2. Urine Formation/Filtration

urine

formed by nephrons and collecting ducts containing wastes, excess water, and electrolytes

urine formed by

glomerular filtration, tubular reabsorption, tubular secretion

glomerular filtration

first step of urine formation, filtrate from golmerular capiilaries enters capsula

capillaries

fenestrated capillaries

basement membrane

size/charge filter (most proteins can’t pass)

podocytes

filtration slits

glomerular filtrate

mostly water and nutrients (amino acids and glucose), wastes (urea, uric acid, creatinine, and creatine), electrolytes (sodium, potassium, chloride)

afferent arterioles diameter > efferent arterioles

blood backs up in glomerular capillaries → increased BP → favors filtration

plasma colloid pressure > glomerular pressure

osmosis moves water into glomerular capillaries → opposes filtration

net filtration pressure =

force favoring filtration - forces opposing filtration (positive = filtration occurs)

glomerula filtration rate

amount of filtrate formed per minute by both kidneys

kidnes received

25% of cardiac output, 205 of blood plasma is filtered by glomerular capillariers

adult GFR

125 mL/min or 180 L/day

blood plasma filtered through glomeruli

60x a day, only small percentage excreted as urine

factors affecting GFR

diameter of arterioles, colloid osmotic pressure of glomerular plasma, hydrostatic pressure in glomerular capsule

afferent arteriole constricts

NFP decreases, GFR drops

efferent arteriole constricts

NFP increases, GFR rises

plasma proteins pull water

into glomerular capillaries, opposing filtration

more plasma proteins

greater osmotic pressure → GFR decreases

fewer pasma proteins

lower osmotic pressure → GFR increases

fluid pressure inside glomerula capsule

opposes filtration, obstruction (kidney stone) causes urine to back up, GFR decreases

GFR remains constant due to

autoregulation, tubuloglomerular feedback, sympathetic nervous system, renin-angiostensin-aldosterone system

autoregulation

afferent arterioles

autoreg blood pressure rises

as afferent stretches, smooth muscle contracts, reduce blood flow, keeps GFR from getting too high

autoreg blood pressure drops

less stretch, arteriole relaxes → increases blood flow → maintain GFR

tubuloglomerular feedback

macula densa, senses NaCl conc

GFR too high (TG feedback)

high NaCl, macula densa sends signal → afferent arteriole constricts → less blood flow → GFR back to normal

GFR too low (TG feedback)

low NaCl, macula densa sends signal → afferent dilates → more blood flow → GFR back to normal

sympathetic system

causes afferent and efferent to constrict simultaneously, net filtration decreases slightly

renin-angiotensin-aldosterone system

hormone system maintaining/restoring GFR, activated by low blood pressure, low blood volume, low NaCl at macula densa, renin released, cascades to angiotensin II

angiotensin II

constricts efferent → blood backs up into glomerulus → GFR increases, increases blood volume, increased thirst

aldosterone

increases Na+ reabsorption → water follows

ADH

increase water reabsorption

tubular reabsorption

substances needed to return back to blood instead of urine (water, electrolytes, amino acids, glucose) occurs in proximal tobules

tubular reabsorption methods

diffusion, osmosis, active transport

diffusion

lipid soluble molecules if present

active transport

movement of substances against conc gradient, needs ATP, each substance has transport maximum

renal plasma threshold

reached when more transported substance in plasma than active transport can handle, excess forms urine

substances reabsorbed by active transport

glucose, electrolytes, Na, Cl, K, Ca) amino acids, urea

70% of Na+

reabsorbed in proximal tubule

Na+

pump gradient allows substances to be reabsorbed by secondary active transport, bicarbonate and Cl- follow Na+ due to charge differences

by time filtrate passes thorugh lenght of tubule

97-99% of Na= and water is reabsorbed