Kidney Function: Urine Production

• Three main mechanisms for waste elimination:– ________________ of blood– ________________of useful substances into bloodstream– ______________ of waste products from blood into tubules of nephron.

Peritubular capillaryStep 3 Step 2 Step 1

Step 1: Filtration of the Blood

• Occurs in ___________ ______________

• Glomerular capillaries contain large ___________________ 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.

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, which is affected by heart rate and blood pressure.

– 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.

Step 2: Reabsorption

• 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

capillaryH2O Ions: Na, K, Ca, MgGlucose, AA, Vit

• 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.

– ____________ 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

Na+ Reabsorptio

n

Reabsorption cont’d

• Potassium - Diffuses out of tubular filtrate by moving between epithelial cells and into interstitial fluid, then the peritubular capillaries.

• Calcium - Influenced by ___________, ___________, ____________

• Magnesium- Influenced by __________

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

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,Certain drugs,Excess K+, H+

Urine Volume Regulation

• Determined by amount of water contained in tubular filtrate when it reaches renal pelvis.

• Regulated by two hormones:

– _______________________________- released by posterior pituitary gland.

• Acts on DCT and collecting ducts to promote water reabsorption and prevent water loss.

• If not present, can result in polyuria.

– ____________________- secreted by adrenal cortex.

• Increases reabsorption of sodium into the bloodstream in the DCT and collecting duct.

• Causes osmotic imbalance, making water follow sodium out of tubular filtrate into blood.

Ureters• Tubes that exit the kidney at the hilus and

connect to the urinary bladder

• Are a continuation of renal ________.

• Outer fibrous layer, middle smooth muscle layer, and inner layer lined with transitional epithelium

– _________________ epithelium allows ureters to stretch as urine passes through on way to urinary bladder

– Smooth muscle layer propels urine through ureter by peristaltic contractions

Function of the Ureters

• Connect the kidney to the urinary bladder at the ________________ (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 the opening shuts to prevent urine from backing up into ureters.– This will not keep ureters from pushing urine

into bladder

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

• 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

Urinary Bladder

Has two parts:

– Muscular sac

• Position and size vary depending on amount of urine it contains.

• Lined with smooth muscle bundles that run in all directions.

– As these muscles contract, urine is squeezed out into urethra.

– Neck

• Contains a sphincter of skeletal muscle fibers.

– Under voluntary control and open to allow urine to leave bladder and go into urethra.

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

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.