7. Physiology of the Urinary System Part I Flashcards
Urine Formation overview (4)
1) Kidney filter entire blood plasma 60 times each day
2) Uses about 25% of body energy to excrete wastes to form urine
3) 47 gallons of glomerular filtrate reduced to 0.5 gallons of urine
4) 99% of water, ions, nutrients returned to blood
Urine Formation Step 1 - Filtration (2)
1) In the glomerulus
2) Substances leave the blood plasma to become filtrate
Urine Formation Step 2 - Reabsorption (2)
1) In the renal tubules
2) Fluid and substances move from filtrate back into the blood plasma
Urine Formation Step 3 - Secretion
Substances entering filtrate from surrounding fluid
Urine formation graphic
URINE FORMATION STEP 1 - FILTRATION
Mechanical Filtration (Glomerular Filtration step 1) (3)
1) Does not require energy
2) Depends on opposing pressures within the glomerular capsule and capillary
3) Filtration based on size:
a) Red blood cells and most proteins too large to pass through
b) Fluids, solutes (water, glucose, amino acids) are small enough to be forced out of the glomerular capillaries
Blood Hydrostatic Pressure (HP) (Glomerular Filtration step 1) (3)
1) Amount of pressure found inside the blood capillaries
2) Drives fluids OUT of the capillaries
3) Varies from peson to person, depending on blood pressure
a) Blood pressure rises, HP rises
Colloid Osmotic Pressure (COP) - Step 1 (4)
1) Also called oncotic pressure
2) Dependent on proteins in the blood plasma
3) Draws water out of the filtrate, into the blood plasma
4) Needs to remain in normal range of 25-32 mmHg
a) Damage occurs to glomerulus if outside of normal range
Capsular Hydrostatic Pressure - Step 1 (2)
1) Mechanical pressure exerted by recoil of glomerular arteries
2) Also opposes blood HP, drives fluid back into the capillaries
Net Filtration Pressure (NFP) - Step 1 (3)
1) Difference in pressures between outgoing and incoming forces at the glomerulus
2) Pressure with which the filtrate enters the PCT
3) HP - (COP + Capsular Pressure)
a) HP should always be largest number
Calculate the NFP for a person with a hydrostatic pressure of 60mmHG, a colloid pressure of 32mmHg and a capsular pressure of 18mmHg
=60-(32+18) = 10mmHg (net out)
Glomerular Filtration Rate (GFR) (4)
1) Amount of blood filtered by the glomerulus over time
2) Normal GFR is 120-125 ml/min or 180 L/day
3) GFR INCREASES with increasing arteriole pressure
a) Increased outgoing forces
4) GFR DECREASES with increasing osmotic pressure
a) Usually caused by dehydration
b) Increased incoming forces
Glomerular Filtration Rate (3)
1) Needs to remain consistent
a) Adequate reabsorption of water
b) Needed filtration of wastes
2) Too fast: needed substances cannot be reabsorbed
3) Too slow: too much filtrate is reabsorbed, including wastes that should be excreted
GFR regulation (2)
1) Regulation of renal blood flow controls the GFR
2) Under control of three mechanisms:
a) Renal autoregulation
b) Nervous system control
c) Hormonal control (RAA system)
Renal Autoregulation (4)
1) Under normal circumstances
2) Control is by the kidneys themselves
3) Kidneys determine their own rate of blood flow
a) Control over diameter of the afferent and efferent arterioles
4) Kidneys can maintain a constant GFR despite blood pressure variates in the rest of the body
Nervous System Control (7)
1) Under emergency circumstances
2) Renal autoregulation is superseded by higher level nervous system controls
3) Diverts blood away from kidneys to vital organs (brain, heart, skeletal muscles)
4) Sympathetic nerve fibers
a) Afferent arteriole diameter is narrowed
5) After narrowing of AA, there is a release of epinephrine by adrenal glands
6) Decrease in renal blood flow, decreases GFR
7) Can only be used for short time
Renin Angiotensin-Aldosterone System (RAA) (3)
1) Responds when the body’s blood pressure drops too low
2) Goal is to bring up the blood pressure to maintain body functions
3) Helps reabsorption of more sodium ions and water from the filtration
a) WATER FOLLOWS SALT
RAA steps
1) Angiotensinogen
a) Pre-enzyme produced by the liver
b) Freely circulates in the blood
2) Renin
a) When blood pressure drops this enzyme is released by the juxtaglomerular cells of the nephron
b) Converts angiotensinogen to angiotensin I
3) Angiotensin I
a) In the lungs is converted to angiotensin II
4) Angiotensin II
a) Triggers thirst mechanism in hypothalamus
b) Causes peripheral vasoconstriction, increasing blood pressure
5) Aldosterone
a) Released when angiotensin II reaches the adrenal cortex
b) Causes renal tubules to reabsorb more sodium ions
c) Increases water retention to increase blood pressure
RAA visual
STEP 2 - REABSORPTION
Tubular Reabsorption Overview (3)
1) Precise control of urine concentrations through hormonally regulated ion channels placed in tubules
2) Most contents of glomerular filtrate that enter tubules are reabsorbed into the blood of the peritubular capillaries
3) If reabsorption did not occur, the entire plasma would be drained away within an hour
Tubular Reabsorption - Active and Passive Process (2)
1) Diffusion: passive process
2) Active: ion pumps are ATP-driven
a) Energy expenditure required
Proximal Convoluted Tubule (PCT) (2)
1) Most reabsorption occurs here
a) Glucose, amino acids
b) 65% Na+, water
c) 90% Bicarbonate
d) 50% Chloride
e) 50% Potassium
f) Most Calcium, Phosphate and Magnesium
Loop of Henle (2)
1) Descending and ascending limbs act differently
a) Water can leave the descending but not the ascending
b) Na+ and K+ can leave the ascending limb but not the descending
2) Reabsorption of additional substances:
a) 25% Na+
b) 15% water
c) 40% of K+
Distal Convoluted Tubule (DCT) (2)
1) Only about 10% of Na+ and Cl- and 20% of water remain in the filtrate once it reaches DCT
2) Ion channels can be placed in this region to control:
a) Abnormal blood pressure
b) Low blood volume
c) Low Na+ concentration
d) High K+ concentration
Reabsorption visual
STEP 3 - SECRETION
Tubular Secretion Overview (3)
1) Substances enter filtrate from the surrounding fluid
2) Allows for elimination of undesirable substances
a) Urea
b) Excess potassium ions
c) Drugs (penicillin)
3) Control over pH
a) Secretion of HCO3- (Bicarbonate ion)
b) Secretion of H+ (Hydrogen ion)
