Circulatory System and Heart
~8 min read
- Human heart: Four chambers — 2 atria + 2 ventricles. Double circulation: pulmonary + systemic.
- Origin of beat: Sinoatrial (SA) node — natural pacemaker.
- Blood pressure: Normal 120/80 mmHg = systolic (ventricular contraction) / diastolic (relaxation).
The cardiovascular system delivers oxygen and nutrients to every cell. CDS/OTA asks about heart chambers, blood vessels and double circulation.
Heart Chamber Count Across Animals
| Animal group | Chambers | Example |
|---|---|---|
| Fishes | Two (1 atrium + 1 ventricle) | Scoliodon, Rohu |
| Amphibians | Three (2 atria + 1 ventricle) | Frog, salamander |
| Most reptiles | Three (incompletely divided ventricle) | Lizard, snake |
| Crocodiles, birds, mammals | Four (2 atria + 2 ventricles) | Pigeon, human |
The Human Heart and Double Circulation
- The heart has four chambers — right atrium, right ventricle, left atrium, left ventricle.
- Right side handles deoxygenated blood; left side handles oxygenated blood. Both sides separated by septum to prevent mixing.
- Pulmonary circulation: Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium.
- Systemic circulation: Left ventricle → aorta → body → vena cava → right atrium.
- Blood goes through the heart twice in one full circuit — hence "double circulation."
- Pulmonary artery carries deoxygenated blood (one of the few arteries to do so). Pulmonary vein carries oxygenated blood.
Heartbeat and Pulse
- The mammalian heart is myogenic — its rhythm originates within the heart muscle itself.
- The Sinoatrial (SA) node in the right atrium is the natural pacemaker — generates the impulse that triggers each heartbeat.
- Pulse is felt in arteries (e.g. wrist, neck) because of the rhythmic contraction and relaxation of the aorta and main arteries as blood is pumped.
- Blood pressure 120/80 mmHg: 120 = systolic (ventricular contraction). 80 = diastolic (ventricular relaxation).
- Coronary arteries supply the heart muscle itself. Blockage causes a heart attack (myocardial infarction).
Blood Vessels
- Arteries: carry blood from heart to body. Thick muscular walls; carry blood under high pressure. Mostly oxygenated except pulmonary artery.
- Veins: return blood to heart. Thinner walls, valves prevent backflow. Mostly deoxygenated except pulmonary vein.
- Capillaries: microscopic — connection between arteries and veins where exchange of gases and nutrients occurs.
- Coronary artery: supplies blood to the heart muscle itself.
CDS/OTA PYQ Examples
Q: Which classes of animals have a three-chambered heart?
(a) Pisces and Amphibia (b) Amphibia and Reptilia (c) Reptilia only (d) Amphibia only
Answer: (b) Amphibia and Reptilia (most reptiles). [CDS-I 2018]
Q: Which one of the following animals has a three-chambered heart?
(a) Scoliodon (b) Salamander (c) Pigeon (d) Human being
Answer: (b) Salamander — amphibian. [CDS-I 2020]
Q: The heartbeat originates from:
(a) sinoatrial node (b) QRS wave (c) T wave (d) hepatic portal system
Answer: (a) Sinoatrial (SA) node — natural pacemaker. [CDS-I 2017]
Q: Pulse is felt due to the rhythmic contraction and relaxation of the:
(a) chambers of heart (b) valves in veins (c) aorta and main arteries (d) valves of heart
Answer: (c) Aorta and main arteries. [CDS-I 2024]
Q: In double circulation, which statement is true?
(a) Oxygenated blood reaches left side of heart from lungs (b) Blood in left side is poor in oxygen (c) Deoxygenated blood from left side goes to lungs (d) Oxygenated blood from right side goes around body
Answer: (a) Oxygenated blood reaches the LEFT side from lungs via pulmonary vein. [CDS-II 2021]
Q: Blood pressure 120/80 mmHg represents:
(a) ventricular contraction and ventricular relaxation (b) ventricular relaxation and contraction (c) auricular contraction and relaxation (d) ventricular contraction and auricular contraction
Answer: (a) Ventricular contraction (systolic) and ventricular relaxation (diastolic). [CDS-I 2023]
Drill Circulatory System and Heart for CDS/OTA
CDS/OTA-pattern items on Circulatory System and Heart with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is double circulation an advantage?
It keeps oxygenated and deoxygenated blood separate, allowing higher pressure in systemic circulation — supports warm-blooded, high-energy lifestyle of mammals and birds.
Why are arteries thicker than veins?
Arteries carry blood under high pressure pumped from the heart; thick muscular walls withstand and maintain this pressure. Veins return blood at low pressure, so thin walls with valves suffice.
What is myocardial infarction?
A heart attack — death of heart muscle due to blockage in a coronary artery, usually by a clot on an atherosclerotic plaque.