Concentration and Dilution of Urine

Urine osmolality can vary from 50-1200 mosmole/kg

Urine volume can vary from .5-20L per day

Total solute excretion is rather constant

ADH is major regulator of urine volume and osmolality

ADH increases cAMP and urea permeability of inner medullary collecting tubule

Rapid response of ADH

Gradient in medulla from corticomedullary border to papillae. At papillae, about 600 NaCl and Urea

After water is extracted, urine 1200, in presence of ADH

Production of medullary gradient of NaCl by countercurrent multiplication

• Ascending LoH is impermeable to H2O, TAL reabsorbs NaCl into medullary interstitium.
  • In TAL, Na/K/2Cl transporter is target of loop diuretics, eg furosemide.
  • The lumen of TAL is electropositive due to K channels.
• Descending limb is highly permeable to H2o and is is osmotically removed due to high NaCl in interstitium.
• At any level, the fluid in descending limb is 200 more concentrated than ascending limb – called single effect, which is multiplied by the LoH.

Countercurrent exchange in the vasa recta helps preserve the medullary osmotic gradient

• As blood descends into medulla, water leaves and NaCL and Urea enter
• As blood rises, water enters and NaCl and Urea leave
• Blood leaving has only slightly higher osmol than entering
• This prevents rapid washout of NaCl and urea

Urea is reabsorbed in PT, and concentrated in thin descending limb

Some urea diffused into interstitium in thin limbs

When ADH high, water reabsorbed in DT and cortical and outer medullary collecting tubule. Also urea diffuses out of inner medullary collecting tubule.

Pt – 2/3 of Na, Cl, water, and 1/2 urea absorbed, but remains at 300

Thin descending limb – water absorbed, urine becomes concentrated

Thin ascending limb – NaCl and urea absorbed

Thick ascending limb – NaCl absorbed and urea becomes major solute. 150.

Early Distal Tubule – NaCl absorbed, more dilute urine

Late Distal Tubule & Cortical Collecting Tubule depends on ADH

• +ADH – permeable to water, equilibrates to 300
• -ADH – fluid remains dilute, more NaCl absorbed. 50.

Medullary Collecting Tubules depends on ADH

• +ADH – water reabsorbed. 1200
• +ADH – urea diffuses out in inner medulla to reach max 600
• -ADH – more NaCl absorbed. 50.

Total solute excretion is UosmV

Osmolar clearance is UosmV/Posm = amount of solute that would have to be cleared each minute.

When urine is isoosmolar to plasma, osmolar clearance equals urine flow rate. Solute and water are being lost at the same proportions. Thus free water clearance is 0. Cwater=V-Cosm.

If urine is hypo-osmolal with respect to plasma, Cosm is smaller than V. Positive free water clearance.

Body Fluid Compartments

• H2o is 60% of body weight – 42L
• 40% is intracellular – 28L
• 20% extracellular – 14L
  • 15% interstitial – 10.5L
  • 5% plasma – 3.5L

Higher body fat means lower water content.

Osmolality is essentially identical between compartments.

Gibbs-Donnan explains unequal distribution in presence of nondiffusible ion. Concentration greatest in compartment containing nondiffusible, negatively charged protein. The side containing the protein will also have the same charge as that protein.

Infusing with normal saline in dehydration will overcompensate ECF and undercompensate ICF. However it is the proper treatment in the case of hemorrhage.

Control of Osmolality and Body Fluid Volume

Osmolality and volume of ECF is tightly regulated, mainly by kidney.

Osmolality regulated by changes in renal water handling – ADH (AVP)

ECF volume regulated by changes in renal Na handling – Renin-angiotension, sympathetics

• Water deprivation is sensed by Hypothalamus which stimulates thirst and ADH secretion.

Excess water is sensed by Hypothalamic Osmoreceptors:

1. stimulate supraoptic and paraventricular nuclei suppress ADH release
2. stimulate lateral pre-optic area supresses thirst

Medullary vasomotor center can also stimulate hypothalamus in response to decreased circulating volume.

ADH synthesized in hypothalamus as precursor, travels down axons to posterior pituitary where it is stored in granules in nerve terminals.

ADH metabolized in kidney and liver, short half-life, varies in plasma osmol range of 280-290

Diabetes Insipidus (tasteless). A defect in ADH secretion or action. There is Central DI and Nephrogenic DI where collecting tubule is unresponsive to ADH.

SIADH – plasma levels higher than normal – leads to negative free water clearance. Caused by cancer and injury.

ECF volume determined mainly by NaCl present.

Dietary Na intake will increase ECF volume, but you will also excrete more from kidney. ANP will decrease Na reabsorption.

Juxtaglomerular Apparatus

Renin secretion is stimulated by lowered afferent arteriolar pressure, increased sympathetic activity, decreased macula densa NaCl delivery.

Renin gets you to Angiotensin 1, ACE gets you to II, II binds to:

• AT1 (ARBs inhibit)
  • increase aldosterone, vasoconstriction
  • proximal Na reabsorption
  • thirst
  • ADH release
  • lowers renal blood flow but maintains GFR
• AT2 receptors – vasodilation

Changes in volume that govern release of renin

• Baroreceptors in afferent arteriole
• Baroreceptors in heart and arteries with regulate sympathetic neural activity and circulating catecholamines
• Cells in macula densa in distal tubule
  • as Cl falls, stimulates renin release via prostaglandins, but volume is dominating factor.

Aldosterone is made in the adrenal cortex and its release is stimulated by Angiotensin II and increased ECF K+.

• stimulates reabsorption of Na from late distal tubule, CCC, and TAL
• promotes K and H secretion
• stimulates Na absorption by colon

SNS activity evoked by reduced arterial pressure and reduced vascular volume. It causes constriction of arterioles, reducing RBF, but only slightly decreasing GFR, increasing FF (enhances reabsorption of Na in PT)

Angiotensin constricts efferent to help keep GFR up.

PGI2 released from JGA is a vasodilator that counteracts SNS and RAA. ACE inhibitors and NSAIDS problem here.

Factors that inhibit Na reabsorption:

• PGE2 in medulla oppose Na reabsorption.
• ANP – released from atria in response to stretch, increases cGMP
  • Decreases reabsorption of Na in distal tubule and collecting tubule by blocking ENaC and inhibiting NaK ATPase.
  • Inhibits release of aldosterone and renin
  • Vasodilates afferent arteriole to increase GFR
• Urodilatin – released by kidney – identical to ANP
• Dopamine released by neurons to proximal tubules to inhibit NaK ATPase and Na/H exchange in proximal tubules.

Regulation of Calcium, Phosphate, Magnesium, Potassium

Calcium

1/2 Ca in body is ionized, the other 1/2 is protein bound, and some is complexed to anions. Alkalosis increases protein binding.

Plasma [Ca] is 5mM

Ca economy regulated by PTH, 1,25-vit D, Calcitonin

Vit D production begins in the skin where a cholesterol derivative is converted to choolecalciferol by UV light. D3 is then converted to vit D precursor in liver, and then stimulated to vit D via PTH and low serum phosphate in kidney.

1/2 Ca filtered in kidney, but only 1% excreted.

Most is reabsorbed in the proximal tubule. This occurs via 2 pathways. 20% is transcellular, with Ca diffusion down gradient into cell, and then extrusion via Ca ATPase or 3Na Ca antiporter. 80% is paracellular, across tight junctions, via solute drag or electro-positive lumen. Thus anything that diminishes Na and H2o reabsorption in proximal tubule will decrease Ca reabsorption.

Therapies to treat hypercalcemia: Volume expansion will limit proximal tubule reabsorption. Loop diuretics inhibit Na K 2Cl pump in TAL and diminish electro-positive lumen.

Calcium Receptors located in PT and TAL. When Ca falls, it activates release of PTH and reabsorption. Activation mutation leads to Hypocalciuric Hypercalcemia.

If hypocalcemia occurs, PTH increases and acts on bone and kidney. In bone, it causes Ca and P release. In kidney, it causes increases P excretion, decreased Ca excretion and increased vit D formation (which increases Ca and P absorption in GI).

Changes in Na and Ca excretion are not parallel in distal tubule. Thiazide diuretics inhibit entry of NaCl resulting in hyperpolarization, which increases time that Ca channels are open, and increases gradient for reabsorbtion. Rx for kidney stones.

Determinants of Ca Handling are:

1. Volume status – ECF depletion enhances proximal reabsorption
2. Level of PTH – enhances TAL and distal tubule reabsorption
3. Calcitriol – enhances distal tubule reabsorption
4. pH – alkalosis enhances reabsorption

Phosphate

Most abundant intracellular anion. Most in bone, lots in intracellular fluid. 10% protein bound.

Plasma [P] is 4mg/dl

Plasma concentration important for bone dynamics, urinary concentration important for buffer to acidity.

Most excreted in proximal tubule via transcellular route

Transport Maximum for phosphate is near the filtered load.

To increase excretion in kidney, PTH stimulates cAMP in PT to decrease reabsorption (opposite from Ca).

Renal Failure

1. P increase due to decreased GFR
2. P complexes with Ca to lower ionized Ca
3. Failure also leads to low calcitriol and low GI absorption
4. All this leads to high PTH which causes osteitis fibrosa cystica

Magnesium

Role in metabolics and transport. NM transmission, cardio tone and rhythm.

1/2 in bone, 1/2 in intracellular space

Plasma [Mg] is 2mM

Most reabsorbed in TAL via paracellular pathway.

PTH stimulates reabsorption

Potassium

Most K is intracellular

Extracellular K regulated between 3.5-5mmol. Consequences of kalemia include paralysis and cardiac.

When you eat a meal, K absorbed into blood, distributed to ECF and rapidly taken into cells to buffer(transcellular effects are short term), over time kidney will excrete (major regulator)

Insulin, catecholamines, aldosterone, bicarbonate, and alkalosis drive K into cell vi Na K ATPase. Rx for Hyperkalemia.

Excercise, increase in ECF osmolality drive K out of cell.

The kidney reabsorbs most K in proximal tube (paracellular route) and LOH

Alterations in distal nephron.

• With normal intake, K is secreted in late distal tubule and cortical collecting duct.
• With low intake, K is reabsorbed in LDT and CCD.
• With high intake, there is increased secretion in LDT and CCD

LDT and CCD have two types of cells

1. Principal cells secrete K and reabsorb Na
2. Intercalated cells reabsorb K (in low state) and secrete H

Principal cells major determinant, governed by

1. Delivery of Na to distal tubule to be exchanged for K (flow rate and [Na])
2. Driving force on K (increased serum K, increased luminal electronegativity)
3. Aldosterone levels – Na reabsorbtion, K secretion

Loop Diuretic use leads to hypokalemia via #1 and #3. This can be counteracted by use of K sparing diuretic such as amiloride or triamterene which blocks the apical Na channels in the distal tubule.

High K intake increases Na K ATPase and aldosterone secretion

Renal Role in Maintaining Acid-Base Homeostasis

Protein catabolism generates H

For each H buffered, 1 HCO3- is consumed

The kidneys reabsorb almost all HCO3- and generate it denovo. They also excrete H 1m/kg/day.

In proximal tubule: Carbonic Anhydrase (CA) converts H2Co3 to H20 adn Co2 which is absorbed from the lumen. These are then converted back to H2Co3, which breaks up into H and HCo3. The Hco3 moves into the blood, while the H is exchanged for Na and reenters the lumen. H is secreted but not excreted, it is being used to reabsorb HCo3. Excretion occurs elsewhere in nephron.

In collecting tubule: Secretion of H from intercalated cell, exchange for K.

Titratable acid excretion and ammonium excretion regernerates lost HCo3 and excretes H.

Most H excreted is buffered by phosphate or NH3

Secreted H is excreted from body

Titratable acid is mostly H2Po4-. Phosphate is a major urinary buffer. At 4.4 almost all phosphate is titrated. At 7.4 most is converted to HPo4-. Almost no H comes off NH4+. The amount of NaOH required to titrate the urine to 7/4 is called titratable acid, an estimate of amount of H excreted bound to phosphate.

Proximal Tubule cells metabolize glutamine to NH4+, alpha ketoglutarate, and HCO3.

NH4 is secreted into proximal tubule, then reabsorbed in descending limb, and actively reabsorbed in TAL, and then diffuses into collecting tubule, trapped due to low pH in collecting tubule giving it charge. 1 pH unit drop gives 10x NH4+

Bicarbonate is major buffer in ECF. Think about ratio of bicarbonate to PCO2. Compensatory change in same direction.

Kidney corrects acidosis by bicarbonate reabsorption, titratable acid excretion, ammonium excretion (takes days), free H excretion

Metabolic vs. Respiratory Acidosis/Alkalosis

All diuretics except those that act on principal cells cause enhanced secretion of H and K.