Blood and Circulatory System – NDA Biology Notes

What is Blood?

Blood is a fluid that flows through the entire body. It carries oxygen, food, hormones, and waste products from one place to another. It also defends the body against infections.

Blood is classified as a connective tissue. It is not epithelial tissue, not muscular tissue, not nervous tissue. [NDA 2019-III]

Blood transports hormones and heat. It also maintains the water balance of the body. [NDA tested via CDS]

The thickness (viscosity) of blood is due to the presence of RBCs and WBCs in it — not proteins alone, not platelets alone, not sodium alone. [NDA tested via CDS]

The normal pH of human blood is 7.5 to 8.0. This means blood is slightly alkaline — not acidic. [NDA tested via CDS]

Components of Blood

Blood has four main components:

Component	Also Called	Type	Main Function
Red Blood Cells	RBC / Erythrocytes	Cell	Carry oxygen to all parts of the body
White Blood Cells	WBC / Leukocytes	Cell	Defend the body against infections
Platelets	Thrombocytes	Cell fragment	Help in blood clotting
Plasma	—	Liquid	Transports everything dissolved in it

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Red Blood Cells (RBC)

Red blood cells carry oxygen from the lungs to all parts of the body. They also carry some carbon dioxide back to the lungs.

RBCs get their red colour from a protein called haemoglobin. Haemoglobin picks up oxygen in the lungs and releases it in the tissues.

Key properties of RBCs — frequently tested:

• RBCs have no nucleus. [NDA 2017-II] [NDA tested via CDS]
• RBCs have no mitochondria. [NDA 2017-II]
• RBCs have no endoplasmic reticulum. [NDA 2017-II]
• The absence of a nucleus gives RBCs more space to pack haemoglobin.
• RBCs are produced in the bone marrow.
• Their lifespan is about 120 days.
• Old and damaged RBCs are destroyed in the spleen.

RBCs in different solutions:

• In plain water (hypotonic) — water enters the RBC by osmosis. The RBC swells and bursts. This is called lysis. [NDA 2012-II] [NDA tested via CDS]
• In normal saline (isotonic) — no net movement of water. RBCs show no change in size. [NDA tested via CDS]
• In detergent solution — the cell membrane is disrupted. RBCs lyse (burst). [NDA 2021-I]
• In hypertonic solution — water leaves the cell. RBC shrinks.

Jaundice:

When RBCs are destroyed, haemoglobin is broken down into a yellow pigment called bilirubin. The liver normally processes bilirubin and removes it from the body.

In jaundice, bilirubin is not fully processed. It builds up in the blood and tissues. The skin and eyes turn yellow. This happens due to incomplete metabolism of bilirubin. [NDA 2009-II]

Sickle-cell anaemia:

In sickle-cell anaemia, the RBCs are shaped like a sickle (crescent) instead of a round disc. These misshapen cells cannot carry oxygen properly. They also block blood vessels. Sickle-cell anaemia is an abnormality in red blood cells. [NDA 2012-II]

White Blood Cells (WBC)

White blood cells defend the body against infections. They are the soldiers of the body’s immune system.

Types of WBC and their functions:

• Lymphocytes — produce antibodies against foreign substances (antigens). [NDA 2010-I] [NDA 2024-II] [NDA 2025-I]
• Neutrophils — engulf and destroy bacteria and other pathogens.
• Monocytes — large cells that engulf foreign particles.

WBCs have a nucleus. They are produced in the bone marrow. Both neutrophils and lymphocytes originate from bone marrow. [NDA tested via CDS]

Antibody formation:

When a foreign substance (antigen) enters the body, lymphocytes become active. They produce specific antibodies against that antigen. Antibodies are proteins that neutralise the threat.

Antibody formation takes place in blood plasma cells (which come from lymphocytes). [NDA 2008-II]

WBCs have the highest number of lysosomes among all cell types. This is because WBCs engulf and digest pathogens constantly. [NDA tested via CDS]

HIV and lymphocytes:

HIV (the virus that causes AIDS) specifically destroys lymphocytes. As lymphocyte count falls, the body loses its ability to produce antibodies. This is why HIV weakens the immune system. [NDA tested via CDS]

Dengue and platelets:

Dengue virus reduces the number of platelets, not lymphocytes. Students often confuse this. [NDA 2017-I]

Platelets (Thrombocytes)

Platelets are small cell fragments. They have no nucleus. [NDA tested via CDS]

When a blood vessel is damaged, platelets gather at the site. They release clotting factors. These clotting factors start a chain reaction that leads to blood clotting.

Platelets initiate blood clotting. [NDA 2015-II] [NDA tested via CDS]

Blood clotting process:

The clotting factors released by platelets convert a protein called fibrinogen into fibrin threads. Fibrin threads form a mesh that traps blood cells and seals the wound. Fibrinogen is the clotting protein. [NDA 2025-I]

Inside healthy blood vessels, blood does not clot. This is because blood contains a natural substance called heparin. Heparin prevents the clotting process from starting inside the vessels. [NDA tested via CDS]

Dengue and platelet count:

Dengue virus reduces platelet count by:

• Interfering with platelet production in bone marrow
• Infecting endothelial cells (the cells lining blood vessels)
• Binding directly with platelets

All three mechanisms are correct. [NDA tested via CDS]

Plasma

Plasma is the liquid part of blood. It makes up about 55% of total blood volume.

Plasma transports:

• Nutrients (glucose, amino acids, fats)
• Hormones
• Waste products (carbon dioxide, urea)
• Proteins (including antibodies, fibrinogen, heparin)

Lymph resembles plasma. Lymph is the fluid that leaks out from blood capillaries into the spaces between cells. It has a composition similar to plasma but contains no RBCs. [NDA 2022-III] [NDA tested via CDS]

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Haemoglobin

Haemoglobin is the protein inside RBCs that carries gases.

Haemoglobin carries both oxygen and carbon dioxide — not just oxygen. [NDA tested via CDS]

Carbon monoxide and haemoglobin:

Carbon monoxide (CO) is a gas produced by vehicle exhausts and incomplete combustion. CO binds to haemoglobin with much greater force than oxygen does. Once CO occupies haemoglobin, oxygen cannot attach. This reduces the blood’s ability to carry oxygen. It can cause suffocation and death. [NDA tested via CDS]

Acidic blood and haemoglobin:

If blood becomes acidic (lower pH), the oxygen-carrying capacity of haemoglobin decreases. [NDA tested via CDS]

Altitude sickness:

At high altitudes, the partial pressure of oxygen is low. There is less oxygen available in the air. Haemoglobin cannot pick up as much oxygen. This causes altitude sickness. [NDA tested via CDS]

Blood Groups

Human blood is classified into four main groups based on the ABO system: A, B, AB, and O. [NDA tested via CDS]

Blood group compatibility is critical for blood transfusion. If incompatible blood is given, the recipient’s immune system attacks the donated blood. This can be fatal.

Blood Group	Can Donate To	Can Receive From	Special Name
O	O, A, B, AB	O only	Universal donor
A	A, AB	A, O	—
B	B, AB	B, O	—
AB	AB only	O, A, B, AB	Universal recipient

[NDA 2011-I] [NDA tested via CDS]

Key transfusion facts:

• Blood group O is the universal donor — it can give blood to any group. But it can receive blood from O only.
• Blood group AB is the universal recipient — it can receive blood from any group. But it can donate to AB only.
• Group AB is NOT a universal donor. This is a common mistake. [NDA tested via CDS]
• Blood group A can donate to A and AB. It can receive from A and O.
• Blood group B can donate to B and AB. It can receive from B and O. [NDA tested via CDS]

Blood groups and genetics — O × AB parents:

Group O has genotype ii. Group AB has genotype IAIB.

Offspring of O × AB parents can only be blood group A or blood group B. They cannot be O or AB. [NDA 2009-I]

The Human Heart

The human heart has four chambers. [NDA tested via CDS]

• Right atrium (also called right auricle)
• Right ventricle
• Left atrium (also called left auricle)
• Left ventricle

The right side of the heart handles deoxygenated blood (blood that has given up its oxygen to the body). The left side handles oxygenated blood (blood that has collected oxygen from the lungs).

Blood flow through the heart:

• Body → Right atrium → Right ventricle → Lungs (via pulmonary artery)
• Lungs → Left atrium → Left ventricle → Body (via aorta)

This complete circuit — one loop through the lungs and one loop through the body — is called double circulation.

The left atrium is the first chamber to receive oxygenated blood coming back from the lungs. [NDA 2018-II] [NDA tested via CDS]

The left ventricle pumps oxygenated blood to the entire body through the aorta. [NDA tested via CDS]

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Arteries and Veins

Arteries carry blood away from the heart.

Veins carry blood towards the heart.

Feature	Arteries	Veins
Direction of flow	Away from heart	Towards heart
Wall thickness	Thick, elastic	Thin
Valves	No valves	Have valves
Blood type (general)	Oxygenated	Deoxygenated

The pulmonary exception — this is the most tested fact:

The general rule is that arteries carry oxygenated blood and veins carry deoxygenated blood. But the pulmonary vessels are the opposite.

• Pulmonary artery — carries deoxygenated blood from the right ventricle to the lungs. [NDA 2012-I] [NDA 2025-I]
• Pulmonary vein — carries oxygenated blood from the lungs to the left atrium. [NDA 2025-I]

The statement “pulmonary arteries always carry oxygenated blood” is wrong. [NDA 2025-I]

Named blood vessels — exam tested:

• Coronary artery — supplies blood to the heart muscle itself. When blocked, it causes a heart attack. [NDA tested via CDS]
• Hepatic artery — supplies blood to the liver (not kidneys). [NDA tested via CDS]
• Renal artery — supplies blood to the kidneys.

Why arteries survive in ancient mummies:

Arteries have elastic fibres in their walls. These fibres are preserved for thousands of years. That is why ancient mummies in Egypt have intact arteries. [NDA 2015-II]

Why face wounds bleed a lot:

The face has very rich blood supply — many blood vessels are packed into a small area. This is called rich vascularity. That is why even small face wounds bleed heavily. [NDA 2009-I]

Why earlobe piercing does not bleed much:

The earlobe contains cartilage, which has very little blood supply. That is why piercing the earlobe causes little or no bleeding. [NDA 2012-II]

Capillaries:

Capillaries are the smallest blood vessels. They connect arteries and veins. Exchange of oxygen, nutrients, and waste happens across capillary walls. [NDA tested via CDS]

Heart Sounds — Lub Dub

The heart makes two sounds when it beats — “lub” and “dub.” This is the familiar heartbeat sound.

These sounds are produced by the opening and closing of heart valves — not by blood flow, not by heart muscle contraction. [NDA 2011-II] [NDA 2015-I]

Abnormal heart sounds are called murmurs.

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Heart Rate and ECG

Normal adult resting heart rate is 72 to 80 beats per minute. [NDA tested via CDS]

The heartbeat starts from a small region called the sinoatrial node (SA node) in the right atrium. The SA node generates the electrical signal that makes the heart beat. This is why the heart is called myogenic — it generates its own beat without any nerve signal. [NDA tested via CDS]

An ECG (Electrocardiogram) is the graphical record of the electrical activity of the heart — not the brain, not the kidneys, not the cornea. [NDA 2019-II]

Adrenaline is the hormone that increases heart rate. It is not insulin (which regulates blood sugar) and not melatonin. [NDA 2022-II]

Pulse is felt in the aorta and main arteries. It is caused by the rhythmic expansion of arteries due to the pumping action of the heart. [NDA tested via CDS]

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Blood Pressure

Blood pressure is the force of blood pushing against the walls of arteries.

Normal blood pressure is 120/80 mmHg (millimetres of mercury). [NDA tested via CDS]

• 120 = systolic pressure — pressure when the ventricles contract (squeeze). [NDA tested via CDS]
• 80 = diastolic pressure — pressure when the ventricles relax. [NDA tested via CDS]

Blood pressure is measured in mmHg — not in mm water, not in mm blood, not in mm air.

The instrument used to measure blood pressure is called a sphygmomanometer. [NDA tested via CDS]

Lymph and Spleen

Lymph:

Lymph is a colourless fluid. It is formed when blood plasma leaks out from capillaries into the spaces between body cells. [NDA tested via CDS]

Lymph resembles plasma in composition. It does not contain RBCs.

Lymph is collected by lymph vessels and returned to the bloodstream. It is not circulated by the heart.

Spleen:

The spleen is an organ in the upper left part of the abdomen.

It stores extra blood. When the body needs more blood suddenly (during heavy blood loss or exercise), the spleen releases this stored blood. [NDA 2007-II]

It also destroys old and damaged RBCs.

Bone marrow:

All blood cells — RBCs, WBCs (including neutrophils and lymphocytes), and platelets — are produced in the bone marrow. [NDA tested via CDS]

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Blood Separation in Labs

In diagnostic laboratories, blood is separated into its components by spinning it at high speed. This process is called centrifugation. The heavier cells (RBCs, WBCs) settle to the bottom. The lighter plasma stays on top. [NDA 2024-III]

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Cholesterol — HDL and LDL

Cholesterol travels in the blood attached to proteins called lipoproteins.

• HDL (High Density Lipoprotein) — called “good cholesterol.” It removes excess cholesterol from blood vessel walls and carries it to the liver. [NDA tested via CDS]
• LDL (Low Density Lipoprotein) — called “bad cholesterol.” It deposits cholesterol in blood vessel walls. This narrows the vessels and increases risk of heart disease.

Potassium plays a role in controlling the water content of blood. [NDA tested via CDS]

Heart Chambers Across Animals

The number of heart chambers differs across animal groups. This comparison is tested directly and repeatedly.

Animal Group	Heart Chambers	Examples
Fish (Pisces)	2 chambers	Scoliodon, Hagfish
Amphibia	3 chambers	Frog, Salamander
Most Reptiles	3 chambers	Lizard, Snake
Crocodile	4 chambers	Exception among reptiles
Birds	4 chambers	Pigeon, Peacock
Mammals	4 chambers	Human, Camel, Whale

[NDA tested via CDS]

Mammals have four chambers — not three. Saying “mammalian heart is three-chambered” is wrong. [NDA tested via CDS]

Salamander (an amphibian) has three chambers. [NDA tested via CDS]

Crocodile has four chambers — it is the exception among reptiles. [NDA tested via CDS]

Birds and mammals are warm-blooded (they maintain constant body temperature). Fish, amphibia, and most reptiles are cold-blooded. Peacock (bird) and camel (mammal) are both warm-blooded. [NDA tested via CDS]

Cardiac muscle structure:

Heart muscle cells are:

• Cylindrical in shape
• Branched — branches allow electrical signals to spread across the heart quickly
• Uninucleate — one nucleus per cell

This is different from skeletal muscle (multinucleate, unbranched) and smooth muscle (spindle-shaped). [NDA tested via CDS]