In the female, the urethra is short, measuring 3 to 5 cm in length from the bladder to the vestibule of the vagina, where it normally terminates just posterior to the clitoris. The mucosa is traditionally described as having longitudinal folds. As in the male urethra, the lining is initially transitional epithelium, a continuation of the bladder epithelium, but changes to stratified squamous epithelium before its termination. Some investigators have reported the presence of stratified columnar and pseudostratified columnar epithelium in the midportion of the female urethra.
Numerous small urethral glands, particularly in the proximal part of the urethra, open into the urethral lumen. Other glands, the paraurethral glands, which are homologous to the prostate gland in the male, secrete into the common paraurethral ducts. These ducts open on each side of the external urethral orifice. They produce an alkaline secretion. The lamina propria is a highly vascularized layer of connective tissue that resembles the corpus spongiosum in the male. Where the urethra penetrates the urogenital diaphragm (membranous part of the urethra), the striated muscle of this structure forms the external (voluntary) urethral sphincter.
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The urinary system consists of the paired kidneys, the paired ureters, which lead from the kidneys to the urinary bladder, and the urethra, which leads from the bladder to the exterior of the body. The kidneys conserve body fluid and electrolytes and remove metabolic wastes such as urea, uric acid, creatinine, and breakdown products of various substances. They produce urine, initially an ultrafiltrate of blood that is modi-fied by selective resorption and specific secretion by kidney tubule cells. The kidneys also function as endocrine organs, producing erythropoi-etin, a growth factor that regulates red blood cell formation, and renin, a hormone involved in blood pressure and blood volume control. They also hydroxylate vitamin D, a steroid prohormone, to produce its active form. Each kidney is a flattened, bean-shaped structure approximately 10 cm long, 6.5 cm wide (from convex to concave border), and 3 cm thick. The concave medial border of each kidney contains a hilum, an indented region through which blood vessels, nerves, and lymphatic vessels enter and leave the kidney. The funnel-shaped origin of the ureter, the renal pelvis, also leaves the kidney at the hilum. A cut, hemi-sected fresh kidney reveals two distinct regions: a cortex, the reddish-brown outer region, and a medulla, a much lighter inner part continu-ous with the renal pelvis. The cortex is characterized by renal corpuscles and their tubules, including the convoluted and straight tubules of the nephron, the collecting tubules, and an extensive vascular supply.
Kidney, human, fresh specimen ×3.