Free MCAT-style practice, 3B
MCAT Cardiovascular Function and Blood Gas Transport Practice Questions
Eight free MCAT-style practice questions on cardiac output, the ECG and AV node, vessel resistance, mean arterial pressure, capillary Starling forces, oxygen-hemoglobin binding and carbon dioxide transport, with explanations.
Quick concept primer
Cardiac output is heart rate times stroke volume, and stroke volume is end-diastolic volume minus end-systolic volume. Electrical activity begins at the sinoatrial node, passes through the AV node (where conduction is deliberately delayed), then spreads through the bundle of His and Purkinje fibers. On an ECG the P wave is atrial depolarization, the QRS complex is ventricular depolarization and the T wave is ventricular repolarization.
Blood flow follows a fluid-flow relationship: flow equals pressure difference divided by resistance, and resistance in a vessel varies with the inverse fourth power of the radius, so small changes in arteriolar radius have large effects. Mean arterial pressure is approximately diastolic pressure plus one third of pulse pressure. In capillaries, hydrostatic pressure pushes fluid out and oncotic pressure pulls it back.
Hemoglobin binds oxygen cooperatively, giving a sigmoidal curve. Acid, CO2, heat and 2,3-BPG shift the curve to the right (lower affinity, easier unloading), while fetal hemoglobin has a left-shifted curve. Most CO2 is carried as bicarbonate, formed in red cells by carbonic anhydrase.
Traps: resistance depends on radius to the fourth power, not the second, and a right shift lowers affinity (raises P50).
8 practice questions
Question 1
A patient has an end-diastolic volume of 140 mL, an end-systolic volume of 60 mL and a heart rate of 75 beats per minute. What is the cardiac output?
- 4.5 L/min
- 8.0 L/min
- 10.5 L/min
- 6.0 L/min
Show answer and explanation
Answer: D. 6.0 L/min
Stroke volume = EDV - ESV = 140 - 60 = 80 mL. Cardiac output = HR x SV = 75 x 80 = 6,000 mL/min = 6.0 L/min. The ejection fraction in this patient is 80/140, about 57 percent, within the normal range.
Question 2
The AV node delays the electrical impulse before it reaches the ventricles. What is the functional benefit of this delay?
- It allows the atria to finish contracting and fill the ventricles before ventricular contraction begins
- It lets the ventricles contract before the atria
- It prevents the SA node from firing
- It causes the QRS complex
Show answer and explanation
Answer: A. It allows the atria to finish contracting and fill the ventricles before ventricular contraction begins
Conduction through the AV node is slow because of small cells and few gap junctions. This delay, visible as the PR interval on the ECG, gives the atria time to complete contraction and top off ventricular filling before the ventricles depolarize. A prolonged PR interval suggests slowed AV conduction (first-degree AV block).
Question 3
An arteriole constricts to half its original radius with no change in blood viscosity or vessel length. By what factor does resistance change?
- 8
- 2
- 16
- 4
Show answer and explanation
Answer: C. 16
Resistance varies as 1/r^4. Halving the radius multiplies the denominator by (1/2)^4 = 1/16, so resistance increases 16-fold. This steep dependence is why arterioles are the main site of regulation of systemic vascular resistance.
Question 4
A patient's blood pressure is 120/80 mmHg. Using the approximation that mean arterial pressure equals diastolic pressure plus one third of the pulse pressure, what is the MAP?
- 80 mmHg
- 93 mmHg
- 100 mmHg
- 87 mmHg
Show answer and explanation
Answer: B. 93 mmHg
Pulse pressure = 120 - 80 = 40 mmHg. One third of 40 is about 13. MAP = 80 + 13 = 93 mmHg. The mean is closer to diastolic than to systolic because the heart spends about two thirds of each cycle in diastole.
Question 5
Pressures at two ends of a capillary are shown.
| Location | Capillary hydrostatic pressure (mmHg) | Capillary oncotic pressure (mmHg) |
|---|---|---|
| Arteriolar end | 35 | 25 |
| Venular end | 15 | 25 |
Assuming interstitial hydrostatic and oncotic pressures are negligible, what is the net movement at the venular end?
- Net reabsorption of 10 mmHg into the capillary
- Net filtration of 40 mmHg
- No net movement
- Net filtration of 10 mmHg out of the capillary
Show answer and explanation
Answer: A. Net reabsorption of 10 mmHg into the capillary
Net pressure favoring outward movement is hydrostatic minus oncotic: 15 - 25 = -10 mmHg. The negative value means fluid is pulled back into the capillary. At the arteriolar end, 35 - 25 = +10 mmHg favors filtration. The lymphatic system returns the small excess of filtered fluid that is not reabsorbed.
Question 6
During intense exercise, muscle tissue produces CO2 and lactic acid and its temperature rises. How does this affect the hemoglobin-oxygen dissociation curve and oxygen delivery?
- The curve is unchanged
- The curve shifts to the right, lowering hemoglobin's oxygen affinity and promoting release of oxygen to the muscle
- The curve becomes hyperbolic like myoglobin
- The curve shifts to the left, increasing hemoglobin's affinity for oxygen
Show answer and explanation
Answer: B. The curve shifts to the right, lowering hemoglobin's oxygen affinity and promoting release of oxygen to the muscle
Increased CO2, H+, temperature and 2,3-BPG all shift the curve to the right (the Bohr effect plus the temperature effect). A rightward shift raises the P50, so at a given PO2 hemoglobin holds less oxygen and unloads more in the active tissue. The sigmoidal shape does not change into a hyperbola.
Question 7
Fetal hemoglobin has a higher oxygen affinity than adult hemoglobin at the same PO2. What is the physiological advantage?
- It lets oxygen move from maternal blood to fetal blood across the placenta
- It binds 2,3-BPG more tightly than adult hemoglobin
- It increases the fetal blood CO2 content
- It prevents the fetus from unloading oxygen in its tissues
Show answer and explanation
Answer: A. It lets oxygen move from maternal blood to fetal blood across the placenta
The fetal oxygen dissociation curve lies to the left of the maternal one. At the placenta, oxygen leaves maternal hemoglobin and is picked up by fetal hemoglobin, which is possible because the fetal protein holds oxygen more tightly. The higher affinity is partly due to weaker binding of 2,3-BPG by fetal hemoglobin, not stronger, so choice B is wrong.
Question 8
In systemic capillaries, CO2 diffuses into red blood cells. What happens next?
- CO2 is exchanged for O2 at the heme iron
- CO2 is converted to CO and carried on hemoglobin
- CO2 is converted by carbonic anhydrase to carbonic acid, which dissociates into H+ and HCO3-; HCO3- leaves the cell in exchange for Cl- (chloride shift) while hemoglobin buffers the H+
- CO2 remains entirely dissolved in the cytoplasm
Show answer and explanation
Answer: C. CO2 is converted by carbonic anhydrase to carbonic acid, which dissociates into H+ and HCO3-; HCO3- leaves the cell in exchange for Cl- (chloride shift) while hemoglobin buffers the H+
About 70 percent of CO2 is carried as bicarbonate. Carbonic anhydrase in red cells accelerates the hydration of CO2, and the H+ released is buffered by hemoglobin (which also promotes oxygen release). Bicarbonate exits through the band 3 anion exchanger in exchange for chloride. Roughly 20 to 25 percent travels bound to hemoglobin as carbamino compounds, not at the heme iron.