Renal Physiology- Chapt 41: Renal Water Balance

Water Gain to Body Per Day: Intake

Liquids: 1400

Foods: 1100

Metabolism: 350

Total: 2850

Water Loss Per Day: Output

Urine 1800

Total: 2850

What does Input Equal?

Output

Input = Output

Kidney Handling of Water- Urine: Kidneys must excrete solute load to body

~600 mosmol/day

50-1200 mOsm/kg H2O

H2O volumes excreted how much?

0.5-18 L/d

What do the kidneys control?

H2O excretion independently of Na+, K+, urea

• Very Important

What are Magical Kidneys?

Critical ability to vary relative proportions of solutes & water excreted in urine to achieve solute & H2O balance

What is Magical Kidney equal to?

HOMEOSTASIS

WHat is Antidiuretic Hormone?

Arginine Vasopressin

What is Diuresis?

urine flow above usual levels

What is Antidiuresis?

Low rate of water excreted (<0.5 ml/min) as hyperosmotic urine

What is normal urine flow?

1 ml/min

What is H2O Permeability Nephron- AQUAPORINE AQP1- Proximal Tubule?

AQP1 AWLAYS OPEN and Present in the membranes

Water Permeability of Nephron in the ABSENCE of ADH:

1. Water Permeable

   • Proximal tubule

   • Thin descending limb of Henle

2. Water Impermeable

   • Thick ascending limb of Henle

   • Distal Tubule

   • Collecting duct

Water Permeability of Nephron in the PRESENCE of ADH:

1. Water Permeable

   • Proximal tubule

   • Thin descending limb of Henle

   • Collecting

2. Water Impermeable

3. - Thick ascending limb of Henle

4. Distal tubule

What is Hypoosmotic urine?

has a lower concentration of solutes (like salt) and a higher concentration of water compared to blood plasma, indicating the kidneys are excreting excess water

What is hyperosmotic Urine?

Has a higher concentration of solutes, and a lower concentration of water compared to blood plasma, indicating the kidneys are excreting low amounts of water

What is Countercurrent Mechanism?

Kidneys maintain plasma osmolarity at 300 mOsm

What is Countercurrent Flow?

Fluid in 1 tube flows in opposite direction to 2nd tube

• Greatly increases opportunity for exchange of H2O & solutes

Countercurrent System: Loop of Henle

Tubule: Hairpin loop

• Thin descending = permeable water, NOT solute

• Thick ascending limb = permeable solute, NOT water

Countercurrent System: Vasa Recta

Capillary: Hairpin loop

• Descending & ascending segments

• Both freely permeable to water & ions, passive exchange based on osmotic gradients and diffusion

Countercurrent System: Hypertonic Medullary

Interstitium

• Serves as osmotic equilibrating device

Countercurrent System: Antidiuretic Hormone

ADH = AVP

• ADH dependent water channels, AQP-2 (aquaporin)

• Passive reabsorption of water

What is concentrated Urine generate?

Generate hypertonic interstitial fluid in confined compartment (renal medulla)

• Water permeable tube: Collecting duct

• Osmosis: to drive water from tubule lumen

  • CONC URINE

What is Dilute urine?

urine becomes dilute as moves through ascending limb loop due to Na+ reabsorption & H2O impermeability

What is Concentrated Urine?

Vasopressin uses cAMP to cause AQP2 insertion into luminal membrane of principle cells of collecting ducts

• Water flows out of tubular fluid of collecting ducts into renal capillaries

• Urine can reach 1200 mOsm (maximum concentration in humans)

Stimuli for ADH Release: Osmoreceptors

Osmolality of Plasma

Posm 1%

Stimuli for ADH Release: Baroreceptors

Sense— receptors

• Low pressure receptors (LT atrium, large pulmonary)

• High Pressure receptors (aortic arch, carotid sinus)

Aquaporins: AQP2, AQP3, AQP4- Collecting Duct

AQP3, 4, AWLAYS OPEEn and Present in the membrane

• AQP2 regulated

Tubular Actions of ADH: AVP/ADH binds V2 Receptor

BL membrane CD

Tubular Actions of ADH: Insert Aquaporin 2- (AQP2)

Apical Membrane

Tubular Actions of ADH: Permeability = H2O) reabsorption

Minimuze dehydration

• Dissociate H2O & Na+ Reabsorption in CD

What does HIGH plasma [ADH] result in?

Increased water reabsorption

• ADH = Vasopressin

Summary: ANTidiuresis

Posm or BP = ADH

Collecting duct epithelial cells

• high permeable to water

• concentrated urine, small volume without major change in solute excretion

What does LOW plasma [ADH] result in?

Decreased water reabsorption

• High water intake- Need Diuresis

Summary: Water Diuresis

Posm- [ADH]

Collecting ducts

• Relatively impermeable to H2O

• Dilute urine, large volume