T3 L4 Formation of urine Flashcards
What molecules are reabsorbed back into the blood in the proximal tubule?
Water Sodium Potassium Chloride Hydrogen carbonate Glucose Proteins & amino acids Urea Calcium Magnesium
What molecules are secreted into the urine at the proximal tubule?
Organic anions & bases Hydrogen Ammonium Urea Creatinine
What happens at the Loop of Henle?
Tubular fluid is further modified here
Aim is to recover fluid & solutes from glomerular filtrate
What are the 2 stages of the process occurring at the loop of Henle?
1) Extraction of water in the thin descending limb
2) Extraction of Na+ & Cl- in the thick ascending limb
What type of nephrons have a longer loop of Henle?
The juxtamedullary nephrons
Describe the thin descending limb
Cells are flat
No active transport of salts
Freely permeable to water via aquaporin-1 channels
Some passive movement of water via tight junctions
Describe the thick ascending limb
Tubular wall is impermeable to water as there are no channels
Specialised Na+/K+/2Cl- cotransporters (NKCC2)
Na+, K+ & Cl- are reabsorbed
Describe the process in the loop of Henle
Fluid entering from the proximal tubule is isotonic
Water is reabsorbed out of the descending loop of henle
By the tip of the loop of Henle, the filtrate is hypertonic
Solutes are pumped out of the ascending limb
By the end of the Loop of Henle the filtrate that enters the distal tubule is hypotonic
What does the counter-current multiplication do?
Creates a large osmotic gradient within the medulla
Facilitated by Na+/K+/2Cl- transport in the ascending limb
Permits the passive reabsorption of water from the tubular fluid in the descending Loop of Henle
Describe the process of the counter-current mechanism
1) Fluid enters tubule
2) Active transport of Na+, K+, Cl- into medullary interstitial fluid
3) Water moves out of descending limb by osmosis
4) Isosmotic state in descending limb creates an osmotic difference between the descending & ascending limb
5) More fluid enters tubule by pushing higher osmolarity fluid through by bulk flow
What happens to urea in the Loop of Henle?
Urea is freely filtered at the glomerulus
Some reabsorption in the PT but LOH & DT are relatively impermeable
Urea can diffuse out of collecting duct into medulla down its concentration gradient
Adds to osmolarity of medullary interstitium
Where does the remainder of mOsm come from?
Active transport of NaCl contributes 600-1000mOsm
The rest comes from urea
What does the distal tubule do?
Performs further adjustment of urine
Active absorption & secretion of solutes occurs here
What happens to sodium & chloride ions in the distal tubule?
They are actively reabsorbed from the tubular fluid & exchanged for potassium or hydrogen ions which are secreted into tubular fluid
How are principal cells involved in the exchange of Na+ for K+?
Cells are sensitive to aldosterone
When aldosterone is released the cells reabsorb more Na+
More water gets reabsorbed
BP increases
Exchange forms post of renin-angiotensin-aldosterone system
What cells are involved in the exchange of Na+ for H+ in the distal tubule & collecting duct?
Intercalated cells
What are the subtypes for intercalated cells?
Alpha intercalated cells
Beta intercalated cells
Describe the pathway from angiotensinogen to increased sodium reabsorption by the distal tubule in exchange for K+
Angiotensinogen –> angiotensin I –> angiotensin II –> increased secretion of aldosterone –> increased sodium reabsorption by distal tubule in exchange for K+
What is the main function of the collecting duct?
Final modification of urine
What is ADH (antidiuretic hormone)?
Most important hormone that regulates water balance
Also known as vasopressin or 8-arginine vasopressin
Released from posterior pituitary subsequent to hypothalamic inputs
Plasma half life is 10-15 minutes
What does ADH do?
Acts on vasopressin V2 receptors on basal membrane of principal cells in distal tubule & collecting duct –> activation of intracellular water channels (AQP2)
What happens if there is the maximal amount of circulating ADH?
Occurs if severely dehydrated
Collecting duct becomes permeable to water due to maximal AQP2 insertion so water reabsorption occurs
Reabsorbs up to 66% of water entering collecting duct
Delivery of fluid to collecting duct is low
Urine concentration can be reduced to 300ml/day
Very concentrated urine
What happens if there is no circulating ADH?
Reabsorption of water occurs at various sites in nephron
Collecting duct becomes impermeable to water as there is no AQP2 so a large volume of water is excreted
Lack of ADH - diabetes insipidus
Dilute urine
How is diabetes insipidus treated?
Give synthetic ADH
