Summary Example document of Human Anatomy and Physiology Course Notes

Overview of Renal Physiology

 3 basic processes:
a. Glomerular filtration
 1st step in urine production
 Water and solutes in blood plasma move across wall of
glomerular capillaries
 Filtered then moved into glomerular capsule then into
renal tubule
b. Tubular reabsorption
 As filtrate moves through renal tubules and collecting ducts,
tubule cells reabsorb 99% of filtered water + useful solutes
 Water + solutes return to blood
 Reabsorption refers to the return of substances into bloodstream
 Absorption = means entry of new substances into the body
c. Tubular secretion
 As filtrate moves through renal tubules and collecting ducts,
cells secrete other materials such as wastes, drugs, excess ions,
into the fluid
 Tubular secretion = removes substances from blood
 The rate of urinary excretion of any solute = rate of glomerular filtration +
rate of secretion - rate of reabsorption

Glomerular Filtration

 Fluid that enters capsular space = glomerular filtrate
 Filtration fraction = the fraction of blood plasma in the afferent arterioles that
becomes glomerular filtrate
o Normally 0.16-0.2 (16-20%)
o Varies in both health and disease
 Daily volume of glomerular filtrate in adults:
o Females = 150L
o Males = 180L
o Only 1-2L is excreted as urine

The Filtration Membrane
 Glomerular capillaries + podocytes = filtration membrane
o Permits filtration of water and small solutes
o Prevents filtration of most plasma proteins and blood cells
 Substances filtered from blood cross 3 barriers:
o Glomerular endothelial cells
 Leaky due to fenestrations
 Permits all solutes
 Prevents blood cells
 Contain mesangial cells - located among glomerular capillaries
 Contractile cells

,  Regulate glomerular filtration
o Basement membrane
 Acellular material between endothelium and podocytes
 Consists of minute collagen fibers and negatively charged
glycoproteins
 Allow water and most small solutes to pass
 Negative charge of glycoproteins repels plasma proteins (most
are anionic)
o Filtration slit formed by a podocyte
 Each podocyte has thousands of footlike processes = pedicels
 Wrap around glomerular capillaries
 Spaces between pedicels = filtration slits
 Slit membrane = extends across each filtration slit
 Allows small molecules (<0.006 um) to pass
 Water, glucose, vitamins, amino acids, very
small plasma proteins, ammonia, urea, ions
 Principle of filtration = use of pressure to force fluids and solutes through a
membrane
o Same as in blood capillaries
o Volume of fluid filtered by renal corpuscle is much larger than in other
blood capillaries for 3 reasons:
 Glomerular capillaries have larger surface area
 Mesangial cells regulate how much SA is available
 When relaxed, SA is maximal = glomerular filtration
is high
 Contraction of mesangial cells = reduced SA =
reduced filtration
 Filtration membrane is thin and porous
 50x leakier than blood capillaries
 Large fenestrations
 Glomerular capillary blood pressure is high
 Efferent arteriole has smaller diameter than afferent
arteriole
 = high resistance to outflow of blood = BP is higher
in glomerular capillaries than in blood capillaries in
other parts of body

Net Filtration Pressure
 Glomerular filtration depends on 3 main pressures:
o 1 promotes filtration
o 2 pressure oppose filtration
1. Glomerular blood hydrostatic pressure (GBHP)
o BP in glomerular capillaries
o Generally about 55 mmHg
o Promotes filtration