D’YOUVILLE COLLEGE
BIOLOGY 108/508 - HUMAN ANATOMY & PHYSIOLOGY II
LECTURE # 21
URINARY SYSTEM II
RENAL PHYSIOLOGY - THE NEPHRON
1.
Functions of Kidneys:
• removal of nitrogenous wastes from blood
• regulation of water, acid/base, and electrolyte balance
• regulation of arterial pressure (renin/angiotensin system)
• regulation of red cell production (erythropoietin mechanism)
• role in activation of vitamin D hormone (calcium regulation)
2.
Functions of the Nephron: glomerular filtration, tubular absorption, & tubular
secretion (fig. 25 - 10)
• Glomerular Filtration: glomerular membrane: consists of capillary
endothelium (very leaky) + inner layer of squamous epithelium (features 'slit pore'
between cells); permeability > 1000X that of normal capillaries (fig. 25 - 9)
• glomerular pressure = 55 mm. Hg (usual is 35 mm.), stabilized by
balanced vasoconstriction of afferent & efferent arterioles
1
• glomerular filtration (non-selective): approx. /5 of plasma flow
(filtration fraction) yields glomerular filtration rate (GFR) = 125 ml./min. (180 l./day!)
• filtrate resembles plasma minus proteins
• Tubular Absorption:
• selective process: sodium recovery (active transport) drives chloride
transport (electrochemical drag); solute recovery provides osmotic gradient for water
recovery; other substances (e.g. glucose, amino acids, vitamins) also recovered by active
transport; urea recovered by diffusion (fig. 25 - 14)
• 65 - 70% absorption in proximal tubule (obligatory water
absorption)
• distal tubule and collecting duct absorption hormonally regulated
(facultative absorption)
• 99+% of water filtered is normally recovered; 100% recovery of glucose,
amino acids; 50% recovery of urea
• 124 ml./min. (178.5 liters/day) normal tubular absorption rate (TA)
results in urine output (UO = GFR - TA) of 1 ml./min. (1.5 liters/day)
• absorbed materials return to blood in peritubular capillaries
• Tubular Secretion:
• active transport of additional substances into renal tubule
• occurs mainly in proximal, distal and collecting tubules
• potassium, hydrogen ion, uric acid, creatinine and ammonia are
substances secreted into tubules
3.
Renal Blood Flow -- Regulation of Glomerular Filtration Rate (fig. 25 -12):
• major influence over GFR and therefore over UO (table illustration)
• regulation of GFR via intrinsic (autoregulation -- adjustments within kidney)
Bio 108
lec. 27 - p. 2
& extrinsic mechanisms: afferent vasoconstriction/vasodilation (autoregulation =
myogenic response, JG apparatus response) promotes drop in GFR/rise in GFR;
extrinsic adjustments alter systemic BP (sympathetic NS, renin-angiotensin system)
• effect of autoregulation: 100% rise in arterial pressure results in only 15%
rise in GFR