A&P Urinary system
Urine production
- Nephrons are considered functional units of the kidney. they are the smallest structural component capable of producing urine -urine formation results from the following three processes: 1) Filtration: filtration is the movement of materials across the filtration membrane into the bowman's capsule to form filtrate 2) Tubular reabsorption: solutes are reabsorbed across the wall of the nephron into the interstitial fluid by transport process. water is reabsorbed across the wall of the nephron by osmosis. water and solute pass from the interstitial fluid into the peritubular capillaries. 3) Tubular secretion: solutes are secreted across the wall of the nephron into the filtrate.
Urea
- Responsible for large part of high osmolarity in medulla -Descending limbs of loops of Henle permeable to urea; urea diffuses into interstitial fluid -ascending limbs and distal tubules impermeable to urea - collecting ducts permeable to urea, some diffuses out into interstitial fluid -urea flows in a cycle maintaining high urea concentration in medulla -urea is excreted in the urine
Medullary concentration gradient
- in order to concentrate urine ( and prevent a large volume of water from being lost), the kidney must maintain a high concentration of solutes in the medulla - interstitial fluid concentration (mOsm/kg) is 300 in the cortical region and gradually increases to 1200 at the tip of the pyramids in the medulla -maintenance of this gradient depends upon:- functions of loops of henle, vasa recta flowing countercurrent to filtrate in loops of henle, distribution and recycling of urea
ADH Mechanisms
- in the presence of ADH, the collecting duct is permeable to water and water is reabsorbed into the interstitial fluid. the result is the production of a small volume of concentrated urine - in the absence of ADH, the collecting duct is impermeable to water and water remains in the collecting duct. the result is the production of a large volume of dilute urine
Tubular reabsorption overview cont.
- substances transported to interstitial fluid and reabsorbed into peritubular capillaries: sodium (active) other ions/molecules (co-transport) water, urea, lipid-soluble, nonpolar (passive transport)
Changes in concentration of solutes in the nephron
- urea: enters glomerular filtrate. as volume of the filtrate decreases, concentration of urea increases. walls of the nephron is not very permeable to urea. only 40-60% is passively reabsorbed -urate ions, creatinine, sulfates, phosphates, nitrates are partially reabsorbed. concentration is high in urine. toxic substances are eliminated
Renal corpuscle
-Bowmans capsule: outer parietal (simple squamous epithelium) and visceral (cells called podocytes) layers -glomerulus: network of capillaries. blood enters through afferent arteriole and exits from efferent arteriole
Filtration, reabsorption, and secretion
-Glomerular filtration: water, salt, nutrient molecules, and waste molecules move from the glomerulus to the inside of the glomerular capsule. These small molecules are called the glomerular filtrate. -Tubular reabsorption: nutrient and salt molecules are actively reabsorbed from the convoluted tubules into the peritubular capillary network, and water flows passively. - Tubular secretions: certain molecules are actively secreted from the peritubular capillary networks into the convoluted tubules
Tubular Reabsorption overview
-Tubular reabsorption: occurs as filtrate flows through the lumens proximal tubule, loop of henle, distal tubule, and collecting ducts - results because of: simple diffusion, facilitated diffusion, active transport, co-transport -thin segments of loop of henle- specialized for passive transport -rest of nephron- active transport, co- transport, and passive transport
Creating/Maintaining high solute concentration in Medulla
-active transport of sodium and cotransport of ions such as potassium and chloride and other ions out of the thick portion of ascending limb into interstitial fluid -impermeability of thin and thick parts of ascending loop of henle to water -vasa recta removes excess water and solutes that enter the medulla without destroying the high concentrations of solutes in interstitial fluid of medulla -active transport of ions from collecting ducts into interstitial fluid of medulla -passive diffusion of urea from collecting ducts into interstitial fluid of medulla, impermeability of the ascending limb and permeability of the descending limb of the loops of henle to urea
Reabsorption in DCT and collecting duct
-active transport of sodium out of tubule cells into interstitial fluid with cotransport of chloride -sodium moves from filtrate into tubule cells due to concentration gradient -collecting ducts extend from cortex (interstitial fluid 300mOsm/kg) through medulla (interstitial fluid very high) -water moves by osmosis from distal tubule and collecting duct into more concentrated interstitial fluid -permeability of wall of distal tubule and collecting ducts have variable permeability to water -urine can vary in concentration from low volume of high concentration to high volume of low concentration
Internal anatomy of kidneys
-cortex: outer area. Renal column: part of the cortical tissue that extends into the medulla -Medulla: inner area, surrounds the renal sinus. Renal pyramids: cone shaped. the base is the boundary between the cortex and the medulla. the apex of the pyramid is the renal papilla which points towards the sinus -calyces: Minor: papilla extend into funnel of minor calyx. Major: converge to form the pelvis -Pelvis: enlarged chamber formed by major calyces -ureter: exits at the hilum. connects to the urinary bladder
The Nephron
-functional and histological unit of the kidney -parts of the nephron: Bowmans capsule, proximal tubule, loop of henle (nephronic loop), distal tubule -urine continues from the nephron to: collecting ducts, papillary ducts, minor calyses, major calyses, the renal pelvis -collecting ducts, parts of the loop of henle, and papillary ducts are part of the renal medulla
urine production cont.
-in proximal convoluted tubules:- sodium and other substances removed. water follows passively. filtrate volume reduced -in descending loop of henle:-water exits passively, solute enters. filtrate volume reduced 15%. - in ascending loop of henle: sodium, chloride, potassium are transported out of filtrate. water remains - in distal convoluted tubules and collecting ducts: water movement out regulated by ADH. if absent, water not reabsorbed and dilute urine produced. If ADH present, water moves out, concentrated urine produced
Reabsorption in loop of Henle
-loop of Henle descends into medulla; interstitial fluid is high in solutes -descending thin segment is highly permeable to water, and moderately permeable to urea, sodium, most other ions (passive) -water moves out of nephron, solutes in. volume of filtrate reduced another 15% - ascending thin segment is not permeable to water, but is permeable to solutes. Solutes diffuse out of tubule and into the more dilute interstitial fluid as the ascending limb projects toward the cortex. solutes diffuse into the descending vasa recta - water moves by osmosis into the interstitial fluid into the vasa recta -the interstitial fluid in the medulla of the kidney, and the blood of the vasa recta have a high solute concentration. solutes diffuse from the vasa recta and interstitial fluid into the tubule.
Filtration
-movement of fluid, derived from blood flowing through the glomerulus, across filtration membrane. -filtrate: water, small molecules, ions that can pass through membrane -pressure difference forces filtrate across filtration membrane -renal fraction: part of total cardiac output that passes through the kidneys. varies from 12-30%, avg is 21% -renal blood flow rate: 1176 ml/min -renal plasma flow rate: renal blood flow rate x fraction of blood that is plasm. 650 ml/min -filtration fraction: part of plasma that is filtered into lumen of bowmans capsules -glomerular filtration rate (GFR): amount of filtrate produced each minute 180 L/day - average urine production: 1-2 L/Day. most of filtrate must be reabsorbed (99%)
Tubular secretion
-moves metabolic byproducts, drugs, molecules not normally produced by the body into tubule of nephron - active or passive -ammonia: produced by epithelial cells of nephron from deamination of amino acids. Diffuses into lumen -hydrogen, potassium, penicillin, and substances such as paraaminohippuric acid (PAH): actively secreted into nephron
Reabsorption of Loop of Henle cont.
-sodium moves across the wall of the basal membrane by active transport, establishing a concentration gradient for sodium. Potassium and chloride are symported with sodium across the apical membrane, and ions pass by facilitated diffusion across the basal membrane of the tubule cells. - at the end of the loop of Henle, inside of nephron is 100mOsm/kg. Interstitial fluid in the cortex is 300mOsm/kg. Filtrate with DCT is much more dilute than the interstitial fluid which surrounds it
Urinary/Excretory system
-the kidneys produce urine -the ureters transport urine -the urinary bladder stores urine -the urethra passes urine to the outside
Urine concentration Mechanism
-when large volume of water is consumed: eliminate excess without losing large amounts of electrolytes. response is that kidneys produce large volumes of dilute urine -when water is not available: kidneys produce small volume of concentrated urine. removes waste and prevents rapid dehydration -mechanisms that create urine of variable concentration:maintenance of high concentration of solutes in medulla. countercurrent functions of loops of henle. control of permeability of distal nephron to water
ADH mechanism
1) ADH binds to peritubular capillaries and binds to ADH receptors in plasma membranes of distal convoluted tubule cells and collecting duct cells 2) when ADH binds to its receptor, a G protein mechanism is activated which in turn activates adenylate cylcase 3) Adenosine cyclase increases the rate of cAMP synthesis. Cyclic AMP promotes the insertion of aquaporin-2 containing cytoplasmic vescicles into the apical membranes of the distal convoluted tubules, and collecting ducts, thereby increasing the permeability to water. Water then moves by osmosis out of the distal convoluted tubules and collecting ducts into the tubule cells through the aquaporin-2 channels 4) water exits the tubule cells and enters the interstitial fluid through aquaporin-3 and aquaporin-4 water channels in the basal membranes
Urine concentrating mechanisms
1) Approximately 180 L of filtrate enters the nephrons each day. The concentration is 300 mOsm/kg 2) aproximately 65% of water and salt is reabsorbed into the proximal convoluted tubule. the filtrate concentration is 300 mOsm/kg 3) aproximately 15% of the water is reabsorbed in the thin segment of the descending limb of the loop of henle, at the tip of the renal pyramid the concentration is 1200 mOsm/kg which is equal to the concentration of interstitial fluid 4) the thin segment of the ascending limp of the loop of henle is not permeable to water. sodium chloride diffuses out of the thin segment 5) the thick segment of the ascending limb of the loop of henle is not permeable to water. Sodium ions are actively transported to the interstitial fluid and chloride follows by diffusion 6) The volume of the filtrate is not changed as it passes through the ascending limb, however the concentration is greatly reduced. by the time the filtrate reaches the cortex the concentration is 100 mOsm/kg, and an additional 25% of NaCl has been reabsorbed 7) the distal convoluted tubules and collecting ducts reabsorb water and NaCl 8) If ADH is present, water moves by osmosis from the less concentrated filtrate into more concentrated interstitial fluid. By the time the filtrate reaches the tip of the renal pyramid an additional 19% of water and 9-10% NaCl has been reabsorbed 9) 1% or less of the filtrate remains as urine when ADH is present
Functions of the urinary system
1) Eliminates waste 2) Filters the blood: has three processes -filtration -reabsorption -secretion 3) regulation of: -blood volume -concentration of blood solutes: Na+ (sodium), Cl-(chloride) ,K+,(potassium), Ca2+ (calcium) ,HPO42- (hydrogen peroxide) -pH of extracellular fluid: secrete H+ -blood cell synthesis 4) synthesis of Vitamin D ( greater PTH= greater vit. D production in Kidney)
Filtration Pressure
1) Glomerular Capillary pressure, the blood pressure (50 mm Hg) within the glomerulus, moves fluid from the blood into the Bowman's capsule 2) Capsule Hydrostatic pressure, the pressure inside Bowmans capsule (10 mm Hg) moves fluid from Bowmans capsule into the blood 3) Blood colloid osmotic pressure, produced by the concentration of blood proteins ( 30 mm Hg) moves fluid from the bowmans capsule to the blood by osmosis 4) filtration pressure is equal to the glomerular capillary pressure minus the capsular hydrostatic and blood colloid osmotic pressure
Types of Nephrons
1) Juxtamedullary Nephrons: Renal corpuscle near the cortical medullary border. Loops of Henle extend deep into medulla 2) cortical nephrons: Renal corpuscle nearer to the periphery of the cortex. Loops of Henle do not extend deep into medulla 3) Renal corpuscle: Bowmans capsule plus the capillary bed of the glomerulus
Movement of flitrate
1) Proximal tubule 2)Descending limb of loop of Henle 3)thin segment of ascending limb 4)Distal tubule 5)collecting duct
loop of henle and vasa recta working together
1) water moves out of the descending limb of the loop of henle and enters the ascending vasa recta 2) solutes diffuse out of the ascending thin segment of the loop of henle and enter the vasa recta 3) solutes transported out of the thick segment of the ascending limb of the loop of henle enter the descending vasa recta. the vasa recta do not dilute the high medullary concentration 4) the concentration of the filtrate is reduced to 100 mOsm/kg by the time it reaches the distal convoluted tubule - water and solutes such as sodium and chloride leave the distal convoluted tubules and collecting duct and enter the ascending vasa recta
Arteries and Veins of the Kidneys
Arterial Supply: 1)Renal arteries: branch from the abdominal aorta 2) Segmented arteries: branch from renal 3) interlobar arteries: ascend with the renal columns from the cortex 4) arcuate arteries: branch and arch over the base of the pyramids 5) interlobular arteries: project into the cortex and give rise to afferent arterioles
Filtration Membrane
Filtration Membrane: filtration barrier. it prevents blood cells and proteins from entering the lumen of Bowman's Capsule. Many times it is more permeable than a capillary. - fenestrated epithelium, basement membrane, and pores formed by podocytes - some albumin and small hormonal proteins enter the filtrate, but they are reabsorbed and metabolized by the cells of the proximal tubule. very little protein is found in the urine - filtration pressure: pressure gradient responsible for filtration; forces fluid from glomerular capillary across membrane into lumen of bowmans capsules
Filtration membrane Cont.
Forces that affect movement of fluid into or out of the lumen of Bowman's capsule: 1) Glomerular capillary pressure (GCP): blood pressure inside of capillary tends to move fluid out of capillary into Bowmans capsule 2) Capsule hydrostatic pressure (CHP): pressure of filtrate already in lumen 3) Blood colloid osmotic pressure (BCOP): osmotic pressure caused by proteins in blood. favors fluid movement into the capillary from the lumen. BCOP greater at end of glomerular capillary than at beginning because of fluid leaving capillary and entering lumen - Filtration pressure (10 mm Hg)= GCP (50 mm Hg)- CHP (10 mm Hg)- BCOP (30 mm Hg)
Location and External anatomy of Kidney
Location of Kidney: -Behind peritoneum (retroperitoneal) on posterior abdominal wall on either side of vertebral column -the lumbar vertebrae and rib cage partially protect it -the right kidney is slightly lower than the left External Anatomy: -Renal Capsule- fibrous connective tissue that surrounds each kidney -Perirenal fat- engulfs the renal capsule and acts as cushioning -Renal fascia- thin layer of loose connective tissue. it anchors the kidneys and surrounding adipose to the abdominal wall -hilum- renal artery and nerves enter, and the renal vein and ureter exit when leaving kidney. it opens into the renal sinus ( cavity filled with fat and loose connective tissue)
loop of henle
Loop of Henle: 1) water diffuses out of the thin segment of the loop of henle 2) the filtrate concentration is 1200 mOsm 3) sodium and other solutes are actively transported out of the loop of henle into the medulla Vasa Recta: 1) water diffuses out and solutes diffuse into the descending portion of the vasa recta 2) the blood concentration is 1200 mOsm 3) water diffuses into and solutes diffuse out of the ascending portion of the vasa recta 4) at the end of the vasa recta, the blood osmolarity is only slightly greater than the osmolarity of the blood at the beginning of the vasa recta
Arteries and Veins of the Kidneys
Part of circulation involved with urine formation: 6) Afferent arterioles: supply blood to glomerulus 7) Glomerulus 8) efferent arterioles: exit the renal corpuscle 9) peritubular capillaries: form a plexus around the proximal and distal tubules 10) vasa recta: specialized parts of peritubular capillaries that course into medulla, along with loops of Henle, then back towards cortex
Anatomy
The bladder is transitional epithelium (it changes in size)
Arteries and Veins of the Kidneys (cont).
Venous Drainage: 11) Peritubular capillaries drain into interlobular veins and lead to 12) arcuate veins 13) interlobar veins 14) renal veins
