The urinary system helps maintain homeostasis by regulating water balance and by removing harmful substances from the blood. The blood is filtered by two kidneys, which produce urine, a fluid containing toxic substances and waste products. From each kidney, the urine flows through a tube, the ureter, to the urinary bladder, where it is stored until it is expelled from the body through another tube, the urethra.
The kidneys are surrounded by three layers of tissue:
The renal fascia is a thin, outer layer of fibrous connective tissue that surrounds each kidney (and the attached adrenal gland) and fastens it to surrounding structures.
The adipose capsule is a middle layer of adipose (fat) tissue that cushions the kidneys.
The renal capsule is an inner fibrous membrane that prevents the entrance of infections.
The loop of Henle of juxtamedullary nephrons is the apparatus that allows the nephron to concentrate urine. The loop is a countercurrent multiplier system in which fluids move in opposite directions through side-by-side, semipermeable tubes. Substances are transported horizontally, by passive or active mechanisms, from one tube to the other. The movement of the transported substances up and down the tubes results in a higher concentration of substances at the bottom of the tubes than at the tops of the tubes.
The ureters, one from each kidney, deliver urine to the bladder. The ureters enter through the back of the bladder, entering at an angle such that when the bladder fills, the ureter openings are forced closed.
The urinary bladder is a muscular sac for storing urine. The triangular base of the bladder, the trigone, is defined by the two ureters that deliver the urine and the one urethra that drains the urine. When empty, the bladder collapses, and folds (called rugae) from in the bladder wall. As it fills, the folds become distended and the bladder becomes spherical. The wall of the...