Physiology 1, Fall 2008, LPC

Chapter 19- Kidneys

I – Function

1. Regulation of extracellular fluid volume & blood pressure (saves or excretes ions, H2O follows, volume of H2O determines blood pressure)
2. Regulation of osmolarity
3. Maintenance of ion balance (mainly Na+, also Ca++ & K+)
4. Homeostatic regulation of pH (remove or conserve H+ or HCO3-)
5. Excretion of wastes (metabolites, hormones, foreign substances)
6. Production of hormones (erythropoietin, renin)

II – Anatomy

kidneys (site of urine formation) -> ureters -> bladder -> urethra

Nephron – functional unit of the kidney

Outer cortex, inner medulla
Blood from arterioles in cortex -> afferent arteriole -> glomerulus -> efferent arteriole -> peritubular capillaries -> renal vein
Hydrostatic force of blood forces mostly everything, except proteins and blood cells into Bowman’s capsule, filtered fluid like plasma
Filtered fluid -> proximal tubule -> Loop of Henle -> distal tubule -> collecting duct

III – Overview of Function

1.  Filtration – movement of fluid from blood into lumen of nephron.

From glomerulus to Bowman’s capsule.
Fluid like plasma.

2. Reabsorption – reclaims 99+% of H2O, salts, amino acids, sugar, more.

Takes place in proximal tubule, Loop of Henle, distal tubule and collecting duct.
Selective and controllable.
Na+ pumped out of tubule into interstitial space.
Cl- and H2O follow (osmosis).

Concentration of osmotically active substances in interstitial space increase as loop of Henle goes toward renal pelvis  

Vasa recta (peritubular capillaries) reabsorb  (main force is low hydrostatic pressure in vasa recta) and secrete substances to form concentrated urine

Glucose transported by secondary active transporter (thanks to Na+ gradient), then facilitated diffusion carrier

3. Secretion – removes selected molecules from blood and adds them to filtrate in tubule lumen

IV – Filtration

3 filtration barriers

1. fenestrated endothelium of capillaries – large pores
2. basal lamina on capillaries – acts as course sieve
3. podocytes on capsule epithelium form filtration slits

Hydrostatic pressure in capillaries -> filtration
Glomerular filtration rate (GFR) = volume of fluid that filters into Bowman’s capsule per unit time.

Influenced by:

1. Net filtration pressure
2. filtration coefficient – depends on surface area available for filtration & permeability of capillary-Bowman’s capsule interface

nearly constant GFR over wide range of arterial blood pressures

primarily controlled by regulating blood flow through the renal arterioles
Myogenic response – when smooth muscle stretches, ->depolarization, -> contraction. Counteracts big changes in blood pressure into glomerulus.

Juxtaglomerlular apparatus – macula densa cells on ascending limb of Loop of Henle are between afferent and efferent arterioles. Increased flow past macula densa sends paracrine message to afferent arterioles to constrict, -> lower GFR

Hormonal and ANS control covered in next chapter.

V – Reabsorption

1. Na+ goes down concentration gradient from tubule lumen into tubular epithelium. Active transport to extracellular fluid
2. anions follow Na+
3. H2O follows salt
4. Concentrations of other solutes increase as fluid volume in lumen decreases.

Permeable solutes are reabsorbed by diffusion.

Glucose reabsorption in proximal tubule driven by secondary active transport of Na+
Urea follows its concentration gradient
If small proteins enter lumen, they are reabsorbed by endocytosis, then transcytosis
Transporters can become saturated, -> unusual chemicals in urine

High osmotic pressure & low hydrostatic pressure in peritubular capillaries favors reabsorption from interstitial fluid

VI – Secretion

Active transport of substrates against concentration gradient from extracellular fluid into lumen of nephron
Secretion of K+ and H+ by nephron important in homeostatic regulation
Metabolites and exogenous substances secreted

VII – Excretion

Excretion = filtration – reabsorption + secretion
Depends on:

1. filtration rate
2. whether substance is reabsorbed, secreted, or both as it passes through tubule

Clearance rate of non-metabolized substance measures GFR. For any substance that is freely filtered, but neither reabsorbed nor secreted, its clearance is equal to GFR.

Creatinine used as tracer molecule for GFR because its concentration is relatively constant in blood

GFR = [urine] X urine production in ml/min or day
                 [plasma]

Can determine whether substance reabsorbed, filtered, or secreted by GFR vs blood concentration and compare with know GFR