# Human Heart and Cardiac Cycle

> Human heart chambers and valves, the cardiac cycle, stroke volume and cardiac output, ECG waves and double circulation for NEET-UG Biology.

- Canonical URL: https://prepelephant.com/topics/neet-ug/biology/human-heart
- Exam / course: NEET-UG · Subject: Biology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Human Heart and Cardiac Cycle", PrepElephant, https://prepelephant.com/topics/neet-ug/biology/human-heart

## Direct answer

The human heart is a mesodermally derived, four-chambered muscular pump — two atria and two ventricles — whose valves guarantee one-way flow: the tricuspid valve on the right and the bicuspid (mitral) valve on the left guard the atrio-ventricular openings, while semilunar valves guard the exits into the pulmonary artery and aorta. It is myogenic, with the sino-atrial node acting as the pacemaker to generate roughly 70 to 75 beats per minute; one cardiac cycle of systole and diastole lasts about 0.8 seconds.

## What you must remember

- **Circuit of blood:** deoxygenated blood enters the right atrium through the venae cavae, is pumped by the right ventricle through the pulmonary artery to the lungs, returns to the left atrium and leaves through the aorta — double circulation keeps oxygenated and deoxygenated blood fully separate.
- **Cardiac cycle numbers:** atrial systole about 0.1 second, ventricular systole about 0.3 seconds and ventricular diastole about 0.5 seconds; stroke volume is about 70 mL per beat, giving a cardiac output of about 5 litres per minute.
- **Conduction pathway:** the SA node, the pacemaker, initiates the impulse, which travels to the AV node, then down the bundle of His and its Purkinje fibres to the ventricular muscle.
- **Heart sounds:** the first (lub) accompanies closure of the atrio-ventricular valves at the start of ventricular systole; the second (dub) accompanies closure of the semilunar valves.
- **ECG:** the P wave denotes atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation; the technique detects arrhythmias and myocardial infarction.
- **Formed elements of blood:** red cells number 5 to 5.5 million per cubic millimetre, white cells 6,000 to 8,000, and platelets 1.5 to 3.5 lakh; plasma forms about 55 per cent and formed elements about 45 per cent of blood volume.
- **Blood groups:** the ABO system, in which NCERT identifies O as the universal donor and AB as the universal recipient, and the Rh factor, critical when an Rh-negative mother carries an Rh-positive foetus.

## Common confusion

Students habitually swap the atrio-ventricular valves: tricuspid on the right side, bicuspid or mitral on the left. The durations also trip candidates — ventricular diastole (0.5 s) is longer than ventricular systole (0.3 s) — and the two heart sounds are matched to the wrong valve pairs in haste.

## Exam-focused takeaway

NEET-UG computes cardiac output from heart rate multiplied by stroke volume (70 mL times about 72 beats gives roughly 5 litres per minute), matches valve names to positions, and asks statement-based questions on ECG wave significance. Assertion-reason items test why double circulation is an advantage — efficient, high-pressure delivery with complete separation of the two bloods.

## Frequently asked questions

### Which node of the heart is called the pacemaker and why?

The sino-atrial node, because it generates its own rhythmic impulses at 70 to 75 per minute and sets the pace for the whole myocardium.

### What are stroke volume and cardiac output?

Stroke volume is the blood ejected by one ventricular contraction, about 70 mL; cardiac output is the volume pumped per minute, about 5 litres at rest.

### Which valves guard the atrio-ventricular openings?

The tricuspid valve on the right side and the bicuspid or mitral valve on the left; semilunar valves guard the aortic and pulmonary exits.

### What do the waves of a normal ECG represent?

The P wave is atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation.

### Why is blood circulation in humans called double circulation?

Because blood passes through the heart twice in one round — through the pulmonary circuit to the lungs and the systemic circuit to the body — keeping oxygenated and deoxygenated blood separate.