Physiology Unit 5 Exam - Urinary

What are the functions of the urinary system?

* regulate plasma ionic composition
* regulate plasma volume
* regulate plasma osmolarity
* regulate plasma pH
* remove metabolic waste products and foreign substances from plasma
* secrete erythroprotein and renin
* activate vitamin D3 to calcitriol
* gluconeogenesis

What is the functional unit of the kidney?

the nephron

* includes renal corpuscle and renal tubules

describe the renal corpuscle

includes glomerulus and glomerular (Bowmans) capsule

Glomerulus is

capillary network for filtration

Glomerular (Bowmans) capsule

* receives filtrate
* inflow to renal tubules

Renal Tubules includes

* proximal tubule
* proximal convoluted tubule
* proximal straight tubule
* loop of henle
* descending limb
* thin ascending limb
* thick ascending limb
* distal convoluted tubule
* connecting tubule
* collecting duct

What is the difference between cortical nephron and juxtamedullary nephron?

cortical:

* short loop of henle
* most numerous, 80-85%

juxtamedullary:

* long loop of hence extends into medulla
* responsible for medullary osmotic gradient

T or F: both the cortical nephron and the juxtramedullary nephron produce urine

true

What are the 2 main distinctive parts of the juxtaglomerular apparatus?

* macula densa
* juxtaglomerular cells

what is the macula densa?

plays a role in blood pressure regulation

What is the juxtaglomerular cells?

secrete renin which plays an important role in regulating blood volume and pressure

Blood supply in the kidney

20% of cardiac output at rest

* accounts for less than 1% of body weight
* accounts for 16% of ATP usage by body

What are 3 renal exchange processes?

1. filtration
2. reabsorption
3. secretion

describe filtration

bulk flow of protein free plasma from the glomerular capillaries into Bowmans capsule

describe reabsorption

selective transport of solute and water from lumen of renal tubule to the interstitial fluid outside the tubule

describe secretion

selective transport of solute and water from interstitial fluid into renal tubule

In glomerular filtration, filtrate must cross -- barriers to enter capsule

3

what are the 3 barriers that that filtrate must cross in glomerular filtration?

1. capillary endothelial layer
2. surrounding epithelial layer (podocytes)
3. basement membrane between these 2 layers

What are Podocytes?

* visceral epithelial cells
* a component of the glomerular filtration barrier
* primary processes
* pedicels (foot processes)

What is the filtration fraction?

fraction of blood filtered by the kidneys

* = GFR / renal plasma flow

\
* if 625 ml plasma enters the kidneys per minute and 125 ml is filtered into the Bowmans capsule then the filtration fraction is 625/125 = 20%

What are the Glomerular Filtration: Starling Forces?

* Glomerular capillary hydrostatic pressure
* Bowmans capsule oncotic pressure
* Bowmans capsule hydrostatic pressure
* glomerular oncotic pressure

what happens when starling forces FAVOR filtration?

* glomerular capillary hydrostatic pressure is about 60 mmHg
* high due to resistance of the efferent arteriole
* capsular oncotic pressure is about 0 mmHg
* no protein in filtrate

what happens when starling forces oppose filtration?

* capsular hydrostatic pressure is about 15 mmHg
* relatively high compared to systemic capillaries
* much smaller volume
* glomerular oncotic pressure is about 29 mmHg
* higher than in systemic capillaries
* plasma proteins in smaller volume of plasma

GFR

COME BACK TO THIS - SLIDE 17

What is the filtered load?

amount of substance filtered by the kidneys

* quantity filtered = filtered load

small molecules that are filtered without impedance are

freely filterable

* depends on plasma concentration of solute and GFR

filtered glucose load =

GFR x \[X\]plasma

\
example:

* GFR = 125 mL/min
* \[glucose\]plasma = 100 mg/dL = 1 mg/mL
* (125)x(1) = 125 mg/min

describe the regulation of GFR

* 180 liters of fluid should be filtered per day
* only 1.5 L urine is excreted per day
* 99% of filtered fluid is reabsorbed
* small increase in GFR leads to large increase in volume of fluid filtered and excreted
* GFR is highly regulated

MAP increases as

GFR increases

Describe myogenic regulation (intrinsic regulation of GFR)

smooth muscle in the wall of afferent arteriole contracts in response to stretch

describe tubuloglomerular feedback (intrinsic regulation of GFR)

* macula densa cells secrete paracrine factors in response to an increase in flow of fluid past them
* smooth muscle of arterioles contract in response to these paracrine

In extrinsic control of GFR, decreases in BP can ------- GFR

decrease

how could GFR be decreased directly

decreased filtration pressure

how could GFR be decreases indirectly

extrinsic control of vascular tone and blood volume

SOLUTE AND WATER REABSORPTION

SLIDES 24 and 25

What is the transport maximum (maximum load, mg/min)

rate of transport when carries are saturated

When a solute is transported across epithelium by carrier protein

saturation of carrier can occur

* if solute in filtrate natures carriers, then some solute is secreted in urine

What is the renal threshold (concentration, mg/ml)

solute concentration in plasma at which carriers are saturated and the solute first appears in the urine.

Describe glucose reabsorption

* freely filtered at glomerulus
* 100% actively reabsorbed in proximal tubules
* no glucose appears in urine in healthy individuals
* carrier proteins for glucose reabsorption
* secondary transport = apical membrane
* facilitated diffusion = basolateral

theoretical renal threshold

GFR x renal threshold = transport max

renal handling of glucose

slide 28 !!!! good info

Describe secretion

* solute moves from peritubular capillaries into tubules
* Barries are the same as for reabsorption
* transport mechanisms are the same but opposite direction

secreted substances in secretion

* potassium
* hydrogen ions
* choline
* creatinine
* penicillin

Non-regulated reabsorption takes place in the

proximal convoluted tubule

regulated reabsorption and secretion takes place in the

distal convoluted tubule and collecting ducts

water conservation takes place in the

nephron loop

In non-regulated reabsorption, the proximal tubule is

a mass reabsorber

* 70% sodium and water
* 100% glucose

In non-regulated reabsorption, the brush border allows for a

larger surface area

In non-regulated reabsorption leaky tight junction allow --- transport

paracellular

In regulated reabsorption & secretion in the DCT and collecting ducts, transport is regulated across

epithelium

In regulated reabsorption & secretion in the DCT and collecting ducts, tight junctions limit

paracellular transport

Whats the purpose of water conversion in the loop of henle?

* nephron loop establishes conditions necessary to concentrate urine
* minimizes water loss

What is the excretion rate?

amount of solute excreted in the urine over time

* Amount of substance excreted = amount filtered + amount secreted - amount reabsorbed
* OR the concentration of the solute in the urine times the volume of the urine production

What 3 factors does the amount of excretion depend on?

1. filtered load
2. secretion rate
3. reabsorption rate

if amount of solute excreted per minute is LESS than filtered load

solute was reabsorbed

if amount of solute excreted per minute is GREATER than filtered load

solute was secreted

Describe Clearance

* Volume of plasma from which a substance has been removed (cleared) by the kidneys per unit time
* Volume of plasma that contains the amount of a substance that has been excreted per unit time

\
* Clearance = excretion rate / plasma concentration = (Ux x V) / Px
* Ux = urine concentration of solute
* V = urine flow rate
* Px = plasma concentration of solute

Describe clearance of Inulin

* clearance of substance freely filtered and neither reabsorbed nor secreted = GFR
* amount of inulin excreted in urine = amount that was filtered = filtered load
* Excretion rate = filtered load = GFR x Pi

\
* Cinulin = (Uinulin)(V) / (Pinulin)

Why would we use creatinine to estimate GFR?

* creatinine is a by product of muscle metabolism
* freely filtered
* not reabsorbed
* small amount secreted
* clearance: “estimate” of GFR
* clearance is a litter greater than GFR: 140 mL/min

The clearance of a substance that is completely reabsorbed is

zero

* example: glucose

Describe the clearance of PAH (para-aminohippuric acid)

* freely filtered
* fully secreted
* not reabsorbed
* PAH clearance = renal plasma flow rate (converts plasma flow to blood flow)
* Amount Excreted = amount contained in volume of plasma that entered the kidneys

If clearance of a solute is greater than GFR then

solute was secreted

If clearance of a solute was less than GFR then

solute was reabsorbed

Micturition is also

urination

urine is formed in the

renal tubules

urine/fluid formed in the renal tubules, drains into

renal pelvis and into the ureter

The ureters lead to the

bladder

the bladder stores urine until it is

excreted

Describe the process leading up to micturition/urination

* increase volume of fluid in the bladder expands the wall and activates stretch receptors
* Sympathetic activity and somatic motor activity is decreased, parasympathetic is increased
* external and internal urethral sphincters relax and detrusor muscle contracts
* sphincters open and urination happens

What factors affect plasma concentration

* kidney regulation of solute and water content (urine volume).
* regulation of acid-base balance
* exchange between different compartment of the body (cells, connective tissue, GI tract, sweating, respiration, etc)

material exchanges affecting plasma content

slide 3

What does balance between water and solute look like?

* solutes and water enter and exit at the same rate
* quantity remains the same

What does positive balance between solute and water look like?

* solute and water enters plasma faster than it exits
* quantity increases

what does negative balance between solute and water look like?

* solute and water exits plasma faster than it enters
* quantity decreases

reabsorption and secretion of solute and water in the PCT are

unregulated

regulation of renal excretion significantly depends on

the late distal tubule and collecting ducts

What type of cells are in the late distal tubules and collecting ducts that regulate solute and water balance?

principal cells

* water
* electrolytes

intercalated cells

* acid-base balance

come back to

slide 6

intake + metabolically produced =

output + water used

normovolemia =

normal blood volume

hypervolemia =

high blood volume due to positive water balance

hypovolemia =

low blood volume due to negative water balance

osmolarity of body fluids =

300 mOsm

* no osmotic force for water to move between fluid compartments

kidneys alter osmolarity of -----

extracellular fluid (ECF)

* compensate for the changes by regulating water reabsorption
* water reabsorption is a passive process
* based on osmotic gradient (osmosis)

during water reabsorption, the proximal tubules

reabsorb 70% of filtered water

not regulated

during water reabsorption, the distal tubules and the collecting ducts

reabsorb the remaining water

this is regulated by ADH (vasopressin) and aldosterone

describe water reabsorption in the proximal tubule

* water reabsorption follows solute reabsorption (primarily sodium)
* Na+ is actively transported across the basolateral membrane; establishes an osmotic gradient for water reabsorption
* Fluid in the PT is 300 most (iso-osmotic)

The osmotic gradient is established by the

countercurrent multiplier

describe the medullary osmotic gradient

* dependent on loop on Henle
* descending limb is permeable to water and transport of Na+, Cl-, or K+ can occur
* Ascending limb is impermeable to water and allows active transport of Na+, Cl-, and K+

The Vasa Recta is

peritubular capillaries

the direction of blood flow in the vasa recta is ---- to the tubular fluid

opposite

* most effective for the exchange, countercurrent multiplier

describe countercurrent exchange

A biological mechanism in which there is an exchange of a substance between fluids flowing in opposite directions. 

20% of water is reabsorbed in the

distal tubules

* regulated by ADH and aldosterone

10% of water is reabsorbed in the

collecting ducts

* regulated by ADH

water reabsorption in the distal tubule and the collecting ducts is dependent on

* the osmotic gradient (established by countercurrent multiplier)
* epithelium permeability to water

water permeability is dependent on

water channels

aquaporin-3 is present in the

basolateral membrane always

* principal cells

aquaporin-2 is present in the

apical membrane only when ADH is present in the blood

when membrane of the late distal tubule and collecting duct is impermeable to water

* water cannot leave the tubules or be reabsorbed
* more water is excreted in urine

Describe the effects of ADH on water reabsorption

* ADH promotes water reabsorption and reduces urine volume (high osmolarity)
* ADH stimulates insertion of aquaporin-2 into apical membrane and water is reabsorbed by osmosis
* The length of loop of Henle also effects water reabsorption
* ADH secreted neurons (body) located in the hypothalamus
* altered ADH production in response to change in ECF osmolarity through osmoreceptors
* increased osmolarity reduces ADH
* ADH production/release is also regulated by baroreceptor activity
* decreased baroreceptor activity = increased ADH secretion