Primary functions of kidneys
regulation of ionic composition (Na+, K+, Ca++, Mg++, Cl-), regulation of plasma volume, regulation of plasma osmolarity, regulation of hydrogen ion concentration, removal of metabolic wastes and foreign substances
secondary functions of kidneys
secretion of hormones and enzymes (erythropoietin, renin), activation of vitamin D3 and gluconeogenesis
function of erythropoietin
erythrocyte production
function of renin
converts angiotensinogen into angiotensinogen II
three basic exchange process of renal nephrons
glomerular filtration, secretion, excretion
forces that drive glomerular filtration
starling forces
three barriers to enter bowman's capsule
1. capillary endothelial cell layer 2. basement membrane 3. epithelial cell membrane of bowman's capsule
capillary endothelial cell layer
fenestrations or pores, INCREASE in movement of fluid through cells by BULK FLOW
basement membrane layer
primary barrier to the filtration of proteins, sandwiched in between capillary endothelial layer and epithelial layer
epithelial cell layer
podocytes-foot processes (interdigitate and form slit pores)
three layers=
the glomerular membrane or filtration barrier-flow of protein-free fluid from blood into lumen of bowman's capsule
glomerular filtration pressure =
net filtration rate due to starling forces
starling forces
1. glomerular capillary hydrostatic pressure 2. bowmans capsule oncotic pressure 60mmHg 3. glomerular oncotic pressure 4. glomerular oncotic pressure
glomerular capillary hydrostatic pressure
60mm Hg, INCREASE in hydrostatic pressure due to high resistance of EFFERENT arteriole
bowmans capsule hydrostatic pressure
OPPOSES filtration=15 mmHg, this pressure LESS than capillary HP & filtration pressure
glomerular oncotic pressure
due to presence of proteins in capillaries, 29 mmHg, OPPOSES filtration
net pressure opposing filtration at renal corpuscle under normal conditions is
44 mmHg
glomerular filtration pressure
16 mmHg
glomerular filtration rate
volume of plasma filtered per unit time (125 ml/min)
glomerular filtration rate(one day)
180 liters filtered every 22 minutes
filtration fraction
glomerular filtration rate (GFR)/ renal plasma flow (RPF), ex 125ml/min/625 ml/min=.20=20%
filtered load and equation and ex w. glucose
quantity of a particular solute that is filtered per unit time, GFR X solute plasma concentration, glucose: 125 ml/min X 1mg/min= 125 mg/min
freely filterable
without restriction when moving across the glomerular membrane
regulation of GFR
less than 1% filtered plasma is secreted, 99% is reabsorbed
regulation of GFR (MAP AND GFR)
an INCREASE in MAP caused and INCREASE in GFR and vice versa
function of intrinsic mechanism
allow for kidneys to maintain the GFR within narrow ranges
three intrinsic mechanisms
1. myogenic regulation 2. tubuloglomerular feedback 3. mesangial cells
myogenic regulation
INCREASE in MAP caused an INCREASE in pressure in afferent arteriole, stretch smooth muscle of afferent arteriole causes reflex contraction of muscle then VASOCONSTRICTION of afferent arteriole, pressure in glomerular capillaries DECREASES-also fall in MA
tubuloglomerular feedback
changes in GFR cause change in tubular fluid flow-change detected by specialized cells of the MACULA DENSA-cells secrete PARACINES-affect contraction of afferent arterioles, INCREASE in GFR=INCREASE in tubular flow=afferent arterioles constrict and reduce
mesangial cells
modified smooth muscle cells-surround glomerular capillaries-when blood pressure INCREASES=mesangial cells STRETCH-cells CONTRACT and DECREASE surface area of the capillaries available for filtration
extrinsic control glomerular filtration
fall of MAP INCREASES sympathetic nervous activity-causes afferent and efferent arterioles to contract, INCREASE in resistance contributes to INCREASE in MAP also DECREASES urine output-helps to conserve water and maintain blood volume
reabsorption
movement of filtered solutes and water from LUMEN of TUBULE back into the PLASMA
two barriers of reabsorption
tubule epithelium and capillary endothelium
two ways in which reabsorption occurs
1. active transporters in basolateral membrane w. carrier proteins present in apical membrane allowing FACILITATE DIFFUSION 2. active transporters can be present in the apical membrane w. carrier proteins located basolateral membrane
water reabsorption (passive)
happens by OSMOSIS, osmolarity in plasma INCREASE because of active transport because of substance from tubule to plasma, creates concentration gradient for water and water flows from region of GREATER OSMOLARITY in the plasma
passive reabsorption via diffusion
need a higher concentration in TUBULAR fluid than in plasma and substance must be permeant through plasma membrane ex. urea
transport maximum
when solute concentration is so high that all carrier proteins and pumps are occupied
urine "spillover
when plasma concentration of the solute rises to the point that the filtration concentration exceeds transport maximum, some of the solutes start to appear @ this point renal threshold is reached
glucose is
transported across the APICAL membrane by SODIUM-LINKED-active transport, a carrier protein moves it across BASOLATERAL membrane into peritubular fluid-diffuses into plasma
glucose(theoretical)
transport max for reabsorption=375 mg/min, normal= 80-100 mg/dL, GFR= 1.25 dL/min, filtered load=125 mg/min
glucose (true)
renal threshold=160-180mg/mL, filtered load=225 mg/min
tubular secretion
molecules move from plasma into the renal tubules to become part of the filtrate, same mechanisms as reabsorption-but in reverse
substances secreted (tubular secretion)
K+, H ions, waste products, Cl-, creatine, and foreign substance(penicillin)
regions of tubules differ in
substances transported and mechanisms of transport
non-regulated reabsorption in proximal tubule
70% H2O and Na+ that is filtered=reabsorbed in proximal tubule, glucose=100% reabsorbed
mass absorber is
proximal tubule-b.c such a large portion of solutes and water is reabsorbed there
three features of proximal tubule that facilitate mass absorption
1. apical membrane has many MICROVILLI that INCREASE surface area 2. cells posses large # of MITOCHONDRIA to supply ATP for active transport 3. the TIGHT JUNCTIONS between epithelia are more PERMEABLE to small solutes and water, which enable DIFFUSION by
distal tubule
1. fewer microvilli on apical membrane 2. epithelial cells have less mitochondria 3. tight junctions are less permeable
regulation of distal tubule
achieved by presence of RECEPTORS to various hormones, affect change in absorption and secretion of various solutes and water
water conservation: Loop of Henle
juxtamedullary nephrons is designed to create an OSMOTIC GRADIENT in the medulla so that the ONCOTIC PRESSURE increases from the boundary between the cortex and the medulla to the renal papilla-enables kidneys to conserve water
excretion
elimination of solute and water as urine
amount secreted =
amount filtered + amount secreted - amount reabsorbed
excretion rate
if filtered load of a solute is calculated and compared to the solute excreted per min., the NET EFFECT of renal processing can be determined
excretion rate (two rules)
1. if the amount of solute excreted is LESS than the filtered load then net reabsorption of the solute occurred 2. if the amount of the solute excreted is GREATER than the filtered load then net secretion of the solute occurred
clearance
a way of measuring excretion rates, IMAGINARY b/c it is the measure of the volume of plasma from which a substance is completely removed, virtual b.c portions of plasma are never completely cleared, useful for describing how the kidneys are handling one s
equation for clearance
excretion rate/ plasma concentration or U X V/P
relative clearance of solutes
indicates how excretion affects the plasma concentration of one solute compared to another
clearance-U=
concentration of substance in Urine
clearance- P=
concentration of substance in plasma
clearance-V
urine flow rate
equation for excretion rate
U X V
estimates of glomerular filtration rate
a substance can be used to calculate the glomerular filtration rate if it is neither absorbed or secreted ex. inulin
excretion rate of inulin =
the filtered load or GFR X Pinulin (concentration of inulin in plasma)
finding clearance of inulin gives you GFR b.c
GFR=excretion rate of inulin/Pinulin=clearance
creatine
natural substance in body that can be used to estimate the GFR, freely filtered, not reabsorbed, slightly secreted, slightly overestimates GFR, but its clearance can be used as a suitable clinical estimate of GFR
two rules to determine the rates of solutes in renal tubules
1. if the clearance of a substance is greater than the GFR then NET SECRETION occurred in the renal tubules 2. if the clearance of a substance is LESS than the GFR then net REABSORPTION occured
clearance of glucose is
0 because it is completely reabsorbed
para-aminohippuric acid (PAH)
freely filtered, not reabsorbed, secreted completely into tubules
micturation
urine is stored in the bladder until it is expelled through this process
detrusor muscle
smooth muscle in the wall of the bladder
internal urethral sphincter
smooth muscle at the neck of the bladder
external urethral sphincter
flow of urine is controlled by the skeletal muscle of the pelvic floor
as expansion of the bladder continues with urine coming in
stretch receptors in the wall of the bladder become activated and trigger the mixturation reflex
parasympathetic neurons innervate
the detrusor muscle
sympathetic neurons innervate
the internal urethral sphincter
somatic motor neurons innervate
the external urethral sphincter
describe how bladder empties
increase in activity of stretch receptors caused PNS to cause contraction of detrusor muscle and SNS relaxation of internal urethral sphincter and Somatic caused relaxation of external urethral sphincter-opening of internal and external urethral sphincter
explain when micturition is overridden by voluntary control
descending pathways from cerebral cortex can inhibit parasympathetic neurons and stimulate motor neurons that excite the external urethral sphincter and this inhibit micturition