Which of the following valves has two leaflets and is closed during the systolic phase of the cardiac cycle?

a. The outer tissue layer consisting of renal corpuscles and convoluted tubules: This describes the renal cortex, not the renal pelvis.

b. The inner tissue layer consisting of the loop of Henle and collecting tubules: This describes the renal medulla, not the renal pelvis.

c. The expanded area of a renal tubule that encloses the glomerulus: This describes Bowman's capsule, not the renal pelvis.

d. The cavity where urine flows from the calyces to enter the ureters: Correct. The renal pelvis is the funnel-shaped cavity that collects urine from the calyces and channels it into the ureter.

a. Secretes fluid to reduce friction between layers: This is correct. The pleural membranes secrete pleural fluid, which reduces friction between the visceral and parietal layers during breathing.

b. Secretes a substance that reduces surface tension of alveoli: This describes the function of surfactant, which is secreted by alveolar cells, not pleural membranes.

c. Secretes a substance to keep epithelial cells wet: This is a general description that doesn't specifically apply to pleural membranes.

d. Secretes fluid involved in protection from infections: While pleural fluid may have some protective roles, its primary function is to reduce friction, not directly to protect from infections.