renal physiology

Question 1

What is osmolality?

Answer 1

Particles per kilo of solution

-a high osmolality is a highly concentrated solution

Question 2

What is osmolarity?

Answer 2

proportional to the number of particles per litre of solution; it is expressed as mmol/L.
-a high osmolarity is a highly concentrated solution

Question 3

What is:

• hypotonic
• hypertonic
• isotonic

Answer 3

hypotonic: solution has a lower conc. than cell
Hypertonic: solution has a higher conc. than cell
isotonic: solution has the same conc. as cell

Question 4

what compartments make up total body water?

Answer 4

Intracellular fluid: 67%

Extracellular fluid: 33%

• plasma 20%
• interstitial fluid 80%
• lymph and transcellular fluid

Question 5

How are body fluid compartments measured?

Answer 5

using tracers - obtain distribution volume from tracer

Question 6

What are the tracers for:

• total body water
• extracellular fluid
• plasma

Answer 6

TBW: 3H2O
ECF: inulin
Plasma: labelled albumin

Question 7

How are body water compartments measured?

Answer 7

Volume in litres = dose (D) / sample conc

e.g. 42mg/1mg per litre = 42litres

e.g. for tracer X
distribution volume = quantity X (mol)/ equilbrium of X in body (mol/l)

Question 8

what are the main ions in extracellular fluid?

Answer 8

Na+ CL- HCO3-

Question 9

what are the main ions in intracellular fluid?

Answer 9

K+ Mg2+ and -vely charged proteins

Question 10

Describe the four ways that fluid homeostasis is challenged? how does it affect intracellular vs extracellular fluid?

Answer 10

1: gain or loss of water - change to both ICF and ECF similarly
2: NaCl gain - water is drawn out of cells into the extracellular fluid
3: NaCl loss - water is drawn into cells out of the extracellular fluid
4: gain or loss of isotonic solution - ECF changes only as this does not affect tonicity

Question 11

Which electrolyte is the main determinant of extracellular fluid?

Answer 11

Na+, therefore it is vital that this is regulated

Question 12

Which electrolyte plays a key role in establishing membrane potential?

Answer 12

Potassium: more than 95% K+ is intracellular and small leakages or increased cellular uptake may affect conc. plasma K+
=muscle weakness
=cardiac irregulations

Question 13

what are the ten kidney functions?

Answer 13

1 - water balance
2 - salt balance
3 - plasma vol. maintenance
4 - plasma osmolarity balance
5 - acid-base balance
6 - excretion metabolic waste products
7 - excretion exogenous foreign compounds
8 - secretion renin
9 - secretion EPO
10 - activated vit D

Question 14

What is the functional unit of the kidney? what are their 3 functions? what two different types do you get and what is the difference? which is the most common type?

Answer 14

Nephron

functions:
1 - filtration
2 - reabsorption
3 - secretion

Juxtamedullary nephrons (20%) have a longer loop of henle which dips far into the medulla

Cortical nephrons (80%) only have a small loop of henle

Question 15

what are the three basic renal processes?

Answer 15

1: glomerular filtration (20% of plasma that enters the glomerulus is filtrated
2: Tubular reabsorption
3: Tubular secretion

Question 16

How is rate of excretion calculated?

Answer 16

Rate of excretion = rate of filtration + rate of secretion - rate of absorption

Question 17

how is filtation rate calculated?

Answer 17

Rate of filtration of X = mass of X filtered into bowmans capsule per unit time

Rate of filtation of X = [X]plasma X GFR

Question 18

How is excretion rate calculated?

Answer 18

rate of excretion of X = mass X excreted per unit time

= [X]urine X urine flow rate

Question 19

How is reabsorption rate measured?

Answer 19

=rate filtration of X - rate excretion X

if so much X has been filtrated but only a smaller amount has been excreted - must have been reabsorbed

Question 20

How is secretion rate measured?

Answer 20

= rate of excretion X - rate of filtration X

more X has been excreted than filtrated and therefore it must have been secreted

Question 21

Describe the 3 filtration barriers in the glomerulus that prevent RBCs/plasma proteins from being filtrated??

Answer 21

glomerular capillary endothelium provides a barrier to RBCs

Basement membrane and slit processes of podocyte provides a barrier to plasma proteins

Question 22

What two factors are involved in net filtration rate in the capillary?

Answer 22

hydrostatic and oncotic pressures in the capillary and in bowmans capsule (although no oncotic pressure in bowmans capsule as no plasma proteins here)
- usually this is about 10mmHg

Question 23

What is GFR and how is this calculated?

Answer 23

rate at which protein free plasma is filtrated to bowmans capsule per unit time
-Kf X net filtration rate

(Kf is filtation coefficient: how holey is glomerular membrane?)

Question 24

How is GFR regulated in the body?

Answer 24

Extrinsic: baroreceptor reflex
Intrinsic: autoregulation in the kidney

Question 25

Describe autoregulation in the kidney?

Answer 25

Myogenic: -if vascular smooth muscle is stretched it contracts and constricts arteriole Tubuloglomerular fneedback -if GFR rises, more NaCl flows through which is detected by the macula densa in the juxtaglomerular apparatus =constriction of afferent arteriole =this ensures renal blood flow is maintained over a wide range of MAP to protect GFR

Question 26

how could: - renal stone - diarrhoea - severe burns - change in surface area avail. for filtration affect GFR

Answer 26

Renal stone: -increase the hydrostatic pressure in the bowmans capsule = GFR decreased Diarrhoea: -increases capillary oncotic pressure (dehydration) =GFR decreased Severe burns: -decreases the oncotic pressure of capillary = increases GFR change in surface area avail. for filtration: -decreases the filtration coefficient = decreases GFR

Question 27

what is plasma clearance?

Answer 27

volume of plasma that is cleared of a particular substance per min =rate of excretion/plasma conc. =[X]urine + volume urine produced ml/min / [X] plasma

Question 28

What substance is used as a clearance marker? what qualities does a substance have to have to be a clearance marker?

Answer 28

Creatinine clearance = GFR -endogeous (inulin can be used too) Qualities: freely filtered but no secreted or reabsorbed

Question 29

``` What is the clearance of: -glucose -urea -H+ -PAH (what can this be used for) compared to GFR ```

Answer 29

Glucose: all is reabsorbed and none is secreted (clearance = zero) Urea: clearance < GFR (is partly reabs. and none secreted) H+: clearance > GFR (not reabsorbed and some secreted) PAH: freely filtered and is all secreted (can use this to calculate renal plasma flow as this is the rate of clearance of PAH)

Question 30

what is the filtration fraction of the kidney?

Answer 30

amount (% or fraction) of plasma that is filtered at the glomerulus =GFR/renal plasma flow

Question 31

Where in the nephron is the main site for water and ion balance regulation?

Answer 31

distal tubule and collecting duct

Question 32

what regulates water and ion balance in the distal tubule and collecting ducts?

Answer 32

mainly hormones: aldosterone - increases sodium reabsorption and increases H+/K+ secretion in the late distal tubule ADH - increases the number of aquaporins in the luminal membrane of the late collecting duct to allow more water reabsorption ANP: decreases sodium reabsorption PTH: Ca2+ reabsorption increases, PO43- reabsorption decreases

Question 33

``` How is: Filtration Secretion/Reabsorption Excretion Regulated? ```

Answer 33

Filtration: changes in blood pressure/changes in size of filtration slits Secretion/reabsorption: changes in solute concentration e.g. ADH/Aldosterone Excretion: bladder function under neural control

Question 34

Describe how ADH is regulated, what does ADH cause

Answer 34

ADH is stimulated by: Hypertonic ECF, decrease in left atrial pressure, nicotine ADH is inhibited by: hypotonic ECF, stretch receptors in upper GI tract, alcohol ADH increases thirst, causes arteriolar constriction and increases water reabsorption in the late distal tubule

Question 35

Describe salt balance in the body:

Answer 35

Amount of sodium reabsorbed is regulated by RAAS Renin released from granular cells in the juxtaglomerular apparatus due to: - decreased pressure in the afferent arteriole - macula densa cells sense a decrease NaCl - increased sympathetic activity due to a decrease in arteriolar BP Aldosterone - stimulated by high potassium or low sodium in the blood or RAAS, acts to reabsorb sodium and and pump out potassium

Question 36

Describe how RAAS is stimulated in heart failure?

Answer 36

Failing heart leads to a decreased cardiac output and decreased blood pressure which causes an increase in salt retention and water

Question 37

What hormone inhibits RAAS? where is this produced and stored?

Answer 37

ANP - causes excretion of sodium Produced by the heart and stored in the atria: if heart stretched (e.g. by high BP due to high salt) ANP is released

Question 38

what is the difference between water diuresis and osmotic diuresis?

Answer 38

Water diuresis: this is an increased urine flow but no increased solute excretion Osmotic diuresis: this is a primary increase in sodium excretion which causes a diuresis

Question 39

How does the kidney regulate acid base balance by regulating H+ ions in plasma?

Answer 39

- in the proximal tubule HCO3- is reabsorbed - if there is a lot of H+ ions in tubular fluid, kidneys make more HCO3- to regenerate buffer stores depleted by acid load

Question 40

If the concentration of H+ ions in the tubular fluid increases - what does this cause?

Answer 40

- drives reabsorption of HCO3 - Forms acid phosphatase - forms NH4+ (ammonium ion)

Question 41

What is the difference between nephrogenic vs neurogenic diabetes insipidus?

Answer 41

Nephrogenic: inability of nephron to respond to ADH Neurogenic: lack of vasopressin release from posterior pituitary - treat with desmopressin