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functions of the kidneys
- regulation of H20 and inorganic ion balance - removal of metabolic waste products from blood and excretion in urine - removal of foreign chemicals in the blood and excretion in urine - gluconeogenesis - endocrine functions
basic concept of the kidneys
excretion equation
= filtration - reabsorption + secretion
filtration occurs
bowman's capsule
reabsorption occurs in
peritubular capillaries
secretion occurs in
peritbular capillaries
function unit of the kidney consisting of: blood vessels, urinary tubules
blood vessels
vasa recta, peritubular capillaries
urinary tubules
PCT (proximal convoluted tubule), LH (loop of Henle), DCT (distal convoluted tubule), and CD (collecting duct)
renal blood vessels
afferent arteriole, glomeruli, efferent arteriole, peritubular capillary
afferent arteriole
delivers blood into the glomeruli, pressure comes from the blood pressure and drives the fluids into the kidney
capillary network that produces filtrate that enters the urinary tubules
efferent arteriole
delivers blood from the glomeruli to the peritbular capillaries
peritubular capilarry
delivers blood to the vasa recta - contains millions of micro villi - involved in secretion and reabsorption
bowman's capsule (glomerular capsule)
- surrounds the glomerulus - filtrate passes into the urinary space into PCT - creates an ionic gradient
loop of henle
- descending limb (water reabsorption - ascending limb (active transport of sodium, impermeable by water)
distal convoluted tubule
involved in secretion and reabsorption
collecting duct
- receives fluid from the DCT of several nephrons - involved in reabsorption and secretion
renal processes
- glomerular filtration - tubular reabsorption - tubular secretion
afferent arteriole
high hydrostatic pressure (PGC) at glomerular capillaries is due to short, wide afferent arteriole (low R to flow)
efferent arteriole
the long, narrow efferent arteriole (high R)
glomeluar filtration (GFR) control
• GFR controlled by diameters of afferent and efferent arterioles
• Sympathetic vasoconstrictor nerves
• ADH and RAAS also have an effect on GFR.
• Autoregulation maintains blood supply and so maintains GFR. Also prevents high pressure surges damaging kidneys.
• Unique system of upstream and downstream arterioles.
GFR in capsular space
– Water (~20%)
– Small proteins
– Sugars
– Amino acids
– Vitamins
– Ions
– Wastes (urea, etc.)
GFR in blood vessels
– Water (~80%)
– Red blood cells
– White blood cells
– Platelets
– Large proteins
GFR filtration rate
• Glomerular filtration rate (GFR) around 120 to 125 mL per min, total combining both kidneys
• Controlled by the hydrostatic pressure of the blood in the glomerulus
– Want to raise GFR?  Then raise the GHP.
Constrict efferent arteriole and dilate afferent
net filtration pressure
BHP (blood hydrostatic pressure) 60 mmHg COP (colloid osmotic pressure) -32 mmHg CP (capsular pressure) -18 mmHg NFP = 10 mmHg
decrease in afferent
RBF increases, GFR increases
increase in afferent
decrease RBF, decrease GFR
decrease in efferent
increase RBF, decrease GFR
increase in efferent
decrease RBF, increase GFR
type of GFR regulation
- autoregulation - hormonal regulation - autonomic regulation
– Constriction and dilation of afferent and efferent
arterioles to regulate blood flow through glomerulus
to raise or lower GH • Ability of kidney to maintain a constant GFR under systemic changes.
– Achieved through effects of locally produced chemicals on the afferent arterioles.
• When MAP drops to 70 mm Hg, afferent arteriole dilates.
• When MAP increases, vasoconstrict afferent arterioles.
• Tubuloglomerular feedback:
– Increased flow of filtrate sensed by macula densa cells in thick ascending LH.
• Signals afferent arterioles to constrict.

hormonal regulation
– Juxtaglomerular cells release renin (raising blood pressure everywhere), atria release ANP (dilates afferent arterioles and so raises GHP)
sympathetic regulation of GFR
• Stimulates vasoconstriction of afferent arterioles.
– Preserves blood volume to muscles and heart.
• Cardiovascular shock:
– Decreases glomerular capillary hydrostatic pressure.
– Decreases urine output (UO)
peritubular reabsorption
• Peritubular capillaries provide nutrients for tubules and retrieve the fluid the tubules reabsorb.
• Oncotic P is greater than hydrostatic P in these capillaries, so therefore get reabsorption NOT filtration
tubular reabsorption
• Hormonal control of water reabsorption:
– ADH:  increases water permeability of DCT and collecting duct
– Aldosterone:  increases reabsorption of Na+ from DCT and collecting duct
– ANP:  inhibits Na+ reabsorption from DCT and collecting duc
Na absorption
• Na+ absorbed by active transport mechanisms, NOT by TM mechanism. Basolateral ATPases establish a gradient across the tubule wall.
• Proximal tubule is very permeable to Na+, so ions flow down gradient, across membranes.
• Microvilli create large surface area for absorption.
• Electrical gradient created also draws Cl- across.
• H2O follows Na+ due to osmotic force.
• Means fluid left in tubule is concentrated
TM (T max)
maximum transport
glucose absorption
• Glucose absorption also relies upon the Na+ gradient.
• Most reabsorbed in proximal tubule.
• At apical membrane, needs Na+/glucose cotransporter (SGLT)
• Crosses basolateral membrane via glucose transporters (GLUT’s), which do not rely upon Na+

countercurrent exchange
• filtrate flows down loop of Henle, and then flows back up
• Thin segment of descending limb is permeable to water, but not solutes
• Thick segment of ascending limb actively pumps NaCl out of filtrate, but is impermeable to wate
• Properties of the different segments of the loop of Henle creates osmotic gradient around loop
• Causes water to move out of filtrate on the way down, and NaCl to exit filtrate on the way up, salvaging a lot of the water still in the filtrate after PC
critical characteristics of the loops
•1. The ascending limb of the loop of Henle actively co-transports Na+ and Cl- ions out of the tubule lumen into the interstitium.  The ascending limb is impermeable to H2O.
•2. The descending limb is freely permeable to H2O but relatively impermeable to NaCl
ADH (anti-diuretic hormone)
• The water potential of the tissue fluid in the medulla is always more negative than that of the filtrate in the collecting duct.
• Whether the water actually leaves the collecting duct (by osmosis) is determined by the hormone ADH (anti-diuretic hormone)
• Osmoreceptors in the hypothalamus detect the low levels of water, so the hypothalamus sends an impulse to the pituitary gland which releases ADH into the bloodstream.
• ADH makes the wall of the collecting duct more permeable to water.
• Therefore, when ADH is present more water is reabsorbed and less is excreted
ADH on urine
• In the absence of ADH, the walls of the
collecting ducts are impermeable to water,
and so the kidneys produce dilute urine
• In the presence of ADH, the collecting
ducts are permeable to water, and so
water is drawn out of filtrate by the
osmotic gradient, producing concentrated
ADH is a
water content in the blood
- too much salt/sweating then water content in the blood is low making the brain produce ADH which makes the urine output low - too much water then water content in the blood is high  making the brain produce less ADH causing a high urine output
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