Body water - distribution and regulation Flashcards
Where is water reabsorbed in the nephron?
There are three important places where water is reabsorbed in the nephron
Proximal convoluted tubule: reabsorbs 67% of all water filtered
Descending limb: reabsorbs 25% of water
Collecting duct: 2-8% of water
<1-6% of filtered load is excreted
What is bulk/obligatory water reabsorption?
Accounts for 92% of water reabsorption, obligatory as it has to happen (automatic process), epithelia in these parts of nephron is leaky epithelia meaning water can be reabsorbed paracellulary or transcellularly
What is regulated/facultative water reabsorption?
Facultative as it is optional, accounts for more variable 2-8% water reabsorption depending on what body needs, regulated under hormonal control by ADH (the more ADH you have the less urine you make). Tight epithelia so water can’t be reabsorbed between cells, only via transcellular pathway
Where is sodium reabsorbed in the nephron?
Sodium reabsorbed in four places in the nephron
PCT reabsorbs 67%, ascending limb reabsorbs 25%, distal convoluted tubule reabsorbs 5% and collecting duct reabsorbs 2-3%
<1% of filtered load is excreted
What is bulk and regulated reabsorption for sodium?
Bulk reabsorption: accounts for 92% of total sodium reabsorption
Regulated reabsorption: accounts for 7-8% of total sodium reabsorption, regulated by aldosterone (RAAS)
Reabsorption of water in proximal convoluted tubule
Water reabsorption in the proximal tubule (67%) is driven by Na+ reabsorption. Transporters such as the sodium glucose co-transporter using the sodium gradient to reabsorb solutes
Glucose and sodium transported through proximal tubule cells, chloride follows via the paracellular pathway, water also follows by the paracellular and transcellular (via aquaporins) pathways
Water reabsorption in the nephron loop
Descending limb is permeable to water as it has aquaporins and is leaky epithelium so H2O is reabsorbed
Ascending limb is permeable to sodium, does not have aquaporins (water can’t be reabsorbed) but does have ion transporters
Water reabsorption in juxtamedullary nephorns
In the juxtamedullary nephrons (loops that go deep into medulla) there are different permeabilities of the descending and ascending parts of JMNs loop allows us to generate the hyper-osmotic medullary gradient
Deeper you go into the medulla = higher osmolality , further down the collecting duct, higher osmolality outside of it
How is the body osmolarity regulated when we lose water?
When we lose water (decrease TBW) osmolarity of ECF increases, osmoreceptors detect this and increase secretion of ADH from posterior pituitary (anti-peeing hormone)
ADH is detected by ADH receptors and aquaporins are inserted into the apical membrane of CD cells, increases water permeability, increases water reabsorption
How is the body osmolarity regulated when we gain water?
When we drink too much water (increase TBW) osmolarity decreases, osmoreceptors detect this and decrease secretion of ADH, this removes aquaporins in apical membrane and decreases water permeability hence decrease in water reabsorption and increase in urine volume
How is the body osmolarity regulated when we gain isosmotic fluid?
Increase in isosmotic fluid (more fluid in ECF but osmolarity normal) activates cardiac muscle cells which releases ANP. ANP makes kidneys reabsorb less sodium and less water, hence we pee out more iso-osmotic fluid
How is the body osmolarity regulated when we lose isosmotic fluid?
Decrease in isosmotic fluid (less fluid in ECF but osmolarity normal) activates receptors in kidneys which activate RAAS leading to increased release of aldosterone from adrenal gland and some ADH. Increased sodium channels in apical membrane of DCT or CD, increases sodium reabsorption, ADH increases water reabsorption, hence replacing isosmotic fluid we lost, peeing out less water and less sodium
Composition of normal urine
95-98% of urine is water
Also composed of creatinine from muscle metabolism, urea from amino acid breakdown, urine acid from purine breakdown, H+, Na+, K+, medications and toxins
Clear, to light or dark amber in colour, pH dependent on diet (4.6-8.0), unremarkable smell
Composition of pathological urine
Can be composed of glucose, protein (especially albumin), red blood cells/erthyrocytes, haemoglobin, white blood cells/leucocytes and bacteria
Orange, red, brown, blue/green in colour
Tastes sweet if glucose present
Smells fruity if ketones present or rotten if infection or tumour present
