NDA Biology · Human Physiology

Circulation — Heart, Vessels and Blood

The heart pumps blood through arteries and veins in a double circuit; blood is a connective tissue, lymph resembles plasma, and clotting seals wounds with fibrin.

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Why this matters

6 PYQs. The heart's four chambers and valves, the artery-vs-vein contrast, and the clotting proteins are all recall staples. The one fact the bank traps most often: the pulmonary artery carries DEOXYGENATED blood — the exception to 'arteries carry oxygenated blood'.

Concept 1 of 3

Heart chambers, valves, and blood flow

Intuition

The human heart has four chambers — two upper auricles (atria) that receive blood and two lower ventricles that pump it out. Valves between and out of the chambers keep blood moving one way only. The single most-tested route: oxygenated blood from the lungs returns to the LEFT auricle.

Definition

Four chambers and their valves:

Right auricle → right ventricle: guarded by the tricuspid valve.
Left auricle → left ventricle: guarded by the bicuspid (mitral) valve.
Ventricle → artery (pulmonary or aorta): guarded by semilunar valves.
Oxygenated blood from the lungs enters the left auricle (via pulmonary veins); deoxygenated blood from the body enters the right auricle.

Worked example

Trace a drop of oxygenated blood from the lungs to the moment it is pumped towards the body. Which chambers and valve does it pass?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Human PhysiologyEASY

Which one of the following valves guards the opening between the right ventricle and the pulmonary artery?

[Q100 · Apr · 2026]

Bicuspid = left, tricuspid = right

The tricuspid valve is on the right (right auricle → right ventricle); the bicuspid / mitral is on the left. Memory hook: 'LAB' — Left Atrium = Bicuspid. Semilunar valves guard the artery exits.

Drill 1 more on heart chambers, valves, and blood flow

Concept 2 of 3

Arteries vs veins

Intuition

Arteries carry blood AWAY from the heart, veins carry it TOWARDS the heart. Arteries have thick elastic walls (they take the pump's pressure) and no valves; veins have thinner walls and valves to stop backflow. The crucial exception: the pulmonary vessels reverse the usual oxygen rule.

Definition

The contrast and its famous exception:

Arteries: carry blood away from the heart; thick, elastic walls; no valves; usually oxygenated.
Veins: carry blood towards the heart; thinner walls; have valves; usually deoxygenated.
Exception: the pulmonary artery carries DEOXYGENATED blood (heart → lungs) and the pulmonary vein carries OXYGENATED blood (lungs → heart).

Feature	Artery	Vein
Direction	Away from heart	Towards heart
Wall	Thick, elastic	Thinner
Valves	Absent	Present
Usual blood	Oxygenated	Deoxygenated EXCEPTION: pulmonary artery = deoxygenated; pulmonary vein = oxygenated.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 2Human PhysiologyEASY

Which one among the following statements about arteries and veins in humans is NOT correct ?

[Q135 · Sep · 2025]

'Arteries carry oxygenated blood' is only USUALLY true

The pulmonary artery carries deoxygenated blood (to the lungs) and the pulmonary vein carries oxygenated blood (back to the heart). Any option claiming pulmonary arteries 'always' carry oxygenated blood is the wrong statement.

Concept 3 of 3

Blood, lymph, and clotting

Intuition

Blood is a connective tissue — plasma carrying red cells, white cells and platelets. When some plasma leaks out into the tissue spaces it becomes lymph. When a vessel is cut, a cascade converts the soluble protein fibrinogen into the insoluble mesh fibrin, which traps cells and seals the wound.

Definition

The facts the bank tests:

Plasma — the fluid part of blood (~55%); carries cells, proteins, nutrients.
Lymph — tissue fluid that resembles plasma but lacks red blood cells and has fewer proteins.
Clotting — fibrinogen (soluble) is converted by thrombin into fibrin (insoluble mesh). Vitamin K and calcium are required.

Worked example

A cut starts to bleed, then a clot forms. Which plasma protein turns into the mesh that traps the blood cells, and which vitamin is required for the process?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Human PhysiologyEASY

Clotting of blood involves which one among the following clotting proteins?

[Q111 · Apr · 2025]

Fibrinogen vs fibrin

Fibrinogen is the soluble plasma protein; fibrin is the insoluble mesh it becomes. The clotting-protein question's answer is fibrinogen — distractors offer non-proteins (pathogen) or cells (macrophage, phagocyte).

Drill 2 more on blood, lymph, and clotting

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Arteries vs veins4 rows

Feature	Artery	Vein
Direction	Away from heart	Towards heart
Wall	Thick, elastic	Thinner
Valves	Absent	Present
Usual blood	Oxygenated	Deoxygenated EXCEPTION: pulmonary artery = deoxygenated; pulmonary vein = oxygenated.

Watch out for (3)

Bicuspid = left, tricuspid = right
→ Heart chambers, valves, and blood flow
'Arteries carry oxygenated blood' is only USUALLY true
→ Arteries vs veins
Fibrinogen vs fibrin
→ Blood, lymph, and clotting

Drill every past-year question on this subtopic

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