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.
Step 3
Peritubular capillary
Step 2
Step 1
• 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.
Step 1: Filtration
of the Blood
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
capillary
H2O
Ions: Na, K, Ca,
Mg
Glucose, AA,
Vit
• 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
• 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+
Reabsorption
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 __________
• 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
Reabsorption
– 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+
• 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.
Urine Volume Regulation
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
• 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
Function of the Ureters
• 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:
Urinary
Bladder
– 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.
•
Urine Accumulation
– Urine is constantly accumulated in bladder until
pressure activates stretch receptors in bladder wall
Process of
Urination
•
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.