Regulation of urine volume and fluid balance Flashcards
What are the signs and symptoms of hypovolaemia?
Symptoms - thirst, dizziness on standing, confusion
Signs - low JVP, postural hypotension, weight loss, dry mucous membranes, reduced skin turgor, reduced urine output
What are the signs and symptoms of hypervolaemia?
Symptoms - ankle swelling, breathlessness
Signs - raised JVP, oedema (peripheral +/- pulmonary), weight gain, hypertension
What ions mainly control ECF osmolarity?
80% total osmolarity of ECF = NaCl-
What ion mainly controls ICF osmolarity?
~50% of total osmolarity of ICF = K+
What are the inputs and outputs of total body water?
Inputs: drinking, food, metabolism
Outputs: lungs, skin, faeces, urine
How do you calculate osmoses excreted/day?
Osmoses excreted/day (Osm) = urine osmolarity (Osm/L) x urine output (L/day)
Define obligatory urine volume
Obligatory urine volume is the minimum urine volume of urine that needs to be produced each day to excrete waste solutes
What things does obligatory urine volume depend on?
1) Maximum urinary concentrating ability of the kidneys (typically 1200mOsm/L)
2) Amount of solutes that need excreting (typically 600 mOsm)
Therefore, typical obligatory urine volume = 0.5L / day
(600 mOsm / 1200 mOsm/L = 0.5 L)
What causes polyuria?
Increased water excretion:
- Excessive water ingestion
- Inability to concentrate urine (tubular damage, diabetes insipidus)
Increased solute excretion:
- Diuretics (or failure to reabsorb sodium)
- Glycosuria (diabetes mellitus)
What causes oliguria?
Decreased water/solute excretion:
- Dehydration/low extracellular volume
- Poor renal perfusion
How is urine concentrated in the collecting duct?
1) Insertion of water channels (aquaporins) - regulated by antidiuretic hormone (ADH)
2) An osmotic gradient - generated by the countercurrent system in loop of Henle
Where is ADH produced?
Produced in supraoptic & paraventricular nuclei of hypothalamus
Transported to the posterior pituitary where it is packaged into storage granules & released by exocytosis
Short plasma half life.
What are the two functions of ADH?
Reduce water excretion (antidiuretic)
Stimulate vasoconstriction
What stimulates ADH release?
Raised plasma osmolarity (hypertonicity) is the main stimulus for ADH release
Hypovolaemia / low blood pressure (triggering release of angiotensin II)
Nausea (‘precaution’ for vomiting & fluid loss)
Drugs (e.g. morphine, nicotine)
What inhibits ADH release?
Reduced plasma osmolarity (hypotonicity)
Hypervolaemia / raised blood pressure
Drugs (e.g. alcohol)
What is the cellular mechanism of ADH?
1) ADH binds to vasopressin 2 receptor
2) cAMP is produced
3) cAMP activates PKA
4) PKA phosphorylates proteins
5) The phosphorylated proteins increase aquaporin synthesis in the nucleus and cause vesicles with inactive water channels to fuse with the luminal membrane
Define the counter current multiplier mechanism and what are the 4 features that contribute to it?
Counter current multiplier mechanism is method by which medullary interstitium is concentrated.
1) Hairpin arrangement of Loop of Henle with both limbs close to each other
2) Fluid travelling in opposite directions in the two limbs
3) Different water permeabilities of the two limbs
4) Ability of the Na / K / 2Cl transporter to actively transport solutes against a concentration gradient
What is the result of the counter current multiplier mechanism?
1) A dilute filtrate entering the distal nephron (allows water to move out of tubule to the circulation by osmosis)
2) Generates a large increase in [NaCl] in the medulla, so contributing to the increase in the osmotic gradient (which allows the movement of water)
What are the vasa recta?
Vasa recta are long thin walled (specialised) peritubular capillaries that parallel the loops of Henle in juxtamedullary nephrons.
For the countercurrent multiplier to work the salt released from the loop of Henle must remain in the medulla, while most of the water is removed.
Low blood flow ~5-10% renal blood flow (just enough to provide nutrients) without ‘washing out’ solutes in medulla.
