Kidney Function: Urine Produciton

1. Kidney Function: Urine Produciton • Three main mechanisms for waste elimination: • ________________ of blood • ________________of useful substances into bloodstream • ______________ of waste products from blood into tubules of nephron. Step 3 Step 2 Step 1 Peritubular capillary

3. Terminology • ____________- passage of water from weaker to a stronger solution across a semi-permeable membrane. • ____________- passage of a substance from a high concentration to a low concentration. • ________________-passage of a substance from the lumen of the renal tubules into the renal capillaries and so back into circulation; this is an active process and requires energy. • ________________- passage of a chemical substance from the renal capillaries into the lumen of the tubules and out of the body in the urine; this is an active process and requires energy.

4. Step 1: Filtration of the Blood • Occurs in ___________ ______________ • Glomerular capillaries contain large fenestrations in the epithelium. Higher blood pressure in the glomerulus, plus the large fenestrations, allow fluid (plasma) to enter the capsular space, but do not allow blood cells or large proteins to pass through. • ______________ _______________ when it enters capsular space. • Similar to plasma except contains very little or no proteins. • Proteins in urine are an abnormality and may be due to capillary damage in the glomerulus.

5. Glomerular Filtration Rate • Speed (ml/min) that plasma is filtered as it passes through the glomerulus. • Depends on rate of blood flow to the kidneys • Reabsorption helps reduce volume of glomerular filtrate. • For every 100L of fluid filtered from the blood only 1 L is produced as urine. • Ex: a 180 lb man produces 280 L of glomerular filtrate daily, but only urinates ~ 2 L daily • 99% of original filtrate is reabsorbed back into blood.

6. Step 2: Reabsorption capillary • __________________takes useful substances from the tubules back into the blood. • Most takes place in the ______ (65%) but may also take place in Loop of Henle (10%), or DCT and collecting ducts (24%). • Many substances in tubular filtrate are still useful • Na+, K+, Ca+, Mg+, Cl-, HCO3-, Glucose, acids, H2O • Substances to be reabsorbed pass out of tubular lumen thru or between tubular epithelial cells • Enter interstitial fluid → peritubular capillaries H2O Ions: Na, K, Ca, Mg Glucose, AA, Vit

7. Na+ Reabsorption • Sodium is the most abundant ion in the tubular filtrate • Attaches to a carrier protein that carries it from the tubular filtrate, into the cytoplasm of the PCT epithelial cell, into the interstitial fluid, and finally the peritubular capillaries. • _______ ions are also reabsorbed in ascending loop of Henle and DCT • ____________ and _______________ attach to same carrier protein and follow Na+ into cell (Na+cotransport; no extra energy required) • _____ then diffuses out thru K+ channels (leakage channels) • _______ follows cations • _______ follows Na+ via osmosis • ________ also gets reabsorbed

8. Potassium Reabsorption • Diffuses out of tubular filtrate by moving between epithelial cells and into interstitial fluid before into peritubular capillaries. • Takes place in the PCT, ascending part of Loop of Henle, and DCT.

9. Reabsorption Continued • Calcium Reabsorption • Takes place in the PCT, the ascending Loop of Henle, and the DCT. • Influenced by ___________, ___________, ____________ • Magnesium Reabsorption • Takes place in the PCT, the ascending Loop of Henle, and the collecting duct. • Influenced by ______

10. Glucose Reabsorption • There is a limit to the amount of glucose that can be reabsorbed by the PCT (_________ Threshold) • Due to a limited number of carrier proteins • In cases of extremely high blood glucose levels (________ ___________), the amount of of glucose in the tubular filtrate exceeds the amount that can be reabsorbed. • Glucose is then seen in the urine (glycosuria/glucosuria) • Glucose pulls water out with it, increasing the total volume of urine, making the animal polyuric • Increased water loss will make the animal polydipsic

11. Step 3: Secretion • Many waste products are not filtered in adequate amounts from the glomerular capillaries. The body still needs to get rid of them, so they are transferred from the peritubular capillaries, to the interstitial fluid, to the tubular epithelial cells, into the tubular filtrate (__________ ___________) • Primarily occurs in the ________. • Hydrogen, potassium, ammonia/urea are eliminated via secretion. • Some medications are eliminated by secretion as well. • Useful for urinary tract infections capillary Secreted substances: urea, uric acid Drugs: Pen, Phenobarb Excess K+, H+

12. Urine Volume Regulation • Determined by amount of water contained in tubular filtrate when it reaches renal pelvis. • Regulated by two hormones: • ____________________(ADH)- released by posterior pituitary gland. • Acts on DCT and collecting ducts to promote water reabsorption and prevent water loss. • If not present will result in polyuria. • ____________________-secreted by adrenal cortex. • Increases reabsorption of sodium into the bloodstream in the DCT and collecting duct. • Causes osmotic imbalance in which water follows sodium out of tubular filtrate into blood. • However, ADH must be sufficient to move water out of DCT.

13. Ureters • Tube that exits the kidney at the hilus and connects to the urinary bladder • Are a continuation of renal ________. • Outer fibrous layer, middle smooth muscle layer, and inner layer lined with transitional epithelium • Transitional epithelium allows ureters to stretch as urine passes thru on way to urinary bladder • Smooth muscle layer propels urine through ureter by peristaltic contractions

14. Function of the Ureters • Connect the kidney to the urinary bladder at the TRIGONE (near the neck of the bladder at its caudal end) • Continuously move urine from the kidneys to the urinary bladder. • Smooth muscle propels urine through peristaltic contractions. • Enables urine to be moved regardless of position of animal’s body • Ureters enter bladder at ___________ angle, so that when bladder is full will collapse to prevent urine from backing up into ureters. • This will not keep ureters from pushing urine into bladder

15. Urinary Bladder • Lined with _____________ epithelium that stretches as the bladder fills • Walls of bladder contain smooth muscle (________________ muscle) • Function is to store, collect and release urine. • Bladder keeps urine from constantly being released from the body • Has two parts: • Muscular sac • Position and size vary depending on amount of urine it contains. • Lined with transitional epithelium. • Lined with smooth muscle bundles that run in all directions. • As these muscles contract, urine is squeezed out into urethra. • Neck • Is a muscular sphincter of skeletal muscle fibers. • Under voluntary control and open and close to allow urine to leave bladder and go into urethra.

16. Urinary Bladder • Rests on pubic bones when empty. • Walls become thinner as the bladder fills • Urination (_____________ or uresis) is the expulsion of urine from the urinary bladder into the urethra

17. Process of Urination • Urine Accumulation • Urine is constantly accumulated in bladder until pressure activates stretch receptors in bladder wall • Muscle contraction • When trigger point is reached, spinal reflex sends impulse to bladder muscles • Muscles in bladder wall contract and give sensation of needing to urinate. • Sphincter Muscle Control • Sphincter at neck of bladder provides temporary control of urine. • If enough pressure is exerted on sphincter, will eventually relax and release urine. • RUPTURE can occur if bladder is not emptied in timely fashion. • Use caution when manually expressing or palpating bladder

18. Urethra • Continuation of the neck of the urinary bladder that runs through pelvic canal. • Carries urine from bladder to external environment. • Lined with transitional epithelium. • Female • _________ and ______________ urethra • Opens on the ventral portion of the vestibule of the vulva. • Only carries urine • Male • _________ and _____________ urethra • Runs down center of penis • Also serves in reproductive role to carry semen, however during ejaculation, sphincter at urinary bladder closes to prevent urine from mixing with seminal fluid.