Chapter 26: The Urinary System.
Functions of the urinary system
The paired kidneys are retroperitoneal organs lying next to the upper lumbar vertebrae.
The renal capsule covers the outer surface of the kidney.
Perinephric fat surrounds the capsule.
Renal fascia or Gerota’s fascia surrounds the perinephric fat.
The above layers have significant importance in protecting the kidney and staging renal cell carcinoma. Renal cancer has a peculiar trait of spreading into the renal vein and up the inferior vena cava as “tumor thrombus”.
The kidneys have a kidney bean shape with a central indentation called the hilus or hilum and are about 4 to 5 inches long.
The kidney is divided into the outer cortex and the inner medulla.
Renal pyramids are part of the medulla and have at their apex a papilla that protrudes into a minor calyx.
Cortical tissues between the pyramids are renal columns. A renal pyramid, the overlying cortex and adjacent columns constitute a renal lobe.
Urine produced in the lobes is emptied into the minor calyx. Several minor calyces converge to form a major calyx and the major calices converge to form the renal pelvis.
At the microscopic level urine is produced in nephrons.
Kidneys are supplied by the renal arteries and drained by the renal veins. Afferent arterioles supply the individual nephrons.
Urine production is primarily auto-regulated but can be modified by both neural and hormonal input.
Histology of kidney
The nephron is the basic structural and functional unit of the kidney.
The renal corpuscle consists of the glomerulus and the surrounding Bowman's capsule. The glomerulus is a vascular tuft fed by an afferent arteriole and drained by an efferent arteriole. The renal corpuscle is continuous with the renal tubule.
Filtration at the glomerulus generates a solution known as the glomerular filtrate. The filtrate then passes through the limbs of the renal tubule which empties into a connecting tubule and finally into a connecting duct. Along the renal tubule important organic compounds and 80% of the water are reabsorbed producing concentrated urine.
There are two types of nephrons, cortical and juxtamedullary. The cortical are the most numerous and are responsible for most of the function of the kidneys but the juxtamedullary are responsible for production of concentrated urine.
The glomerulus protrudes into Bowman’s capsule and is surrounded by a capsular space. Blood pressure forces the liquid portion of blood, minus large proteins into the capsular space. Blood cells cells and large proteins are not included in this filtrate and are always abnormal when found in urine.
Proximal Convoluted Tubule: The first portion of the renal tubule. Most nutrients, ions and proteins are actively absorbed here.
Loop of Henle: The loop of Henle dips into the renal medulla and then returns to the cortex. The loop actively transports sodium and potassium ions out of the loop and into the interstitial fluid. The resulting osmotic gradient results in movement of water out of the loop and into the interstitium where it is absorbed by the vasa recta and returned to the general circulation.
Distal Convoluted Tubule: Actively secretes ions to be removed via urine and further absorbs water and some ions.
The Juxtaglomerular Apparatus: Monitors electrolyte concentration and secretes the hormones renin and erythropoietin.
Some water may further be reabsorbed in the collecting system in the collecting system in response to the hormone ADH.
Structures of urine transport, storage and elimination:
The pelvocalyceal system: included the major and minor calyces and the renal pelvis. These drain into the . . .
Ureters: Muscular tubes that extend from the renal pelvis (UPJ) to the bladder (UVJ). These are retroperitoneal structures. They enter the bladder at an oblique angle which helps to prevent backflow of urine. Smooth muscle in the wall of the ureters rhythmically contracts (peristalsis) to move urine into the bladder.
Histology: transitional epithelium surrounded by smooth muscle layer and outer adventitial layer.
The Bladder: A muscular, roughly round container that temporarily holds urine. In the male it lies between the rectum and pubic symphysis. In the female it lies between vagina/uterus and the pubic symphysis. The superior surface of the bladder is covered by peritoneum; the bladder is an extra-peritoneal structure. The area between the ureteral orifices and the opening of the urethra is known as the trigone. An internal urethral sphincter controls the exit of urine from the bladder.
Histology of the bladder: mucosa of transitional epithelium, Submucosa, and thick muscular layer know as the detrusor muscle
The urethra:
Female: relatively short, exits just anterior to the vagina
Male: longer, divided into three sections the prostatic, membranous and spongy urethra. The prostatic urethra is enclosed in the prostate gland. The membranous urethra is a short section that penetrates the urogenital diaphragm. The spongy urethra or penile urethra extends from the membranous urethra to the external urethral orifice (meatus).
As the urethra passes through the UG diaphragm it is incased in a circular band of skeletal muscle; the external urethral sphincter. This is under voluntary control via the pudendal nerve. The muscle has a resting tone and requires voluntary control for relaxation.
Histology: In both male and females the urethra starts out as transitional cell but quickly becomes stratified squamous in the female. The male urethra is more variable but ends up stratifies squamous as well.
