By Dr Hemant Upadhyay, Zoology faculty at Pramana Institute (18 years teaching NEET, ex-Allen)
"Body Fluids and Circulation" is one of the most reliable NCERT chapters in NEET. The questions come almost word for word from NCERT lines on the heart, its nodal tissue, the cardiac cycle and the ECG. Ten minutes on this page before a test is worth more than an hour with a new reference book.
The heart: 12 points to know
- The heart is mesodermal in origin. It lies in the thoracic cavity between the two lungs, slightly tilted to the left, and is about the size of a clenched fist.
- It is protected by a double-walled membranous pericardium enclosing pericardial fluid.
- It has four chambers: two thin-walled atria above and two thick-walled ventricles below.
- The inter-atrial septum separates the atria, the inter-ventricular septum separates the ventricles, and the atrio-ventricular septum separates each atrium from its ventricle.
- The tricuspid valve guards the opening between the right atrium and right ventricle. The bicuspid (mitral) valve guards the opening between the left atrium and left ventricle.
- Semilunar valves guard the openings of the right ventricle into the pulmonary artery and the left ventricle into the aorta. All valves allow blood to flow one way only.
- The SA node lies in the upper right corner of the right atrium. It is the pacemaker and generates 70–75 action potentials a minute.
- The AV node lies in the lower left corner of the right atrium, close to the atrio-ventricular septum.
- From the AV node, the AV bundle (bundle of His) passes through the septum and divides into right and left bundle branches, which end in Purkinje fibres in the ventricle walls.
- Cardiac muscle is myogenic: the heartbeat starts in the heart itself (the SA node), not in a nerve.
- The medulla oblongata can moderate the heart: sympathetic nerves raise the rate and force of contraction, while parasympathetic nerves lower them. Adrenal medullary hormones also raise cardiac output.
- Humans have double circulation: pulmonary (right ventricle → lungs → left atrium) and systemic (left ventricle → body → right atrium).
Cardiac cycle numbers
| Quantity | Value (NCERT) |
|---|---|
| Heart rate | 72 beats per minute |
| Duration of one cardiac cycle | 0.8 seconds |
| Stroke volume | About 70 mL per ventricle per beat |
| Cardiac output | Stroke volume × heart rate ≈ 5,000 mL (5 L) per minute |
| Extra filling from atrial systole | About 30% |
Heart sounds and the ECG
| Event | What causes it |
|---|---|
| First sound, "lub" | Closure of the tricuspid and bicuspid valves |
| Second sound, "dub" | Closure of the semilunar valves |
| P wave | Depolarisation of the atria, which leads to atrial contraction |
| QRS complex | Depolarisation of the ventricles, which starts ventricular contraction |
| T wave | Return of the ventricles from the excited to the normal state (repolarisation); its end marks the end of systole |
Quick trick: to find heart rate from an ECG, count the QRS complexes in a given time period.
10-point self-check
- Which node is called the pacemaker? SA node
- Where exactly is the AV node? Lower left corner of the right atrium, near the atrio-ventricular septum
- Which valve lies between the left atrium and left ventricle? Bicuspid (mitral)
- What causes the "dub" sound? Closure of the semilunar valves
- How long is one cardiac cycle at 72 beats a minute? 0.8 s
- What is the normal cardiac output? About 5 L per minute
- Which ECG wave shows ventricular depolarisation? QRS complex
- What does the end of the T wave mark? The end of systole
- By how much does atrial systole increase ventricular filling? About 30%
- Is the human heart neurogenic or myogenic? Myogenic
FAQs
Is "Body Fluids and Circulation" still in the NEET syllabus? Yes. Check the latest NEET UG syllabus on nmc.org.in and the NTA notice each year, as chapters are sometimes trimmed.
Do I need anything beyond NCERT for this chapter? For NEET, NCERT text, figures and tables are enough. Spend the extra time on practice questions, not another book.
What is the most common mistake here? Mixing up the positions of the SA and AV nodes, and swapping the causes of the two heart sounds.
Revise NCERT Biology with weekly tests in a batch of at most 40 at Pramana, Geeta Bhawan Square, Indore. See our NEET program and the NEET 90-Day Revision Program.
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