7 Sodium, Chloride and Water along the Nephron Flashcards
In simple terms, how does the kidney maintain the volume of ECF within a very narrow margin?
Regulating excretion of sodium and chloride ions
What is the ‘sodium ion balance’?
Matching process- kidneys balancing amount of sodium excretion and amount of sodium ingestion (amount NOT concentration)
(Glomerular tubular balance)
What happens within the body if the sodium ion content of the ECF increases? (patient in positive balance)
Increase in ECF volume (water from nephron drawn out), blood volume increase, blood pressure increase, oedema may follow
What happens within the body if the sodium ion content of the ECF decreases? (patient in negative balance)
Water remains in nephron, ECF volume decreases, Blood volume decreases, blood pressure decreases
What is natriuresis?
excretion of sodium in the urine
A decrease in filling of the pulmonary vascualture and cardiac atria increases sympathetic nerve activity and causes ADH secretion and therefore more water uptake. What causes the increase in sympathetic nerve activity?
Low- pressure Baroreceptors in atria and pulmonary vascualture
Send signals to brainstem via vagus nerve- modulates sympathetic nerve outflow
What % change in pressure in the blood vessels is required to evoke a response (from baroreceptors)?
5-10% pressure change
What is the bainbridge reflex?
(More important than baroreceptor reflex when blood volume raised, less important when blood volume= diminished)
Increases in right atrial pressure increases HR
High pressure baroreceptors- arterial side- (carotid sinus and aortic arch) respond to pressure
Send impulses via afferent fibres of:
- Vagus nerve
- Glossopharyngeal nerve
Decrease in BP increases sympathetic nerve activity and secretion of ADH
Ingestion of sodium daily varies between what range (usually)?
0.5 g/d and 20-25 g/d
Why is it important that we change the amount of sodium in ECF (eg blood plasma) rather than the amount of water?
Because changing the amount of water would change plasma osmolarity
What’s the difference between transcellular and paracellular absorption?
Transcellular: through cell
Paracellular: between (next to) cell
Name a transporter which is found on the apical membrane at each of these segments in the tubule within the kidney:
- Proximal convoluted tuble
- Loop of Henle
- Early distal convoluted tubule
- Late distal convoluted tubule and Collecting Duct
Where in the nephron will we not find aquaporin channels? Why?
- Ascending limb of loop of henle
- Distal convoluted tubule
Regions do not absorb water
What are the early and late segments of the proximal convoluted tubules also called?
Early: S1
Late: S2
Why is it that the peritubular capillaries and the efferent arterial have a high potential for reabsorption of filtrate?
High oncotic pressure as 20% filtered out but still proteins and large molecules
Name the important transporter that can be found on the basement membrane in S1 in the PCT and state its function.
3Na-2K-ATPase
Removing sodium ions from inside epithelial cell
Establishes concentration gradient
Sodium in lumen moves into cell
In S1 of the PCT, 3Na-2K-ATPase is transporting sodium out of the basolateral membrane, creating a concentration gradient and causing sodium ions to drawn in from the lumen of the PCT to the tubular epithelia cell. Give an example of a transporter bringing sodium into the cell.
What happens to glucose molecules in S1 of the PCT? What other molecules can be transported in the same way that glucose is here?
- Transported with sodium into tubular epithelial cell
- Diffuses into interstitium
- Diffuses from interstitium into capillary
(some of transport into tubular epithelial cell is active as glucose concentration decreases)
Other molecules:
- Carboxylic acids
- Amino acids
- Phosphate
What happens to the water in S1 of the PCT?
- Water follows sodium through aquaporin channel
- Moves into interstitium
- Moves into capillary due to hydrostatic pressure
(ie Bulk reabsorption volume)
Identify two substances/ions which are left behind in the lumen of the PCT in S1.
- Urea
- Cl-
Why is it that if a patient is diabetic, glucose might be found in their urine?
Transport maximum reached
How do we transport bicarbonate from the lumen of the PCT S1 to the capillary?
Carbonic anhydrase- dissociates bicarbonate ion
CO2 and water transported across
Anion exchanger exchanges bicarbonate for chloride
What is the action of the diuretic amiloride in the PCT (S1) and in the DCT?
PCT: Blocks Na+/H+ antiporter
DCT: Inhibits epithelial sodium channels (ENaC)
What is the main process that occurs in the S1 of the proximal convoluted tubule (whats its main function)?
Isosmotic reabsorption: bulk transport
What happens to chloride ions in the lumen of the proximal convoluted tubule in S2-3? What makes this possible?
(freebie)
Majority- diffuse paracellularly into interstitium
Due to build up of Cl- ions in lumen in S1 of PCT so concentration gradient AND charge gradient
DON’T NEED TO EXPEND ENERGY
Water follows chloride ions
What % of each of the following is reabsorbed in the PCT:
- Water
- Glucose
- Amino acids
- Na+
- Water: 65%
- Glucose: 100%
- Amino acids: 100%
- Na+: 67%
How is Na+ reabsorption stimulated in the proximal convoluted tubule if someone has a low BP?
Stimulated by angiotensin II (RAAS)
What happens within the proximal convulted tubule is BP increase (in renal artery)?
- Reduce Na-H anitporter and Na-K ATPase activity
- Reduced sodium reabsorption
- Increased sodium and water excretion
- Pressure natruresis (sodium excretion)
- Pressure diuresis (water excretion)
What is the main process (movement of molecules) that occurs in the ascending and descending limbs in the loop of henle?
Descending: water moves out
Ascending: reabsorption of sodium (diluting segment)
(concentrates sodium and chloride ions in lumen of descending limb ready for active transport in ascending
At what point in the loop of henle are active transporters required for the transport of sodium?
Thick ascending limb
Name the main transporter of sodium in the thick ascending limb of the loop of henle. Where else can it be found?
NKCC2
Also in: Macula densa
What effects do loop diuretics have on the NKCC22 transporter?
Blocks channel
Therefore lots of fluid lost
(Doesn’t affect ROMK)
(also blocks NKCC22 in macula densa) –> not constricting afferent arteriole
Why is the ROMK (renal outer medullary potassium channel) so important?
Allows NKCC2 to keep functioning- transports K+ back into lumen
How do K+ sparing diuretics work? (eg spirondactone)
(less fluid lost than with loop diuretics)
Block function of ROMK
The distal convoluted tubule has an important function. What is this function?
Volume determination- ie need more ECF
if volume to low- reabsorb more sodium
The reuptake of Na+ in the distal convoluted tubule is under the control of which system?
RAAS
Renin-angiotensin-aldosterone-system
Aldosterone- upregulates reuptake of sodium (take more water back in collecting duct)
NCCT (sodium chloride chloride transporter) can be found in the DCT and also the cortical collecting duct. Name a diuretic it is sensitive to.
Thiazide
Name a diuretic which the ENaC channels are sensitive to:
Amiloride
How does the reuptake of calcium occur in the DCT? What regulates this reuptake process?
Binds to calbidin
Then antiporter
Regulated by: Parathyroid hormone
What are the 2 distinct cell types which can be found in the collecting duct?
- Principal cells
- Intercalated cells
Describe principle cells (found in collecting ducts).
Have 2 channels: ENaC and ROMK
(and 3Na-2K-ATPase)
Depends on ADH - variable uptake of water
What are the 2 cell types of intercalated cells?
A-IC or B-IC
A-IC- secrets hydrogen ions
B-IC- secretes bicarbonate ions
(For info)
