CBSE Class 11 Biology Revision Notes Chapter 15 Body Fluids and Circulation

Body Fluids and Circulation explains how blood and lymph transport nutrients, gases, hormones and waste materials in the body.

In CBSE Class 11 Biology, this chapter covers blood, lymph, blood groups, clotting, heart, cardiac cycle, ECG and double circulation.

Body Fluids and Circulation explains how essential substances reach cells and how waste materials are removed from tissues. Simple animals may use water from their surroundings for transport, but higher organisms use body fluids such as blood and lymph.

Use CBSE Class 11 Biology Revision Notes Chapter 15 for 2026–27 to revise blood composition, plasma, formed elements, ABO and Rh blood groups, coagulation, lymph, circulatory pathways, human heart, cardiac cycle, ECG, double circulation and disorders of the circulatory system.

Key Takeaways

• Blood: A special connective tissue made of plasma and formed elements.
• Plasma: Straw-coloured fluid that forms nearly 55% of blood.
• Formed elements: RBCs, WBCs and platelets form nearly 45% of blood.
• Double circulation: Blood passes through the heart twice in one complete circuit.

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Body Fluids and Circulation Class 11 Biology Notes: Chapter Overview

Living cells need nutrients, oxygen and other essential substances. They also need continuous removal of waste and harmful substances.

Blood is the most commonly used transport fluid in higher organisms. Lymph also helps in transport and immune responses.

Blood in CBSE Class 11 Biology Notes Chapter 15

Blood is a special connective tissue. It has a fluid matrix called plasma and cellular components called formed elements.

Blood helps in transport, defence, clotting and maintenance of internal balance.

Plasma in Class 11 Biology Chapter 15 Notes

Plasma is a straw-coloured, viscous fluid. It forms nearly 55% of blood.

Plasma also contains clotting factors in inactive form. Plasma without clotting factors is called serum.

Formed Elements in Body Fluids and Circulation Class 11 Notes

Formed elements include erythrocytes, leucocytes and platelets.

Together, formed elements constitute nearly 45% of blood.

Erythrocytes or Red Blood Cells

RBCs are the most abundant blood cells.

Haemoglobin is an iron-containing red pigment. It helps in the transport of respiratory gases.

A healthy individual has about 12 to 16 g of haemoglobin in every 100 mL of blood.

Leucocytes or White Blood Cells

WBCs are colourless because they lack haemoglobin. They are nucleated and fewer than RBCs.

WBCs are grouped into granulocytes and agranulocytes.

Types of White Blood Cells

Neutrophils are the most abundant WBCs. Basophils are the least abundant WBCs.

WBC Percentage

B and T lymphocytes are responsible for immune responses of the body.

Platelets in Class 11 Biology Chapter 15 Notes

Platelets are cell fragments produced from megakaryocytes in the bone marrow.

A decrease in platelet count may cause clotting disorders and excessive blood loss.

Blood Groups in CBSE Class 11 Biology Revision Notes Chapter 15

Human blood differs based on antigens present on RBCs. Two major blood grouping systems are ABO and Rh.

Blood groups must be matched before transfusion.

ABO Blood Grouping System

ABO grouping depends on A and B antigens on RBCs and antibodies in plasma.

Group O is called the universal donor. Group AB is called the universal recipient.

Rh Blood Grouping

Rh antigen is similar to one found in Rhesus monkeys. It is present on RBCs of nearly 80% of humans.

Rh group must be matched before blood transfusion. An Rh-negative person exposed to Rh-positive blood can form antibodies against Rh antigen.

Erythroblastosis Foetalis

Rh incompatibility may occur between an Rh-negative mother and an Rh-positive foetus.

This condition is called erythroblastosis foetalis. It can be prevented by giving anti-Rh antibodies to the mother immediately after delivery of the first child.

Coagulation of Blood in Body Fluids and Circulation Class 11 Notes

Coagulation is blood clotting in response to injury or trauma. It prevents excessive blood loss.

A clot is formed mainly of fibrin threads. Dead and damaged formed elements get trapped in this fibrin network.

Simple Sequence of Blood Clotting

Clotting is a cascade process involving many inactive factors in plasma.

Lymph or Tissue Fluid in Class 11 Biology Chapter 15 Notes

As blood passes through tissue capillaries, some water and small water-soluble substances move out into spaces between cells. This fluid is called interstitial fluid or tissue fluid.

Lymph has the same mineral distribution as plasma but contains fewer proteins and formed elements.

Functions of Lymph

Lymph acts as a drainage and defence system.

Circulatory Pathways in CBSE Class 11 Biology Notes Chapter 15

Circulatory systems may be open or closed.

Closed circulation is more efficient because blood flow can be regulated more precisely.

Heart Types in Vertebrates

All vertebrates have a muscular chambered heart, but the number of chambers differs.

Birds and mammals have complete separation of oxygenated and deoxygenated blood.

Human Circulatory System

The human circulatory system is also called the blood vascular system. It includes the heart, blood vessels and blood.

The human heart is present in the thoracic cavity between the two lungs and is slightly tilted to the left.

Structure of Human Heart in Body Fluids and Circulation Class 11 Notes

The heart is a mesodermally derived organ. It is protected by a double-walled pericardium containing pericardial fluid.

Ventricular walls are thicker than atrial walls because ventricles pump blood out of the heart.

Valves of Human Heart

Heart valves maintain one-way flow of blood.

Valves allow blood to flow from atria to ventricles and from ventricles to pulmonary artery or aorta.

Nodal Tissue and Pacemaker

The heart has specialised cardiac musculature called nodal tissue. It can generate action potentials without external stimulus.

The sino-atrial node can generate 70–75 action potentials per minute. It initiates and maintains the rhythmic activity of the heart, so it is called the pacemaker.

The human heart normally beats 70–75 times per minute, with an average of 72 beats per minute.

Cardiac Cycle in Class 11 Biology Chapter 15 Notes

The cardiac cycle is the sequence of events in one heartbeat. It includes systole and diastole of atria and ventricles.

Steps of Cardiac Cycle

Atrial systole increases blood flow into ventricles by about 30%.

Duration and Output of Cardiac Cycle

Cardiac output = Stroke volume × Heart rate.

Heart Sounds

During each cardiac cycle, two main sounds are produced.

These sounds can be heard using a stethoscope and are clinically important.

Electrocardiogram or ECG in Body Fluids and Circulation Class 11 Notes

An electrocardiogram is a graphical record of the electrical activity of the heart during a cardiac cycle.

The QRS complex helps determine the heartbeat rate. Any deviation from a normal ECG pattern may indicate abnormality or disease.

Double Circulation in CBSE Class 11 Biology Notes Chapter 15

In double circulation, blood passes through the heart twice in one complete circuit. It includes pulmonary circulation and systemic circulation.

Double circulation prevents mixing of oxygenated and deoxygenated blood.

Blood Vessels in Human Circulation

Blood vessels carry blood through a fixed route.

Layers of Blood Vessels

Veins have a comparatively thinner tunica media than arteries.

Hepatic Portal System and Coronary Circulation

Some special circulatory routes support specific organs.

The coronary system is important because heart muscles need continuous oxygen and nutrients.

Regulation of Cardiac Activity

The heart is myogenic because normal activities are regulated intrinsically by nodal tissue.

The autonomic nervous system helps adjust cardiac activity according to body needs.

Disorders of Circulatory System

Circulatory disorders affect heart, blood vessels and blood pressure.

Hypertension

Normal blood pressure is about 120/80 mm Hg. If repeated checks show 140/90 mm Hg or higher, it indicates hypertension.

High blood pressure may lead to heart disease and affect organs such as brain and kidneys.

Coronary Artery Disease

Coronary Artery Disease is also called atherosclerosis. It affects blood vessels that supply the heart muscle.

This reduces blood supply to heart muscles.

Angina

Angina pectoris causes acute chest pain when enough oxygen does not reach the heart muscle.

It may occur in both men and women of any age, but it is more common among middle-aged and elderly people.

Heart Failure

Heart failure occurs when the heart cannot pump enough blood to meet the needs of the body.

It is sometimes called congestive heart failure because lung congestion is one of its main symptoms.

Blood and Lymph Difference

Open and Closed Circulatory System Difference

Pulmonary and Systemic Circulation Difference

Important Terms from Body Fluids and Circulation Class 11 Notes

NCERT-Based Exam Points

• Blood is a special connective tissue.
• Plasma forms nearly 55% of blood.
• Formed elements form nearly 45% of blood.
• Plasma has 90–92% water.
• Plasma proteins include fibrinogen, globulins and albumins.
• Serum is plasma without clotting factors.
• RBCs are formed in red bone marrow in adults.
• RBCs are biconcave and enucleated in most mammals.
• Haemoglobin helps transport respiratory gases.
• RBCs live for about 120 days.
• Spleen is called the graveyard of RBCs.
• WBCs are colourless because they lack haemoglobin.
• Neutrophils are the most abundant WBCs.
• Basophils are the least abundant WBCs.
• B and T lymphocytes help in immune responses.
• Platelets are produced from megakaryocytes.
• ABO grouping depends on A and B antigens.
• O group is the universal donor.
• AB group is the universal recipient.
• Rh mismatch may cause erythroblastosis foetalis.
• Fibrin threads form the blood clot.
• Calcium ions are important in clotting.
• Lymph returns tissue fluid to blood.
• Fats are absorbed through lymph in lacteals.
• Arthropods and molluscs have open circulation.
• Annelids and chordates have closed circulation.
• Fishes have a 2-chambered heart.
• Birds and mammals have a 4-chambered heart.
• SAN is the pacemaker of the heart.
• Average heartbeat is about 72 beats per minute.
• One cardiac cycle takes about 0.8 seconds.
• Stroke volume is about 70 mL.
• Cardiac output is about 5 litres per minute.
• P wave represents atrial excitation.
• QRS complex represents ventricular depolarisation.
• T wave represents ventricular repolarisation.
• Double circulation includes pulmonary and systemic circulation.
• The heart is myogenic.
• Hypertension means blood pressure higher than normal.
• Coronary Artery Disease is linked with narrowing of coronary arteries.

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