2. salt and water balance Flashcards
Q Describe the distribution of water and sodium in the main fluid compartments
Water makes up 60% of the body water volume
42 L
28 L – intracellular
14 L – extracellular (3L plasma, 11L interstitial space)
Sodium – 140mmol/L extracellular, 5mmol/L intracellular
Potassium (main intracellular ion) – 5mmol/L extracellular, 140mmol/L intracellular
Calcium – 2.25 – 2.5 extracellular
Albumin – 35-50g/L
Sodium chloride, calcium and bicarbonate are the main extracellular ions
K+, Phosphate and magnesium are intracellular
Q Define the term osmolality, oncotic pressure and osmosis.
Osmolarity = the solute concentration (higher amount of solutes, higher Osmolarity) – measured in g/L
The formula for osmolality = (2 x Na+ conc) + urea + glucose
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The formula for osmolality = (2 x Na+ conc) + urea + glucose
The reason for multiplying by 2 is to account for the presence of chloride
Water potential = the opposite
Oncotic pressure = a pulling force on fluid exerted by proteins
Osmotic pressure = the amount of solutes required to prevent osmosis from happening
Osmosis = the passive movement of water from an area of high concentration of water to an area of low water concentration across a partially permeable membrane
Q Explain how low albumin may lead to oedema
Albumin is needed to pull fluid back into the blood vessel via oncotic pressure after the fluid has bathed the surrounding tissue
If albumin levels are low in the blood, then you don’t have a high enough oncotic pressure to pull fluid back into the venule end of the capillary
This means the fluid will remain in the interstitial fluid = odema
Q What are the key routes of water loss from the body? Define insensible losses.
Insensible loss = losses that cannot be measured/occur unconsciously: - Sweating - Exhaled water vapour - Menstrual blood - GI losses Sensible loss – measurable - urine - faeces - sweat Daily section - 2L gastric - 1500ml pancreas - 1500ml salivary glands - 500ml bile - 1500ml small intestine - 6700ml reabsorption in small intestine too
Q Explain water homeostasis with reference to ADH
- Low water volume – high osmolarity (high conc)
- High osmolarity = causes water to move out of the osmoreceptor cells (towards the area of high salt conc) water moves in and out of the osmoreceptor via aquaporin
- This causes the cell to shrink, which causes a generator potential to occur
- This is the initial potential generated in a receptor and it becomes an action potential if it reaches the threshold
- This action potential triggers the suprapoptic nucleus of the hypothalamus to release vasopressin
- This is then secreted from the posterior pituitary to act on the collecting ducts on the kidneys by binding to V2 receptors on the Basolateral membrane of the collecting ducts
- The V2 receptors –
- V2 receptors are G-protein coupled receptors. G-proteins express adenylyl cyclase to convert ADP to cAMP. cAMP then activates protein kinases which:
- Then increase protein synthesis of aquaporins in the kidney tubular cells
- This then increases the number of aquaporin 2 in the apical membrane of the kidney tubule by fusing the vesicle
- On the basolateral membrane water moves through aquaporin 3 and 4 (which are already present)
- So then water reabsorption increases into the blood – to increase blood volume and decrease urine output
- Vasopressin and ADH have the same action but vasopressin is more potent – because it’s also a vasoconstrictor whereas ADH only acts on the kidneys to increase resorption
EQ RAAS SYSTEM
diagram
F Total body water (TBW) is distributed into three compartments: intracellular, interstitial and intravascular. How is the TBW distributed by volume? Assuming a man of 70kg.
d) 28/11/3
F A diagrammatic representation of the relative amounts of intracellular cations and anions is shown. Which of the following are most likely to be the letters Y (most of +ve) and Z (small of -ve)?
a) Na and K
b) Na and lactate
c) K and lactate
d) K and HCO3
e) Na and HCO3
d) K and HCO3
+ve= Mg2+, Ca2+, and???
-ve=Cl-. HPO43-
SO42-, Proteins
F Osmolality can be measured directly in the laboratory. Osmolality can be calculated from blood results. Which of the following is a factor in calculated osmolality?
a) Bicarbonate
b) Urea
c) Chloride
d) Magnesium
e) Phosphate
b) Urea
F Which of the following is an example of insensible loss?
a) Water from respiration
b) Vomiting
c) Diarrhoea
d) Fluid in abdominal surgical drain
e) Urine
a) Water from respiration
F So you examine a patient and note he has lower leg pitting oedema. You also note that his blood albumin level is low. Which statement correctly describes the relationship between albumin and oedema?
d) A low albumin causes a decrease in oncotic pressure and water diffuses from the blood into the interstitial fluid
F What is most likely represented by the x axis on the graph? Y=plasma conc. of ADH, Y is low until X>280, increases proportionally. >295=thirst
a) Blood pressure
b) Renin
c) Sodium concentration
d) Plasma osmolality
e) Potassium concentration
d) Plasma osmolality
F What is the site of synthesis of ADH?
a) Adrenal cortex
b) Distal tubules and collecting ducts of the nephron
c) Juxtaglomerular cells
d) Posterior pituitary
e) Hypothalamus
f) Anterior pituitary
e) Hypothalamus
9) What is the site of synthesis of aldosterone?
a) Adrenal cortex
b) Distal tubules and collecting ducts of the nephron
c) Juxtaglomerular cells
d) Posterior pituitary
e) Hypothalamus
f) Anterior pituitary
a) Adrenal cortex
F What is the principal site of renin production?
a) Adrenal cortex
b) Distal tubules and collecting ducts of the nephron
c) Juxtaglomerular cells
d) Posterior pituitary
e) Hypothalamus
f) Anterior pituitary
c) Juxtaglomerular cells
