Renal Physiology

Macroscopic Anatomy

• Kidneys:– Cortex– Medulla– Pelvis

• Ureter• Urinary bladder• Renal arteries and veins

Microscopic Anatomy

• Nephrons– Vascular components: Afferent arteriole,

Glomerular capillary, Efferent arteriole, Peritubilar capillaries, venules

– Tubular component: Bowman’s capsule, proximal tubule, Loop of Henle (descending and acsending), distal tubule, collecting duct

Urine formation

• Glomerular filtration• Tubular reabsorption• Tubular secretion• Excretion

– Excretion rate = filtration rate - reabsorption rate + secretion rate

Glomerular Filtration

• GFR = filtration coefficient * net filtration pressure– Kf = permeability * surface area

• The glomerular membrane– Capillary wall: fenestrated and very permeable– Basement membrane restricts plasma proteins– Podocytes: create filtration slits

GFR and tubule pressures

• Glomerular hydrostatic pressure: favors filtration• Glomerular osmotic pressure: opposes filtration• Bowman’s capsule pressure: opposes filtration• Net filtration pressure = 10 mm Hg

Regulation of GFR• Factors:

– blood pressure in the capillaries

– Capillary surface area

– Membrane permeablility

• Purpose :– Keep GFR

constant

• Increase Kf; increases GFR

• Increase capsule hydrostatic pressure: decrease GFR

• Increase glomerular colloid osmotic pressure; decrease GFR

• Increase glomerularcapillary hydrostatic pressure, increases GFR

Regulation of GFR

• Autoregulation of GFR– Keeps GFR constant in spite of arterial BP– Allows precise control over renal excretion (salt

and water)– Vasoconstriction/vasodilation adjust capillary

pressure back to 60 mmHg• Extrinsic control: Sympathetic inputs• Hormonal /Chemical control

Reabsorption and SecretionReabsorption - movement of solutes from the tubule lumen back toThe capillary

Reabsorption and Secretion

Active transport:Na,glucose, amino acids, Phosphate and calcium

Passive transport:Chloride and water, urea

Secondary active transport

Reabsorption of substancesIs linked to Na by passive transport

Proximal tubule:65% NA H20 and Cl Reabsorbed hereExtensive brush boarderLots of mitochondria

Loop of Henle:Descending thin, ascending thin,no brush boarderDescending thin is site of water reabsorption

Loop of Henleascending thickHi metabolic activityImpermeable to water

Distal tubule

Early:Juxtaglomerular apparatus:Feedback control over GFR and blood flow, ion reabsorptionDiluting segment!

Late and Cortical:Principal and cortical cellsUrea not reabsorbed hereNa reabsorbed under control of aldosteroneH secreted for acid -base balanceWater permeability controlled by ADH

Medullary Collecting Duct

• Reabsorbs < 10% water• Determines final output of water and solutes

in urine• ADH has large control over urine volume

here• Tubular urea absorbed here - helps form a

concentrated urine• H secreted for acid base balance

Regulation of reabsorption• Glomerulotubular balance- rate of reabsorption

matches tubular inflow

Regulation of reabsorption• Hormonal control

Regulation of reabsorption• Sympathetic control• Decreases sodium and water excretion by

by vasoconstriction which reduces GFR

Chapter 22

• Excreting a dilute urine– Counter current exchange

• Excreting a concentrated urine– Counter current exchange and the vasa recta

•Excreting a dilute urine

• Start with 300• Dilute filtrate• PT- solutes and water• Dloh- osmosis• Aloh-reabsorbtion solutes• DT-dilute• CD- no ADH!-> dilute urine!

Renal medulla becomes Salty!

• Na, K, Cl pumped out of Aloh• Active transport of ions from CD to medulla• Passive diffusion of urea to medulla• Little diffusion of water to medulla from

CD

Renal medulla becomes Salty!

Excreting a concentrated urine

• Start with 300• Dilute filtrate• PT- solutes and water• Dloh- osmosis• Aloh-reabsorbtion solutes• DT-dilute• CD- High ADH!-> concentrated urine!

Tloh pumps solutes to medulla

Dloh loses waterpassively

Hyper osmotic fluidMoves to Tloh,

Tloh pumps solutes to medulla, again

Cycle again

Countercurrent multiplier

• Repetitive reabsorption of NaCl from Tloh• Continued inflow of new NaCl from PT

Urea

• Urea builds up in the DT then diffuses to the medulla when it reaches the CT

High permeabilityprevents dissipationof the salty medulla!

• Design a system to monitor and keep blood osmolaratity at 300 mosm

Sketch an osmoreceptor