Water And Homeostasis Flashcards
How much of the body’s weight is water and where is this water divided between?
60% of the body’s weight is water
40% of its weight is ICF (the fluid found in cells)
20% of its weight is ECF - 80% of this is interstital (fluid found surrounding cells) and 20% is intravascular (in the blood)
What is the concentration of ions in the ECF
NA+ are the main factor to osmolality and volume - 140mmol/l
Low levels of K+ = 10mmol/l
Anions Cl- and HC03- found in ECF
Glucose, urea and proteins also effect osmolality
What is the ICF osmolality factors?
High K+ concentration = 140mmol/l
Low Na+ = 10mmol/l
Similar other ions to the ECF
Will the ICF and ECF always be isotonic
Yes they water will also flow between to make the two fluids isotonic
Plasma osmolality formula and what happens when it changes?
Plasma osmolality = 2(Na) + 2(K) + urea + glucose in mmol/l
X 2 ions to make up for associated anions
When it changes water will be pushed or pulled across cell membranes
Water are the 2 types of water loss and give examples of each:
Sensible - fluid loss you can measure e.g. urination, faces, vomit
Insensible - fluid loss you can not measure e.g. sweating, breathing
What fluid is usually given intravenously?
Most commonly glucose or a saline solution is given intravenously. Water can not be given due to its low osmolality it would be absorbed by blood cells and cause them to burst = haemolysis
Note protein is also too large a molecule to be given intravenously
What is the normal ECF osmolality?
275-295 mmol/Kg
Describe the events after water deprivation:
Increased ECF osmolality is detected by osmoreceptors in the hypothalamus, which then causes the release of ADH from the posterior pituitary gland. There is also movement of water from the ICF to the ECF and stimulation of the thirst centre in the hypothalamus, which leads to more water consumed. The ADH travels in the blood to the kidneys where they attach to specific receptors on the collecting duct and cause aquaporins to fuse. These aquaporins allow water to be reabsorbed via the collecting duct. Hence ECF osmolality returns to normal.
Describe the renin-angiotensin-aldosterone system:
The kidneys detect a fall in the ECF volume (could be due to a low NA ion concentration) and release renin. Renin acts on angiotensin (produced by the liver) to produce angiotensin I. ACE is produced by the units and acts on angiotensin to produce angiotensin II. Angiotensin II cause increased sympathetic activity and also acts on the adrenal glands causing release of Aldosterone. This acts on the distal tubule of the kidney and increases absorption of NA ions and water and excretion of K ions and hence increases ECF volume and also leads to more NA ions.
Slow response compared to ADH
Describe the sequence of events after over hydration:
Decreased ECF osmolality is detected by the osmoreceptors in the hypothalamus and ADH is not produced by the posterior pituitary gland. The hypothalamus does also not stimulate the thirst centre.
This results in less water being reabsorbed as the collecting duct is not permeable. Hence a large volume of dilute urine is produced. This helps to decrease the excess fluid.
Describe the dangers of over hydration:
Low Na concentration — hyponatremia - cerebral over hydration — nausea and vomiting.
Cerebral overhydration - brain cells are squeezed against the skull.
Heart failure
Kidney damage
Signs of dehydration:
Thirst, dry moth, swollen eyes, hypotension, inelastic skin
Oedema, what it is and what are its causes?
Oedema is excess fluid in the interstital space.
Lymphatic oedema - when the lymphatic system becomes blocked and hence the fluid in the interstital space builds up.
Hypoalbuminaemic oedema - low plasma albumin - lower oncotic pressure can reabsorb all water from venous end.
Inflammatory oedema - increase in size of epithelial pores in capillary, this allows albumin escapes leading to decreased oncotic pressure to reabsorb at venous end — build up
Venous oedema - there’s a blockage in the vein, build up in pressure of the vein at venous end, this back pressure means less interstital fluid is reabsorbed leading to a build up.
What is hydrostatic pressure?
What is oncotic pressure?
Pressure difference between plasma and interstital fluid water.
Water moves from plasma to interstital fluid due to higher hydrostatic pressure in the blood.
Pressure caused by the difference in protein concentration between plasma and interstital fluid.
Due to protein in blood (albumin) it increases its osmolality and hence water will move by osmosis from the interstital fluid to the plasma.
