Step-by-step solved numerical problems on cardiac output, BMR, GFR, lung volumes and more from human body systems for NEET and boards.

Human Body Systems: Numerical Problems Solved Step-by-Step

Biology is not only theory. Human body systems throw up neat numericals — cardiac output, GFR, lung capacities, BMR — and NEET loves them because they test whether you actually understood the definition. We will walk through five short problems that cover the classics.

Question 1 — Cardiac output

A person has a stroke volume of $70\ \text{mL}$ and a heart rate of $72\ \text{beats/min}$ . Find the cardiac output.

\text{CO} = \text{Stroke Volume} \times \text{Heart Rate}

$\text{CO} = 70 \times 72 = 5040\ \text{mL/min} \approx 5.04\ \text{L/min}$ .

Answer: ~5 L/min, which matches the textbook value for a resting adult. Remember this baseline — NEET uses it as a reference.

Question 2 — Pulmonary ventilation

Tidal volume is $500\ \text{mL}$ and respiratory rate is $15\ \text{per min}$ . Dead space volume is $150\ \text{mL}$ . Find (a) pulmonary ventilation and (b) alveolar ventilation.

$\text{PV} = \text{TV} \times \text{RR} = 500 \times 15 = 7500\ \text{mL/min}$ .

Only $(TV - \text{dead space})$ actually reaches the alveoli. $\text{AV} = (500 - 150) \times 15 = 350 \times 15 = 5250\ \text{mL/min}$ .

(a) $7500\ \text{mL/min}$ (b) $5250\ \text{mL/min}$ . The gap is why shallow-fast breathing is inefficient — dead space steals a bigger fraction.

Question 3 — GFR calculation

The kidneys filter about $125\ \text{mL}$ of plasma per minute. How much filtrate is formed per day, and how much urine if 99% is reabsorbed?

$125 \times 60 \times 24 = 180{,}000\ \text{mL} = 180\ \text{L/day}$ .

$1\%$ of $180\ \text{L} = 1.8\ \text{L/day}$ .

Filtrate ≈ 180 L/day, urine ≈ 1.8 L/day. NCERT quotes these exact numbers — memorise them.

Question 4 — Haemoglobin oxygen capacity

If $1\ \text{g}$ of haemoglobin carries $1.34\ \text{mL}$ of $\text{O}_2$ and blood has $15\ \text{g/dL}$ of Hb, find the oxygen content per $100\ \text{mL}$ of fully saturated blood.

$15 \times 1.34 = 20.1\ \text{mL O}_2$ per $100\ \text{mL}$ blood.

~20 mL O₂ / 100 mL blood at full saturation. Dissolved $\text{O}_2$ adds only $\sim 0.3\ \text{mL}$ — negligible compared to Hb-bound.

Question 5 — BMR estimate

A $60\ \text{kg}$ adult male has a BMR of about $1\ \text{kcal/kg/h}$ . Find total BMR per day.

$\text{BMR} = 60 \times 1 \times 24 = 1440\ \text{kcal/day}$ .

~1440 kcal/day, close to the standard 1500 kcal figure. Females run ~10% lower because of lower muscle mass.

NEET repeatedly asks “cardiac output at rest” and “daily glomerular filtrate”. Keep $5\ \text{L/min}$ and $180\ \text{L/day}$ on the tip of your tongue.

Students often multiply tidal volume with RR and call it “alveolar ventilation”. That is pulmonary ventilation. Subtract dead space first.

Wrap up — the five numbers that come back again

If you remember only five numbers from human body systems, make them these: cardiac output 5 L/min, daily glomerular filtrate 180 L, urine output 1.8 L/day, resting pulmonary ventilation 6 L/min and oxygen capacity of blood 20 mL per 100 mL. Almost every numerical in NEET and CBSE boards is a one-line arithmetic step on top of one of these baselines.

A good habit: before picking up your calculator, write the formula, circle the unknown, and sanity-check the unit of your final answer. If the unit is wrong, the number is wrong — no matter how clean the arithmetic looks. And if a result comes out far from the NCERT baseline, retrace the last conversion step first; that is where the factor of 60 or 1000 usually disappears.

Physiology numericals are really vocabulary tests in disguise. Know the definitions — the maths is trivial afterwards.