QUESTION: 185

Which of the following compounds directly functions in excitation contraction coupling in smooth but not skeletal muscle?

actin
calcium
myosin
myosin light chain kinase
troponin

Answer(s): D

Explanation:

In smooth muscle, increased cytoplasmic [ ] activates myosin light chain kinase, which acts to
phosphorylate the light chain of the myosin molecule. Phosphorylation of the light chain activates myosin ATPase activity, initiating contraction. Troponin (choice E) is a calciumbinding protein that functions in both skeletal and cardiac muscles to regulate contractile activity. Troponin is not expressed in smooth muscle.
Calcium (choice B) does regulate contractile activity. Increased intracellular [ ] serves to activate contraction in both skeletal and smooth muscles. Myosin (choice C) and actin (choice A) are contractile proteins present in both skeletal and smooth muscles. Though essential to muscle contraction, these proteins function in contraction in both muscle types.

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QUESTION: 186

In which of the following conditions would the gas composition of both end pulmonary capillary blood and alveolar air approximate that of mixed venous blood?

airway obstruction
breathing 100% oxygen
hyperventilation
normal breathing
pulmonary capillary embolus

Answer(s): A

Explanation:

When the airway is obstructed, an alveolus will not be adequately ventilated. In this condition, effective gas exchange does not occur and alveolar gas composition and end pulmonary blood gas compositions approach 46 mm Hg and 44 mm Hg C --the gas composition of mixed venous blood. In normal breathing (choice D) alveolar air equilibrates with pulmonary capillary blood, with a gas composition of approximately 100 mm Hg and 40 mm Hg C . With hyperventilation (choice C), C partial pressure will fall and partial pressure may increase relative to the normal values quoted above. With a pulmonary capillary embolus (choice E), blood flow to an alveolus or to a group of alveoli is blocked.
In this case, efficient gas exchange does not occur and gas composition of alveolar air approximates that of tracheal air (150 mm Hg and 0 mm Hg C ). This condition is referred to as
wasted ventilation. In a healthy individual, breathing 100% (choice B) will increase alveolar partial pressure and thereby increase end pulmonary capillary partial pressure.

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QUESTION: 187

Below figure shows pulmonary function test tracings from a healthy person and a patient with pulmonary fibrosis. Of the following measures which would you expect to be increased in this patient, as compared to the expected norms?

Forced expiratory volume (FEV)/forced vital capacity (FVC)
FRC
FVC
tidal volume
vital capacity

Answer(s): A

Explanation:

Pulmonary fibrosis is a restrictive lung disease. below figure illustrates pulmonary function testing data in a normal subject and a patient with fibrosis. Volume-time curves reveal and in both subjects. The ratio, which is typically about 0.8, is increased in patients with pulmonary
fibrosis. Lung volumes such as vital capacity (choice E) are decreased. Patients with restrictive lung disease tend to exhibit shallow breathing, thus tidal volume is decreased (choice D). Increased elasticity causes a decrease in FRC, the mechanical balance point between lungs and chest wall (choice B). Though as noted above and illustrated in below figure the ratio is increased in patients with pulmonary fibrosis, itself is decreased (choice C).

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QUESTION: 188

A patient with chronic renal insufficiency due to renal vascular disease has a net functional loss of nephrons. If we assume that production of urea and creatinine is constant and that the patient is in a steady state, a 50% decrease in the normal GFR will cause which of the following to occur?

decrease plasma urea concentration
greatly increase plasma
increase the percent of filtered Na+ excreted
not affect plasma creatinine
significantly decrease plasma

Answer(s): C

Explanation:

Both and excretion are tightly regulated. Thus, as GFR decreases in disease, the percentage
of filtered or excreted increases to maintain a normal amount of or excretion (assuming Na+ and intake remain the same). Substances like urea (choice A) (some reabsorption) and creatinine (choice D) (almost exclusively excreted by glomerular filtration) have no adaptive mechanisms to regulate plasma levels. Thus, a significant decrease in GFR results in significant increases in plasma creatinine and urea (assuming production of both substances remains constant). This is because the amount of substance x that is excreted (Ux·V) equals the amount produced. Furthermore, · V = GFR · Px. If GFR decreases, increases. Because of the increase in percent
filtered and that is excreted, an increase in plasma (choice B) or a decrease in plasma (choice E) would not be expected with a GFR that is 50% of normal. ÷ urine concentration of x; ÷ plasma concentration of ; V ÷ urine volume.

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QUESTION: 185

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