Which of the following describes the endocardium?

a. Acetylcholine: This is the primary neurotransmitter at neuromuscular junctions. It binds to receptors on muscle cells (specifically nicotinic acetylcholine receptors) and triggers an action potential, leading to muscle contraction.

b. Calcium: While calcium ions are essential for muscle contraction, they are not neurotransmitters. Instead, they play a role in the release of acetylcholine from the nerve terminal and in the contraction process within muscle cells.

c. Adenosine triphosphate (ATP): ATP is the primary energy carrier in cells but is not a neurotransmitter. It is crucial for muscle contraction, as it provides the energy for various steps in the contraction cycle.

d. Sodium: Sodium ions are involved in generating action potentials and depolarizing muscle cells, but they are not neurotransmitters.

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.