ventricles.
What is the cardiac cycle?
It is one complete sequence of heart relaxation and contraction during a heartbeat.
What is cardiac output?
It is the volume of blood pumped by one ventricle per minute: heart rate multiplied by stroke volume.
What is double circulation?
Blood passes through the heart twice in one complete trip: once through pulmonary and once through systemic circulation.
What does an ECG record?
An electrocardiogram records electrical changes associated with the heart’s activity.
What is the main role of lymph?
Lymph returns excess tissue fluid to blood and participates in immune defence and fat transport.
What is hypertension?
It is persistently elevated blood pressure; diagnosis depends on properly measured readings and clinical criteria.