Wk 4 L24

Cortex: medulla ratio in dogs and cats

1:2 dogs

1:3 cats

At end of the proximal tube,

the osmolarity(solute concentration) of the filtrate fluid is still iso-osmolar to plasma: ~280 - 300 mosm/L

thin descending limb: osmolarity changes

permeable to water but impermeable to solutes(NaCl)

As water leaves, urine osmolarity increases as urine flows toward highly concentrated medulla

What contributes to the highly concentrated medullary interstitium?

Thick ascending limb: osmolarity changes

impermeable to water but actively reabsorbs NaCl to the tubular cell and then to the interstitial compartment (medulla)

By the end of the loop of Henle,

the filtrate fluid has a lower osmolarity than plasma: < 220 mosm/L (water is retained and solutes reabsorbed)

Loop diuretics: furosemide

Inhibits the Na+ K+ 2Cl-cotransporter in thick Ascending LOH

Electrolytes stay in the urine → water follows the sodium, increasing urine volume and the excretion of excess fluid from the body

Overall effects of furosemide

Decrease blood volume in cardiac insufficiency, pulmonary edema, high blood pressure, kidney disease

Major function of the kidney

to produce concentrated urine and preserve water in the body

which explains why osmolarity of urine is higher than plasma in different animals, particularly those who subsist in the desert.

Components of the countercurrent mechanism

Countercurrent multiplier system in the loop of Henle of juxtaglomerular nephrons.

Recycling of urea; involving collective ducts and descending LOH

Countercurrent exchanger system in the vasa recta

Steps 1 and 2 of the Countercurrent multiplier system

Steps 3 and 4 of the countercurrent multiplier system

Urea resorption

reabsorbed from the collective duct and papilla ducts due to a high permeability (mediated by ADH) and down its concentration gradient towards the interstitial (medulla) compartment, thus contributes to increasing the osmolarity of the medulla

Some urea is recycled by entering the descending limb of the loop of Henle

Urea-recycling “saves” NaCl

Fluid balance function of sodium

Sodium regulates the amount of water inside and outside your cells, maintaining proper hydration and blood volume

Blood pressure function of sodium

It plays a key role in controlling blood pressure, though too much can raise it

Nerve transmission function of sodium

It helps nerve cells send electrical signals (impulses) throughout the body, crucial for brain-body communication

Muscle function of sodium

Sodium is essential for muscle contraction and relaxation, including your heart muscle

Nutrient transport function of sodium

It aids in absorbing other nutrients like glucose (sugar) and amino acids into intestinal & renal cells

Formation of concentrated urine: ADH

Release from the hypothalamus/neurohypophysis (posterior pituitary).

Stimulus: plasma osmolarity (dehydration)

High osmolarity stimulates the release of ADH

How does ADH(vasopressin) work?

causes the expression and translocation of aquaporins 2 in the apical membrane of the collective duct cells, promoting the reabsorption of FACULTATIVE WATER towards the high osmolarity renal medulla