Water Balance

Reabsorbed Molecules

Water, sodium, potassium, calcium/phosphate, acid/base

Keeps ECF volume stable

Granular Cells

Specialized SM cells that synthesize & secrete renin (activate beta-adrenergic receptor)

Macula Densa

Tubular cells sensitive to [NaCl] in Loop of Henle (ascending loop)

Renin Angiotensin Aldosterone System (RAAS)

1. Dehydration or Na+ deficiency

2. Decreased BV and BP

3. Juxtaglomerular cells (granular cells) of the kidney secrete renin

4. Liver produces angiotensinogen → increasing angiotensin I

5. Angiotensin I causes lungs (ACE) to secrete angiotensin II (vasoconstriction) going to adrenal cortex

6. Increased aldosterone 

7. In kidneys, increased Na+ and H2O reabsorption and increased secretion of H+ and K+ in urine

8. Increased BV and BP

Osmoregulation

Adjusments of thirst and water excretion by kidneys through vasopressin aka ADH or arginine vasopressin (AVP)

Aortic Baroreceptors

Sense decrease BP 

Stretch Receptors

Sense decrease arterial stretch

Hypothalamic Osmoreceptors

Sense osmolarity

ADH Release

1. ADH made and packaged in cell body of neuron

2. Vesicles are transported from hypothalamus to posterior pituitary gland 

3. Released into blood when needed

Vasopressin Function

Increase H2O permeability of late distal tubule & principal cells of collecting ducts (aquaporins)

Increase activity of Na+/K+/2Cl- cotransporter of thick ascending tubule, enhancing countercurrent multiplication

Increase urea permeability in inner medullary collecting ducts, enhancing urea recycling & size of cortico-papillary osmolarity gradient

Hypoosmotic Urine (Dilute)

Low ADH

Water content in the body increases, causing inhibition of ADH secretion → reabsorption of water in tubules does not occur

PCT → filtrate is isoosmotic

DCT/Collecting duct → minerals are reabsorbed in the absence of ADH = impermeable to H2O

Hyperosmotic (Concentrated) Urine

High ADH

Water content in body decreases, causing reabsorption of water in the tubules 

Requires medullary gradient → urea reabsorption

50% of urea is absorbed by PCT (simple diffusion)

Diabetes Insipidus

Disease state where the secretion of ADH or the response to ADH is impared

Central Diabetes Insipidus

Damage to hypothalamus or pituiotary gland

Nephrogenic Diabetes Insipidus

Often hereditary (faulty receptors), or other disorder that affect the kidneys

Dilute urine

Free Water Clearance (CH2O)

Amount of solute free water excreted per day 

Osmolar Clearance

Amount of urine excreted per day that contains all the solute that is isoomotic to plasma 

Nephron

Generates free-H2O in diluting segments, where solutes are reabsorbed without H2O (thick ascending tubule and early distal tubule)

Measurement of Free-H2O

Assess kidneys’ ability to dilute/concentrate urine