Renal Regulation of Ion and Water Balance Flashcards
How does the body gain water?
Via ingestion and internal production
How does the body lose water?
Via urine, the GIT, and evaporation from the skin and respiratory tract (insensible water loss and sweat)
How does the body gain Na+ and Cl-?
By ingestion
How does the body lose Na+ and Cl-?
Via the skin (sweat), the GIT and urine
What is the major homeostatic control point for both Na+ and water?
Renal excretion
What is the average daily water gain/loss in adults?
2850ml
What is the daily sodium chloride intake and output?
8.50g
What is the basic renal process for sodium and water?
- Na+ is freely filterable at the glomerulus; its reabsorption is a primary active process dependent upon Na+/K+ ATPase
- Na+ is not secreted
Where are the Na+/K+ ATPase found?
In the basolateral membranes of the tubular epithelium
What is the Na+ entry into tubular epithelial cells from the tubular lumen like?
It is always passive, either through ion channels or by co-transport or countertransport with other substances
What are the methods of sodium reabsorption like in proximal tubule?
- Apical membrane transport: co-transport or countertransport
- Basolateral membrane transport
- Sodium recyclcing
What is co-transport across the apical membrane like?
Cotransport (with other molecules like glucose or amino acids) moves Na+ into the cell alongside other substances
What is the Na+ counter-transport process like?
Countertransport (or antiport) exchanges Na+ for hydrogen ions (H+), meaning Na+ enters the cell while H + is secreted back into the lumen.
What is the basolateral membrane transport like?
Once inside the proximal tubule cell, Na+ is pumped out across the basolateral membrane into the interstitial fluid.
This is done by the Na+/K+ ATPase pump (Na+/K+ pump), which uses ATP to actively transport Na+ out of the cell and K+ into the cell. This pump maintains a low Na+ concentration inside the cell, enabling continuous Na+ reabsorption from the lumen.
What is potassium recycling like?
Potassium ions brought into the cell by the Na+/K+ pump are recycled back out through potassium channels in the basolateral membrane, maintaining cellular K+ balance.
What is the apical membrane transport of sodium like in the cortical collecting duct?
Sodium ions (Na+) enter the cortical collecting duct cell from the tubular lumen through sodium channels located on the apical membrane. This movement occurs via diffusion, driven by the concentration gradient of Na+ across the membrane.
What is the effect of sodium reabsorption in regards to water?
It creates an osmotic difference across the tubule, which drives water reabsorption, largely through water channels
What are the water channels called?
Aquaporins
Up until which part is water reabsorption independent of the posterior pituitary hormone vasopressin?
Until the point where it reaches the collecting-duct system, where vasopressin increases water permeability
What is the posterior pituitary hormone vasopressin also known as?
ADH; anti-diuretic hormone
What is the effect of low vasopressin and, hence, low water reabsorption?
A large volume of dilute urine
What is the coupling of water and Na+ reabsorption process like?
- Sodium enters the tubular epithelial cells from the tubular lumen passively, “downhill”
- As Na+ accumulates in the epithelial cell, the local osmolarity within the cell rises, and the osmolarity in the lumen decreases.
The increase in osmolarity inside the cell and in the surrounding interstitial fluid (once Na+ is pumped out of the cell) drives water reabsorption by osmosis. - Water moves from the tubular lumen into tubular epithelial cells via osmosis due to the osmotic pressure created by the Na+
- Na+ is actively transported from the tubular epithelial cells into the interstitial fluid, through Na+/K+ ATPase.
- This active transport of Na+ into the interstitial space further raises the osmolarity there, drawing water from the cell and interstitial space into the peritubular capillaries by bulk flow.
What drives bulk flow?
High osmolarity and oncotic pressure in the interstitial fluid, helping maintain blood volume and pressure.
What causes the increased urine excretion?
low vasopressin (ADH) –> water diuresis
What pathology is associated with the consequences of losing control or response to vasopressin?
Diabetes insipidus
What are the two types of diabetes insipidus?
Central diabetes insipidus
Nephrogenic diabetes insipidus
What causes central diabetes insipidus?
The failure of the posterior pituitary gland to release vasopressin
What causes nephrogenic diabetes insipidus?
Inability of the kidneys to respond to vasopressin
What is the term osmotic diuresis?
Any loss of solute in the urine must be accompanied by water loss
What happens to the urine volume when plasma vasopressin concentration is high?
A small volume of concentrated urine is produced by the renal countercurrent multiplier system
Which part of the loop of Henle is relatively impermeable to water?
The ascending loop of Henle
What is the effect of active transport of sodium chloride by the ascending loop of Henle?
Causes increased osmolarity of the interstitial fluid of the medulla but a dilution of the luminal fluid
What is the permeability of the descending loop of Henle?
Relatively impermeable to solutes and freely permeable to water
How does vasopressin increase the permeability to water of the cortical collecting ducts?
By increasing the number of AQP2 water channels inserted into the apical membrane
Water is reabsorbed by this segment until the luminal fluid is isoosmotic to the interstitial fluid and peritubular plasma of the cortex
What happens the tubular fluid becomes isoosmotic?
It enters and flows through the medullary collecting ducts, and the concentrated medullary interstitium causes water to move out of these ducts –> made highly permeable to water by vasopressin
Where is the countercurrent multiplier system found?
Henle’s loop
What is the purpose of the countercurrent multiplier system and urea trapping?
Establish a hypertonic medullary interstitium
How does the structure of the blood vessels in the medullas help maintain hypertonic medullary interstitium?
Prevents the countercurrent gradient from being washed away
What are the blood vessels in the medulla?
Vasa recta
What happens to the salt and water being reabsorbed from both loops of Henle and collecting ducts?
Carried away by the venous blood
What is the purpose of the vasa recta in terms of maintaining a hypertonic medullary interstitium?
Minimizes the washout of the hypertonic interstitium of the medulla
How are GFR and the filtered load of Na+ controlled?
By baroreceptor reflexes
What is the effect of decreased vascular pressures?
Decreased baroreceptor firing –> increased sympathetic outflow to renal arterioles –> vasoconstriction –> decrease in GFR
What is the sodium loss and its effect on GFR like? (13)
- Increase Na+ and H2O loss due to diarrhea
- Decrease in plasma volume
- Decrease in venous pressure
- Decrease in venous return
- Decrease in atrial pressure
- Decrease in stroke volume
- Decrease in cardiac output
- Decrease in arterial blood pressure
- (4 to 8) cause increased activity of renal sympathetic nerves
- In the kidneys, there is increased constriction of afferent renal arterioles
- Decreased net glomerular filtration pressure
- Decreased GFR
- Decreased Na+ and H2O excreted
What is the direct effect of decreased arterial blood pressure to the kidneys?
Decrease net glomerular pressure –> Decrease in GFR –> Decrease in Na+ and H2O excreted
What is the major control of tubular Na+ reabsorption?
Adrenal cortcial hormone; aldosterone
What is the effect of aldosterone in renal sodium regulation?
Stimulates Na+ reabsorption by the distal convoluted tubule and the cortical collecting ducts
What controls aldosterone secretion?
The renin-angiotensin system (RAAS) is one of the major controllers
What are the effects of RAAS?
Vasoconstriction & Na+ and H2O retention –> Increase in BP
What are the three inputs that stimulate renin secretion when extracellular volume decreases?
- Stimulation of renal sympathetic nerves to the juxtaglomerular cells by external baroreceptor reflexes
- Pressure decreases sensed by juxtaglomerular cells, themselves acting as intrarenal baroreceptors
- A signal generated by low Na+ or Cl- concentration in the lumen of the macula densa
What is the RAAS like? (9)
- Decrease in the plasma volume
- Increased activity of renal sympathetic nerves OR decreased arterial pressure OR Decreased GFR, which causes decreased flow to macula densa (decreased NaCl delivery to macula dense)
- Increased renin secretion (from the renal juxtaglomerular cells)
- Increased plasma renin
- Increased angiotensin II
- Increased aldosterone secretion in the adrenal cortex
- Increased plasma aldosterone
- Increased Na+ and H2O reabsorption in cortical collecting ducts
- Decreased Na+ and H2O excretion
What is another factor that influences Na+ reabsorption?
Atrial natriuretic peptide (ANP)
How is ANP secreted?
By cells in the atria in response to atrial distention
What is the effect of ANP?
It inhibits Na+ reabsorption and increases GFR
Where does atrial pressure act?
It acts locally on the renal tubules to influence Na+ reabsorption
What is the effect of an increased atrial pressure?
Causes decreased reabsorption and increased excretion
What is water excretion?
The difference between the volume of water filtered (GFR) and the volume reabsorbed
What are the factors that regulate water excretion?
GFR regulation via baroreceptor reflexes, and the major one being the vasopressin-mediated control
What controls the vasopressin secretion?
Osmoreceptors and non-osmotic sensors such as cardiovascular baroreceptors in the hypothalamus
How do the hypothalamic osmoreceptors help with renal water regulation?
A high body fluid osmolarity stimulates vasopressin secretion, and a low osmolarity inhibits it
What is the effect of a low extracellular volume on vasopressin?
Stimulates vasopressin secretion via the baroreceptor reflexes
What is the effect of a high extracellular volume on vasopressin?
It inhibits it
What is another effect of vasopressin? How does this help?
Arteriolar constriction –> helps restore the arterial blood pressure towards normal
Which factor has a greater impact on real water regulation: osmotic or baroreceptor?
Osmoreceptor but baroreceptor reflexes can become very important in pathological states
In which kind of pathological states can baroreceptor reflexes become very important?
Hemorrhage
What is the body’s response to sweating?
- Severe sweating can lead to a decrease in plasma volume and an increase in plasma osmolarity (because Na+ and H2O are lost)
- This will result in a decrease in GFR and an increase in aldosterone, which together decrease Na+ excretion and an increase in vasopressin, which decreases H2O excretion.
- The net result of the renal retention of Na+ and H2O is to minimize hypovolemia and maintain plasma osmolarity
What stimulates thirst?
An increase in extracellular fluid osmolarity and a decrease in extracellular fluid volume
Angiotensin II?
Plasma osmolarity –> the most important, under physiological conditions
Is salt appetite a major regulatory factor when it comes to thirst?
No, not in human beings
How does a person remain in potassium balance?
By excreting an amount of potassium in the urine equal to the amount ingested minus the amounts lost in the feces and sweat
What is the permeability of K+ like?
Freely filterable at the renal corpuscle and undergoes both reabsorption and secretion
What are the major factors determining K+ excretion?
K+ secretion that occurs in the cortical collecting ducts
What happens when the body potassium incraeses?
The extracellular potassium concentration also increases –> acts directly on the cortical ducts to increase K+ secretion and also stimulate aldosterone secretion
What is the effect of increased plasma aldosterone?
Stimulates K+ secretion
What is the most common cause of hyperaldosteronism?
Noncancerous adrenal tumor (adenoma) that secretes aldosterone in the absence of stimulation from angiotensin II
What is the effect of excess aldosterone in the cases of hyperaldosteronism?
It causes increased renal K+ secretion, Na+ reabsorption, and fluid retention
What does hyperaldosteronism often cause?
Endocrine hypertension
What portion of the plasma calcium is ionized and filterable?
About half of the calcium plasma
Where does most of the calcium and phosphate ion reabsorption take place?
In the proximal tubule
What is the effect of PTH on calcium ion reabsorption?
Increases calcium ion reabsorption in the DISTAL CONVOLUTED TUBULE and early cortical collecting duct
What is the effect of PTH on the phosphate ion reabsorption?
It decreases phosphate ion reabsorption in the PROXIMAL TUBULE
What is the function of the proximal tubule of the nephron?
Responsible for the bulk reabsorption of solute and water
What is the function of the loop of Henle of the nephron?
Generates the medullary osmotic gradient that allows for the passive reabsorption of water in the collecting ducts
What is the function of the distal tubules and collecting ducts of the nephron?
Site of most regulation (fine-tuning) of the excretion of solutes and water
