The urinary system consists of the paired kidneys and ureters, the bladder, and the urethra. This system’s primary role is to ensure optimal properties of the blood, which the kidneys continuously monitor. This general role of the kidneys involves a complex combination of renal functions:
Regulation of the balance between water and electrolytes (inorganic ions) and the acid-base balance;
Excretion of metabolic wastes along with excess water and electrolytes in urine, the kidneys’ excretory product which passes through the ureters for temporary storage in the bladder before its release to the exterior by the urethra;
Excretion of many bioactive substances, including many drugs;
Secretion of renin, a protease important for regulation of blood pressure by cleaving circulating angiotensinogen to angiotensin I;
Secretion of erythropoietin, a glycoprotein growth factor that stimulates erythrocyte production in red marrow when the blood O2 level is low;
Conversion of the steroid prohormone vitamin D, initially produced in the skin, to the active form (1,25-dihydroxyvitamin D3 or calcitriol); and
Gluconeogenesis during starvation or periods of prolonged fasting, making glucose from amino acids to supplement this process in the liver.
Approximately 12-cm long, 6-cm wide, and 2.5-cm thick in adults, each kidney has a concave medial border, the hilum—where nerves enter, the ureter exits, and blood and lymph vessels enter and exit—and a convex lateral surface, both covered by a thin fibrous capsule (Figure 19–1). Within the hilum the upper end of the ureter expands as the renal pelvis and divides into two or three major calyces. Smaller branches, the minor calyces, arise from each major calyx. The area surrounding the renal pelvis and calyces contains adipose tissue.
Each kidney is bean-shaped, with a concave hilum where the ureter and the renal artery and vein enter. The ureter divides and subdivides into several major and minor calyces, around which is located the renal sinus containing adipose tissue. Attached to each minor calyx is a renal pyramid, a conical region of medulla delimited by extensions of cortex. The cortex and hilum are covered with a fibrous capsule.
The parenchyma of each kidney has an outer renal cortex, a darker stained region with many round corpuscles and tubule cross sections, and an inner renal medulla consisting mostly of aligned linear tubules and ducts (Figure 19–1). The renal medulla in humans consists of 8-15 conical structures called renal pyramids, all with their bases meeting the cortex (at the corticomedullary junction) and separated from each other by extensions of the cortex called renal columns. Each pyramid plus the cortical tissue at its base and extending along its sides constitutes a renal lobe. Parallel ducts and tubules extending from the medulla into the ...