CDS Biology · Human Physiology
Heart and Circulation
The four chambers of the heart, the double circulation through the lungs and the body, the main blood vessels, and what blood pressure and the pulse measure.
Why this matters
Seven CDS questions from 2021 (II) to 2026 (I). Four are about blood pressure and the pulse, two of them in 2025 (I); three trace the path of blood between the heart and the lungs.
Concept 1 of 2: Double circulation and the blood vessels
The heart is two pumps side by side. The right side takes used, oxygen-poor blood from the body and sends it to the lungs. The lungs fill it with oxygen and send it back to the left side, which pumps it out to the whole body. So blood passes through the heart twice in one round: that is double circulation. Arteries carry blood away from the heart, veins bring it back, and capillaries join the two.
Definition
The heart and the path of blood:
- Four chambers: two atria (receive blood) above, two ventricles (pump blood) below. The left ventricle has the thickest wall, since it pumps to the whole body.
- Right side, oxygen-poor: body → vena cava → right atrium → right ventricle → pulmonary artery → lungs.
- Left side, oxygen-rich: lungs → pulmonary vein → left atrium → left ventricle → aorta → body.
- Arteries carry blood away from the heart (thick, elastic walls). Veins bring it back (thinner walls, valves). Capillaries are the thin vessels that connect arteries to veins, where exchange with the tissues happens.
- The coronary arteries supply the heart muscle itself.
- The pulmonary artery is the one artery with oxygen-poor blood, and the pulmonary vein the one vein with oxygen-rich blood.
| Vessel | Carries blood from | To |
|---|---|---|
| Pulmonary artery | Right ventricle | Lungs (oxygen-poor blood) |
| Pulmonary vein | Lungs | Left atrium (oxygen-rich blood)Q Asked twice, CDS 2021 (II) and 2026 (I): oxygen-rich blood from the lungs enters the left atrium first. |
| Aorta | Left ventricle | The whole body |
| Vena cava | The whole body | Right atrium |
| Coronary artery | Aorta | The heart muscle |
| Capillaries | Arteries | Veins |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (I) 2022 — General Knowledge · Q19Moderate
Example 1 · Human Physiology · Heart and Circulation
Match List I with List II and select the correct answer using the code given below the lists :
Code :
| List I (Blood Vessel) | List II (Function) |
|---|---|
| A. Pulmonary artery | 1. Carries blood from lungs to heart |
| B. Capillaries | 2. Carries blood to heart muscle |
| C. Coronary artery | 3. Connection between arteries and veins |
| D. Pulmonary vein | 4. Carries blood from heart to lungs |
Artery means away, not oxygen-rich
An artery carries blood away from the heart, a vein towards it. So the pulmonary artery carries oxygen-poor blood and the pulmonary vein oxygen-rich blood.
Left is oxygen-rich
Oxygen-rich blood arrives on the left side (left atrium) from the lungs and leaves from the left ventricle to the body. Options that put oxygen-poor blood on the left, or send oxygen-rich blood out from the right, are wrong.
Concept 2 of 2: Blood pressure and the pulse
Each time the ventricles squeeze, they push a surge of blood into the arteries and the pressure inside rises; as they relax and refill, it falls. Blood pressure is written as these two numbers, the high one over the low one. The surge also stretches the elastic wall of the aorta and the big arteries, which then spring back. You feel that stretch and recoil as the pulse.
Definition
Blood pressure:
- Written as systolic / diastolic; normal is about 120/80 mm Hg.
- Systolic (the higher number): pressure while the ventricles contract.
- Diastolic (the lower number): pressure while the ventricles relax.
- Measured with a sphygmomanometer. Pressure that stays high is hypertension.
The pulse:
- The rhythmic expansion and recoil of the aorta and the main arteries after each heartbeat, felt where an artery lies close to the skin, as at the wrist.
- The resting pulse of an adult is about 72 beats a minute, the same as the heart rate.
| Term | Meaning |
|---|---|
| Systolic pressure | About 120 mm Hg, while the ventricles contractQ CDS 2025 (I): the normal systolic pressure is 120 mm Hg. |
| Diastolic pressure | About 80 mm Hg, while the ventricles relax |
| Sphygmomanometer | The instrument that measures blood pressureQ CDS 2025 (I): blood pressure is measured manually with a sphygmomanometer. |
| Pulse | Stretch and recoil of the aorta and main arteriesQ CDS 2024 (I): the pulse comes from the aorta and main arteries, not the heart chambers or valves. |
| Resting pulse rate | About 72 per minute |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q5Moderate
Example 2 · Human Physiology · Heart and Circulation
The blood pressure of a normal human being is found to be 120/80 mmHg. These 'numbers' represent the blood pressure at
Both numbers are about the ventricles
120 is the pressure while the ventricles contract and 80 while they relax. Options about the atria (auricles) contracting, or with the order reversed, are wrong.
The pulse is in the arteries
The pulse is the aorta and arteries stretching and recoiling. It is not the heart's chambers, the heart valves or the valves in veins.
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 (2)
Double circulation and the blood vessels6 rows
| Vessel | Carries blood from | To |
|---|---|---|
| Pulmonary artery | Right ventricle | Lungs (oxygen-poor blood) |
| Pulmonary vein | Lungs | Left atrium (oxygen-rich blood)Q Asked twice, CDS 2021 (II) and 2026 (I): oxygen-rich blood from the lungs enters the left atrium first. |
| Aorta | Left ventricle | The whole body |
| Vena cava | The whole body | Right atrium |
| Coronary artery | Aorta | The heart muscle |
| Capillaries | Arteries | Veins |
Blood pressure and the pulse5 rows
| Term | Meaning |
|---|---|
| Systolic pressure | About 120 mm Hg, while the ventricles contractQ CDS 2025 (I): the normal systolic pressure is 120 mm Hg. |
| Diastolic pressure | About 80 mm Hg, while the ventricles relax |
| Sphygmomanometer | The instrument that measures blood pressureQ CDS 2025 (I): blood pressure is measured manually with a sphygmomanometer. |
| Pulse | Stretch and recoil of the aorta and main arteriesQ CDS 2024 (I): the pulse comes from the aorta and main arteries, not the heart chambers or valves. |
| Resting pulse rate | About 72 per minute |
Watch out for (4)
- Artery means away, not oxygen-rich→ Double circulation and the blood vessels
- Left is oxygen-rich→ Double circulation and the blood vessels
- Both numbers are about the ventricles→ Blood pressure and the pulse
- The pulse is in the arteries→ Blood pressure and the pulse
Test yourself on Human Physiology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.