Lecture 52 – Tubular function and hormonal control Flashcards
Electrolyte balance across tubular epithelium
o Na+ is reabsorbed by active transport
o Electrochemical gradient drives anion reabsorption
o Water moves by osmosis, following solute reabsorption
o Concentrations of other solutes increase as fluid volume in lumen decreases. Permeable solutes are reabsorbed by diffusion
Nephron blood supply
Each nephron has two arterioles and two sets of capillaries associated with it
Na+ reabsorbed
o Na+ enters cell through membrane proteins, moving down its electrochemical gradient
o Na+ is pumped out the basolateral side of cell by the Na+-K+-ATPase
Glucose and Na+ reabsorbed
o Na+ moving down its electrochemical gradient using the SFLT protein pulls glucose into the cell against its concentration gradient
o Glucose diffuses out of the basolateral side of the cell using the GLUT protein
o Na+ is pumped out by Na+-K+-ATPase
Filtration rate of glucose vs plasma glucose concentration
Linear proportional positive relationship
Sodium reabsorption
o 80% of oxygen consumed by the kidney
o Tied to reabsorption
Water, chloride
Glucose, amino acids, urea
o Tied to secretion of
Potassium
Hydrogen ions
Sodium reabsorption table
refer to notes
Renin-angiotensin system
o Renin release from granular cells is stimulated by EC volume depletion
Fall in pressure at preglomerular arteriole
Reduction in sodium chloride delivery to macula densa
Sympathetic nerve activation
Sodium balance
o Input
Diet (food and drink) – 150 mmol
Intravenous (normal saline) – 0 mmol
o Output
Urine – 140 mmol
Faeces – 8 mmol
Skin – 2mmol
water balance
o Input
Diet (food and drink) – 2200 mL
Metabolism 300mL
Intravenous (5% dextrose) 0 mL
o Output
Urine 1500 mL
Lungs 500 mL
Skin 400 mL
Faeces 100mL
Detecting changes in osmolarity
o osmoreceptor cells
Anterior hypothalamus
Response to changes in cell size
Vasopressin and osmolarity
Linear proportional relationship that hits a plateau
Vasopressin and ECF volume
Negative linear relationship
Water and the nephron
o Proximal tubule
Bulk absorption
o Descending limb of loop of Henle
Dilution of filtrate
o Collecting ducts
Fine-tuning according to needs
Urine osmolality
o Ranges from 3-1200 mOsM/kgH2O
o Must excrete 600 mOsM of solute per day
o Must excrete at least 500mL of water per day
Concentration of urine along the nephron
o Isosmotic fluid leaving the proximal tubule becomes progressively more concentrated in. the descending limb
o Removal of solute in the thick ascending limb creates hypoosmotic fluid
o Hormones control distal nephron permeability to water and solutes
o Urine osmolarity depends on reabsorption in the collecting duct
Vasopressin binds to membrane receptor
o Receptor activates cAMP second messenger system
o Cell inserts AQP2 water pores into apical membrane
o Water is absorbed by osmosis into the blood
Concentrated urine
With maximal vasopressin, the collecting duct is freely permeable to water. Water leaves by osmosis and is carried away by the base recta capillaries
Dilute urine
In the absence of vasopressin, the collecting duct is impermeable to water and the urine is dilute.
Movement of water across collecting duct lumen to medullary interstitial fluid to vasa recta (blood)
o Vasopressin binds to membrane receptor
o Receptor activates cAMP second messenger system
o Cell inserts AQP2 water pores into apical membrane
o Water is absorbed by osmosis into the blood
Is reabsorption of secretion a bigger tubular activity?
reabsorption
Where is reabsorption greatest along the nephron?
proximal tube
What is the most important solute to be reabsorbed?
sodium
Which solutes are actively reabsorbed?
Sodium, amino acids, glucose, lactate