Body Fluids and Circulation NEET Notes 2027 and PDF

Body Fluids and Circulation is Chapter 15 of NCERT Class 11 Biology and part of Unit 5, Human Physiology, of the NEET UG syllabus. These notes cover plasma and formed elements, ABO and Rh blood groups, coagulation, lymph, circulatory pathways, the human heart, cardiac cycle, ECG, double circulation, regulation and disorders, with a free notes PDF.

Body Fluids and Circulation NEET Notes

Section-wise key points from NCERT Chapter 15, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Body Fluids and Circulation at a Glance

Item	Details
NEET UG syllabus unit	Unit 5: Human Physiology
NCERT textbook	Biology, Class 11, Chapter 15 (rationalised book)
Other chapters in the unit	Breathing and Exchange of Gases (Chapter 14), Excretory Products and their Elimination (Chapter 16), Locomotion and Movement (Chapter 17), Neural Control and Coordination (Chapter 18), Chemical Coordination and Integration (Chapter 19)
Syllabus topics	Composition of blood, blood groups, coagulation of blood; composition of lymph and its function; human circulatory system, structure of human heart and blood vessels; cardiac cycle, cardiac output, ECG; double circulation; regulation of cardiac activity; disorders of circulatory system (hypertension, coronary artery disease, angina pectoris, heart failure)
Official chapter	NCERT Chapter 15
Notes PDF	View PDF

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

What is blood made of?

Blood is a special connective tissue made of a fluid matrix, plasma, and formed elements.

Plasma

Plasma is a straw coloured, viscous fluid forming nearly 55% of blood.
90-92% of plasma is water and proteins form 6-8% of it.
Major plasma proteins: fibrinogen (clotting), globulins (defence mechanisms) and albumins (osmotic balance).
Plasma also has small amounts of minerals (Na+, Ca++, Mg++, HCO3-, Cl-) and glucose, amino acids and lipids in transit.
Clotting factors are present in plasma in an inactive form. Plasma without the clotting factors is serum.

Formed Elements

Erythrocytes, leucocytes and platelets are the formed elements and make up nearly 45% of blood.

Formed element	Count	Key points
Erythrocytes (RBCs)	5 to 5.5 million per cubic mm of blood (healthy adult man)	Most abundant cells; formed in red bone marrow in adults; no nucleus in most mammals; biconcave; 12-16 g haemoglobin per 100 ml blood; life span 120 days; destroyed in the spleen (graveyard of RBCs)
Leucocytes (WBCs)	6000-8000 per cubic mm	Colourless (no haemoglobin), nucleated, generally short-lived; granulocytes and agranulocytes
Platelets (thrombocytes)	1,50,000-3,50,000 per cubic mm	Cell fragments from megakaryocytes in bone marrow; release substances for clotting; a low count causes clotting disorders and excessive blood loss

WBC type	Share of total WBCs	Role
Neutrophils (granulocyte)	60-65% (most abundant)	Phagocytic; destroy foreign organisms
Lymphocytes (agranulocyte)	20-25%	B and T lymphocytes; immune responses
Monocytes (agranulocyte)	6-8%	Phagocytic
Eosinophils (granulocyte)	2-3%	Resist infections; associated with allergic reactions
Basophils (granulocyte)	0.5-1% (least)	Secrete histamine, serotonin, heparin; involved in inflammatory reactions

Blood Groups: ABO and Rh

ABO grouping depends on the presence or absence of antigens A and B on RBCs, and Rh grouping on the Rh antigen.

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	nil	anti-A, B	O

Group O people are universal donors; group AB people are universal recipients.
Unmatched transfusion causes clumping (destruction of RBCs).
The Rh antigen, similar to one in Rhesus monkeys, is on the RBCs of nearly 80% of humans (Rh+ve). An Rh-ve person exposed to Rh+ve blood forms antibodies, so Rh must also be matched.
Erythroblastosis foetalis: an Rh-ve mother carrying an Rh+ve foetus. There is no exposure in the first pregnancy because the placenta separates the bloods, but exposure is possible during the first delivery. In later pregnancies the mother’s anti-Rh antibodies can destroy foetal RBCs, causing severe anaemia and jaundice or death.
It is avoided by giving anti-Rh antibodies to the mother immediately after the delivery of the first child.

How does blood clot?

Coagulation stops excessive blood loss after an injury; the clot (coagulum) is a network of fibrin threads in which dead and damaged formed elements are trapped.

An injury stimulates platelets to release factors that start coagulation; factors released by injured tissues can also start it.
A cascade of linked enzymic reactions involving inactive plasma factors forms the enzyme complex thrombokinase.
With thrombokinase, inactive prothrombin is converted into the enzyme thrombin.
Thrombin converts inactive fibrinogen into fibrin threads.
Calcium ions play a very important role in clotting.

Lymph (tissue Fluid)

Lymph is the fluid in the lymphatic system, formed from the tissue fluid that leaks out of blood capillaries.

As blood passes through capillaries, water and small water-soluble substances move into the spaces between cells, leaving larger proteins and most formed elements in the vessels. This is interstitial or tissue fluid.
Tissue fluid has the same mineral distribution as plasma, and all exchange of nutrients and gases between blood and cells happens through it.
The lymphatic system collects this fluid and drains it back into the major veins.
Lymph is colourless and has specialised lymphocytes for immune responses. It also carries nutrients and hormones.
Fats are absorbed through lymph in the lacteals of the intestinal villi.

Circulatory Pathways in Animals

Circulation is either open or closed, and the number of heart chambers decides whether oxygenated and deoxygenated blood mix.

Type	How it works	Animals
Open circulation	Blood is pumped through large vessels into open spaces or body cavities called sinuses	Arthropods, molluscs
Closed circulation	Blood always flows through a closed network of vessels; flow can be regulated more precisely	Annelids, chordates

Animals	Heart	Circulation
Fishes	2 chambers: one atrium, one ventricle	Single circulation: the heart pumps deoxygenated blood, which is oxygenated by the gills
Amphibians and reptiles (except crocodiles)	3 chambers: two atria, one ventricle	Incomplete double circulation: blood mixes in the single ventricle
Crocodiles, birds and mammals	4 chambers: two atria, two ventricles	Birds and mammals: double circulation with no mixing

Human Heart: Structure and Nodal Tissue

The human heart is a mesodermally derived, four-chambered pump in the thoracic cavity between the lungs, slightly tilted to the left, about the size of a clenched fist.

It is protected by the double-walled pericardium, which encloses pericardial fluid.
Two small upper atria and two larger lower ventricles. A thin muscular inter-atrial septum separates the atria and a thick-walled inter-ventricular septum separates the ventricles. A thick fibrous atrio-ventricular septum separates the atrium and ventricle of each side.
Tricuspid valve (three cusps): right atrium to right ventricle. Bicuspid or mitral valve: left atrium to left ventricle. Semilunar valves guard the openings of the ventricles into the pulmonary artery and aorta.
Valves allow blood to flow only one way and prevent backflow.
The whole heart is made of cardiac muscle; the walls of the ventricles are much thicker than those of the atria.

Nodal tissue	Location	Role
Sino-atrial node (SAN)	Right upper corner of the right atrium	Generates the most action potentials (70-75 per minute); starts and maintains the rhythm, so it is the pacemaker
Atrio-ventricular node (AVN)	Lower left corner of the right atrium, close to the atrio-ventricular septum	Conducts the impulse to the ventricular side
AV bundle (bundle of His)	From the AVN through the atrio-ventricular septa to the top of the inter-ventricular septum, where it divides into right and left bundles	Transmits the impulse through the ventricular musculature
Purkinje fibres	Minute fibres throughout the ventricular musculature	Spread the impulse through each ventricle

Nodal musculature is autoexcitable: it generates action potentials without external stimuli.
The heart normally beats 70-75 times a minute (average 72 beats per minute).

Cardiac Cycle, Cardiac Output and Heart Sounds

The cardiac cycle is the sequence of systole and diastole of the atria and ventricles that repeats with every heartbeat.

Joint diastole: all four chambers relax; tricuspid and bicuspid valves are open and blood flows from the pulmonary veins and vena cava into the ventricles; semilunar valves are closed.
Atrial systole: the SAN fires and both atria contract, increasing blood flow into the ventricles by about 30%.
Ventricular systole: the impulse passes through the AVN, AV bundle and bundle of His; the ventricles contract while the atria relax. The tricuspid and bicuspid valves close and the semilunar valves open, sending blood into the pulmonary artery and aorta.
Ventricular diastole: ventricular pressure falls, the semilunar valves close, the tricuspid and bicuspid valves are pushed open, and the heart returns to joint diastole.

Term	Value or meaning
Heart rate	72 beats per minute (average)
Duration of one cardiac cycle	0.8 seconds
Stroke volume	About 70 mL pumped by each ventricle per cycle
Cardiac output	Stroke volume x heart rate: about 5000 mL (5 litres) per minute per ventricle; higher in athletes
First heart sound (lub)	Closure of the tricuspid and bicuspid valves
Second heart sound (dub)	Closure of the semilunar valves

Electrocardiogram (ECG)

An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.

A standard ECG uses three electrical leads: one to each wrist and one to the left ankle. Multiple chest leads are used for detailed evaluation.

Wave	What it represents
P wave	Depolarisation (electrical excitation) of the atria, leading to contraction of both atria
QRS complex	Depolarisation of the ventricles, which starts ventricular contraction; contraction begins shortly after Q, marking the start of systole
T wave	Repolarisation of the ventricles (return to normal state); the end of the T wave marks the end of systole

Counting QRS complexes in a given time gives the heart rate.
Any deviation from the usual shape suggests a possible abnormality, so the ECG has great clinical significance.

Double Circulation and Blood Vessels

Humans have complete double circulation: separate pulmonary and systemic pathways with no mixing of blood.

Arteries and veins have three layers: tunica intima (squamous endothelium), tunica media (smooth muscle and elastic fibres) and tunica externa (fibrous connective tissue with collagen fibres). The tunica media is comparatively thin in veins.
Pulmonary circulation: right ventricle, pulmonary artery, lungs, pulmonary veins, left atrium.
Systemic circulation: left ventricle, aorta, arteries, arterioles and capillaries to the tissues, then venules, veins and vena cava to the right atrium.
Hepatic portal system: the hepatic portal vein carries blood from the intestine to the liver before it enters the systemic circulation.
Coronary system: blood vessels exclusively for circulation to and from the cardiac musculature.

Regulation of Cardiac Activity

The heart is myogenic: its normal activity is regulated intrinsically by the nodal tissue, and nerves and hormones only moderate it.

Regulator	Effect on the heart
Neural centre in the medulla oblongata	Moderates cardiac function through the autonomic nervous system (ANS)
Sympathetic nerves	Increase heart rate, strength of ventricular contraction and cardiac output
Parasympathetic nerves	Decrease heart rate, speed of conduction of action potential and cardiac output
Adrenal medullary hormones	Increase cardiac output

Disorders of the Circulatory System

The syllabus names four circulatory disorders, all described briefly in NCERT.

Disorder	Key points
Hypertension (high blood pressure)	Normal is 120/80 mm Hg (systolic/diastolic). Repeated readings of 140/90 or higher show hypertension, which leads to heart disease and affects the brain and kidney.
Coronary artery disease (CAD)	Often called atherosclerosis; deposits of calcium, fat, cholesterol and fibrous tissue narrow the lumen of the arteries that supply heart muscle.
Angina (angina pectoris)	Acute chest pain when not enough oxygen reaches the heart muscle; more common in the middle-aged and elderly.
Heart failure	The heart does not pump blood effectively enough for the body’s needs; also called congestive heart failure because lung congestion is a main symptom. It is not the same as cardiac arrest or a heart attack.

Most-asked Facts for NEET: Body Fluids and Circulation

Revise these one-line facts from the NCERT chapter just before the exam.

Plasma is about 55% of blood; formed elements about 45%.
Fibrinogen: clotting; globulins: defence; albumins: osmotic balance. Plasma without clotting factors is serum.
RBCs: 5 to 5.5 million per cubic mm, life span 120 days, destroyed in the spleen.
Haemoglobin: 12-16 g per 100 ml of blood.
Neutrophils are the most abundant WBCs (60-65%); basophils the least (0.5-1%).
Neutrophils and monocytes are phagocytic; basophils secrete histamine, serotonin and heparin.
Platelets come from megakaryocytes: 1,50,000-3,50,000 per cubic mm.
Group O: universal donor; group AB: universal recipient.
About 80% of humans are Rh+ve.
Thrombin converts fibrinogen into fibrin; calcium ions are essential for clotting.
Fats are absorbed through lymph in the lacteals of intestinal villi.
Open circulation: arthropods and molluscs; closed circulation: annelids and chordates.
SAN is the pacemaker (70-75 action potentials per minute).
Cardiac cycle 0.8 s; stroke volume about 70 mL; cardiac output about 5 litres per minute.
Lub: closure of AV (tricuspid and bicuspid) valves; dub: closure of semilunar valves.
ECG: P wave is atrial depolarisation, QRS is ventricular depolarisation, T wave is ventricular repolarisation.
Hepatic portal vein carries blood from the intestine to the liver.
Hypertension: repeated readings of 140/90 mm Hg or higher.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

Saying lymph has the same protein content as blood. Larger proteins and most formed elements stay in the blood vessels.
Mixing up universal donor and universal recipient: O is the universal donor, AB the universal recipient.
Saying erythroblastosis foetalis affects the first child. The first pregnancy is normally safe; the danger is in later pregnancies of an Rh-ve mother with an Rh+ve foetus.
Placing the SAN in the left atrium. Both the SAN and AVN are in the right atrium.
Linking the first heart sound with semilunar valves. Lub is closure of the tricuspid and bicuspid valves; dub is closure of the semilunar valves.
Reading the T wave as atrial repolarisation. The T wave is ventricular repolarisation; the P wave is atrial depolarisation.
Calling the heart neurogenic. It is myogenic; the medulla and ANS only moderate its activity.
Saying parasympathetic signals increase heart rate. Sympathetic signals increase it; parasympathetic signals decrease it.
Saying all reptiles have a 3-chambered heart. Crocodiles have a 4-chambered heart.
Treating heart failure, cardiac arrest and heart attack as the same. NCERT separates all three.

Download the AglaSem notes for Body Fluids and Circulation (key notes and important questions with answers) free: View PDF

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Full topic list: NEET syllabus
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Source text: NCERT Chapter 15 (NCERT, Class 11 Biology)

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Body Fluids and Circulation NEET Notes FAQs

Which NCERT chapter is Body Fluids and Circulation?

It is Chapter 15 of the NCERT Class 11 Biology textbook (rationalised edition), in Unit 5, Human Physiology. In the NEET UG syllabus it is part of Unit 5, Human Physiology.

Is Body Fluids and Circulation in the NEET 2027 syllabus?

It is in the latest NEET UG syllabus (2026, released by NMC on 22 December 2025) under Unit 5, Human Physiology. NMC is expected to release the 2027 syllabus later; check the syllabus page once it is out.

How is cardiac output calculated?

Cardiac output is stroke volume multiplied by heart rate. With a stroke volume of about 70 mL and 72 beats per minute, it is about 5000 mL or 5 litres per minute for each ventricle in a healthy person.

Why is the SAN called the pacemaker?

The SAN, in the right upper corner of the right atrium, generates the maximum number of action potentials (70-75 per minute), so it starts and maintains the rhythmic contraction of the heart.

Are these notes enough for NEET questions on circulation?

No. Read NCERT Chapter 15 line by line first, because NEET Biology questions closely follow the NCERT textbooks. Use these notes and the PDF for quick revision, then practise previous year questions.

Body Fluids and Circulation NEET Notes 2027: Key Points and PDF