Heart: Structure and Function
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Direct answer
Four chambers, four valves, a self-exciting pacemaker and a 0.8-second cycle summarise the human heart. Mesodermally derived and fist-sized, it sits in the thoracic cavity wrapped in a double-walled pericardium lubricated by pericardial fluid. The right atrium receives deoxygenated blood from the venae cavae and passes it through the tricuspid valve to the right ventricle, which sends it via the pulmonary artery to the lungs; returning pulmonary veins fill the left atrium, and the left ventricle — the thickest chamber — drives oxygenated blood through the aorta to the whole body. The sinoatrial node, the pacemaker, generates 70 to 75 action potentials per minute, conducted through the atrioventricular node, bundle of His and Purkinje fibres. Each cardiac cycle of 0.8 seconds ejects about 70 mL per beat (stroke volume), giving a cardiac output near 5 litres per minute.
What you must remember
- Anatomy: two atria and two ventricles separated by interatrial and interventricular septa; mesodermal origin; pericardium with lubricating pericardial fluid.
- Valve map: tricuspid (right atrioventricular), bicuspid or mitral (left atrioventricular), and two semilunar valves guarding the pulmonary artery and aorta — all preventing backflow.
- Conduction cascade: sinoatrial node (the pacemaker, 70–75 beats per minute) to atrioventricular node to bundle of His to Purkinje fibres; the human heart is myogenic — its rhythm arises from specialized cardiac muscle, not nerves.
- Cycle timing: one cardiac cycle is 0.8 s — atrial systole 0.1 s, ventricular systole 0.3 s, and joint diastole 0.4 s.
- Output numbers: stroke volume about 70 mL per ventricle per beat; cardiac output = stroke volume × heart rate, roughly 70 × 72 ≈ 5000 mL per minute at rest.
- Heart sounds: the first sound (LUBB) closes the atrioventricular valves at the start of ventricular systole; the second (DUPP) closes the semilunar valves as the ventricles relax.
- Circuit logic: pulmonary circulation is the low-pressure lung loop, systemic circulation the high-pressure body loop — double circulation, and the left ventricle is thick because it pushes the longer circuit.
Follow one drop of blood around the circuit
Start in the superior vena cava with a drop of deoxygenated blood from the head. It drops into the right atrium during diastole, waits out the 0.1-second atrial contraction that tops up the ventricle, and crosses the tricuspid valve — the LUBB now snaps it shut as ventricular systole begins. Squeezed into the pulmonary artery past its semilunar valve, the drop reaches a lung capillary, unloads carbon dioxide, loads oxygen, and returns in a pulmonary vein (the only veins carrying oxygenated blood) to the left atrium. Through the mitral valve into the left ventricle, and the 0.3-second left-sided systole fires it through the aortic valve into the aorta — the DUPP sealing behind it as the ventricles relax into their 0.4-second joint diastole.
The wiring makes the plumbing honest. The sinoatrial node fires autonomically; the wave spreads over the atria, is deliberately delayed at the atrioventricular node so the ventricles fill fully, then races down the bundle of His and Purkinje fibres so both ventricles contract as one. That 70-75-per-minute spark, propagated mechanically, is why a heart beats in a dish — myogenic, self-driven, merely tuned by nerves.
How the exam frames it
Valve-side questions are the reliable trap: tricuspid on the right, bicuspid or mitral on the left, and options habitually swap them. Heart-sound attribution follows — first sound to atrioventricular closure, second to semilunar closure. The pacemaker identity (sinoatrial node) and its 70–75 per minute rate are NCERT figures quoted verbatim, and the arithmetic item is cardiac output: 70 mL times roughly 72 beats equals about 5 litres per minute, sometimes dressed up with exercise heart rates. Note also that pulmonary artery and pulmonary veins are exceptions to the usual artery–vein oxygen rule; statement questions lean on exactly those two reversals.
Frequently asked questions
Which structure is called the pacemaker of the heart?
The sinoatrial node in the right atrium, generating 70 to 75 action potentials per minute on its own.
What closes to produce the first and second heart sounds?
The first sound (LUBB) marks closure of the atrioventricular valves; the second (DUPP) marks closure of the semilunar valves.
Why is the left ventricle thicker than the right?
It must pump blood through the systemic circuit at higher pressure, requiring a more muscular wall.
How long is one cardiac cycle, and what are its parts?
About 0.8 seconds: atrial systole 0.1 s, ventricular systole 0.3 s and joint diastole 0.4 s.
What is cardiac output at rest?
Stroke volume of about 70 mL multiplied by heart rate of about 72 beats per minute, giving roughly 5 litres per minute.