Kidney Function

Question 1

what is the role of the kidney in water/ electrolyte homeostasis

Answer 1

intake/ loss must be in balance (over any significant period)
- approximate
- expenditure of a lot of metabolic energy
- temperature regulation

Question 2

what are the conditions that normal intake/ output are measured in

Answer 2

adult
male
23 degrees + normal daily activities

Question 3

what is the typical intake of water per day

Answer 3

water ~1200ml
food ~1000ml
metabolic ~300ml
total ~2.5L/day

Question 4

what is the typical output of water per day

Answer 4

urine ~1500ml
sweat ~100ml
faeces ~200ml
insensible loss ~700ml
- respiratory loss
total ~2.5ml

Question 5

what is the sweat loss /hour and /day during heavy exercise in hot/ humid conditions

Answer 5

> 2L/ hour
10L/ day
urine output may well be reduced in these conditions

Question 6

how is the role of salt controversial

Answer 6

intake is hard to measure
excretion normally measured
UK recommendations is 6g/ day
American Guidelines are similar but drop to 3.75g/ day
- if you are >51, have ^ BP, diabetes, chronic kidney disease

Question 7

what is renal blood flow

Answer 7

25% of cardiac output
- about 625 ml/ 100g/ min

Question 8

what is the BP is glomerular capillaries

Answer 8

50-60mmHg
- renal artery is short and relatively large radius

Question 9

what is the passage of blood through the kidney

Answer 9

afferent arteriole -> glomerular capillaries -> efferent arteriole -> tubular capillaries -> venuole

Question 10

what are the two types of nephron

Answer 10

superficial and juxta-medullary
- superficial just dips into the medulla
- juxta-medullary extend to papilla
- water reabsorption more effective in the longer juxta-medullar nephrons q

Question 11

what are the 4 sections of the nephron

Answer 11

PCT, loop of Henle, DCT, collecting duct
continuous layer of epithelium
the cell shapes in the wall are very different
- reflects activity; SA; ion pumping etc.

Question 12

what are the three basic principles of the kidney

Answer 12

ultrafiltration
reabsorption
secretion

Question 13

describe ultrafiltration in the kidney

Answer 13

driven by blood pressure in glomerular capillaries
- high renal blood flow
- high filtration rate (90-140ml/ min)

Question 14

describe reabsorption in the kidney

Answer 14

in the PCT and DCT
active pumping from filtrate in tubules
- for substances to be retained: water, glucose, amino acids, electrolytes

Question 15

describe secretion in the kidney

Answer 15

active pumping into tubules
for substances to be eliminated fast then filtration alone allows: H+, ammonia, uric acid, some drugs (e.g. antibiotics -> need to take them every few hours)
foreign, unusual substances

Question 16

how are the pumping rates controlled in the kidney

Answer 16

hormones
e.g. aldosterone can adjust the rates of Na+ and K+ excretion
aldosterone promotes salt retention

Question 17

what are podocyte cells

Answer 17

the gap between the podocyte processes determines what gets filtered out
if the molecule is small enough, it will be filtrated

Question 18

what molecules are filtered by podocyte cells

Answer 18

filtration of water and small molecules through slits between the podocytes
- ions, urea, glucose, amino acids, small proteins etc.
- cut off at ~67KD -> the size of haemoglobin

Question 19

what does the presence of haemoglobin in urine suggest

Answer 19

either kidney infection/ inflammation or ^BP

Question 20

describe active reabsorption in the PCT

Answer 20

brush border
active reabsorption of glucose, amino acids, Na+ and K+ ions
Co-transporters, aqueous channels, membrane pumps
- lots of mitochondria to promote this
substantial water reabsorption

Question 21

what is the result of absorption by the end of the PCT

Answer 21

complete reabsorption of glucose, amino acids
substantial reabsorption of Na+ an water
volume of filtrate reduced by 2/3rds

Question 22

what is the structure of the loop of Henle

Answer 22

thinner wall during descent into the medulla
thicker wall during ascent from the medulla

Question 23

what is the function of the loop of Henle

Answer 23

solute diffuses into descending tubule: counter-current mechanism ‘recycles’ solutes
ion pumping develops high osmotic pressure at the tip of the loop
- no net re-absorption here
- longer the loop, higher the osmotic pressure

Question 24

what is the difference in structure of the DCT to the PCT

Answer 24

more solute reabsorption and secretion
DCT pumps are under hormone control
- fine tuning the product
less intense electrolytes and water reabsorption
DCT ion pumping can be controlled by hormones like aldosterone

Question 25

what is the purpose of ADH/ AVP

Answer 25

CDs pass close to tips of the loop
- if CDs are permeable to water, then moves out of the duct to concentrate filtrate
ADH increases presence of aquaporins in the lumina membrane
- allows water movement
- quick response

Question 26

describe process of control of blood volume

Answer 26

1- water intake restricted
2- plasma osmolarity ^
3- more ADH is secreted by the hypothalamus
4- ADH ^ water permeability of CDs
5- ^ water absorption
6- concentrated urine is produced

Question 27

what is normal plasma osmolarity

Answer 27

300 mOsm (275-290 mOsm - UK value)

Question 28

what is the maximum concentration of urine

Answer 28

~1200 mOsm

Question 29

what is the minimum urine output

Answer 29

~1 ml/ min

Question 30

describe control of blood pressure

Answer 30

renin/ angiotensin/ aldosterone system
hypo-filtration initiates secretion of renin by the juxtaglomerular apparatus
renin splits angiotensinogen
- angiotensin I produces -> converted to angiotensin II (powerful vasoconstrictor)
this system regulates renal blood flow and glomerular filtrations rate (low BP, low renal flow, hypofiltration)
sympathetic nerves enhance this action

Question 31

describe control of salt balance

Answer 31

aldosterone ^ when electrolyte concentrations fall (secreted by golerulosa cells of the adrenal cortex)
aldonsterone ^ reabsorption of Na+ and Cl- ions from Loop, DCT and duct cells
- also ^ K+ secretion
when electrolyte reabsorption increases; water reabsorption increases