11.2 kidney function

processes involved in producing urine

1. filtration (ultrafiltration)

2. reabsorption of filtered materials (back into the blood)

3. secretion into filtrate (to the urine)

at its proximal end, the nephron forms a cuplike structure — __, which is around a ball of capillaries (__)

Bowman’s capsule, glomerulus

process of filtration (think of where it occurs)

• water, amino acids, ions, glucose, and nitrogenous waste molecules are forced from the glomerulus into Bowman’s capsule. the fluid in the Bowman’s capsule is called: filtrate

• filtrate travels thru the nephron (where reabsorption and secretion occur) and drains into the collecting ducts and renal pelvis → ureter → urinary bladder → urethra

• afferent arterioles deliver blood to the glomerulus

• there are small spaces btwn the endothelial cells of the glomerular capillaries and the cells of the Bowman’s capsule. blood cells and proteins are too big to pass thru these spaces, but water, amino acids, ions, glucose, and nitrogenous waste molecules in fluid can pass through

• pressure drives the mvmt of this fluid into the Bowman’s capsule

__ of arteries and glomerular capillaries maintains the __ needed to drive fluid into __

larger diameter (less resistance), pressure, Bowman’s capsule

efferent arterioles

receives blood from the glomerulus; has smaller diameter → blood “backs up” (dam) in glomerulus, keeping pressure high

the __ forces small molecules such as water, glucose, amino acids, sodium chloride, and urea through the filter, from the blood in the __ across the basement membrane of the __ and into the __. THIS TYPE OF HIGH PRESSURE FILTRATION IS __. THE FLUID FORMED IN THIS WAY IS CALLED __.

high pressure, glomerular capsule, Bowman’s capsule, nephron, ULTRAFILTRATION, GLOMERULAR FILTRATE

filtration in Bowman’s capsule begins the process of

excretion

__ and __ take place in the remainder of the __

reabsorption, secretion, nephron

proximal tubule

• secretes H+ into the filtrate

• reabsorbs Na+, Cl-, and K+ along w water, HCO3-, and nutrients

• main function: reabsorption of water, ions and nutrients back into the interstitial fluid

• Na+/H+ exchangers and Na+/K+ pumps in epithelium of the tubule move Na+ from the filtrate into the interstitial fluid

• movement of these positive ions results in a voltage gradient which causes Cl- ions to follow (that is, move from the filtrate into the interstitial fluid or get reabsorbed)

• specific membrane proteins actively transport glucose, amino acids, and nutrients from the filtrate in the tubule to the interstitial fluid (reabsorption)

• filtrate is now hypoosmotic to the interstitial fluid, so water moves from the tubule to the interstitial fluid via aquaporins) due to osmosis. water “follows” salt

• nutrients, water, and HCO3- move from the interstitial fluid into the peritubular capillaries, and return to the circulation

loop of Henle

• sets up a gradient of solutes in interstitial fluid

• very high solute concentration towards pelvis of kidney

• descending segment is for water reabsorption by osmosis

• ascending segment is for Na+ and Cl- reabsorption

• allows production of urine that is hyperosmotic to blood

descending loop of Henle

• tubule passes thru increasingly higher regions of solute concentrations of interstitial fluid in medulla

• thus, water moves out from the tubule

• this, then, concentrates the filtrate in the tubule

• by the bottom of the loop, osmolarity of fluid in tubule = osmolarity of medulla fluid

ascending loop of henle

• Na+ and Cl- are reabsorbed (move from tubule to interstitial fluid)

• as tubule ascends, Na+ and Cl- are actively transported out of the tubule, fluid in tubule has a lower osmolarity as tubule ascends toward Cortex

• ascending limb is IMPERMEABLE to water bc of a lack of aquaporins

the design of the Loop of Henle and vasa recta give rise to the __ which enables interstitial fluid in kidney (cortex and medulla) to maintain a __. this concentration keeps the blood coming from the __ to the __ to not be too __, allows for __, and for __

counter-current multiplier, concentration gradient, vasa recta, renal vein, hyperosmotic, water reabsorption, the production of concentrated urine

counter current multiplier (counter current + multiplier)

counter current

• flow of filtrate thru descending LoH is opposite to the flow thru the ascending LoH

• flow of filtrate thru the LoH is in the opposite direction from the flow of blood thru the vasa recta

multiplier

• as salt is transported out of the ascending LoH, the surrounding interstitial fluid becomes more concentrated, which causes water to move out of the descending loop (permeable to water but not salt)

• this mvmt of water out of the filtrate of the descending LoH into the interstitial fluid causes the filtrate in the descending LoH to be more “salty”/concentrated, which makes the pumps in the ascending LoH pump out more salt

• this positive feedback gives rise to the term “multiplier”, and it ends when max concentration is achieved, which is dependent on the capacity of the pumps on the ascending LoH

why doesn’t the water that moves out of the descending LoH dilute the concentration of the interstitial fluid?

because the vasa recta is reabsorbing the water (due to oncotic pressure of blood)

descending LoH is permeable to __, not __. as filtrate moves down, __ moves into __, thereby concentrating the filtrate in the part of the tubule that is deep in the __. here, blood flow in __ is moving __ and the water that is being __ by the descending LoH is being __ the bloodstream, so this water is not __ the interstitial fluid and ruining the concentration gradient in medulla.

water, salt, water, interstitial fluid, medulla, vasa recta, up, released, reabsorbed into, diluting

vasa recta capillaries have __ that make them highly permeable to __ and __, allowing for equilibration. thus the blood in the vasa recta remains nearly __ to the __ at each level so it doesn’t undo the osmotic gradient in the interstitial fluid.

fenestrae (pores), water, solutes, isosmotic, interstitial fluid

urea

• contributes to the total osmolality of the interstitial fluid

• the main nitrogen-containing substance in the urine of mammals; is highly soluble in water, and practically non-toxic

• vasa recta also has urea transporters (for facilitated diffusion)

• solutes (such as salt) and urea diffuse into the descending vasa recta. these same solutes then diffuse out of the ascending vasa recta and back into the interstitial fluid

• why? bc at each lvl of the medulla, the concentration of solutes is higher in the ascending vasa recta vessels than the interstitial fluid AND the concentration of solutes is higher in the interstitial fluid than in the descending vasa recta vessels

what is reabsorbed and secreted by the distal convoluted tubule?

Na+, Cl-, H2O, and HCO3- are reabsorbed

K+ and H+ is secreted

collecting duct

• function: to concentrate urine

• as duct descends, there is an increasing solute gradient in the medulla

• H2O moves out of the duct

• some urea is passively transported out of the duct, which adds to the increasing solute gradient in the medulla

• H+ is secreted into the duct