NEET · Biology
Human Circulatory System: NEET Biology Notes
Heart structure, cardiac cycle, blood vessels, and double circulation explained for NEET biology with diagrams and NCERT-aligned facts.
Structure of the Human Heart
The human heart is a four-chambered muscular organ — two atria and two ventricles — enclosed in a double-layered membrane called the pericardium.
Cardiac Cycle
One complete heartbeat consists of:
- Atrial systole — atria contract, pushing blood into ventricles.
- Ventricular systole — ventricles contract, pushing blood into pulmonary artery and aorta.
- Joint diastole — both chambers relax and refill.
A healthy adult heart beats roughly 72 times per minute at rest, giving a cardiac cycle duration of about 0.8 seconds.
Double Circulation
Humans have double circulation: blood passes through the heart twice in one complete cycle — once through the pulmonary circuit (heart → lungs → heart) and once through the systemic circuit (heart → body → heart). This ensures oxygenated and deoxygenated blood never mix, which is essential for warm-blooded, high-metabolism organisms.
NEET-Specific Facts to Remember
- The SA node is the natural pacemaker of the heart, located in the right atrium.
- ECG waves: P (atrial depolarization), QRS (ventricular depolarization), T (ventricular repolarization).
- Blood pressure is typically written as systolic/diastolic, e.g., 120/80 mmHg.
Frequently Confused Terms
| Term | Meaning |
|---|---|
| Systole | Contraction phase |
| Diastole | Relaxation phase |
| Tachycardia | Heart rate above 100 bpm |
| Bradycardia | Heart rate below 60 bpm |