Reabsorption of sodium along the tubule Flashcards
Where is glucose reabsorbed?
- All glucose is reabsorbed at proximal convoluted tubule
How is glucose reabsorbed?
- Secondary active transport
- Driven by energy released from Na+ down its concentration gradient
- Na+/K+ ATPase creates a low conc of Na+ within the cells
- More Na+ therefore diffuses into cell
- Glucose is transported alongside Na+
What is Tm?
- Maximum tubular resorptive capacity for a solute
Why does glycosuria develop?
- There is a limited number of Na+/glucose carriers, so resorption of glucose is limited
- When plasma glucose rises above 10mmol/L, there are not enough transporters to remove all the glucose from the filtrate
- This results in glycosuria
Which demographic is glycosuria common in?
- Pregnant women
- Tm for glucose falls and glucose is excreted into urine
How are amino acids reabsorbed?
- Filter easily through glomerulus
- Reabsorbed in PCT by secondary active transport
- Symport with Na+ (driven by Na+/K+ ATPase)
- Tm limited process
Outline how sodium is reabsorbed in the PCT
- Na+/K+ ATPase - 3Na+ is pumped into blood and 2 K+ brought into cell
- Lots of mitochondria generate energy for ATPase
- Co-transport with glucose, amino acids, phosphates, carboxylic acids
What ions are reabsorbed at the PCT?
- Na+
- Cl- diffuses via Cl-/base exchanger
- HCO3- and H+
- Water moves through aquaporins
Outline how HCO3- is reabsorbed at the PCT
- HCO3- combines with H+ to form H2CO3
- H2CO3 splits to form CO2 and H2O
- CO2 and H2O diffuse into cell
- Carbonic anhydrase converts them back to H2CO3
- H2CO3 dissociates back to H+ and HCO3-
- HCO3- resorption into blood is coupled with Na+
- Na+ is removed to balance charge of H+
What transporters are present in the cells surrounding the loop of Henle?
- Na+/K+/2Cl- co-transporter - bring these ions out of lumen and into cell
- Na+/K+ ATPase
- ROMK
- Cl- channels
What is the function of the ROMK channels in the loop of Henle?
- Allow some K+ to trickle back into loop of Henle
- This allows K+ to still be used in the Na+/K+/2Cl- transporter
- Required because K+ has a lower conc than the other 2 ions
What allows electrolytes such as Mg+ and Ca2+ to be reabsorbed from the loop of Henle?
- Transport of Na+, K+ and 2Cl- makes lumen of loop of Henle more positive
- Positively charged ions are repelled out of the lumen and between tight junctions
- They are then reabsorbed
Outline the movement of ions at the distal convoluted tubule
- Na+/Cl- transporter moves ions out of DCT
- ENac channels allow active resorption of Na+
-Ca2+ resorbed - Na+/K+ ATPase on basolateral membrane
- ROMK moves K+ into blood
- Cl- channels allow Cl- into blood
- Ca2+ moved into blood and exchanged for Na+
- NO WATER REABSORBED
What is the function of aldosterone?
- Upregulate the number of ENac channels in the cell membrane
- Upregulates number of ROMK channels in cell membrane
- Inhibited by ANP
Outline the channels found in the collecting duct cells and their functions
- Aquaporins allow H2O through - ADH binding to V2 receptor upregulates number of aquaporins in the membrane
- ROMK allows K+ back into collecting duct
- ENac brings Na+ into cell
- Na+/K+ ATPase on basolateral membrane
What causes central diabetes insipidus?
Damage to hypothalamus or pituitary gland due to:
- Brain injury e.g. fracture to base of skull
- Tumour
- Sarcoidosis or tuberculosis
- Aneurysm
- Some forms of encephalitis or meningitis
What is central diabetes insipidus?
- Impaired ADH secretion by the hypothalamus
- Water is inadequately resorbed from collecting ducts
- Large quantities of urine produced
How is central diabetes insipidus managed?
- ADH injection s or ADH nasal spray treatments
What is nephrogenic diabetes insipidus?
- Acquired insensitivity of kidney to ADH
- Plasma ADH levels are normal
- Water inadequately reabsorbed from collecting ducts
- Large quantity of urine produced
How is nephrogenic diabetes insipidus managed?
- Low-salt, low protein diet reduces urine output
- No current treatment to correct deficit
What are the causes of nephrogenic diabetes insipidus?
- Mutation in gene coding for V2 receptors
- Chronic pyelonephritis
- Polycystic kidneys
- Drugs such as lithium
What are the causes of nephrogenic diabetes insipidus?
- Mutation in gene coding for V2 receptors
- Chronic pyelonephritis
- Polycystic kidneys
- Drugs such as lithium
What characterises Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH)?
- Excessive release of ADH from posterior pituitary gland
- Dilutional hyponatraemia
- Plasma Na+ conc lowered
- Total body fluid increased
