Cardiovascular, respiratory and renal systems sample question 1

the filtered glucose load exceeds the proximal tubule's transport maximum.

Cardiovascular, respiratory and renal systems sample question 2

Prolonged prothrombin time and prolonged activated partial thromboplastin time, with a normal platelet count and bleeding time

Cardiovascular, respiratory and renal systems sample question 4

It converts plasminogen to plasmin, which digests the fibrin mesh and releases fragments such as D-dimer.

Cardiovascular, respiratory and renal systems sample question 5

2,3-BPG binds the central cavity of deoxyhemoglobin and stabilizes the T state, shifting the curve right to help unload O₂.

MCAT · Biological and Biochemical Foundations · Biology

Cardiovascular, respiratory and renal systems

Cardiovascular, respiratory and renal systems is part of Biology in the MCAT Biological and Biochemical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.

Question types you will see here include cardiac output and Starling mechanism, flow, resistance and pressure, hemoglobin saturation and oxygen content and ventilation control and acid-base data.

Common traps: confusing preload with afterload; forgetting that very high rate shortens filling time; reading cardiac output as heart rate alone.

Sample questions

Question 1
Glucose appears in the urine of a patient with poorly controlled diabetes mellitus mainly because:
- A.
  the collecting duct secretes glucose into the urine.
- B.
  glucose transporters in the loop of Henle are destroyed.
- C.
  the filtered glucose load exceeds the proximal tubule's transport maximum.
- D.
  insulin deficiency makes the glomerular filtration barrier more permeable to glucose.
Answer: C. the filtered glucose load exceeds the proximal tubule's transport maximum.
Glucose is filtered freely, and when plasma glucose is high enough the amount filtered saturates the SGLT transporters in the proximal tubule, so the excess stays in the urine. Glucose is normally filtered anyway; the problem is saturation of reabsorption, not increased glomerular permeability or tubular secretion.
Question 2
A newborn with severe vitamin K deficiency begins to bleed from the umbilical stump. Which laboratory pattern is most likely?
- A.
  Normal prothrombin time and normal partial thromboplastin time, with a prolonged bleeding time
- B.
  Low platelet count and prolonged bleeding time, with normal prothrombin time
- C.
  Normal prothrombin time, with a prolonged partial thromboplastin time and a normal platelet count, as in a deficiency of an intrinsic-pathway factor
- D.
  Prolonged prothrombin time and prolonged activated partial thromboplastin time, with a normal platelet count and bleeding time
Answer: D. Prolonged prothrombin time and prolonged activated partial thromboplastin time, with a normal platelet count and bleeding time
D is correct: vitamin K is needed to carboxylate factors II, VII, IX and X, so both the extrinsic and intrinsic tests are prolonged, whereas the platelet plug is normal. A and B describe platelet defects; C would require loss of only an intrinsic-pathway factor, but factor VII is also vitamin K dependent.
Question 3
A patient has a blood pressure of 126/84 mmHg. What is the approximate mean arterial pressure?
- A.
  84 mmHg
- B.
  98 mmHg
- C.
  105 mmHg
- D.
  126 mmHg
Answer: B. 98 mmHg
MAP ≈ diastolic + one-third of pulse pressure = 84 + (126 − 84)/3 = 84 + 14 = 98 mmHg. A is the diastolic pressure, C is the simple average of systolic and diastolic, which overweights systole, and D is the systolic pressure.
Question 4
Within hours of an ischemic stroke caused by a blood clot, a patient receives tissue plasminogen activator (tPA). Which statement describes its action?
- A.
  It inhibits thrombin, so that no new fibrin forms and the existing clot remains unchanged in the cerebral vessel.
- B.
  It converts plasminogen to plasmin, which digests the fibrin mesh and releases fragments such as D-dimer.
- C.
  It binds Ca²⁺ in the blood, which stops the coagulation cascade at several steps.
- D.
  It activates platelets, which retract the clot and release it from the vessel wall.
Answer: B. It converts plasminogen to plasmin, which digests the fibrin mesh and releases fragments such as D-dimer.
tPA cleaves plasminogen to plasmin, a protease that digests cross-linked fibrin, so the clot dissolves and D-dimer fragments appear; this is fibrinolysis. Thrombin inhibition (A) would not remove an existing clot, calcium chelation (C) is how citrate works in tubes, and activated platelets (D) promote clotting.
Question 5
After two weeks at high altitude, a climber's red cells contain more 2,3-bisphosphoglycerate (2,3-BPG). What is the effect on hemoglobin and why is it useful?
- A.
  2,3-BPG binds the heme iron and competes with O₂, which shifts the curve left and helps load O₂ in the lungs in the thin air at altitude.
- B.
  2,3-BPG binds the central cavity of deoxyhemoglobin and stabilizes the T state, shifting the curve right to help unload O₂.
- C.
  2,3-BPG stabilizes the R state, which shifts the curve left and lowers P50 so that more O₂ is bound.
- D.
  2,3-BPG binds only oxyhemoglobin, which lowers P50 and so raises saturation in the lungs.
Answer: B. 2,3-BPG binds the central cavity of deoxyhemoglobin and stabilizes the T state, shifting the curve right to help unload O₂.
The negatively charged 2,3-BPG fits into the central cavity of the T-state tetramer and lowers oxygen affinity (higher P50), so each red cell gives up more O₂ to tissues, an adaptation to low oxygen availability. A puts the binding at the heme and shifts left; C and D describe a left shift that would hold O₂ more tightly.

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