TestAS · Medicine
How organ systems work and regulate themselves: heart and circulation, breathing, kidneys, hormones and the immune system.
testas.de lists this topic for the Medicine module. In the test it comes as testlets: a short text, table or formula, then 4 to 8 questions you answer from it.
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Full exam-length testlets from the Kollego course, generated the same way as our mocks. Answer from the material, then open each answer to check.
Cardiac output at rest, in exercise and in heart failure
The heart's output per minute, the cardiac output (CO), is the product of heart rate (HR) and stroke volume (SV), the volume ejected by one ventricle with each beat. Stroke volume is the difference between the volume in the ventricle at the end of filling, the end-diastolic volume (EDV), and the volume left at the end of ejection, the end-systolic volume (ESV). The ejection fraction (EF) is the share of the end-diastolic volume that is ejected; in healthy adults it lies between about 55 and 70 percent.
CO = HR × SV SV = EDV − ESV EF = SV / EDV MAP ≈ CO × TPR
Two mechanisms increase stroke volume during exercise. First, greater venous return fills the ventricle more, and a more stretched heart muscle contracts more strongly (the Frank-Starling mechanism). Second, sympathetic nerves and adrenaline raise contractility, so that the ventricle empties more completely at the same filling. Sympathetic activity also raises heart rate. Together, these changes can increase cardiac output four- to fivefold in a trained person.
Mean arterial pressure (MAP) depends on cardiac output and on total peripheral resistance (TPR), which is set mainly by the diameter of small arteries and arterioles. During heavy exercise, arterioles in working muscle widen strongly under the influence of local metabolites, while vessels in the gut and kidneys narrow. Because total resistance falls, mean arterial pressure rises only moderately although cardiac output rises several times over. Blood flow to the brain stays almost constant in absolute terms.
In systolic heart failure, the damaged ventricle contracts weakly. It empties poorly, so the end-systolic volume rises, and the ventricle dilates over time. The body compensates at first with a faster heart rate and by retaining salt and water, which increases filling. These responses can keep resting cardiac output close to normal, but the reserve for exercise is small, and the extra fluid can back up into the lungs and cause breathlessness.
Endurance training works in the opposite direction. The ventricle of a trained athlete becomes larger and fills more, so it ejects a bigger stroke volume. The same resting cardiac output is then reached with a lower heart rate, often below 50 beats per minute.
| Person | Heart rate | EDV | ESV |
|---|---|---|---|
| Healthy, at rest | 70 /min | 120 mL | 50 mL |
| Healthy, heavy exercise | 170 /min | 140 mL | 30 mL |
| Patient with heart failure, at rest | 95 /min | 200 mL | 150 mL |
1. What is the stroke volume of the healthy person at rest?
C: 70 mL
SV = EDV − ESV = 120 − 50 = 70 mL.
2. Approximately what cardiac output does the healthy person reach in heavy exercise?
A: 18.7 L/min
SV = 140 − 30 = 110 mL; 170 × 110 mL = 18,700 mL/min.
3. What is the ejection fraction of the patient with heart failure?
D: 25%
SV = 200 − 150 = 50 mL; 50 / 200 = 0.25.
4. How does the patient keep resting cardiac output near normal?
B: With a higher heart rate and a larger filling volume
Patient: SV = 200 − 150 = 50 mL, CO = 95 × 50 mL ≈ 4.75 L/min. Healthy at rest: 70 × 70 mL = 4.9 L/min. The text says the body compensates "with a faster heart rate and by retaining salt and water, which increases filling" (EDV 200 mL).
5. In heavy exercise, cardiac output rises about fourfold but mean arterial pressure rises only moderately. According to the text, this is because…
B: total peripheral resistance falls
MAP ≈ CO × TPR. The text says in heavy exercise muscle arterioles "widen strongly", and "Because total resistance falls, mean arterial pressure rises only moderately". CO goes up, TPR goes down, so the product rises only a little.
6. In the healthy person, the lower ESV during exercise mainly reflects…
B: increased contractility from sympathetic activity
ESV falls from 50 to 30 mL in exercise, so the ventricle empties more. The text says sympathetic nerves and adrenaline "raise contractility, so that the ventricle empties more completely at the same filling".
Oxygen transport and the dissociation curve
Almost all oxygen in the blood is bound to haemoglobin. Each gram of haemoglobin can bind about 1.34 mL of oxygen when it is fully saturated. Only a small amount is physically dissolved in plasma: about 0.03 mL per litre of blood for each mmHg of oxygen partial pressure (PO2). With a normal haemoglobin concentration of 150 g per litre, arterial blood therefore carries about 200 mL of oxygen per litre, of which only about 3 mL is dissolved.
The link between PO2 and haemoglobin saturation is described by the oxygen dissociation curve, which is S-shaped. At the top, between about 60 and 100 mmHg, the curve is flat: even a clear fall of PO2 in the lungs changes saturation only a little, which protects oxygen uptake at moderate altitude or in mild lung disease. Below about 60 mmHg the curve becomes steep, so small falls in PO2 in the tissues release large amounts of oxygen. The PO2 at which haemoglobin is half saturated is called the P50; it is about 27 mmHg in adults.
The curve is not fixed. A rise in carbon dioxide, a fall in pH, a rise in temperature and more 2,3-bisphosphoglycerate (2,3-BPG) in red cells shift it to the right. The P50 then increases, meaning that haemoglobin releases oxygen more easily at a given PO2. These conditions are typical of working muscle. The opposite changes shift the curve to the left, as in the lungs, where carbon dioxide is being removed.
Oxygen delivery can fail in different ways. In anaemia, the haemoglobin concentration is low, so the oxygen content of arterial blood falls although PO2 and saturation are normal. A pulse oximeter, which measures saturation, will then look reassuring. Carbon monoxide binds to haemoglobin about 200 to 250 times more strongly than oxygen. It blocks binding sites and shifts the curve of the remaining haemoglobin to the left, so less oxygen is released in the tissues.
Altitude lowers the partial pressure of oxygen in the air. At 4,000 m it is only about 60 percent of the value at sea level, and arterial PO2 can fall to about 50 mmHg. The body responds at once with faster breathing and, over several weeks, with more red blood cells, stimulated by the hormone erythropoietin from the kidneys.
| PO2 (mmHg) | 100 | 80 | 60 | 40 | 27 | 20 |
|---|---|---|---|---|---|---|
| Saturation | 98% | 95% | 90% | 75% | 50% | 35% |
1. About how much oxygen does one litre of arterial blood carry when haemoglobin is 150 g/L and saturation is 98%?
C: About 200 mL
Bound O₂ = haemoglobin × 1.34 mL/g × saturation = 150 × 1.34 × 0.98 ≈ 197 mL per litre. The first paragraph adds about 3 mL dissolved, so about 200 mL.
2. Mixed venous blood has a PO2 of 40 mmHg. What share of the oxygen bound in arterial blood (PO2 100 mmHg) has been released?
D: About a quarter
In the table, saturation is 98% at 100 mmHg and 75% at 40 mmHg. Released share = (98 − 75) ÷ 98 = 23 ÷ 98 ≈ 0.23, about a quarter.
3. A climber's arterial PO2 falls from 100 to 60 mmHg. Why is oxygen content only slightly lower?
B: The curve is flat in this range
The text says the curve is flat "between about 60 and 100 mmHg". The table shows saturation only falls from 98% to 90% over this range, so oxygen content drops only a little.
4. In warm, acidic working muscle, compared with resting tissue, haemoglobin…
B: gives up more oxygen at the same PO2
The third paragraph says more CO₂, lower pH and higher temperature shift the curve right, and "haemoglobin releases oxygen more easily at a given PO2". These conditions are "typical of working muscle".
5. A patient has haemoglobin 75 g/L, arterial PO2 95 mmHg and saturation 97%. Which statement is correct?
D: Oxygen content is about half of normal
Bound O₂ = Hb × 1.34 × saturation. With 75 g/L instead of 150, the amount is halved: 75 × 1.34 × 0.97 ≈ 97 mL/L instead of about 197. The text says in anaemia content falls "although PO2 and saturation are normal".
6. Why is carbon monoxide poisoning more dangerous than the loss of binding sites alone would suggest?
C: The remaining haemoglobin releases oxygen less easily
The text says CO "blocks binding sites and shifts the curve of the remaining haemoglobin to the left, so less oxygen is released in the tissues". So both fewer sites and poorer release cut delivery.
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