ch. 28 renal tubular reabsorption and secretion

From bowmans capsule...

glomerular filtrate enters renal tubule.

flowing through proximal, loop of henle, distal, collectingduct.

Along the course...

some substances are reabsorbed from tubules back to blood. others are secreted from blood into tubular lumen.

basic mech of urine formation

filtration

reabsorbtion

secretion

excretion

Tubular reabsorption

HIGHLY SELECTIVE

Completely reabsorbed: glucose, A.A.

Some: Na+, Cl-, Bicarb (variable rates)

Others: Urea, Creatine (poorly)

Kidneys regulate excretion of solutes...

independently of one another

Reabsorption across tubular epithelium into interstitial fluid can occur via...

active or passive transport by same mechs used to transport across any cell membrane.

Once water/ solutes are in interstitium...

they are transported through peritubular cap. walls by bulky flowmediated by hydrostatic and colloid osmotic pressures.

primary active transport

moves solute against concentration gradient.

uses ATP

Na+/K+ pumps

at rest 6% total body ATP use

Secondary active transport

moving against concentration gradient. energy stored in ions electrochemical gradient, rather than hydrolysis of ATP, drives another substance across memebrane

Movement of ion down concentration gradient provides energy up hill movement of a second sustance.

Symporters

move substances in same direction. glucose

Na+ and glucose

antiporters

move substances in opposite direction. H+

Na in H+ out

Water reabsorption occurs...

Via osmosis

Water following sodium

passive

Obligatory (on all time) water reabsorption

90%

water follows solutes reabsorbed

occurs in proximal tubule

Facultative (if needed) water reabsorption

10%

Regulated by ADH Determined by bodys hydration status.

occurs in distal tubule

water still follows sodium if able. (permeable)

Proximal tubule

largest solute and water rabsorption

obligatory water reabsorption as Na+ is reabsorbed.

Glucose, AA, other nutrients as well.

Na and osmolarity....

stay together

Loop of henle reabsorption

osmolarity (concentration of fluid salt/water ) is similar to blood

no glucose, AA or nutrients remain in tubular fluid.

Ascending limb of loop

Impermeable to water, but solutes still being reabsorbed.

Ascending limb sets stage for...

independent regulation of volume and osmolarity of body fluids.

osmolarity of tubular fluid decreases ( becomes more watery) as it flows toward end of ascending limb.

Descending limb

permeable to water, not salts

Distal convuluted tubule/ collecting duct

tubular fluid is modified depending on bodys needs.

Receptors for ADH and aldosterone

Na+ and H20 reabsorption varies

K+ may be secreted to tubular fluid depending on blood potassium levels

early distal tubule

not permeable to water

permeable to Urea

Late distal tubule

permeability to H2O depends on ADH.

not permeable to Urea

Permeable if...

ADH present

what hormones saves Na and water?

angiotensin II

aldosterone

antidiuretic ADH

What hormone gets rid of Na and water?

atrial natriuretic peptide ANP

What hormone saves Ca+?

Parathyroid hormone PTH

When BP and volume decrease...

walls of afferent arterioles are stretched less... juxtaglomerular cells secrete renin.

Renin clips some AA off angiotensinogen to form agiotensin I.

ACE clips off another couple AA forming angiotensin II (active form of the hormone)

Angiotensin II enheances reabsorption of Na+ and water, It also stimulates the adrenal cortex to produce aldosterone

aldosterone enhances the reabsorption of Na+ and water, and secretion of K+.

Antidiuretic hormone

increases water reabsorption by signaling the insertion aquaporin channels into the apical membrane of cells lining the distal tubule and collecting duct.

without ADH...

cells are impermeable to water

Level of ADH released by...

posterior pituitary depends on bodys hydration status.

ADH is the major determinant of...

Volume and osmolarity of urine.

Atrial natriuretic peptide (ANP)

produced by heart in response to increased blood volume.

ANP inhibits reabsorption of Na+ and suppresses secretion of aldosterone and ADH.

results in diuresis. increased urine output.

Parathyroid hormone

not involved in Na+/H2O.

secreted by parathyroid glands in response to decrease CA++ levels in blood.

PTH causes kidneys to reabsorb more Ca++