Urine Formation 2 – Reabsorption and Secretion

Tubular Secretion

Selected substances moved from capillary blood into tubular fluid

Mainly occurs in PCT

Functions:

• Disposal of substances bound to plasma proteins

• Elimination of passively reabsorbed wastes

• Excretion of excess K+

• Regulation of blood pH

Aldosterone

Secreted by adrenal gland in response to:

• Low blood Na+

• High blood K+•

Low blood pressure -> RAA mechanism

Stimulates reabsorption of Na+ and secretion of K+

Promotes retention of NaCl and water

Reduced urine output w/high K+

Antidiuretic Hormone (ADH)

Direct mechanism of water retention

Secreted by posterior pituitary in response to high blood osmolarity

• E.g. dehydration

Induces aquaporin presence in collecting duct•

Reduces urine output

Reduced blood osmolarity -> reduced ADH secretion

•Increased urine output

Atrial Natriuretic Peptide (ANP)

Secreted by atrial myocardium in response to high blood pressure

Dilates afferent arteriole, constricts efferent arteriole -> increased GFR

Inhibits renin and aldosterone secretion

Inhibits ADH secretion

More salt and water excreted -> reduced blood volume

Parathyroid Hormone (PTH)

Secreted from parathyroid glands in response to hypocalcemia

• Increases calcium reabsorption

Stimulates calcitriol synthesis

• Enhances Ca2+ absorption in small intestine

Increases phosphate secretion at PCT

• Allows reabsorbed calcium to stay in circulation

impacts both the kidneys and small intestines

Urine Concentration

Kidneys can vary urine concentration/volume

Occurs in collecting ducts

Osmotic gradient required for reabsorption of water!

Medullary osmotic gradient

Driven by 2 mechanisms:

• Countercurrent multiplier creates gradient

• Countercurrent exchanger preserves gradient

Medullary osmotic gradient

salinity gradient in renal medulla

Osmotic Gradient

Water requires osmotic gradient to be reabsorbed

Osmotic movement of water stops once equilibrium has been reached

Concentration of urine via reabsorption of water requires:

• Dilute filtrate within collecting duct

• Concentrated interstitial fluid outside of collecting duct

Countercurrent multiplier -> Descending

Juxtamedullary nephron loops

Descending limb only permeable to water

• H2O reabsorbed along osmotic gradient

• NaCl retained within nephron loop

• Tubular fluid osmolarity increases

Countercurrent multiplier -> Ascending

Ascending limb only permeable to solutes

NaCl actively pumped out

H2O retained within nephron loop

Interstitial osmolarity increases

Tubular fluid becomes more dilute

Countercurrent Multiplier all together