kidney physiology Flashcards
water and electrolyte homeostasis
- intake/loss must be in a balance (over any significant period)
- problems: haemorrhage, unusual eating/drinking
typical intakes/outputs
Intake: water ~1200 ml, food ~1000ml, metabolic ~300 ml. Total ~2.5L
Total ~2.5L/day
Output: urine ~1500 ml, sweat ~100ml, faeces ~200ml, insensible loss ~700 ml (included respiratory water loss)
total ~2.5l
salt intakes/outputs
- Uk, no more than 6g
renal blood flow
At rest 25% of cardiac output supplies about 200g of tissue 1 so about 625 ml/100g/min.
This is a much larger flow that kidney metabolism requires. Flow supplies tissue metabolism and is filtered.
The blood pressure in glomerular capillaries is high 50-60mmHg. Renal artery is short and relatively large radius. The sequence of blood vessels are also unusual: afferent arteriole → glomerular capillaries → efferent arteriole → tubular capillaries → venule
nephron
Each nephron is a tube.
The nephron wall is a continuous layer of epithelium.
Nephron: 4 sectionsa. Proximal Convoluted Tubuleb. Loopc. Distal Convoluted Tubule d. Collecting Duct.
The cell shapes in the wall are very different. This reflects activity: surface area, ion pumping etc.
ultrafiltration
Ultrafiltration (driven by blood pressure in glomerular capillaries)
High renal blood flow, (25% of resting cardiac output, ~ 1.25 L/min of blood, ~650 ml/min of renal plasma flow), High filtration rate (90-140 ml/min)
reabsorption
Reabsorption (active pumping from filtrate in tubules) For substances to be retained: water, glucose, amino acids, electrolytes…
secretion
Secretion (active pumping into tubules) For substances to be eliminated faster than filtration alone allows: e..g. H+, ammonia, uric acid, some drugs……
pumping rates
Pumping rates can be controlled by hormones. For example, aldosterone can adjust the rates of Na+ and K + excretion.
glomerulus utrafiltration
High glomerular capillary pressure: 55mmHg
Filtration of water and small molecules through slits between podocytes. (ions, urea, glucose, amino acids, small proteins etc, Cut off at ~ 67KD. Hb?)
Limit is the space between podocyte processes.
Normal Glomerular Filtration rate: 90-140 ml/min
proximal convoluted tubule active reabsorption
Brush border
Active reabsorption of glucose, amino acids, Na + and K+ ions etc.
Co-transporters, aqueous channels, membrane pumps.
Substantial water reabsorption
By the end of PCT
Complete reabsorption of glucose, amino acids
Substantial reabsorption of Na+ and water
Volume of filtrate reduced by 2/3rds
loop of Henle, counter-current concentration
Thinner wall during descent into the medulla
Thicker wall during ascent from medulla
Thicker wall on ascent, active pumping out of tubule.
Solute diffuses into descending tubule: Counter-current mechanism ‘recycles’ solutes.
Ion pumping develops high osmotic pressure at the tip of the loop.
No net re-absorption here.
distal convoluted tubule
Similar structure and function to Proximal Tubule. Compare A and C. No need for glucose transporters
Less intense electrolytes and water re-absorption
DCT ion pumping can be controlled by hormones like aldosterone to ‘fine tune’ Na + and K + exchange. .
collecting duct, concentration of urine
CDs pass close to tips of Loop
If CDs are permeable to water, then moves out of the duct to concentrate filtrate.
Duct permeability set by ADH/AVP.
renal glomerular filtration system
Each human kidney contains ~1,000,000 glomeruli. An afferent arteriole branches into capillaries (glomerular tuft), the walls of which constitute the glomerular filter.
The plasma filtrate, the primary urine, is led to the proximal tubules while unfiltrated blood is returned to the blood circulation.
The filtration barrier contains fenestrated endothelial cells, the glomerular basement membrane (GBM), and podocytes with their interdigitating foot processes. The slit diaphragm is uniformly wide, porous filter structure containing specific components.
