23.3 Urine Formation I: Glomerular Filtration

glomerular filtration, tubular reabsorption, tubular secretion, and water conservation

what are the four stages that convert blood plasma into urine?

glomerular filtrate

the fluid in the capsular space; similar to blood plasma except it has almost no protein

tubular fluid

fluid from the PCT through the DCT; substances have been removed added by tubular cells

urine

fluid within the collecting duct and beyond; undergoes little alteration beyond this point except for changes in water content

glomerular filtration

water and some solutes pass from blood within glomerulus into capsular space of the nephron

filtration membrane

barrier through which filtered fluid passes; contains three components which are fenestrated endothelium of the capillary, the basement membrane, and filtration slits; can be damaged by kidney infections and trauma

fenestrated endothelium of the capillary

contains large filtration pores that are highly permeable but small enough to exclude blood cells

basement membrane

proteoglycan gel with negative charge; excludes molecules greater than 8 nm; smaller albumin repelled by negative charge

flitration slits

podocyte foot processes (pedicels) wrap around the capillaries; have negatively charged ________ _____ between them

proteinuria (albuminuria)

albumin in urine

hematuria

blood in urine

blood hydrostatic pressure (BHP)

60 mmHg inward; high in glomerular cavities because afferent arteriole is larger than efferent arteriole

hydrostatic pressure in capsular space

18 mmHg outward; created by high filtration rate and accumulation of fluid in the capsule

colloid osmotic pressure (COP)

32 mmHg outward; glomerular filtrate is almost protein-free so there is no significant ____ _______ ______

net filtration pressure (NFP)

the balance of BHP, COP, and HP of glomerular cavity; should add up to 10 mm Hg outward

glomerular filtration rate (GFR)

amount of filtrate formed per minute by both kidneys; equation is NFP (net filtration pressure) * K_f (filtration coefficient); 125 mL/min or 180 L/day in males and 105 mL/min 150 L/day in women

99%

what percentage of filtrate is reabsorbed

high GFR

causes fluid to flow through renal tubule too rapidly for it to reabsorb the usual amount it does; urine output rises, increasing chance of dehydration and electrolyte depletion

low GFR

causes wastes to be reabsorbed, azotemia may occur

change glomerular blood pressure

how to adjust GFR from moment to moment?

renal autoregulation

the ability of the nephrons to adjust their own blood flow and GFR without external (nervous or hormonal) control; allows kidneys to remain relatively stable with changes to mean arterial BP

myogenic mechanism

stabilizing GFR based on the tendency of smooth muscle to contract when stretched; if arterial BP increases afferent arteriole constricts to prevent blood flow from changing, if arterial BP falls afferent arteriole dilates for same reason

tubuloglomerular feedback

glomerulus receives feedback on the status of downstream tubular fluid and adjusts filtration rate accordingly; regulates filtrate composition, stabilizes kidney performance, and compensates for BP fluctuations

juxtaglomerular apparatus

complex structure found at the end of the nephron loop where it has just reentered the renal cortex; ascending nephron loop comes into contact with arterioles of the renal corpuscle

macula densa

patch of slender, closely spaced sensory cells in nephron loop; when GFR is high, filtrate contains more NaCl; when macula densa absorbs more NaCl, it secretes ATP which stimulates nearby granular cells

granular (juxtaglomerular) cells

modified smooth muscle cells wrapping around arterioles and close to the macula densa; respond to adenosine by constricting afferent arterioles, reducing blood flow and correcting GFR

RAAS granular cells

contain granules of renin which are secreted in response to a BP drop

glomerular sympathetic control

innervate renal blood vessels; constrict the afferent arterioles in strenuous exercise or acute conditions like circulatory shock