Fluid & Electrolyte Balance Flashcards
Systemic Response to Decrease in Blood Pressure / Volume
- Receptors in aorta and carotid bodies and atria fire
- Cardiovascular: up vasoconstriction (up pressure)
- Behavior: Drink water (up volume, up pressure)
- Kidney: Conserve water (up volume, up pressure)
- Pressure restored!
Systemic Response to Increase in Blood Pressure / Volume
- Baroreceptors + volume and endocrine cells in atria fire
- Cardiovascular: up vasodilation (down pressure)
- Behavior: N/A! (Don’t want to intake salt b/c would also intake fluid)
- Kidneys: Excrete fluid (down volume, down pressure)
- Pressure restored!
How do kidneys maintain fluid balance?
- Most of water that is filtered is reabsorbed
- Final concentration of urine tailored to current water needs
- Kidney decides whether ions needed or should be excreted with urine
PCT is permeable to…
Almost everything! Glucose, amino acids, vitamins, ions, water.
Descending loop of Henle is permeable to…
Water
Ascending loop of Henle is permeable to…
NaCl
DCT is permeable to…
NaCl, Water
Collecting Duct is permeable to…
Water, UREA
Apical Membrane of PCT Epithelial Cell has…?
Villi!
Main method of transport in PCT
Symports! Using Na+ gradient (low Na+ concentration in cell) to bring in other molecules / ions up gradient
The renal medulla contains a…
Concentration gradient
What creates the concentration gradient of the renal medulla?
The Loops of Henle of juxtamedullary nephrons
The concentration gradient is exploited by…
The collecting duct, to concentrate the urine
What happens in the ascending Loop of Henle?
- High concentration filtrate passes by.
- NKCC transporter moves Na+, K+, and 2 Cl- ions into the cell. These cells all move out of cell, either via channels, via a K+/Cl- hsymport, or via sodium potassium pump.
- WATER CANNOT FOLLOW SOLUTE!
- Filtrate leaving ascending loop is less concentrated.
Why is ascending loop of Henle impermeable to water?
Layer of glycoproteins
Loop diuretics inhibit…
The NKCC symport. More ions in the urine means more water will dilute the urine, making them diuretics!
Are there paracellular pathways in the ascending Loop of Henle?
NO, tight junctions only!!
What shape are the cells of the ascending loop? Why?
Cuboidal: This allows for all the necessary membrane proteins to pump / transport ions across the epithelium.
Keep in mind that cells toward the bottom of the loop can rely on diffusion, while cells higher in the loop must use pumps / symports and LOTS of ATP.
What drives water reabsorption in descending Loop of Henle?
Na+ reabsorption in ascending loop!
What cells make up the descending loop of Henle?
Squamous cells - flat b/c NO transport proteins or pumps… only permeable to water
What is the direction of water in the descending loop of Henle determined by?
Osmotic gradient!
How does water diffuse in descending loop of Henle?
- Through membranes of squamous cells
2. Via paracellular pathway
Vasopressin / ADH
“Anti Diuretic Hormone”
Controls water reabsorption in collecting duct
Released by posterior pituitary near hypothalamus
What stimulates ADH release?
Rise in blood osmolarity / decreased atrial stretch / decreased blood pressure, communicates to hypothalamus, which releases ADH
What inhibits ADH release?
Rise in blood pressure / decrease osmolarity / increase atrial stretch communicates to hypothalamus and stop sending out ADH
How does ADH work in the collecting duct
- Vasopressin (peptide hormone) binds to GPCR
- Activates cAMP messenger system
- This results in exocytosis of vesicles containing Aquaporin 2 membrane proteins, these are delivered to apical membrane of cell
- Water from lumen comes into cell via these pores, diffuses across cell and back into bloodstream
Max Vasopressin
Super concentrated urine! Yellow pee!!
No vasopressin
Dilute urine - the color of a beautiful pale buttercup
What signals for the release of ADH?
- Decrease in blood pressure (carotid and aortic baroreceptors)
- Decrease in atrial stretch (atrial stretch receptor in heart)
- Increase in osmolarity (hypothalamic osmoreceptors)
What controls release of ADH?
Hypothalamus, which then talks to the posterior pituitary
How does high blood glucose affect a diabetic?
- Peeing out glucose, so body excretes more water to dilute the urine.
- This leads to greater osmolarity of the blood, increased thirst, and ADH secretion.
- Blood volume and blood pressure decrease to make up for lost volume.
- Heart attempts to restore blood pressure by beating faster…
- COMA OR DEATH!
Countercurrent Exchanger
Leverage the opposite flow of two systems to save energy
e.g. in the Loop of Henle: have blood and filtrate flow in opposite direction, then leverage medulla concentration gradient to tune the osmolarity of each
Aldosterone
Steroid hormone, controls sodium balance
Controls sodium reabsorption in DCT and collecting duct; controls K+ secretion as well - does both by acting on Na+/K+ pump
Homeostatic response to salt ingestion
- Up osmolarity, same volume
- Vasopressin, thirst
- Up renal H2O absorption, up volume in blood
- Kidneys have to excrete salt AND water. Cardiovascular system lowers blood pressure.
- Pressure & volume return to normal.
What signals for aldosterone?
- Hyperkalemia - elevated K+ levels
2. Decreased blood pressure (proxy for decreased osmolarity)
What produces aldosterone?
Adrenal cortex
Where does aldosterone act?
the P cells : in last third of DCT and cortical portion of collecting duct
How does aldosterone act?
- Aldosterone diffuses across membrane and binds to receptor in cytoplasm (it’s a steroid!)
- This initiates transcription in nucleus
- Translation and protein synthesis makes new Na+/K+ pumps and ion channels
- Aldosterone-induced proteins modulate existing channels and pumps
- Na+ reabsorbed and K+ secreted into lumen via channels; Na+ pumped out of cell by Na+K+ pump
Renin-Angiotensin System
Potent vasoconstrictor, but many complex effects: increases vasopressin and aldosterone release (NOTE: affects PCT!!!)
Renin secreted by granular cells of afferent arteriole in response to signaling from macula densa, in response to decreased GFR / decreased blood pressure
Angiotensin produced by liver, becomes angiotensin II when combined with renin - INCREASES PRESSURE
ANP and BNP
ANP released due to atrial stretch, results in excretion of sodium and water (opposite to all the hormones we’ve been looking at - REDUCES blood pressure)
BNP released in response to ventricular stretch, aka heart failure
This is a complex system that also controls the others ,but DECREASES vasopressin, aldosterone, and renin
Dilates afferent arteriole to increase GFR; increases NaCl and H2O excretion in collecting duct
What happens to neurons at low pH?
They become less excitable…. confused, disoriented, loss of consciousness
What happens to neurons at high pH?
Too excitable - random firing, sustained contraction
Why is acidosis more common than alkalosis?
More acids in our diet, more acids as metabolic byproducts
What buffers do we have to respond to acid?
- Bicarbonate in ECF
- Proteins, hemoglobin, phosphates in cells
- Phosphates, ammonia in urine
What responses do we have to acidosis?
- Ventilation: breathe off more CO2
2. Excretion: pee off H+
What responses do we have to alkalosis?
- Maybe breathe a little less
2. Excretion: Pee off bicarbonate
pH homeostasis depends on….
- Ventilation
- Buffers (bicarbonate, hemoglobin, phosphate ions)
- Kidneys (additional buffers as well w/ ammonia and phosphate)
Renal Compensation for Acidosis
- Carbonic anhydrase in cells of nephron turn CO2 and H2O into carbonic acid
- Release bicarbonate ion (negative) into blood stream; release H+ ion (positive) into urine (charge balance)
- Acidic urine!
Respiratory acidosis
pH is low, but bicarbonate is high
Elevated CO2 shifts Le Chatelier’s to the right, producing more H+ and more bicarbonate
Metabolic acidosis
pH is low, but bicarbonate is also low
Elevated H+ pushes reaction to the left, so H+ is up but this shifts reaction toward reactants : aka CO2 and H2O, pulling bicarbonate out of solution
