Lecture 31: Sodium and Water Homeostasis Flashcards
What percent of the brain is made up by water?
75%
What percent of the blood is made up by water?
83%
What percent of the heart is made up by water?
79%
What percent of the bones is made up by water?
22%
What percent of the muscles is made up by water?
75%
What percent of the Liver is made up by water?
85%
What percent of the kidneys is made up by water?
83%
Give three factors that affect body water distribution:
1) age
2) gender
3) fat content
What percentage of weight does total body water make up in males?
60%
What is the average total body water in a 70kg male?
42L
What fraction of total body water is intracellular fluid?
2/3
What fraction of total body water is extracellular fluid?
1/3
What fraction of extracellular fluid is plasma?
1/4
What fraction of extracellular fluid is extravascular fluid?
3/4
What is hydrostatic pressure?
the pressure exerted by the fluid against the membrane
What is osmosis?
The movement of water across a semipermeable membrane from low solute to high solute
What is osmotic pressure?
The ability of a solution to cause osmosis measured in mmHg
What is oncotic pressure?
The osmotic effect caused by large colloid particles/ protein such as albumin
Describe the distributive effects of water IV:
has a hydrostatic effect and is evenly distributed in intracellular, extracellular and intravascular compartments
Describe the distributive effects of saline IV:
has osmotic effect and is distributed in interstitial and vascular fluid
What percent of body weight in females is attributable to water?
55%
Describe the distributive effects of colloid IV:
has an oncotic effect and is distributed in intravascular fluid only
What are the two types of passive transport?
1) diffusion
2) fascilitated diffusion
What is facilitated diffusion?
Movement of specific molecules across cell membranes through protein channels down a concentration gradient
What is active transport?
movement against a concentration gradient requiring ATP and specialised transport proteins
What ion does water follow?
Na+
What other solute might water follow?
Glucose
Is sodium found more abundantly is extracellular or intracellular fluid?
extracellular fluid
Is potassium found more abundantly is extracellular or intracellular fluid?
Intracellular fluid
What is the name of a protein transporter that transports ions/ molecules in the same direction?
cotransporter
What is the name of a protein transporter that transports ions/ molecules in the opposite direction?
Exchanger
Describe the SGLT protein as an example as a cotransporter:
it moves both 2Na+ and one glucose molecule across the membrane
Describe the sodium-calcium exchanger:
it moves 3Na+ in one direction for every Ca2+ pumped in the opposite direction
Define osmolality:
The number of osmoles of solute particles per unit Kg (Weight of solvent)
What are the units of osmolality?
mmol/Kg
Define osmolarity:
the number of osmoles of solute particles per unit litre (volume of solution)
What are the units of osmolarity?
mmol/L
True or false: osmolality can be measured directly
True
True or false: osmolarity can be measured directly
false (it has to be calculated)
How is osmolarity calculated?
by adding all electrolytes and molecules that have an effect on water movement: 2x(Na+K) + urea + glucose (if abnormal)
How is the osmolar gap calculated?
measured osmolality - calculated osmolarity
What does an osmolar gap greater than 10 suggest?
the presence of other osmotically active solutes in the plasma that perhaps should not be there
What is a hypertonic solution?
A solution with a higher solute concentration than the cell so water moves out of the cell and into the solution
What is a hypotonic solution?
a solution with a lower solute concentration than the cell so water moves into the cell
What is an isotonic solution?
a solution contains equal concentrations of solutes to a neighbouring solution resulting in equilibrium of movement
Give three general ways in which water is lost:
1) insensible losses (500 ml)
2) GI loss (100 ml)
3) urinary loss (1.5-2.5L)
Give two examples of insensible losses of water:
1) sweat
2) respiration
True or false: where sodium goes, H2O will ALWAYS follow
True
What is the rule about water and osmolality?
H2O WILL MOVE FROM AN AREA OF LOW OSMOLALITY TO AN AREA OF HIGH OSMOLALITY
Describe how osmoreceptors help control water balance:
they detect changes in osmolality in extracellular fluid
Which are more sensitive in the control of water balance, baroreceptors or osmoreceptors?
Osmoreceptors
Describe how baroreceptors help control water balance:
they detect changes in intravascular volume
Give three locations that baroreceptors are located in:
1) carotid sinus
2) main veins
3) right atrium
When intracellular fluid is equal to extracellular fluid, what does osmolality equal?
285mmol/kg
What control structures control the release of ADH?
hypothalamic osmoreceptors
What is another name for anti-dieretic hormone?
arginine vasopressin (AVN)
What structures produce ADH?
Supraoptic and paraventricular nuclei
Where is ADH stored?
posterior pituitary gland
Give two stimuli that alter stimulate ADH release:
1) shrinkage caused by loss of extracellular fluid
2) hypovolaemia (more potent stimulus)
How does ADH increase water uptake?
it acts on the collecting duct to increase the secretion of aquaporin-2
What effect does mercury have on aquaporin expression?
inhibition
What 7 things is ADH secreted in response to?
- Hypovolaemia
- Hyperosmolality
- Stress
- Pain
- Trauma
- Drugs
- Nausea
What effect does lithium have on aqauporin expression?
decreased
What is the single most powerful non-osmotic stimulus of ADH?
Nausea
What triggers thirst?
loss of extracellular fluid (but not intracellular fluid)
Where does most (80%) of sodium reabsorption take place?
proximal tubule of the kidney
Where does ‘fine tuning’ of sodium reabsorption take place?
distal convoluted tubules
What hormone converts angiotensinogen to angiotensin I?
Renin
What hormone converts angiotensin I to angiotensin II in the lungs?
ACE
What stimulates renin release from kidneys?
systemic volume contraction
Describe how aldosterone helps with Na+ uptake in the collecting duct of the kidney?
1) it stimulates the Na+/K+ ATPase transporter of collecting duct cells to generate intracellular electronegativity by pumping 3Na+ out for 2K+ in
2) the electrogenic Na+ channel on the apical boarder of the collecting duct cell allows Na+ entry from the lumen
3) K+ is secreted simultaneously to balance charge
What two effects do natriuretic peptides cause?
1) vasodilation
2) increased loss of urinary sodium and water
When are natriuretic peptides released?
When myocytes are mechanically stretched by increased plasma volume
Give three examples of natruretic peptides:
1) atrial natriuretic peptide (ANP)
2) brain natriuretic peptide (BNP)
3) c-type natriuretic peptide (CNP)
What is hyponatraemia?
where sodium levels are found below the normal range of 133-146mmol/L
Give 8 symptoms of hyponatraemia: (SALT LOSS)
1) stupor (coma)
2) anorexia
3) lethargy
4) tendon reflexes decreased
5) limp muscles
6) orthostatic hypotension
7) seizures and headache
8) stomach cramping
What diseases may present is hypovolaemia presents with hyponatraemia? (3)
1) heart failure
2) renal disorders
3) liver cirrhosis
Give two causes of pseudohyponatraemia
1) hyperproteinaemia
2) hypertriglyceridemia
(both cause water imbalances)
Give three causes of syndrome of inappropriate ADH (SIADH)?
1) malignant disease
2) neurological disorders
3) pulmonary disorders
Give two treatments used for SIADH:
1) fluid restriction
2) demeclocycline (ADH antagonist)
Give 3 presentations of SIADH:
1) hyponatraemia
2) high sodium in urine
3) normal renal function
Give 6 fluids that Na+ can be lost from:
1) gastric juice
2) pancreatic juice
3) bile
4) ileal fluid
5) stool water
6) sweat
What is hypernatraemia?
sodium levels found above the normal range of 133-146mmol/L
Give 9 symptoms of hypernatraemia: FRIED SALT
1) flushed skin
2) restless, anxious
3) increased blood pressure
4) edema (peripheral and pitting)
5) decreased urine output
6) skin flushed
7) agitation
8) low grade fever
9) thirst
How does Addison’s disease affect sodium levels?
the hypoaldosteronism causes sodium loss
How does Conn’s syndrome affect sodium levels?
the primary hyperaldosteronism causes Na+ excess
What is diabetes insipidus?
a disease where there is no effective ADH meaning no aquaporin uptake and subsequent water loss
Give 2 general causes of diabetes insipidus:
1) pituitary or hypothalamus damage
2) issues with ADH receptors or aquaporins
What test is used to check for diabetes insipidus?
water deprivation test
What is the water deprivation test?
where a patient has restricted fluids for 12 hours while serum osmolarity and urine volume is regularly measured
