BIOL 3542 Lecture Notes - Lecture 19: Facilitated Diffusion, Distal Convoluted Tubule, Macula Densa

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Human Physiology II
Chapter 19: The Kidneys
Functions of the Kidneys
most important function is homeostatic regulation of water, ion content of blood (a.k.a. salt
and water balance of fluid and electrolyte balance)
kidneys maintain normal blood concentrations of ions, water by balancing intake with
excretion in urine, obeying principle of mass balance
6 general areas of kidney function:
1. Regulation of extracellular fluid volume and blood pressure
when ECF volume decreases, blood pressure decreases
if blood pressure falls too low, body cannot maintain adequate blood flow to brain,
other essential organs
kidneys work with cardiovascular system to ensure blood pressure, tissue perfusion
remain within acceptable range
2. Regulation of osmolarity
body integrates kidney function with behavioural drives (ex. thirst) to maintain blood
osmolarity at 290 mOsM
3. Maintenance of ion balance
kidneys keep concentrations of sodium, potassium, calcium in normal range by
balancing intake, excretion
4. Homeostatic regulation of pH
if ECF too acidic, kidneys remove H+, conserve bicarbonate to act as buffer
if ECF too alkaline, kidneys remove bicarbonate, conserve H+
kidneys don’t correct pH disturbances as fast as lungs
5. Excretion of wastes
remove metabolic wastes (ex. creatinine from muscle metabolism, nitrogenous wastes
urea and uric acid), xenobiotics (foreign substances ex. drugs, environmental toxins)
metabolite urobilinogen gives urine yellow colour
also remove hormones from blood
6. Production of hormones
erythropoietin cytokine/hormone regulates rbc synthesis
renin enzyme regulates production of hormones involved in sodium balance, blood
pressure homeostasis
renal enzymes convert vitamin D3 into hormone that regulates calcium balance
Anatomy of the Urinary System
Urinary System: kidneys, accessory structures
Renal Physiology: study of kidney function
The Urinary System Consists of Kidneys, Ureters, Bladder, and Urethra
urine production begins when water, solutes move from plasma into microscopic nephrons of
kidneys
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nephrons modify fluid composition; modified fluid = urine
urine leaves kidney, passes into hollow tube (ureter)
2 ureter, 1 from each kidney to urinary bladder
bladder expands, fills with urine until micturition/urination reflex during which bladder
contracts, expels urine through urethra
urine sample from UTI patient contains many red, white blood cells not normally found in
urine
The Kidneys
lie on either side of spine at level of 11th, 12th ribs, just above waist
below diaphragm, outside abdominal cavity, sandwiched between membranous peritoneum
that lines abdomen, and bones and muscles of back
renal blood vessels, nerves, lymphatics, ureters emerge from concave surface
renal arteries branch off abdominal aorta, supply blood to kidneys
renal veins carry blood from kidneys to inferior vena cava
The Nephron Is the Functional Unit of the Kidney
interior of kidney arranged in 2 layers formed by arrangement of nephrons:
outer cortex: contains 80% of nephrons
inner medulla: contains 20% of nephrons (juxtamedullary nephrons)
nephron functional unit of kidney (smallest structure that can perform all functions of organ)
nephrons divided into sections associated with specialized blood vessels
Vascular Elements of the Kidney
blood enters kidney through renal artery, flows into smaller arteries, then arterioles in cortex
blood vessel arranged in portal system (2 capillary beds in series; 1 of 3 in body)
in renal portal system, blood flows from renal arteries into afferent arteriole, then into 1st
capillary bed (glomerulus), then into efferent arteriole, then into 2nd capillary bed
(peritubular capillaries) that surround nephron
peritubular capillaries converge to form venules, small veins, sending blood out of kidney
through renal vein
Vasa Recta: long peritubular capillaries that dip into medulla in juxtamedullary nephrons
function of renal portal system to filter fluid out of blood, into lumen of nephron at
glomerular capillaries, then reabsorb fluid from tubule lumen back into blood at peritubular
capillaries
Tubular Elements of the Kidney
kidney nephron consists of single layer of epithelia cells connected near apical surface which
is folded into microvilli
basal side of polarized epithelium rests of basement membrane/basal lamina
cell-cell junctions mostly tight, some have selective permeability for ions
nephron begins with hollow, ball-like Bowman’s capsule that surrounds glomerulus
endothelium of glomerulus fused to epithelium of Bowman’s capsule so fluid filtering out
of capillaries passes directly into lumen of nephron
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Renal Corpuscle: combination of glomerulus, Bowman’s capsule
fluid flows from Bowman’s capsule into proximal tubule, then loop of Henle which dips
down toward medulla, then back up
loop of Henle divided into 2 limbs:
thin descending limb
ascending limb with thin and thick segments
fluid then passes into distal tubule, up to 8 of which drain into single larger tube called
collecting duct
Distal Nephron: distal tubule and its collecting duct together
collecting ducts pass from cortex through medulla, drain into renal pelvis
filtered, modified fluid (now urine) flows from renal pelvis into ureter
Juxtaglomerular Apparatus: final part of ascending limb of loop of Henle that passes between
afferent and efferent arterioles
proximity of ascending limb, arterioles allows paracrine communication between
them, key feature in kidney autoregulation
Overview of Kidney Function
>99% of fluid that enters nephrons must be reabsorbed back into blood or body will
dehydrate
Kidneys Filter, Reabsorb, and Secrete
3 basic processes in nephron:
1. Filtration: movement of fluid from blood into lumen of nephron
only takes place in renal corpuscle, where walls of glomerular capillaries, Bowman’s
capsule are modified to allow bulk flow of fluid
once filtered, filtrate passes into lumen of nephron, becomes part of body’s external
environment
anything that filters into nephron destined for excretion unless it’s reabsorbed into
body
after filtrate leaves Bowman’s capsule, it’s modified by reabsorption, secretion
2. Reabsorption: process of moving substances in filtrate from lumen of tubule back into
blood flowing through peritubular capillaries
3. Secretion: selectively removing molecules from blood, adding them to filtrate in tubule
lumen
more selective than filtration, uses membrane proteins to move molecules across
tubule epithelium
The Nephron Modifies Fluid Volume and Osmolarity
180L of fluid filtered into Bowman’s capsule/day identical in composition to plasma, nearly
isosmotic (~300 mOsM)
70% reabsorbed in proximal tubule, leaving 54L in lumen
reabsorption occurs when proximal tubule cells transport solutes out of lumen, water follows
by osmosis
filtrate leaving proximal tubule has same osmolarity as filtrate that entered
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Document Summary

6 general areas of kidney function: regulation of extracellular fluid volume and blood pressure. When ecf volume decreases, blood pressure decreases. If blood pressure falls too low, body cannot maintain adequate blood flow to brain, other essential organs. Kidneys work with cardiovascular system to ensure blood pressure, tissue perfusion remain within acceptable range: regulation of osmolarity. Body integrates kidney function with behavioural drives (ex. thirst) to maintain blood osmolarity at 290 mosm: maintenance of ion balance. Kidneys keep concentrations of sodium, potassium, calcium in normal range by balancing intake, excretion: homeostatic regulation of ph. If ecf too acidic, kidneys remove h+, conserve bicarbonate to act as buffer. If ecf too alkaline, kidneys remove bicarbonate, conserve h+ Kidneys don"t correct ph disturbances as fast as lungs: excretion of wastes. Remove metabolic wastes (ex. creatinine from muscle metabolism, nitrogenous wastes urea and uric acid), xenobiotics (foreign substances ex. drugs, environmental toxins) Also remove hormones from blood: production of hormones.

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