Fluid & Electrolyte Homeostasis Flashcards
What percentage of total body weight does fluid make up?
60% - 40% intracellular fluid and 20% extracellular fluid
What percentage of extracellular fluid is interstitial fluid?
80%
What type of tissue contains the least amount of fluid?
Adipose tissue
Why are salts important?
- Neuromuscular excitability - Secretory activity - Membrane permeability - Controlling fluid movements
What are two qualities of electrolytes?
They dissociate in water & are electrically charged
What type of salts account for 90-95% of all solutes in the ECF and how many mOsm do they contribute to the total 300mOsm ECF solute concentration?
Sodium salts; 280mOsm
Why does sodium play a role in controlling ECF volume and water distribution in the body?
- Sodium is the only cation to exert significant osmotic pressure - Sodium ions being leaked into cells/pumped out against their electrochemical gradient
True or false: Sodium concentration in the ECF normally remains stable
True
What do changes in plasma sodium levels affect?
Plasma volume/blood pressure & ICF/interstitial fluid volumes
What is the meaning of ‘renal acid-base control mechanisms are coupled to sodium ion transport’?
H+ and Na+ are swapped - where sodium goes, water actively follows
What are the normal and maximal (dehydrated) urine concentrations?
Normal: 50-70 mOsm/L Maximal: 1200-1400 mOsm/L
What type of relationship exists between urine osmolarity and urine specific gravity?
Linear
What is specific gravity?
A measure of the weight of solutes in urine
What is specific gravity influenced by and why?
Glucose & protein in urine - not normal constituents of urine (increased solutes = increased weight = increased SG)
What happens to urinary solute excretion and plasma osmolarity following ingestion of 1L of water?
They both remain relatively stable
What happens to urine flow and urine osmolarity following ingestion of 1L of water?
Urine flow: Sharp increase, plateau, sharp decrease Urine osmolarity: Sharp decrease (more diluted), plateau, sharp increase
What is diuresis?
An increase in urine output
How is a dilute urine formed?
Continue electrolyte reabsorption, decrease water reabsorption
What is the mechanism behind the formation of a dilute urine?
Decreased ADH release (no aquaporins formed = more water in filtrate) & water permeability in distal/collecting tubules
How is a concentrated urine formed?
Continue electrolyte reabsorption, increase water reabsorption
What is the mechanism behind the formation of a concentrated urine?
Increased ADH release increases water permeability in distal/collecting tubules (opens aquaporins & allows water to leak out), high osmolarity of renal medulla, countercurrent flow of tubular fluid
What is obligatory urine volume and what is the formula?
The minimum urine volume in which the excreted solute can be dissolved and excreted
Amount of solute that must be excreted each day to maintain electrolyte balance/Max urine osmolarity
i.e. 600mOsm/d / 1200mOsm/L = 0.5L/day
During renal disease, urine concentrating ability is impaired, resulting in
a) decreased obligatory urine volume
b) increased obligatory urine volume
b) increased obligatory urine volume
What is the osmoreceptor-ADH feedback mechanism for regulating extracellur fluid osmolarity?
Water deficit
= increased osmolarity
= increased ADH secretion
= increased plasma ADH
= increased H2O permeability in distal/collecting tubules
= decreased H2O excreted
*negative feedback to water deficit*
Where is ADH synthesised, released and actioned?
Synthesised in the magnocellular neurons of the hypothalamus, released by the posterior pituitary, actioned on the kidneys
What are the stimuli for ADH secretion?
Increased osmolarity, decreased blood pressure/volume, input from cerebral cortex (e.g. fear), nausea, angiotensin II, nicotine, morphine
Is plasma osmolarity or blood volume more sensitive to change?
Plasma osmolarity
What are the stimuli for decreased ADH secretion?
Decreased osmolarity, increased blood pressure/volume, alcohol
What are the stimuli for thirst?
Increased osmolarity, decreased blood pressure/volume, increased angiotensin II, dryness of mouth
What is angiotensin II?
The body’s most potent vasoconstrictor
What are the stimuli for decreased thirst?
Decreased osmolarity, increased blood pressure/volume, decreased angiotensin II, gastric distention
When the ADH-thirst system is blocked and sodium is ingested, what happens to plasma sodium levels and why?
They dramatically increase, as the body is unable to counteract the change and control plasma sodium concentration
What is the renal-body fluid feedback system?
Fluid intake
= increased ECF volume
= increased blood volume
= increased mean circulatory filling pressure
= increased venous return
= increased cardiac output
= increased total peripheral resistance
= increased arterial pressure
= dramatic increase in renal fluid excretion
Where is aldosterone produced?
Adrenal cortex
How does aldosterone regulate sodium balance?
- Sodium reabsorption (65% in proximal tubules, 25% in loop of Henle)
- When aldosterone levels are high, remaining sodium is actively reabsorbed from filtrate
What are the two triggers for the release of aldosterone to regulate sodium balance?
The renin-angiotensin mechanism & elevated potassium/decreased sodium levels in ECF
How does the renin-angiotensin mechanism trigger aldosterone release?
- Juxtaglomerular apparatus releases renin in response to SNS stimulation, decreased filtrate osmolarity & decreased stretch
- Renin catalyses production of angiotensin II
- Angiotensin II stimulates aldosterone release
How does aldosterone increase ECF sodium levels?
Increased water reabsorption, reduced urine output, increased blood volume
What happens when baroreceptors alert the brain of increased blood volume/pressure?
SNS impulses to kidneys decline, afferent arterioles dilate, glomerular filtration rate increases, sodium & water output increases
What is pressure diuresis and what does it do?
The baroreceptor-renal system mechanism, decreases blood pressure
What happens when systemic blood pressure decreases?
Increased SNS impulses to the kidneys, afferent arterioles constrict, glomerular filtration rate decreases, sodium and water output decreases
How does atrial natriuretic peptide (ANP) reduce blood pressure/volume?
By inhibiting sodium/water retention and events that promote vasoconstriction
What does ANP target in order to reduce blood pressure?
JG apparatus of kidney (inhibits angiotensin II system), hypothalamus/posterior pituitary (decreased ADH release) & adrenal cortex (decreased aldosterone release)
How does relative ICF-ECF potassium ion concentration affect a cell’s resting membrane potential?
Excessive ECF potasisum decrease membrane potential, too little potassium causes hyperpolarisation
What are the consequences of hyperkalemia and hypokalemia?
Diruption of electrical conduction in the heart and sudden death
Where and how is potassium balance controlled?
In the collecting ducts of the cortex, by changing the amount of potassium secreted into filtrate
What happens when ECF potassium levels are high?
Principal cells secrete potassium into filtrate for excretion, aldosterone is released
What effect does aldosterone have on potassium balance?
Stimulates potassium secretion by the principal cells
In the cortical collecting ducts, what is the ratio of sodium reabsorbed to potassium secreted?
1:1
How does potassium control its own ECF concentration?
By negative feedback regulation of aldosterone release
What is ionic calcium in ECF important for?
Blood clotting, cell membrane permeability, secretory behaviour
What does hypocalcemia cause?
Increased excitability of muscles & muscle tetany (cramps)
What does hypercalcemia cause?
Inhibitionn of neurons and muscle cells and heart arrhythmias
What hormones control calcium balance?
Parathyroid hormone and calcitonin
How does PTH promote increases in calcium levels?
By targeting bones (activates osteoclasts), small intestine (enhances absorption) & kidneys (enhances reabsorption/decreases phosphate reabsorption)
What does calcium reabsorption go hand in hand with?
Phosphate excretion
Where is filtered phosphate actively reabsorbed?
Proximal tubules
What happens when ECF calcium levels are normal or high?
PTH secretion is inhibited, osteoclasts are inhibited, calcium is excreted & phosphate is reabsorbed
