Renal 2: Urine Formation 18/4 Flashcards
Transport across cell membranes
- only active transport can move substances against concentration gradient. It requires input of energy in the form of ATP
- all other forms of transport move substances along their electro-chemical gradient.
- osmosis is the name given to passive diffusion of water though water channels (aqua portions) along the water concentration gradient
Reabsorption
Transport pathways across the tubules
To be reabsorbed substances must cross the tubular epithelial cell layer and the wall of the peri tubular capillary. 2 main pathways to get through epithelial layer.
1. Transcellular pathway: substance simply move through straight through. Must cross luminal membrane and bassolateral membrane.
- Para cellular
Goes in between the tight junctions. These tubular epithelial cells are joined by tight junctions. Particularly important in proximal tubule.
Movement across peri tubular capillary walls by bulk flow
Active reabsorption eg sodium
The whole process is active if either part of the transport is active.
Slide 10
Starts on basolateral surface sodium potassium pump
3 Sodium gets pumped out of cell, 2 potassium gets pumped in. With low sodium inside the cells we then have a concentration gradient that allows passive diffusion of sodium.
Secondary active co-transport with Na+ (symporters)
-the active step is to the sodium, other substance are the glucose, and phosphates etc.
on the basolateral membrane we have sodium potassium pump so Na is lower inside cell so then have conc. gradient.
When sodium flows in it carries with it other substances like glucose. Also occurs with amino acids and phosphates.
Important: the carriers can be saturated. Exhibit a transport maximum.
Check out slide 12.
Passive reabsorption of water
-always passive
-requires osmotic gradient set up by reabsorption of sodium or other solutes.
-either transcellular pathway (via aquaporins) or para cellular pathway.
Slide 13
I’m pretty sure it’s passive because the proteins can’t pass through the glomerular capsule creating a concentration gradient outside of tubule. Water want to go to we’re all the proteins are to dilute them?
Also because water moves by osmosis only if there is an osmotic gradient. Gradient is set up by the reabsorption of solutes such as Na+
Passive reabsorption of solutes
-water reabsorption from tubule increases concentration of other solutes left behind (eg urea, Cl-, K+).
-This sets up concentration gradient for passive diffusion of these solutes.
-reabsorption of Na+ sets up electrical gradient for reabsorption of anions eg Cl-
-passive reabsorption of solutes can occur via transcellular or paracellular pathways.
Slide 14
Check out slide 15 and study it
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Movement into peri tubular capillaries
-movement of molecules from renal interstitial fluid into peri tubular capillaries.
-by bulk flow- movement of water and solutes between endothelial cells of capillary wall along pressure gradient.
-pressure gradient is the balance of hydrostatic pressure and colloid osmotic pressures across the capillary wall.
Hydrostatic Pressure favours filtration at arterial end.
-high hydrostatic pressure in renal interstitium and low hydrostatic pressure in peri tubular gradient is always into peri tubular capillaries. Hydrostatic pressure in peritubular capillaries is quite low because its just been through glomerulus and all RBC etc have been removed
Secretion
- Secretion is the movement of solutes from the peritubular capillaries into the tubular lumen so they can be excreted.
- total amount of solute secreted is much less that the amount reabsorbed
- allows more excretion than filtration alone eg wastes and foreign chemicals
- secretion is important for homeostatic regulation eg K+ and H+
Tell me how the active secretion of K+ works?
- in principle it happens in cells of distal tubule and collecting duct
- K+ secretion is linked to Na+ reabsorption via Na+/K+ pump on basolateral membrane
- K+ moves out through channels in the luminal membrane
- Na+ moves in through channels in luminal membrane.
Tell me about Active secretion of H+.
Then tell me about secondary active secretion of H+
-occurs in the PT and intercalated cells of the DT and CD
-active transport of H+ across luminal membrane via a H+ ATPase or H+ pump
Slide 20
Secondary active secretion
-occurs only in the proximal tubule
-secondary active counter-transport with Na+
-carrier on luminal membrane binds H+ on inside and Na+ on outside
-H+ is pushed out as Na+ is drawn in along its concentration gradient.
Slide 21
Secondary active secretion of Organic anions and cations
- many metabolic wastes and foreign chemicals are secreted by secondary active counter transport with Na+
- bile salts, Uric acid, choline, foreign chemicals and drugs including penicillin and salicylates
- mainly in the proximal tubule
Division of labour in nephron What is reabsorbed/ added in the Renal corpuscle Proximal tubule Loop of Henle Distal tubule and collecting duct
Renal corpuscle (glomerulus and bow mans capsule) -filtration
Proximal tubule:
- mass reabsorption: most of filtered water, Na+ and other ions, all of the glucose and amino acids
- also most secretion: metabolic wastes, foreign chemicals and H+
- reabsorption and secretion here are obligatory or unregulated.
- activation of vitamin D3 (in tubular epithelial cells)
Loop of Henle:
- main function is creating an increased osmolarity in medullary intertitium allowing concentration of urine
- descending limb: water reabsorption but no solute reabsorption
- ascending limb: solute reabsorption but no water reabsorption.
Distal tubule and collecting duct:
- fine tuning or reabsorption and secretion for homeostatic control
- regulated- mostly by hormones,
- Na+ reabsorption and K+ secretion regulated by aldosterone
- Ca2+ reabsorption regulated by parathyroid hormone
- H+ secretion and HCO3- reabsorption regulated by PCO2 and H+
Micturition (Urination)
Also what is the micturition reflex and voluntary control
- process by which urine is excreted from the body
- urine formed in renal tubules
- urine drains into renal pelvis and into ureter
- ureter leads to bladder
- bladder stores urine until excreted
Also what is the micturition reflex and voluntary control?
-when bladder fills to a certain level, stretch receptors initiate reflex emptying.
-stretch receptors also send info to brain making is aware of bladder fullness
-with learned behaviour, reflex can be overridden by voluntary control over somatic motor neurons…up to a certain point
Slide 32
