At the instant following the second heart sound, which heart valves are open?
A. Both atrioventricular valves and semilunar valves
B. Atrioventricular valves only
C. All valves are closed
D. Semilunar valves only
Answer: C. All valves are closed
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Why is there variation in skin color among modern humans?
A. People have different numbers of melanocytes. B. People have a different distribution of carotene in their skin. C. People have a different distribution of melanin in their epidermis. D. People are exposed to different chemicals over their lifetime. E. People have different hemoglobin in vessels close to their skin.
The ascending limb of the loop of Henle is where
a. NaCl passively leaves the tubular fluid down its concentration gradient b. NaCl is actively transported into the interstitial fluid, leaving water behind because the tubular cells are not permeable to water c. K+ is secreted d. aldosterone stimulates Na+ reabsorption e. none of the above occur
true or false? mRNA can be made from particular introns that have been removed from pre-mRNA.
a. true b. false
Impaired motor coordination in Parkinson disease is often due to ________.
A. degeneration of the red nucleus B. degeneration of the substantia nigra C. degeneration of the mesolimbic system D. lack of regulation of the cerebral peduncle