About Human Anatomy and Physiology
Human anatomy and physiology questions in a recruitment paper cover the whole breadth of the subject - the organisation of the body from cells to systems, the skeleton and the muscles, the nervous and endocrine control systems, the circulation, the lungs, the gut, the kidneys, the blood and the immune defences - and many of them are answered by a number rather than a description. This topic covers that breadth on one page, with the physiology that can be worked out.
What you need to understand
- The body is organised in levels: cells, tissues, organs, systems. Each level has properties the level below does not, which is why a system cannot be understood from its cells alone.
- Homeostasis is the maintenance of the internal environment within narrow limits - temperature, pH, glucose, water and electrolytes - and it works by negative feedback: a sensor, a control centre and an effector that opposes the change.
- Cardiac output is the volume of blood the heart moves in a minute, and it is the stroke volume multiplied by the heart rate. Resting it is about five litres a minute, which is the whole blood volume passing round once.
- The stroke volume is the end diastolic volume less the end systolic volume, because the ventricle never empties. The ejection fraction is the stroke volume as a proportion of the end diastolic volume, and it is a better measure of pump function than the stroke volume alone.
- Blood pressure is the product of cardiac output and peripheral resistance, so a rise in either raises the pressure. The resistance is controlled mostly by the diameter of the arterioles.
- Minute ventilation is the tidal volume times the respiratory rate. Alveolar ventilation is smaller, because the anatomical dead space - about 150 mL in an adult - takes no part in gas exchange.
- Breathing deeply raises the alveolar ventilation much more than breathing quickly does, because a rapid shallow breath spends most of its volume refilling the dead space.
- Oxygen is carried almost entirely on haemoglobin, at about 1.34 mL per gram when fully saturated, with a very small amount dissolved in the plasma. Anaemia lowers the content far more than a modest fall in saturation does.
- The glomerular filtration rate is about 125 mL a minute, and a clearance measures the volume of plasma cleared of a substance in a minute: the amount excreted divided by the plasma concentration.
- The kidney filters a large volume and then reabsorbs almost all of it, which is why a clearance can be used to measure filtration and why a substance that is reabsorbed has a clearance below the filtration rate.
- The nephron handles different substances differently: glucose is reabsorbed completely until the transport is saturated, which is where the renal threshold and the glucose in the urine of an untreated diabetic come from.
- The nervous system carries fast, addressed signals along axons, while the endocrine system carries slow, broadcast signals in the blood. The two work together, which is why the adrenal medulla is controlled by nerves.
- Negative feedback opposes a change; positive feedback amplifies one, and in the body it is used only where a rapid finish is wanted, such as the clotting cascade and childbirth.
- Body surface area follows height and weight more closely than weight alone, which is why drugs that act throughout the body are dosed by surface area in children.
- Surface area matters wherever exchange happens: the alveoli, the gut with its villi and the tubules of the kidney all achieve a large area in a small volume for the same reason.
How to work through these questions
- Decide which quantity the question is asking for, and whether the numbers given are volumes, rates or concentrations. A rate is a volume divided by a time, and mixing the two up is the commonest error.
- For anything about the heart, keep the three quantities apart: the volume ejected at a beat, the number of beats, and the volume moved in a minute. Each is one multiplication or division from the others.
- For ventilation, take the dead space off the tidal volume before multiplying by the rate. Multiplying first gives the minute ventilation, which is a different quantity and is usually one of the options.
- For a clearance, sort the three numbers by their units: two concentrations and a flow. The concentrations cancel against each other and what is left is a volume per minute.
- For a surface area, square the height and multiply by the weight before dividing - and remember that the answer is a square root, not a division.
- Check the answer against what the body does: a cardiac output around five litres a minute, a glomerular filtration rate around 125 mL a minute, a tidal volume around 500 mL and a surface area around 1.7 square metres are all worth knowing as reference points.
Mistakes that cost marks
- Leaving the answer in millilitres per minute when litres per minute were asked for, or the other way round.
- Subtracting the end diastolic volume from the end systolic volume, which gives a negative stroke volume - the larger is always the diastolic one.
- Using the minute ventilation where the alveolar ventilation was asked for, which means leaving the dead space in.
- Inverting the clearance formula, dividing the plasma by the product of the urine concentration and the flow.
- Multiplying height by weight for a body surface area and forgetting both the division by 3600 and the square root.
- Forgetting that the dissolved oxygen is a very small part of the total, and reporting it as the whole answer.
- Confusing the ejection fraction with the fraction left in the ventricle: the two add to a hundred, and answering with the wrong one is a common and expensive slip.
- Treating the respiratory rate as the ventilation. A rate is how often, and a ventilation is how much, and a question can ask about either.
Worked example
A patient has a heart rate of 70 beats per minute and a stroke volume of 70 mL. What is the cardiac output? The same patient has an end diastolic volume of 140 mL: what is the ejection fraction?
- For the cardiac output, multiply the stroke volume by the heart rate: 70 mL x 70 beats gives 4900 mL per minute.
- A litre is a thousand millilitres, so the output is 4.9 litres a minute - just under the five litres expected at rest, which is the check that the answer is of the right size.
- For the ejection fraction, the stroke volume is 70 mL and the end diastolic volume is 140 mL, so the fraction ejected is 70 / 140, which is one half.
- As a percentage that is 50 per cent. A healthy ventricle ejects a little over half of what it holds, so 50 per cent is at the lower end of normal but not a cause for alarm on its own.
- Notice how the two answers relate: the same 70 mL appears in both, once as a volume per beat and once as a fraction of what was in the ventricle. Keeping the two ideas apart is most of the work in this topic.
Answer: Cardiac output 4.9 litres per minute; ejection fraction 50 per cent
Practice questions with answers
A few Human Anatomy and Physiology questions with the full solution shown, so you can see
how the method is applied before you attempt the timed set.
Question 1
A resting adult has a heart rate of 75 beats per minute and a stroke volume of 110 mL. What is the cardiac output, in litres per minute?
-
A
4.12
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B
8.25
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C
16.50
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D
8250
Answer: Option B — with explanation
Cardiac output is the volume the heart moves in a minute, which is the stroke volume multiplied by the number of strokes in that minute: the amount ejected at each beat, times the beats.
That is 110 mL x 75 beats = 8250 mL per minute, which is 8.25 litres per minute.
The body can raise the output by raising either factor. Raising the rate is quick - the heart simply beats faster - while raising the stroke volume depends on how much blood is coming back to the heart, which is why the two are controlled separately.
Common mistakes
- 8250 is not the answer: 8250
- 4.12 is not the answer: 4.12
- 16.50 is not the answer: 16.50
Question 2
An\vechocardiogram shows an end diastolic volume of 140 mL and an end systolic volume of 80 mL. What is the stroke volume, in millilitres?
Answer: Option C — with explanation
The ventricle does not empty completely, so the volume ejected at each beat is the difference between how full it was and how much is left: the end diastolic volume less the end systolic volume.
That is 140 - 80 = 60 mL.
The volume left behind is not waste. It is what keeps the flow continuous while the ventricle is refilling, and it is the reserve that lets the heart eject more when it has to.
Common mistakes
- 80 is not the answer: 80
- 220 is not the answer: 220
- 140 is not the answer: 140
Question 3
A ventricle has an end diastolic volume of 200 mL and ejects 60 mL at each beat. What is the ejection fraction, as a percentage?
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A
0.30
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B
333.33
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C
70
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D
30
Answer: Option D — with explanation
The ejection fraction is the proportion of the blood in the ventricle that is ejected at each beat, so it is the stroke volume divided by the end diastolic volume, written as a percentage.
That is 60 / 200 x 100 = 30 per cent.
It is the measure of how well the pump is working, and it is used in preference to the stroke volume alone because it takes account of the size of the heart. A healthy ventricle ejects a little over half of what it holds; a figure well below that is one of the signs of heart failure.
Common mistakes
- 333.33 is not the answer: 333.33
- 70 is not the answer: 70
- 0.30 is not the answer: 0.30
Question 4
With a tidal volume of 400 mL and a respiratory rate of 15 breaths per minute, what is the minute ventilation, in litres per minute?
Answer: Option A — with explanation
Minute ventilation is the volume moved in and out of the lungs in a minute, so it is the volume of one breath multiplied by the number of breaths.
That is 400 mL x 15 = 6000 mL per minute, which is 6 litres per minute.
It is a measure of how much work the muscles of breathing are doing, not of how well the lungs are exchanging gas - most of a normal breath never reaches the alveoli at all, which is why the alveolar figure is the one that matters for gas exchange.
Common mistakes
- 415 is not the answer: 415
- 15 is not the answer: 15
- 6000 is not the answer: 6000
Question 5
With a tidal volume of 900 mL, a respiratory rate of 16 breaths per minute and a dead space of 150 mL, what is the alveolar ventilation, in litres per minute?
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A
2.40
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B
16.80
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C
14.40
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D
12
Answer: Option D — with explanation
Not all of each breath reaches the alveoli. The first part of it fills the nose, the trachea and the bronchi, where no gas exchange happens at all, and that volume - the anatomical dead space, about 150 mL in an adult - is breathed out again unchanged.
So the volume taking part in exchange at each breath is 900 - 150 = 750 mL, and at 16 breaths a minute the alveolar ventilation is 12 litres per minute.
This is why breathing deeply is more effective than breathing quickly: a rapid shallow breath spends most of its volume on the dead space, and the alveolar ventilation can fall even though the minute ventilation has risen.
Common mistakes
- 14.40 is not the answer: 14.40
- 16.80 is not the answer: 16.80
- 2.40 is not the answer: 2.40
Frequently asked questions
Why does breathing deeply put more air into the alveoli than breathing quickly?
Because the dead space has to be refilled in every breath, whatever its size. A deep breath of 800 mL delivers 650 mL to the alveoli, while a shallow one of 250 mL delivers only 100 mL - and at twice the rate, that is still less than half as much.
What is the difference between filtration rate and clearance?
Filtration is what the glomerulus does - about 125 mL of plasma filtered a minute. Clearance is a calculation from what appears in the urine, and it equals the filtration rate only for a substance that is freely filtered and neither reabsorbed nor secreted, which is why inulin is used as the reference.
Why is the ejection fraction more useful than the stroke volume?
Because it takes account of the size of the heart. A large heart can have a normal stroke volume while ejecting only a third of what it holds, so the stroke volume alone would look reassuring. The fraction shows the pump is failing.
Why does blood pressure rise if the arterioles narrow?
Because pressure is cardiac output times resistance. Narrowing the arterioles raises the resistance without changing the output, so the pressure has to rise. That is the mechanism the body uses to divert blood from one organ to another.
Why are drugs sometimes dosed by body surface area rather than weight?
Because surface area follows the metabolic rate, and the metabolic rate is what determines how fast a drug is cleared. Two patients of the same weight but very different heights can need quite different doses, and surface area accounts for that.
What is the renal threshold?
The blood glucose above which the kidney can no longer reabsorb it all, so glucose starts to appear in the urine. It corresponds to a blood glucose of about 10 mmol per litre, and it is why glucose in the urine is a sign of diabetes.
