7. Physiology of the Urinary System Part I

Question 1

Urine Formation overview (4)

Answer 1

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

Question 2

Urine Formation Step 1 - Filtration (2)

Answer 2

1) In the glomerulus

2) Substances leave the blood plasma to become filtrate

Question 3

Urine Formation Step 2 - Reabsorption (2)

Answer 3

1) In the renal tubules

2) Fluid and substances move from filtrate back into the blood plasma

Question 4

Urine Formation Step 3 - Secretion

Answer 4

Substances entering filtrate from surrounding fluid

Question 5

Urine formation graphic

Answer 5

Question 6

URINE FORMATION STEP 1 - FILTRATION

Question 7

Mechanical Filtration (Glomerular Filtration step 1) (3)

Answer 7

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

Question 8

Blood Hydrostatic Pressure (HP) (Glomerular Filtration step 1) (3)

Answer 8

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

Question 9

Colloid Osmotic Pressure (COP) - Step 1 (4)

Answer 9

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

Question 10

Capsular Hydrostatic Pressure - Step 1 (2)

Answer 10

1) Mechanical pressure exerted by recoil of glomerular arteries

2) Also opposes blood HP, drives fluid back into the capillaries

Question 11

Net Filtration Pressure (NFP) - Step 1 (3)

Answer 11

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

Question 12

Calculate the NFP for a person with a hydrostatic pressure of 60mmHG, a colloid pressure of 32mmHg and a capsular pressure of 18mmHg

Answer 12

=60-(32+18) = 10mmHg (net out)

Question 13

Glomerular Filtration Rate (GFR) (4)

Answer 13

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

Question 14

Glomerular Filtration Rate (3)

Answer 14

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

Question 15

GFR regulation (2)

Answer 15

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)

Question 16

Renal Autoregulation (4)

Answer 16

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

Question 17

Nervous System Control (7)

Answer 17

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

Question 18

Renin Angiotensin-Aldosterone System (RAA) (3)

Answer 18

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

Question 19

RAA steps

Answer 19

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

Question 20

RAA visual

Answer 20

Question 21

STEP 2 - REABSORPTION

Question 22

Tubular Reabsorption Overview (3)

Answer 22

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

Question 23

Tubular Reabsorption - Active and Passive Process (2)

Answer 23

1) Diffusion: passive process

2) Active: ion pumps are ATP-driven
a) Energy expenditure required

Question 24

Proximal Convoluted Tubule (PCT) (2)

Answer 24

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

Question 25

Loop of Henle (2)

Answer 25

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+

Question 26

Distal Convoluted Tubule (DCT) (2)

Answer 26

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

Question 27

Reabsorption visual

Answer 27

Question 28

STEP 3 - SECRETION

Question 29

Tubular Secretion Overview (3)

Answer 29

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)