Blood and Circulatory System – NDA Biology Notes

Every cell in the body needs oxygen. Every cell produces waste. Every cell needs nutrients delivered and hormones received. Blood does all of this. It does so continuously, without pause, from before birth until death.

Blood is the body’s primary transport system. It carries oxygen from the lungs to every cell. It carries carbon dioxide back to the lungs. It moves nutrients from the gut to tissues, carries hormones from glands to target organs, distributes heat, and maintains the water balance of the body.

For NDA, this chapter delivers some of the most frequently tested facts in biology: the pulmonary paradox, blood group compatibility, haemolysis, haemoglobin and carbon monoxide, cardiac muscle structure, and the comparative heart chamber table. Each section here marks where exam questions come from.

1. What is Blood?

Blood is a living tissue that serves as the body’s internal transport medium. It carries oxygen, removes carbon dioxide, delivers nutrients, transports hormones, distributes heat, and maintains water balance.

Blood is not just a simple fluid. It is composed of several distinct components, each with a specific and non-interchangeable function. Understanding these components individually is the foundation of the entire chapter.

Blood as Connective Tissue

Most students think of blood as a liquid. In biology, blood is classified as a connective tissue. It is not epithelial tissue, not muscular tissue, and not nervous tissue.  [NDA 2019-II]

What makes it connective tissue? All connective tissues share one feature: cells scattered within a non-cellular matrix. In blood, the matrix is plasma, the liquid component. The cellular elements, red blood cells, white blood cells, and platelets, float within it.

Blood is also viscous: thicker than water. This viscosity comes from red blood cells and white blood cells suspended in plasma. Proteins alone do not cause viscosity. Sodium does not cause it. Platelets alone do not cause it. Both RBCs and WBCs together are responsible.

The normal pH of human blood is slightly alkaline, between 7.5 and 8.0. When blood becomes acidic (pH drops), haemoglobin carries less oxygen. Low blood pH directly reduces oxygen transport.

When blood needs to be separated into its components in a laboratory, the technique used is centrifugation. Centrifugation spins the blood at high speed. Heavier components settle to the bottom; lighter components rise to the top. Filtration, chromatography, and crystallisation are not used for this.  [NDA 2024-II]

2. Red Blood Cells (Erythrocytes)

Structure: What Makes RBCs Unique

Red blood cells are unlike almost every other cell in the human body. Mature human RBCs have no nucleus, no mitochondria, and no endoplasmic reticulum. They have shed all their organelles completely.  [NDA 2017-II]

The reason is functional. The job of an RBC is to carry haemoglobin. Every bit of internal space must be available for haemoglobin molecules. Organelles would take up that space. So, the body removes them as RBCs mature.

RBC Behaviour in Different Solutions

What happens to an RBC depends entirely on the solution it is placed in. Three scenarios are tested directly and repeatedly. All three must be known.

Solution	What Happens	Why	Term
Water (hypotonic)	Cell swells and bursts	Water enters by osmosis: lower solute outside	Haemolysis [NDA 2012-II | NDA 2021-I]
Normal saline (isotonic)	No change in size or shape	Equal solute concentration: no net water movement
Detergent solution	Membrane disrupts: contents spill out	Mechanical disruption, not osmosis	[NDA 2021-I]

Haemoglobin and Gas Transport

Haemoglobin is the protein inside RBCs that carries gases. It carries oxygen from the lungs to the body’s cells. It also carries carbon dioxide from cells back to the lungs. It is incorrect to say haemoglobin carries only oxygen. It transports both gases.

Carbon monoxide poisoning.  Carbon monoxide (CO) is produced by incomplete combustion: vehicle exhaust, poorly ventilated fires. CO binds to haemoglobin approximately 200 times more strongly than oxygen.  When CO occupies haemoglobin, oxygen cannot bind. Cells receive no oxygen. This causes suffocation and can cause death.  

Altitude sickness.  At high altitude, atmospheric oxygen is lower. The partial pressure of oxygen is reduced. Haemoglobin cannot saturate with oxygen as easily. Less oxygen reaches body cells. The result is altitude sickness: breathlessness, headache, and fatigue.

3. White Blood Cells (Leukocytes)

White blood cells are the body’s defence force. Their role is to protect against infection and foreign agents. They do not carry oxygen. They do not clot blood. They do not provide energy. Their function is immune defence.  [NDA 2012-II]

Among all body cells, white blood cells have the highest concentration of lysosomes, organelles packed with digestive enzymes. WBCs use them to break down pathogens, debris, and foreign particles after engulfing them.

Both neutrophils and lymphocytes, two major types of WBCs, are produced in the bone marrow. Bone marrow is the primary site of blood cell production in adults.

Lymphocytes and Antibody Production

Among all WBC types, lymphocytes are the specific cells responsible for producing antibodies. When a foreign antigen enters the body, lymphocytes recognise it and produce antibodies, proteins that neutralise that specific antigen.  [NDA 2010-I | NDA 2024-II | NDA 2025-I]

The precise exam-level answer is: lymphocytes produce antibodies. Students sometimes hear that plasma cells produce antibodies. Plasma cells are a mature form of B lymphocytes. At NDA level, the correct and consistent answer is lymphocytes.

HIV targets and destroys lymphocytes. When lymphocyte numbers fall, the body loses its ability to produce antibodies. This is why HIV infection leads to AIDS. The immune system collapses.

4. Platelets and Blood Clotting

Platelets (thrombocytes) are small cell fragments in the blood. Like mature RBCs, they have no nucleus: they are enucleate. When a blood vessel is damaged, platelets rush to the site, clump together at the injury, and release clotting factors that trigger the coagulation cascade.  [NDA 2015-II]

The Clotting Process

Fibrinogen is the key clotting protein, dissolved in plasma in its inactive form. When clotting begins, fibrinogen is converted into fibrin. Fibrin forms long sticky threads that create a mesh. This mesh traps blood cells and forms a clot that seals the wound.  [NDA 2025-I]

Heparin is the natural anticoagulant in blood. It prevents clotting inside blood vessels under normal conditions. Without heparin, blood would clot spontaneously inside vessels. It is not haemoglobin, not fibrin, and not plasma that prevents spontaneous clotting. It is heparin.

Dengue and Platelet Reduction

Dengue virus dramatically reduces platelet count through three mechanisms: it interferes with platelet production in bone marrow, it infects endothelial cells lining blood vessels, and it binds directly with circulating platelets. Low platelet count increases the risk of dangerous bleeding.  [NDA 2017-I]

5. Blood Groups: ABO System

The human ABO system has exactly four blood groups: A, B, AB, and O. No other designations are valid. Blood groups are determined by antigens on the surface of red blood cells. Group A has A antigens. Group B has B antigens. Group AB has both. Group O has neither.

Universal Donor and Universal Recipient

Blood group O is the universal donor. Group O RBCs carry no A or B antigens and therefore do not trigger immune rejection in any recipient. Group O can donate to A, B, AB, or O.

Blood group AB is the universal recipient. Group AB individuals already have both A and B antigens. Their immune system does not attack either antigen type, so they can receive blood from any group.

★ IMPORTANT: The most frequently reversed answer in this chapter: Blood group O = universal DONOR (no A or B antigens: safe for all) Blood group AB = universal RECIPIENT (accepts all groups: not the donor) AB donates to AB only. O donates to everyone.

Donor-Recipient Compatibility

Blood Group	Can Donate To	Can Receive From
A	A, AB	A, O
B	B, AB	B, O
AB	AB only	A, B, AB, O (universal recipient)
O	A, B, AB, O (universal donor)	O only

 [NDA 2011-I]

Blood Group Inheritance

When one parent has blood group O and the other has blood group AB, what blood groups can the children have?

The O parent contributes only a recessive i allele. The AB parent contributes either an I^A or I^B allele. The possible combinations are I^A i (blood group A) or I^B i (blood group B). Neither O nor AB is possible in the children.  [NDA 2009-I]

6. Blood Pressure

Blood pressure is a three-fact cluster. These facts connect and are tested together. Learn them as one unit.

Fact	Detail
Normal reading	120/80 mmHg: unit is millimetres of mercury, not water, not blood
Systolic (120 mmHg)	Pressure during ventricular contraction: when blood is pushed out
Diastolic (80 mmHg)	Pressure during ventricular relaxation: when chambers refill
Instrument	Sphygmomanometer: not centrifuge, not electrophoresis machine
Pulse	Rhythmic expansion of aorta and main arteries: not from heart chambers, valves, or veins

HDL (High Density Lipoprotein) is the “good cholesterol”, associated with reduced cardiovascular disease risk. LDL (Low Density Lipoprotein) is the “bad cholesterol”. It builds up in artery walls and raises the risk of heart attack and hypertension.

7. The Heart

The human heart has four chambers: two atria (upper) and two ventricles (lower). The right side handles deoxygenated blood. The left side handles oxygenated blood. The left ventricle is the most muscular chamber. It must push blood against the greatest resistance in the entire body.

Normal heart rate in a healthy adult is 72–80 beats per minute. Values like 80–97 or 82–87 are deliberate distractors in exam options.

Heart Sounds: Lub and Dub

The heartbeat produces two sounds: “lub” and “dub.” These sounds are produced by the opening and closing of heart valves. Not by blood flowing through vessels. Not by heart muscle contracting. Only by valve movement.  [NDA 2011-II | NDA 2015-I]

Abnormal heart sounds are called murmurs. The specific term is murmur: not simply “different sounds.”

Cardiac Muscle: Unique Structure

The heart is made of cardiac muscle. Cardiac muscle cells have a structure that is unique among all muscle types in the body.

Property	Cardiac Muscle	Skeletal Muscle	Smooth Muscle
Shape	Cylindrical	Cylindrical	Spindle-shaped
Branching	Branched	Unbranched	Unbranched
Nuclei per cell	Uninucleate (1 nucleus)	Multinucleate	Uninucleate

The combination of cylindrical + branched + uninucleate belongs only to cardiac muscle. This is the identifying set.

The Pacemaker: SA Node

The heartbeat does not start from the brain. It starts from within the heart itself. A cluster of specialised cells in the right atrium called the sinoatrial (SA) node generates electrical impulses that spread across the heart, triggering each beat.

Because the heartbeat originates from cardiac muscle itself, not from a nerve, the heart is described as myogenic.

Adrenaline is the hormone that increases heart rate. It is released by the adrenal glands during stress or excitement and acts directly on the SA node, making it fire faster.  [NDA 2022-II]

ECG and EEG

An ECG (Electrocardiogram) records the electrical activity of the heart. It does not record the brain, the kidneys, or the cornea.  [NDA 2019-II]

An EEG (Electroencephalogram) records the electrical activity of the brain. These two are tested as a pair: ECG = heart, EEG = brain. Do not confuse them.

8. Double Circulation

Humans, and all mammals, have double circulation. Blood passes through the heart twice in every complete circuit. This double-pass ensures oxygenated and deoxygenated blood never mix. It is the most efficient circulatory arrangement.

Circuit	Path	Blood Type Carried
Pulmonary circulation	Right ventricle → Lungs → Left atrium	Deoxygenated → becomes oxygenated
Systemic circulation	Left ventricle → Body → Right atrium	Oxygenated → becomes deoxygenated

Oxygenated blood returning from the lungs arrives at the left atrium first.  [NDA 2018-II] From there it moves to the left ventricle, which pumps it into the aorta for distribution to the entire body.

The Pulmonary Paradox

There is a general rule about blood vessels: arteries carry oxygenated blood and veins carry deoxygenated blood. This rule has one critical exception, and frequently asked in exams.

★ IMPORTANT: EXCEPTION: The pulmonary artery carries deoxygenated blood. → It goes from the right ventricle TO the lungs: carrying blood that needs to be oxygenated. EXCEPTION: The pulmonary vein carries oxygenated blood. → It returns FROM the lungs to the left atrium: carrying freshly oxygenated blood. Naming follows direction of flow (toward/away from lungs): not oxygen content. [NDA 2012-I | NDA 2025-I]

9. Blood Vessels

The cardiovascular system has three types of blood vessels, each with a distinct structure matched to its function.

Vessel	Direction	Wall	Special Feature
Arteries	Away from heart	Thick, elastic	Withstand high pressure from each heartbeat [NDA 2025-I]
Veins	Toward heart	Thin	Valves prevent backflow
Capillaries	Connect arteries to veins	One cell thick	Site of gas and nutrient exchange with tissues

The elastic fibres in arterial walls are structurally durable. They have even been found preserved in ancient Egyptian mummies.  [NDA 2015-II]

Other Vessel Facts

The coronary artery supplies oxygenated blood to the heart muscle itself. If a coronary artery is blocked, the heart muscle is starved of oxygen. This is a heart attack.

Facial wounds bleed profusely because the face has an exceptionally dense network of blood vessels very close to the skin surface. It is not because the face is near the heart, and not because facial skin is soft. Rich vascularity is the reason.  [NDA 2009-I]

Earlobe piercing causes minimal bleeding because the earlobe is made of cartilage. Cartilage has comparatively fewer blood vessels than most body tissues.  [NDA 2012-II]

Potassium controls the water content of the blood and maintains osmotic balance.

10. Jaundice and Bilirubin

Red blood cells have a lifespan of about 120 days. Old RBCs are broken down, mainly in the spleen. When haemoglobin breaks down, it produces a yellow pigment called bilirubin. The liver processes bilirubin and removes it from the blood.

When the liver fails to process bilirubin properly, it accumulates and deposits in the skin and the whites of the eyes. This produces the characteristic yellow colour of jaundice.  [NDA 2009-II]

Jaundice is caused by the accumulation of bilirubin, not biliverdin. Biliverdin is a green pigment that is quickly converted to bilirubin. Jaundice is yellow, not green, because bilirubin is the accumulating compound.  [NDA 2009-II]

11. The Spleen

The spleen is an organ in the upper left abdomen. One of its most important roles is acting as a blood reservoir. It stores extra blood and releases it into circulation when the body needs more rapidly: during haemorrhage or intense physical exertion.  [NDA 2007-II]

The liver, pancreas, and lymph nodes do not serve this blood-storage function. Only the spleen does.

12. Lymph and the Lymphatic System

Blood plasma seeps out of capillary walls into the spaces between cells. This fluid is called lymph. Lymph occupies the intercellular (interstitial) spaces of the body.  [NDA 2022-II]

Lymph is similar in composition to blood plasma but does not contain red blood cells. It is a clear, colourless fluid. Lymph does not circulate through the heart. It travels through its own separate system, the lymphatic vessels, driven by skeletal muscle contractions and breathing movements. Eventually it drains back into the bloodstream.

13. Sickle-Cell Anaemia

Sickle-cell anaemia is a genetic disease caused by an abnormality in red blood cells. A mutated form of haemoglobin (HbS) causes RBCs to take a rigid, crescent (sickle) shape when oxygen levels are low.  [NDA 2012-II]

The disease affects RBCs only, not WBCs, not platelets, not plasma. Sickle-shaped cells block small blood vessels and are destroyed faster than normal RBCs, causing anaemia.

14. Heart Chambers Across Vertebrate Classes

The number of heart chambers is one of the most tested comparative facts in biology. Different vertebrate classes have different numbers. Several specific species are tested by name.

Vertebrate Class	Heart Chambers	Example Animals
Pisces (Fish)	2 (1 atrium + 1 ventricle)	Shark, Scoliodon, Rohu
Amphibia	3 (2 atria + 1 ventricle)	Frog, Toad, Salamander
Reptilia (most)	3 (2 atria + 1 ventricle)	Lizard, Snake, Tortoise
Crocodile (exception)	4	The only reptile with 4 chambers
Aves (Birds)	4 (2 atria + 2 ventricles)	Pigeon, Peacock, Ostrich
Mammalia	4 (2 atria + 2 ventricles)	Human, Dog, Whale, Camel

Warm-blooded (maintain constant body temperature): Birds and Mammals: Peacock, Camel. Cold-blooded (body temperature varies with environment): Fish, Amphibia, Reptiles: Crocodile, Hagfish, Dogfish, Tortoise.

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