NCERT Notes Class 11 Science Body Fluids and Circulation

Our body mainly consists of various fluids. Body fluids are essential for the smooth functioning of our tissues and carry out the important function of providing living cells with nutrition and eliminating harmful substances produced in our bodies. Different organisms have developed other mechanisms to facilitate these tasks. The study of bloody fluids covers various branches of biology. Complex organisms such as human beings, however, make use of fluids such as blood and lymph in order to execute the aforementioned processes.

Here in this blog, we find out more about body fluids and circulation. It is an important part of the NCERT Biology Class 11 and will also help you prepare for medical entrance exams such as NEET and JIPMER. 

What is Blood?

Before we find out about body fluids and circulation, let us talk about the most important fluid in our body, blood! Blood is a specialized connective tissue composed of:

• Fluid matrix
• Plasma
• Formed elements

We will now talk about these important components of blood. 

Fluid matrix 

The fluid matrix is the watery medium inside connective tissues in our body. It surrounds the cells and fibres, helping in the exchange of nutrients, gases, and waste.

• In cartilage, it’s a gel-like matrix.
• In blood, the fluid matrix is called plasma.
• In bone, it’s the calcified matrix.

Plasma

It is a thick, straw-colored fluid that makes up about 50–55% of the total blood volume. It serves as the transport medium for various substances throughout the body. Plasma consists of:

• 90-92% water
• 5-8% proteins, including: fibrinogens, globulins, and albumins.
• Small quantities of minerals, such as: Na+, Ca++, Mg++, HCO3 –, Cl–.
• Other dissolved substances, including: Amino acids, glucose, lipids, etc.

Formed Elements of Blood

The formed elements make up approximately 45% of the total blood volume. They include red blood cells, white blood cells, and platelets, each with a distinct structure and function. Here is an overview of all three elements:

1. Erythrocytes (Red Blood Cells / RBCs)
   • RBCs are biconcave, disc-shaped cells that lack a nucleus.
   • They are produced in the red bone marrow.
   • A healthy adult has about 5.0 – 5.5 million RBCs per cubic millimetre (mm³) of blood.
   • RBCs contain hemoglobin, a complex protein responsible for transporting oxygen and carbon dioxide.
   • Their average lifespan is 120 days.
2. Leukocytes (White Blood Cells / WBCs)
   • WBCs are nucleated cells and are much fewer in number than RBCs, with an average count of 6000 – 8000 cells per mm³ of blood.
   • They play a central role in defense and immunity.
   • WBCs are classified into:
     • Granulocytes
     • Agranulocytes
   • Two major functional types are:
3. Thrombocytes (Platelets)
   • Platelets are small, non-nucleated cell fragments.
   • A normal platelet count ranges from 1,50,000 – 3,50,000 per mm³ of blood.
   • They play a crucial role in blood clotting, as they release substances that aid in coagulation and the prevention of excessive bleeding.

Blood Groups

Blood groups are an essential topic under body fluids and circulation. Worldwide, two major systems are used for blood group classification: ABO and Rh.

The ABO blood group system is based on the presence or absence of two antigens—A and B—on the surface of red blood cells (RBCs). Depending on which antigens are present (or absent), human blood is classified into four groups:

• A
• B
• AB
• O

Each blood group also contains specific antibodies in the plasma, and the combination of antigens and antibodies determines compatibility during blood transfusion.

The table below summarizes the details of the ABO blood groups:

Blood Group	Antigens on RBCs	Antibodies in Plasma	Donor’s Group
A	A	anti-b	A, O
B	B	anti- a	B, O
AB	A, B	nil	AB, A, B, O
O	nill	anti-a,b	O

People with the O blood group are known as Universal Donors because their blood can be safely given to individuals of any other blood group. In contrast, people with the AB blood group are called Universal Recipients, as they can receive blood from all other blood groups without complications.

Also Read: 20 NEET Zoology Chapter-Wise Questions and Answers

RH Group

The Rh factor is an important component of blood and plays a major role in circulation and pregnancy. About 80% of people have the Rh antigen on their red blood cells.

• Individuals with the antigen are Rh-positive
• Individuals without the antigen are Rh-negative

Rh Incompatibility

Problems may arise during pregnancy when:

• The mother is Rh-negative
• The foetus is Rh-positive

If the blood of the mother and foetus mixes (usually during childbirth), the mother’s immune system may produce antibodies against the Rh antigen. These antibodies can enter the foetus’s circulation in subsequent pregnancies and destroy foetal RBCs, which can lead to a serious and potentially fatal condition known as erythroblastosis fetalis.

This condition can be prevented by administering anti-Rh antibodies (Rh immunoglobulin) to the mother shortly after delivery.

Coagulation of Blood

Blood clotting (coagulation) is a protective mechanism that prevents excessive blood loss after an injury. Steps involved in clot formation

1. Injury or trauma occurs.
2. A clot (coagulum) forms at the site due to a mesh of protein threads called fibrin.
3. Thrombin, an enzyme, converts inactive fibrinogen → fibrin.
4. Prothrombin, present in the plasma, is converted into thrombin.
5. The conversion requires thrombokinase, an enzyme complex formed through a chain of reactions.

Lymph (Tissue Fluid)

Lymph is a fluid connective tissue formed when blood filters through capillaries into tissues. The characteristics of Lymph are:

• Has a mineral composition similar to plasma
• Participates in the exchange of gases, nutrients, and waste between blood and body cells
• Contains lymphocytes, contributing to immune defence
• Transports nutrients, hormones, etc., to tissues

Circulatory Pathway

Moving on to the topic of body fluids and circulation, let’s discuss the circulatory pathways. These pathways are broadly classified into two main types: open circulatory system and closed circulatory system. Here are the types:

Type	Description
Open circulation	Blood is not confined to vessels; it flows through open cavities (sinuses), and organs lie directly in the sinuses
Closed circulation	Blood circulates through an interconnected network of vessels

Circulation in different organisms

Name	Type of Heart	Circulation
Fish	2-chambered (1 atrium, 1 ventricle)	Single circulation
Amphibian and Reptiles	3-chambered, 1-atrium and 1-ventricle	Incomplete double circulation
Crocodiles, Birds and Mammals	4-chambered, 2-atrium and 1-ventricle	Double circulation

Human Circulatory System

The heart, mesodermal in origin, lies in the thoracic cavity between the lungs and is slightly tilted to the left. It is roughly the size of a fist.

Valves

• Bicuspid (mitral) valve — between the left atrium and left ventricle
• Tricuspid valve — between the right atrium and right ventricle
• The left ventricle opens to the aorta, while the right ventricle opens to the pulmonary artery

Nodal tissues (conducting system)

• Sinoatrial node (SAN) — located in the upper right corner of the right atrium; acts as the pacemaker of the heart and generates 70–75 action potentials per minute
• Atrioventricular node (AVN) — located in the lower left corner of the right atrium
• AV bundles + Purkinje fibres form the Bundle of His, which conducts impulses throughout the ventricles and helps regulate heartbeat

Structure of blood vessels

Arteries and veins consist of three layers:

1. Tunica intima (inner layer)
2. Tunica media (middle layer)
3. Tunica externa (outer layer)

Cardiac Cycle

In the study of body fluids and circulation, the cardiac cycle refers to the sequence of events that occur during one complete heartbeat. The cycle begins with joint diastole, a phase where all four chambers of the heart are relaxed. Stages of the Cardiac Cycle

As ventricular pressure decreases after contraction, the valves reset, and the cycle begins again.

• During joint diastole, the bicuspid (mitral) and tricuspid valves open, allowing blood to flow into the left and right ventricles.
• The semilunar valves remain closed at this stage.
• The SA (sinoatrial) node generates an action potential, triggering the contraction of both atria.
• This electrical impulse then reaches the AV (atrioventricular) node.
• From the AV node, the impulse travels through the Bundle of His and then to the ventricular muscle fibres.
• This causes ventricular systole (ventricular contraction).
• While the ventricles contract, the atria enter a relaxation phase. The bicuspid and tricuspid valves close, and the semilunar valves open, allowing blood to enter the circulatory system.
• The closing of valves prevents the backflow of blood, ensuring one-way circulation.

Heart Sounds

Each cardiac cycle produces two characteristic sounds:

• “Lub” – caused by the closing of the bicuspid and tricuspid valves.
• “Dub” – caused by the closing of the semilunar valves.

Electrocardiogram (ECG)

An ECG is a graphical recording of the electrical activity of the heart during a cardiac cycle. It is a key tool in understanding heart function and diagnosing cardiac issues. Components of an ECG Waveform

• P wave: Represents atrial depolarisation, which leads to atrial contraction.
• QRS complex: Represents ventricular depolarisation, triggering ventricular contraction.
• T wave: Represents ventricular repolarisation, where the ventricles return to their resting state. The end of the T wave marks the end of systole.

The number of QRS complexes per unit time is used to calculate the heart rate.

Double Circulation

Moving ahead in body fluids and circulation, let us now find out about double circulation. The blood flows twice through the heart, once in an oxygenated state and the other in a deoxygenated state. This is called Double circulation and includes systemic and pulmonary circulation.

• Systemic circulation– Flow of oxygenated blood from the left ventricle to all parts of the body and deoxygenated blood from other parts of the body to the atrium.
• Pulmonary circulation – It is the flow of deoxygenated blood from the right ventricle to the lungs and oxygenated blood from the lungs to the left atrium.

Regulation of Cardiac Activity

In the chapter on body fluids and circulation, an important topic is how cardiac activity is regulated, since the heart must work continuously to pump blood throughout the body.

The functioning of the heart is controlled by the autonomic nervous system (ANS), with its regulatory centre located in the medulla oblongata of the brain.

• Sympathetic nerves and hormones released by the adrenal medulla increase heart rate and enhance cardiac output.
• Parasympathetic nerves, on the other hand, act to reduce the heart rate.

Disorder of the Circulatory System

Disorders of the circulatory system can disrupt the normal flow of blood and other body fluids, affecting the functioning of vital organs. Some common circulatory system disorders include:

1. High Blood Pressure (Hypertension): When blood pressure remains consistently above 140/90 mm Hg, it can strain the heart and blood vessels. Over time, this may lead to heart disease and can also harm organs such as the brain and kidneys.
2. Coronary Artery Disease (CAD): Also known as atherosclerosis, CAD occurs when the coronary arteries supplying blood to the heart become narrow. This narrowing is caused by the buildup of cholesterol, fats, calcium, or fibrous tissue, reducing blood flow to the heart muscle.
3. Angina (Angina Pectoris): Angina is severe chest pain that occurs when the heart muscles do not receive enough oxygen. It is usually a result of reduced or interrupted blood flow to the heart.
4. Heart Failure: Heart failure is a condition in which the heart is unable to pump sufficient blood to meet the body’s needs. This is different from a heart attack, which is caused by the sudden blockage of blood flow leading to damage of the heart muscle, or cardiac arrest, where the heart stops beating altogether.
5. Coronary Thrombosis: This condition involves the formation of a blood clot within a coronary artery. It most commonly occurs in the left anterior descending (LAD) artery, which can severely block blood flow to the heart.

Also Read: What is the Main Function of Capillaries?

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