Urinary System Shiping Ding Zhejiang University

In many texts this is considered as part of the Excretory System. It is the system involved with cleaning the blood of the end products of metabolic activities. Also functions to adjust the composition of body fluids by selective re-absorption and secretion. secrete some bioactive factors-renin, erythropoietin.

The outside covering of the kidney is the renal capsule. Kidney is comprised of an outer region, the renal cortex and an inner region, the renal medulla. Renal medulla also contains the cup or funnel- like connective tissue calyces which open into the renal pelvis and exit by way of the ureter. ---hilum: BV, LV, N and ureters enter or out *renal pelvis: funnel-shaped expension of upper end of ureter *calyx: branches of renal pelvis

Cortex contains all of the structure of cortical nephrons and part of the structure of juxta- medullary nephrons. Renal medulla contains parts of descending and ascending tubules and loops of juxta-medullary nephrons and the collecting tubules of all nephrons. Collecting tubules are arranged in bundles called pyramids. Spaces between pyramids called renal columns. Nephrons are the structural and functional unit of the kidney. Cortical nephrons have short tubules and loops of Henle and are retained within the cortex layer Juxta-medullary nephrons have much longer tubules and extend deep into medulla layer.

Nephron Major components of the nephron include: Renal corpuscle consists of glomerulus and capsules Proximal convoluted tubule Distal convoluted tubule Peritubular capillaries

Nephron Renal Corpuscle consists of the glomerulus and the capsule. Glomerulus is a capillary ball. Blood is supplied to the glomerulus by way of the afferent arteriole and leaves the glomerulus by the efferent arteriole. This is different in that blood travels from arteriole to capillary bed back to arteriole. The capsule consists of two layers of epithelium. The visceral layer fits like a glove over the glomerulus. This can not be seen with light microscope. Outer layer the parietal layer can be seen with light microscope.

Podocyte ： cells with many processes ( primary and secondary processes-foot processes) Foot processes: interdigitated with each other and embraced the capillaries slit pore: -narrow intercellular space between foot processes -25 nm, with 4-6 nm diaphragm-slit membrane ---function: produce filtrate by filtration * filtration barrier or membrane: the structure for filtration is called filtration barrier or membrane, including: fenestrated endothelial cell: negative ions basal lamina: type IV collagen, proteoglycan, laminin-negative ions (sulphate heparin) slit membrane-nephrin(size selective filter: negative ions

*intraglomerular mesangium: /mesangial cell: -small, irregular, with processes -small dark N -EM: RER, Golgi, lysosome, phagocytic vesicles, cytoskeleton-MF, MT,IF and secretory granules -functions: i. produce ground substance ii. phagocytosis iii. contract iv. secrete renin and enzymes

Renal tubule: a. proximal tubule: 50-60um in D, 14 mm long ---structure L/M: E/M: pyramidal cuboidal eosinophilic round N brush-liked border microvilli(apical canaliculi) longitudinal striation plasma membrane infolding no clear boundary lateral extension(rich in Na + K + ATPase ) Function: Re-absorption and secretion: All glucose, all amino acids and 85% of mineral ions are reabsorbed by active transport from the filtrate to the tissue fluid. They then diffuse into the blood capillaries. Small proteins are reabsorbed by pinocytosis, digested, and the amino acids diffuse into the blood. 80% of the water is reabsorbed to the blood by osmosis. As urea molecules are so small and carry no charge that they diffuse passively through the cell membrane. In part this explains why not all urea is excreted as blood passes through the kidney.

b. thin segment: /10-15 um, simple squamous epi, /facilitate the passage of water and ions /reabsorb Na + and Cl - and 5% water

c. distal tubule: ---structure: L/M E/M cuboidal slight-stained round N no brush-liked border less microvilli well-developed longitudinal striation plasma membrane infolding ---function: i. reabsorption of 8% water, Na + ions ii. excretion of K +, H +,NH 3 iii. regulated by aldosterone(adrenal gland) and antidiuretic hormone (vasopressin) (pituitary gland)

Collecting tubule: -simple cuboidal epi. to simple columnar epi. -slight –stained -have clear boundary -reabsorb 4% water

Juxta-glomerular apparatus Is consists of following components: Juxtaglomerular cells: the tunica media of the afferent arteriole has modified smooth mucle cells. Macula densa – a specialized segment of distal tubule, a ion sensor Extraglomerular mesangial cells:a signal sender Function: Allows to sample the composition of blood as it enters nephron and as it is ready to leave. Plays important role in regulation of sodium excretion by kidney. Secretes hormone renin which then trigers the angiotensin II pathway.

a. juxtaglomerular cell: ---a groups of modified SM cell of afferent arterioles ---structure: -larger, cuboidal in shaped, with round N -contain secretory granules ---function : i.secrete renin→adrenal gland→aldosterone→blood pressure↑ ↑ angiotensinogen→angiotensin I→angiotensin II→contraction of SM of BV ii. secrete erythropoietin to promote erythropoiesis

b. macula densa ---a group of cells derived from epi. of distal tubule ---cell becomes taller, narrow, with round N apical part arranged ---cells have processes connecting with other cells ---function: chemoreceptors- feel the Na+ ions concentration c. extraglomerular mesangial cell(polar cushion cell) ---similar to intraglomerular mesangial cell ---transfer the information d. peripolar cell ---structure: EM: -microvilli -junctional complexes -RER, Golgi, and granules ---function: regulate the reabsorption and secretion of renal tubule

Renal interstitial: CT ---fibers: type I,III,IV collagen ---matrix ---cell: fibroblast macrophage lipid-laden interstitial cell: -stellate cell with processes -osmiophilic lipid droplets: -function: i. involve in formation of F and matrix ii. secret prostaglandin

Blood supply to the kidney i. Very large blood flow (1.2L/min) ii. To form cap.two times iii. Glomerular cap. have a high blood pressure iv. Form vasa recta loop near medullary loop v. More larger blood flow in renal cortex

Interlobar arteries → Arcuate arteries →Interlobular arteries →Capsular cap. ↑ ↓ ↓ Renal artery Afferent arterioles Afferent arterioles ↓ ↓ Juxtamedullary nephron Cortical nephron glomerulus glomerulus ↓ ↓ Efferent arterioles Efferent arterioles ↓ ↓ Vasa recta(artery and vein)← Capillary network Capillary network ↓ ↓ ↓ Renal vein←Interlobar vein← Arcuate vein ← Interlobular vein ← Stellate vein

Questions: 1. Describe the fine structure of renal corpuscle in detail. 2. Draw the ultra-structures of the filtration barrier of kidney and describe it in detail. 3. Compare the structures and functions of the proximal convoluted tubule with that of distal convoluted tubule. 4. Compare the structure and function of the juxtaglomerular cells with that of the macula densa. 5. Describe the blood supply of the kidney.

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