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BIOB33H3 Lecture Notes - Distal Convoluted Tubule, Macula Densa, Proximal Tubule

Biological Sciences
Course Code
Connie Soros

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Chapter 14 Urinary System
The Kidney is the major functional organ, other organs carry urine out of the body
- basic functions
1. water balance and osmolarity
2. electrolyte (ion) balance
3. maintain plasma volume, long term regulation of blood pressure
4. acid/base balance
5. excrete wastes (urea, uric acid, creatinine) and foreign materials
6. secrete erythropoietin
7. secrete renin (Na+ balance)
8. converts vitamin D to its active form
- nephron is the functional unit of the kidney
1. arrangement forms cortex and medulla
2. glomerulus
a. tuft of capillaries that filters blood
b. renal artery branches to form afferent and efferent arteriole for each nephron
(1) efferent arteriole divides to form peritubular capillaries (supply renal
tissue with blood)
3. tubule
a. Bowman's capsule surrounds glomerulus and collects filtrate
b. proximal convoluted tubule (PCT)
c. loop of Henle
d. distal convoluted tubule (DCT)
e. collecting duct/tubule - drains fluid from several nephrons to renal pelvis
4. juxtaglomerular apparatus
a. regulates kidney function
b. macula densa - specialized cells of DCT as it passes by glomerulus
c. granular cells (juxtaglomerular cells or JG cells) are specialized smooth muscle
cells of arterioles
5. 2 types of nephrons
a. cortical - lie mainly in cortex (80%)
b. juxtamedullary - loops dip to end of medulla (important in urine
concentration/conserving water)
(1) vasa recta are blood vessels that run near long loop

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- 3 renal processes
1. glomerular filtration
a. about 20% of plasma entering glomerulus is filtered
b. entire plasma volume filtered 65 times/day
c. nonselective process - everything but cells and plasma proteins are filtered
2. tubular reabsorption
a. selective recovery of filtered substances
3. tubular secretion
a. selective transfer of materials from plasma in peritubular capillaries to filtrate
Glomerular Filtration
- filtered substances pass through highly permeable filtration membrane
1. glomerular capillaries are 100x more permeable than other capillaries
2. basement membrane (collagen for strength and glycoproteins with a negative charge that
repels plasma proteins)
3. inner layer of Bowman's capsule
a. podocytes wrap around capillaries and form filtration slits
- glomerular capillary blood pressure forces fluid through filtration membrane
1. higher than in other capillaries
a. diameter of afferent arteriole larger than efferent arteriole, blood dams up and
filtration occurs throughout glomerulus
2. glomerular filtration rate (GFR)
a. GFR = Kf x net filtration pressure
(Kf= filtration coefficient, collective properties of filtration membrane)
b. autoregulation
(1) allows GFR to remain constant despite changes in BP
(vasoconstriction/dilation of afferent arteriole)
(2) myogenic mechanism - arteriolar smooth muscle constricts when stretched,
relaxes with decreased pressure
(3) tubuloglomerular feedback mechanism - macula densa detects changes in
rate of filtrate flow or osmotic changes and signals granular cells to release
vasoactive substances
increased flow vasoconstriction decreased GFR
decreased flow vasodilation increased GFR
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