Excretory System in Human Being: Gross Anatomy

In humans, the excretory system is built around a pair of bean-shaped kidneys, positioned one on either side of the vertebral column, spanning roughly from the 12th thoracic vertebra to the 3rd lumbar vertebra. Because the kidneys lie behind (dorsal to) the peritoneum rather than inside the peritoneal cavity, they are described as retroperitoneal. Each adult kidney is roughly 10 cm long, with a convex outer surface and a concave inner surface; the notch on the concave inner surface is the hilus, the single point through which the renal artery enters and both the renal vein and the ureter leave the kidney. Each kidney contains close to one million nephrons, its basic functional units.

From each kidney's hilus arises a ureter — a long, muscular tube about 25-30 cm long — which carries urine down to the urinary bladder. The two ureters enter the bladder through separate openings that are not guarded by valves; instead, each ureter runs obliquely through the wall of the bladder, and this oblique path is what prevents urine from being pushed back up (refluxed) into the ureter when the filling bladder compresses it.

The urinary bladder is a hollow, muscular, roughly pear-shaped sac lying in the pelvic cavity, just behind the pubic symphysis, that serves as a temporary reservoir for urine (with an average capacity of about 700 ml). At the base of the bladder is a small, inverted-triangular area called the trigone, whose two basal corners receive the two ureteric openings, while its apex opens into the urethra. The bladder wall's outer covering is peritoneum; beneath that is a thick muscular layer, the detrusor muscle, made of three layers of smooth muscle (longitudinal-circular-longitudinal); and its innermost lining is transitional epithelium, specially suited to stretching as the bladder fills. …