THE URINARY SYSTEM

Urinary System 2 kidneys Urinary Passages Urethra 2 ureters Urinary Bladder

The kidney: Is a compound tubular gland. Is made of cortex and medulla. Its unit structure is the uriniferous tubule. Functions:  The kidney performs the following functions: Formation & excretion of urine. Filtration of blood & excretion of waste products. Regulation of the concentration of body fluids. Secretion of renin and erythropoietin.

Structure of kidney The kidney is formed of : Cortex. Medulla.

Renal Cortex: It is outer portion of the kidney. It is red (due to glomeruli) and granular (due to convoluted tubules). It arches over the pyramids of the medulla and dips in between adjacent pyramids, forming columns. The cortex is subdivided into: Cortex proper: narrow area underneath the capsule. Columns of Bertin: found between the medullary pyramids. Labyrinth of the cortex: found between medullary rays.

Renal Medulla: It appears yellowish and radially striated due to radial arrangement of its tubules & blood vessels. It is made of 8-18 pyramids. The apices of the renal pyramids (renal papilla) project into the minor calyces and their bases project toward the cortex. Renal pelvis: The renal papillae are surrounded by minor calyces, which fuse together forming 2-3 major calyces. The later collect into the renal pelvis, which is a funnel- shaped sac that occupies the renal sinus and forms the upper expanded end of the ureter. Medullary rays: are straight tubules extending from the pyramids to the cortex. The renal lobe: is made of a pyramid and the corresponding cortical tissue, while the renal lobule includes a medullary ray and the surrounding cortical tissue.

Renal papilla (apex of renal papilla) showing the openings of Ducts of Bellini

Renal lobe: A single pyramid with its associated overlying cortex. Renal lobule: It is defined within cortex and involves a single medullary ray (central axis of lobule) with adjacent cortical labyrinth. It is the functional unit that consists of a collecting duct and all the nephrons that it drains. Renal lobe Distribution of the nephron

The Uriniferous Tubule It is the structural and functional unit of kidney. It consists of: nephron and collecting tubule. Uriniferous Tubule Nephron (functional secretory part) Collecting Tubule (Excretory part)

Nephron It is the functioning part of uriniferous tubule. There are 1-4 millions of nephrons in the kidney. Each nephrons nephron consists of: Renal corpuscle. Proximal convoluted tubule. Loop of Henle. Distal convoluted tubule. Nephrons are either cortical or juxtamedullary, according to the location of the renal corpuscle.

Renal Corpuscle It is a spherical body ( um in diameter) found in the renal cortex. Its function is filtration of plasma minus its proteins. It is made of Bowman's capsule and glomerulus.

It is a double walled cup- shaped structure. It has 2 poles: Vascular pole  in contact with the glomerulus. Urinary pole  continuous with the rest of the nephron. It is formed of visceral & parietal layers, enclosing a capsular space. The parietal layer (continuous with the proximal convoluted tubule) is lined with simple squamous epithelium. The visceral layer (in contact with the glomerulus) is lined by modified cells, the podocytes. Bowman's Capsule

 It is formed of a tuft of about 50 blood capillaries, in the concavity of the Bowman's capsule.  The capillaries arise form the afferent arteriole.  They unite again and leave as the efferent arteriole.  The capillaries are lined by endothelial cells, resting on a thick continuous basement membrane:  Endothelial cells: are fenestrated and joined together by tight junctions.  Basement membrane: is thick and continuous. Glomerulus They are modified flat cells lining the visceral layer of the Bowman's capsule. They have major processes (feet), form which minor processes arise, to terminate around the basement membranes of the glomerular capillaries. The minor processes interdigitate with those of the adjacent podocytes. The spaces between the minor processes are called filtration slits. They are closed by slit diaphragms (membranes). Podocytes

Lumen of capillary

PODOCYTE 1° process 2° pedicels

Blood Renal Barrier (Filtration Barrier) It is the barrier which separates the blood (in glomerular capillaries) form the capsular space of the Bowman's capsule. It regulates the process of filtration and the passage of molecules from blood stream to the Bowman's capsule. 130 ml of glomerular filtrate enters the urinary space every minute. It consists of: The pores (fenestrations) of capillary endothelium. The thick continuous basement membrane of glomerular capillaries. The filtration slits (between the minor processes of the podocytes) & their closing diaphragms. The barrier allows passage of the blood constituents except the large size blood cells and plasma proteins. Maximum of the filtrate is reabsorbed by the convoluted tubules and loops of Henle.

Blood Renal Barrier (Filtration Barrier)

Mesangial cells In addition to capillary endothelial cells and podocytes, renal corpuscles also contain mesangial cells, which resemble pericytes. They and their surrounding matrix comprise the mesangium, which fills interstices between capillaries. Functions of the mesangium : 1.Structural support to the glomerulus and contraction like pericytes, and so decreasing the surface area available for filtration. 2. Phagocytosis: mesangial cells phagocytose protein aggregates that adhere to the glomerular filter, including antibody-antigen complexes abundant in many pathological conditions. 3.Secretion: the cells synthesize and secrete several cytokines, prostaglandins, and other factors important for immune defense and repair in the glomerulus.

Proximal convoluted tubule (PCT) It is a cortical structure which is continuous with the loop of Henle. It is 15 mm long and highly convoluted (that is why it appears more numerous, in section, than the distal convoluted tubules). It has a large diameter (60 um) and narrow lumen. It is lined with 3-4 high cuboidal cells with indistinct cell boundaries and central rounded nuclei. These cells have: Basal enfolding causing clear basal striations. Prominent apical brush (striated) borders due to numerous microvilli. The cytoplasm contains numerous mitochondria and is darkly acidophilic. The PCT is concerned with reabsorption of water, glucose, amino acids, K and most the Na.

Loop of Henele It is a U-shaped tubule, mm long connecting the proximal with the distal convoluted tubules. It is partially cortical and partially medullary. Cortical nephrons have short loops, while juxta-glomerular nephrons have long loops. It consists of 4 parts: Thick descending part: It is similar to the proximal convoluted tubule (i.e. lined with simple cubical epithelium). Thin descending part: lined with simple squamous epithelium. Thin ascending part: lined with simple squamous epithelium. Thick ascending part: similar to distal convoluted tubule.

Distal convoluted tubule (DCT) It is a cortical structure, that opens into the collecting tubule. It is 5 mm long and less convoluted than PCT. It has a smaller diameter (40 um) and wide lumen. It lined with 5-8 cubical cells with indistinct cell boundaries and apical rounded nuclei. Its apical surfaces of cells show few microvilli. Its cells have basal striations due basal enfolding. Its cytoplasm is lightly acidophilic. cytoplasm contains fewer mitochondria. It is concerned with reabsorption of H20, CI & Na and secretion of ammonia. It is the site of action of ADH.

Proximal Convoluted Tubule (PCT) Distal Convoluted Tubule (DCT) 1. Lining- Simple high cuboidal epithelium-Simple low cuboidal epithelium 2. lumen- Narrow- Wide 3. Cytoplasm- Highly acidophilic - Basal numerous mitochondria - Contain vacuoles, lysosomes & canaliculi - Less acidophilic - Less mitochondria 4. Nucleus- Rounded central- Rounded apical 5. Apical brush border - Contains long numerous microvilli to increase surface area for reabsorption - Absent or less marked brush border with few & short microvilli 6. Lateral cell membrane - Interdigitation with adjacent cells 7. Basal cell membrane - Has basal enfolding

It is a medullary structure, mm long, concerned with excretion of urine. Each collecting tubule receives 5-10 nephrons. It is lined with cubical cells with central rounded nuclei and pale acidophilic cytoplasm. Larger collecting tubules (ducts of Billini) are lined with simple columnar epithelium. Collecting tubules are under the control of ADH. Collecting tubule

Renal tubules histology 1- Proximal tubule. 2- Loop of Henle. 3- Distal tubule. 4- Collecting tubule.

Juxta-glomerular apparatus (JGA) It is formed of 3 components in direct contact with each other: Juxta-glomerular cells: They are modified smooth muscle cells found in the wall of the afferent arteriole, which contain granules rich in the enzyme renin. They are sensitive to blood pressure of the afferent arterioles. Macula densa: When the distal convoluted tubule comes in contact with the glomerulus, its lining cells become tall, thin and columnar. These cells are called macula densa because they appear as a dense spot under the microscope. They are sensitive to Na ion contents of glomerular filtrate in the distal tubules. Extraglomerular mesangial cells (Lacis cells): They are masses of small cells with pale nuclei, found in the concavity between the afferent & efferent arterioles.

Juxta glomerular apparatus

Function of the JG Apparatus: Secretion of renin: when blood pressure falls and in response to a decrease in extracellular fluid volume (perhaps detected by the macula densa) 1) 1) Regulation of extracellular fluid volume. 2) 2) Regulation of blood volume. 3) 3) Regulation of blood pressure.   Decrease blood pressure and decrease in Na ion contents  release of renin  conversion of angiotensinogen  angiotensin I (with converting enzymes)  angiotensin II  which 1. release of aldosterone  increase reabsorption of Na ions and water and 2. vasoconstriction  increase blood pressure. Other endocrine function of the kidney Secretion of erythropoietin: it is produced by the peritubular capillary endothelial cells. It stimulates the bone marrow to form new red blood corpuscles.

Blood supply of the kidney Renal artery  Interlobar arteries (between the pyramids)  Arcuate arteries (between cortex & medulla)  Interlobular arteries  Afferent arterioles  Glomerulus  Efferent arterioles  Peritubular capillaries (for supply of proximal & distal convoluted tubules)  Venules  Arcuate vein. In the inner 1/3 of the cortex, the efferent arterioles are large and give the Arteriola Recta, for the supply of the loop of Henle. The Arteriola Recta Vera arise directly form the arcuate arteries and extend to the medulla for supply of the collecting tubules.

Blood supply

Ureters The wall of the ureter is formed of: Mucosa: It appears folded and consists of transitional epithelium, resting on a C.T. corium. There is no submucosa. Musculosa: It is formed of smooth muscle fibres, arranged in 2 layers, inner longitudinal & outer circular. In the lower part of the ureter, musculosa adds a third outer longitudinal layer. Adventitia: is made of loose C.T.

Mucosa: is highly folded and formed of: Transitional epithelium: is thicker than that of ureter. When the bladder is empty the epithelium is made of many cell layers. As the bladder fills the number of cell layers is reduced and the superficial cells become flat. Lamina propria (corium): made of C.T. Musculosa: Smooth muscle fibres are arranged in 3 layers (as in lower ureter): inner longitudinal, middle circular and outer longitudinal. A prominent circular bundle of smooth muscle surrounds the urethral outlet, forming the internal sphincter. Adventitia: is made of loose C.T. However, the adventitia of the upper portion of the bladder is covered by mesothelium (peritoneum). Urinary Bladder

Male Urethra: acts as a passage for both urine and semen. It is about 20 cm long. It consists of: Prostatic urethra: (4 cm long) It passes through the prostate. It receives sperms from the ejaculatory ducts and urine from urinary bladder. It is lined by transitional epithelium. The epithelium rests on a highly vascular lamina propria (corium). Corium is surrounded by inner longitudinal and outer circular layers of smooth muscle fibers. Membranous urethra: (1 cm long) It is lined by pseudostratified or stratified columnar epithelium. It is surrounded by skeletal muscle fibers which form the external sphincter. Urethra

Penile (cavernous) urethra: (15 cm long) It passes through the corpus spongiosum of penis. It is lined by stratified columnar epithelium, which changes to stratified squamous epithelium at the opening (fossa navicularis). It receives the ducts of Cowper's glands (see male genital system). It is also associated with small mucoid glands (glands of Littre). Female Urethra: It is about 4 cm long. Along most of its length, it is lined by stratified squamous epithelium. However, its upper portion is lined by either transitional or pseudostratified columnar epithelium.