Reabsorption and Secretion Flashcards
Filtration without resorption takes place in which regions of the nephron?
Glomerular capillaries
Why is pressure in the peritubular capillaries low?
Hydrostatic pressure overcoming frictional resistance in the efferent arterioles
Why is the oncotic pressure high in the peritubular capillaries?
Loss of 20% of plasma concentration causing a relative increase in concentration of plasma proteins
Starling forces in the peritubular capillaries favour what?
Reabsorption
What % of urea filtered at the glomerulus is reabsorbed in the tubule?
50%
What % of glucose filtered at the glomerulus is reabsorbed in the tubule?
100% (normally)
What % of water filtered at the glomerulus is reabsorbed in the tubule?
99%
What % of Na+ filtered at the glomerulus is reabsorbed in the tubule?
99.5%
What are the mechanisms of resorption?
Carrier mediated transport systems
What determines the maximum transport capacity of a substrate?
Level of Saturation of the carriers
If carriers are saturated, what happens to excess substrates?
The excess enters the urine
What happens when a substrate binds with a carrier protein?
The carrier protein changes shape and the substrate leaves
What is the renal threshold?
Plasma threshold at which saturation occurs
All glucose will be reabsorbed with glucose levels up to what?
<10mmoles/l
What happens when plasma glucose >10mmoles/l?
Excess over 10 appears in the urine
How much glucose will be secreted if [glucose] is 17.5mmoles/l?
7.5mmoles
What is normal plasma [glucose]?
5mmoles/l
Glycosuria is due to a failure in what?
INSULIN
What is the maximum transport capacity (Tm) of amino acids?
High - regulated by insulin
Why is the maximum transport capacity (Tm) of glucose so high?
Because non-diabetic patients should never exceed 10mmoles/l
What is the maximum transport capacity (Tm) of glucose?
10mmoles/l
How does maximum transport capacity (Tm) function to regulate substance concentrations?
Tm is set at a level where normal [plasma] causes saturation
What proportion of Na+ reabsorption occurs in the proximal tubule?
65-75%
How is Na+ reabsorbed?
Active transport
ATP pumps
Na+ ATP pumps are located where?
Basolateral surfaces of the tubule
How do Na+ ions enter the tubule cell from the lumen?
Passively down the concentration gradient
How do Na+ ions interstitial fluid from the tubule cell?
Na+K+ ATP pumps
Up the concentration gradient
Why is Na+ able to penetrate the brush border membrane of the PCT so easily?
Microvilli on the surface and large number of Na+ ion channels
How do negative ions cross the proximal tubular membrane?
Follow the electrical gradient established by the active transport of Na+
What is the net effect of Na+ and Cl- leaving the tubule?
Osmotic force - water follows the ions out
What is the net effect of water following ions out of the tubule?
Concentration of the substances left in the tubule
How is Na+ reabsorbed?
Active transport
How is H2O reabsorbed?
Osmosis
How are permeable solutes reabsorbed?
Diffusion
The rate of reabsorption of non-actively reabsorbed solutes depends on what?
Amount of H2O removed - determines extent of concentration gdt
Permeability of the membrane to the solute
How permeable to urea is the tubule membrane?
Moderately
What process establishes the gradient for reabsorption?
Active transport of Sodium in the basolateral membrane
Other than the generation of an osmotic gradient, why is the active transport of Na+ in the tubules important?
Na+ is essential for carrier mediated transport systems
What is the effect of high tubule [Na+] on glucose?
Facilitated glucose reabsorption
What is the effect of low tubule [Na+] on glucose?
Inhibition of glucose reabsorption
Na+ reabsorption is linked to the reabsorption of what?
HCO3-
Glucose
Amino acids
What transporter is used to take glucose from lumen → cell?
Sodium dependent glucose transporter
What transporter is used to take glucose from cell → ISF?
GLUT facilitated diffusion
What transporter is used to take Na+ from cell → ISF?
Na+/K+ ATPase
How do protein-bound substances enter the lumen?
Peritubular capillaries → lumen
Tm-limited carrier mediated secretory mechanisms
The Organic Acid mechanism is used to excrete what?
Lactic/uric acid
Penicillin
Aspirin
Para-amino-hippuric acid
Why are carrier mechanisms able to excrete exogenous substances?
They are not specific
The Organic base mechanism is used to excrete what?
Choline, Creatinine
Morphine
Atropine
Protein-bound substances are secreted where?
Proximal convoluted tubule
What is the normal ECF [K+]?
4mmoles
Hyperkalemia is defined as what?
> 5.5mmoles/l
Hypokalemia is defined as what?
<3.5mmoles/l
How does hyperkalaemia cause injury?
Decrease in resting membrane potential –> ventricular fibrillation
How does hypokalemia cause injury?
Increased resting membrane potential
Hyperpolarised muscle
Cardiac arrhythmia
Where is the majority of K+ reabsorbed?
Proximal tubule
Changes in K+ excretion are due to what?
Changes in secretion in the distal parts of the tubule
Increased renal tubule cell [K+] will cause what?
Increased K+ secretion
K+ secretion is regulated by what?
Adrenal cortex (Zona glomerulosa) ALDOSTERONE
Increased Aldosterone causes what?
Stimulation to increase renal tubule cell K+ secretion
Na+ reabsorption at distal tubule
H+ ions are secreted by what?
Actively secreted into the lumen by TUBULE CELLS
Increased K+ causes what?
Increased plasma [K+]
Increased aldosterone
Increased K+ secretion
Decreased [K+]
