Reabsorption and Secretion Flashcards
Describe the Starling’s forces in the peritubular capillaries
Ppc very low as hydrostatic pressure is overcoming frictional resistance in efferent arterioles
IIp (oncotic) high compared to normal as loss of 20% plasma conc.
IIp > Ppc so favours reabsorption
Explain substances reabsorbed by carrier mediated transport systems
Carriers have a maximum transport capacity - Tm which is due to saturation of carriers
If Tm exceeded then substrate enters urine
Ex. glucose, aa, organic acids, sulphate and phosphate ions
What is renal threshold?
Plasma threshold at which saturation occurs
What do carrier proteins enable?
Larger molecules such as glucose to cross the membrane but capacity is limited by number of carriers
Explain the titration curve of glucose
Glucose is freely filtered so whatever it’s plasma conc. is how much is filtered
Plasma glucose can go up to 10mmoles/l - renal threshold for glucose
Beyond this then it appears in urine
What is responsible for the regulation of glucose?
Insulin and counter regulatory hormone responsible
Kidneys do not regulate
Why is Tm for glucose set so high?
All valuable nutrient is normally reabsorbed
Normal glucose conc. is 5mmoles/l
What is the reason for appearance of glucose in urine in a diabetic?
Failure of insulin and not the kidney
What substrates does the kidney regulate by means of the Tm mechanism?
Sulphate and phosphate ions
Tm is set at level of normal plasma conc. causes saturation
Anything above is excreted - plasma regulation
Also, PTH decreases reabsorption of phosphate ions
How is Na reabsorbed?
By active transport - not Tm mechanism
This establishes a gradient for Na ions across the tubule wall
How much Na is reabsorbed each day?
99.5% is reabsorbed
25560mmoles/day
65-75% in proximal tubule
Explain active Na+ pumps
Located on basolateral surfaces where there is a high density of mitochondria
This decreases Na in epithelial cells which increases Na gradient to move ions into cells passively across luminal membrane
How does Na passively move into cell if not permeable at cell membranes?
Brush border of proximal tubule cells has a higher permeability for Na ions than other membranes in the body - due to massive surface area and Na channels
What is the reabsorption of Na ions the key to?
Reabsorption of other components of the filtrate
Explain the passive movement of Cl- ions across tubular membrane
Negative ions like Cl diffuse passively down the electrical gradient established and maintained by active transport of Na ions
Describe the osmotic force in tubule
Active transport of Na out of tubule followed by Cl creates an osmotic force which draws H2O out of tubule
H2O removed by osmosis
What happens to fluid in lumen after reabsorption of Na, anions, and H2O?
Conc. of other solutes increase as fluid volume in lumen decreases - creates outgoing conc. gradients
Permeable solutes are reabsorbed by diffusion
What is the rate of reabsorption of non-actively reabsorbed solutes dependent on?
Amount of H2O removed which determines the extent of conc. gradient
Permeability of membrane to particular solute
Is the tubular membrane permeable to urea?
Moderately - 50% reabsorbed and remainder stays in tubule
What substances are impermeable to the tubular membrane?
Insulin and mannitol
What could decrease active transport of Na?
Decreased blood flow can disrupt renal function
Active transport establishes the gradients down with other ions
How is active transport of Na ions important for carrier mediated transport systems?
Substances such as glucose and aa share the same carrier molecules as Na (symport)
High Na in tubule facilitates and low Na conc. in tubule inhibits glucose transport
Describe the SGLT - sodium dependant glucose transporter
Na moving down its electrochemical gradient using the SGLT protein pulls glucose into the cell against conc.
Glucose diffuses out basolateral membrane using GLUT protein into ISF
Na is pumped out to ISF by Na/K/ATPase
What is tubular secretion?
Third renal process
Secretory mechanisms transport substances from peritubular capillaries into the tubule lumen and provide second route into tubule
What substances are secreted at proximal tubule?
Important for substances which are protein bound as filtration in glomerulus is restricted
Also for harmful substances are eliminated
Tm limited carrier mediated secretory mechanisms
What is the effect of carrier mechanism not being specific?
Ex. organic acid mechanism which secretes lactic acid and uric acid can also be used for substances like penicillin, aspirin and PAH
Organic base mechanism can be used for morphine and atropine
How is the amount of solute excreted calculated?
Amount filtered - amount reabsorbed + amount secreted
What is normal ECF K+ conc.?
Around 4mmoles/l
Maintenance of K+ balance is essential for life
What happens if K+ conc. is up to 5.5mmoles/l?
Hyperkalaemia so decreased resting membrane potential of excitable cells and eventually ventricular fibrillation and death
What happens if K+ conc. is lower than 3.5mmoles/l?
Hypokalaemia - increases resting membrane potential so hyperpolarises muscle, cardiac cells so cardiac arrhythmias and eventually death
How is renal handling of K+ complex?
K+ is filtered at glomerulus is reabsorbed primarily at proximal tubule
Changes in K+ excretion are due to secretion in distal parts of tubule
What does increased renal tubule cell K+ conc. do?
Due to increased ingestion of K+
This causes K+ secretion while any decrease in intracellular K+ conc. causes reduced secretion
What is K+ secretion also regulated by?
Adrenal cortical hormone - aldosterone
What does an increase of K+ conc. cause to aldosterone?
K+ in ECF then stimulates the aldosterone secreting cells which cause aldosterone release which circulates to kidneys and stimulates increase in renal tubule cell K+ secretion
What does aldosterone also stimulate?
Na reabsorption in distal tubule but by a different reflex pathway
Describe H+ secretion
H+ ions are actively secreted from tubule cells into lumen
Explain a summary of K+ regulation by aldosterone
Increased plasma K+ - stimulates release of aldosterone from adrenal cortex - K+ excreted so K+ loss
