The Urinary System The urinary system consist of the paired kidney and ureters and the unpaired bladder and urethra .This system contributes to the maintenance of homeostasis by a complex process that involves filtration , active absorption , passive absorption , and secretion

Dr. Reem A.R.Al Saad
Medicine College of university Babylon
The Urinary System
The urinary system consist of the paired kidney and ureters and the unpaired bladder and urethra .This system contributes to the maintenance of homeostasis by a complex process that involves filtration , active absorption , passive absorption , and secretion .The result is the production of urine , in which various metabolic waste products are eliminated , Urine produced in the kidneys passes through the ureters to the bladder , where it is temporarily stored and the released to the exterior through the urethra .The two kidneys produce about 125 m L of filtrate per minute.
Kidneys
Each kidney has a concave medial border , the hilum – where nerves enter , blood and lymph vessels enter and exit , and the ureter exits and a convex lateral surface

The renal pelvis , the expanded upper end of the ureter , is divided into two or three major calyces . Several small branches , the minor calyces , arise from each major calyx.
The kidney can be divided into an outer cortex and an inner medulla



In humans , the renal medulla consists of 10-18 conical or pyramidal structures the medullary pyramids . from the base of each medullary pyramid , parallel arrays of tubules , the medullary rays penetrate the cortex. Each kidney composed of 1-4 million nephrons ,each nephron consists of a dilated portion the renal corpuscle , the proximal convoluted tubules , the thin and thick limbs of Henles loop , the distal convoluted tubule and the collecting tubules and ducts.
Each renal corpuscle consists of a tuft of capillaries , the glomerulus , surrounded by a double walled epithelial capsule called glomerular (Bowman s ) capsule

The internal layer ( the visceral layer) of the capsule envelops the capillaries of the glomerulus. The external layer forms the outer limit of the renal corpuscle and is called the parietal layer of Bowman s capsule , between the two layers of Bowman s capsule is the urinary space, which receives the fluid filtered through the capillary wall and the visceral layer, each renal corpuscle has a vascular pole, where the afferent arteriole enters and the efferent arteriole leaves, the parietal layer of Bowman s capsule consists of a simple squamous epithelium supported by a basal lamina and a thin layer of reticular fibers.
During embryonic development , the epithelium of the parietal layer remains relatively unchanged , whereas the internal or visceral , layer is greatly modified the cells of this internal layer, the podocytes have cell body from which arise several primary processes each primary process gives rise to numerous secondary process , called pedicels , the secondary processes of podocytes interdigitate , between the fenestrated endothelial cells of the glomerular capillaries and the podocytes that cover their external surfaces is a thick (0.1 µm) basement membrane .

Proximal Convoluted Tubule
At the urinary pole of the renal corpuscle , the squamous epithelium of the parietal of Bowman s capsule is continuous with the cuboidal or low columnar , epithelium of the proximal convoluted tubule , the tubule is longer than the distal convoluted tubule and is therefore more frequently seen near renal corpuscles in the renal cortex. the cells of this cuboidal epithelium have an acidophilic cytoplasm because of the presence of numerous elongated mitochondria, the cell apex has abundant microvilli about 1 µm in length , which form a brush border because the cells are large .

Function of Proximal convoluted tube
The proximal convoluted tubule absorbs all the glucose and amino acids and about 85%of the sodium chloride and water contained in the filtrate , in addition to phosphate and calcium .Glucose , amino acids , and sodium are absorbed by these tubular cells through an active process involving Na /K ATP ase(sodium pump)located in the basolateral cell membranes .Water diffuses passively , following the osmotic gradient .In addition to these activates , the proximal convoluted tubule secretes creatinine and substances foreign to the organism , such as para- aminohippuric acid and penicillin , from the interstitial plasma into the filtrate



Henle s Loop
It is a U-shaped structure consisting of a thick descending limb , a thin descending limb .The thick limbs are very similar in structure to the distal convoluted tubule The ascending and descending loops lie next to each other, and there is an increasing osmotic gradient from the cortex to the tip. Urea, sodium and chloride ions are actively transported into the interstitial space of the medulla, by the ascending limb, and these are taken up by the descending limb, which has a lower osmotic concentration, This means that moving from the cortex into the medulla, the salt concentration in the intestinal space increases becoming more and more hypertonic, relative to the fluid passing through the collecting ducts, helping to extract water from the filtrate, and concentrate the urine.
The lumen is made up of simple squamous epithelium. This part of the nephron is hard to tell apart from adjacent capillaries, except that there are no red blood cells in the lumen.





Distal Convoluted Tubule
The thick ascending limb of Henles loop penetrates the cortex after descending a certain trajectory , it becomes tortuous and is called the distal convoluted tubule this tubule , like the ascending limb , is lined with simple cuboidal epithelium


The distal convoluted tubule differ from the proximal convoluted tubule (both found in the cortex) because they have no brush border , no apical cananliculi and smaller cells .the distal tubule is modified , as is the afferent arteriole .In this juxtaglomerular region , cells of the distal convoluted tubule usually become columnar , and their nuclei are closely packed together . Most of the cells have a Golgi complex in the basal region . This modified segment of the wall of the distal tubule , which appear darker in microscopic preparations because of the close proximity of its nuclei , is called the macula densa.

Collecting Tubules & Ducts
Urine passes from the distal convoluted tubules to collecting tubules that join each other to form larger , straight collecting ducts , which widen gradually as they approach the tips of the medullary pyramids , the smaller collecting tubules are lined with cuboidal epithelium .the epithelium of collecting ducts is responsive to arginine vasopressin , or antidiuretic hormone, secreted by the posterior pituitary .
Juxtaglomerular Apparatus
Adjacent to the renal corpuscle , the tunica media of the afferent arteriole has modified smooth muscle cells .These cells are called juxtaglomerular (JG) cells and have a cytoplasm full of secretory granules .Secretion of JG cells play a role in the maintenance of blood pressure .The macula densa of the distal convoluted tubule is usually located near the region of the afferent arteriole that contains the JG cells. together , this portion of the arteriole and the macula densa form the JG apparatus .JG cells produce the enzyme renin , which acts on a plasma portion-angiotensinogen – to produce an inactive decapeptide , angiotensin I , as result of the action of a converting enzyme present in high concentration in lung endothelial cells ,this substance loses two amino acids and becomes an active vasopressive octapeptide, angiotensin II.



Blood Circulation
Each kidney receives blood from its renal artery , which usually divides into two branches before entering the organ .One branch goes to the anterior part of the kidney and the other to the posterior part .While still in the hilum , these branches give rise to arteries that branch again to form the interlobar arteries located between the renal pyramids .At the level of the corticomedullary junction, the interlobar arteries form the arcuate arteries .Interlobular arteries branch off at right angles from the arcuate arteries and follow a course in the cortex perpendicular to the renal capsule .Interlobular arteries form the boundaries of the renal lobules . which consist of a medullary ray and the adjacent cortical labyrinth.


Bladder & Urinary passages
The bladder and the urinary passages store the urine formed in the kidneys and conduct it to the exterior .The calyces , renal pelvis , ureter , and bladder have the same basic histological structure , with the walls of the ureters becoming gradually thicker as proximity to the bladder increases.
The mucosa of these organ consists of transitional epithelium and a lamina properia of loose –to- dense connective tissue .Surrounding the lamina propria of these organs is a dense woven sheath of smooth muscle .the transitional epithelium of the bladder in the undescended state is five or six cells in thickness; the superficial cells are rounded and bulge into the lumen .These cells are frequently polyploidy or binucleate. When the epithelium is stretched as when the bladder is full of urine , the epithelium is only three or four cells in thickness , and the superficial cells of the transitional cells become squamous .

Urethra
The urethra is a tube that carries the urine from the bladder to the exterior . duct that transmits urine from the bladder to the exterior of the body during urination. The urethra is held closed by the urethral sphincter, a muscular structure that helps keep urine in the bladder until voiding can occur. in men sperm also pass through it during ejaculation , in woman , the urethra is exclusively a urinary organ .The male urethra consists of four parts ; prostatic , membranous ,bulbous and pendulous . the female urethra is a 4-5 cm long , lined with stratified squamous epithelium and areas of pseudo stratified columnar epithelium .