Tubular Reabsorption and Secretion
Tubular Reabsorption
- substances are returned to blood
- involves transfer of substances from tubular lumen into peritubular capillaries
- high selective and variable process
- 125mL/min filtered; 124 reabsorbed
- waste products are highly concentrated
Transepithelial transport
5 barriers
1. leave tubular fluid crossing luminal membrane of tubular cell
2. pass through cytosol from one tubular cell to other
3. croos basolateral membrane of tubular cell to ebnter interstitial fluid
4. diffuse through interstitial fluid
5. penetrate capillary wall to enter blood plasma
Passive reabsorption
- no energy required
- occurs down elctrochemical or osmotic gradients
Active Reabsorption
- occurs if any one of steps in transepithelial transport of substance requires energy
- movement occurs against electrochemical gradient
Na/K ATPase Pump
- active Na/K ATPase pump in basolateral membrane essential for Na reabsorption
- 80% kidney energy used for Na transport
- Na not reabsorbed in descending limb of loop of henle
- Na is reabsorbed throughout most of tubule
- Water follows reabsorbed sodium by osmosis
- 99.5% reabsorbed; 67% proximal tubule; 25% loop of Henle; 8% distal and collecting ducts
Na Reabsorption in Proximal Tubule
- pump transports Na out of tubular cell into interstitial space
- keeps tubular cells concentration low maintaining a gradient from the lumen to the tubular cell to promote Na retention
Water Reabsorption Proximal Tubule
- water is passively reabsorbed throughout tubule
- 80% of water is uncontrolled; 65% in prox tubule, 15% loop of henle
Renin-angiotensin-aldosterone system
- hormone regulating Na
- granular cells of juxtaglomerular apparatus secrete enzymatic hormone renin in response to a fall in NaCl/ECF vol/BP
- Renin increases Na reabsorption, promoting water retention
RAAS Activation
1. Renin is released response to low NaCl/decreased ECF volume/BP
2. Renin activates angiotensinogen to angiotensin I
3. Ang I -> Ang II by ACE
4. Ang II stimulates release of aldosterone to increase Na+ retention
5. Na+ reabsorption increases as well as H2O retention
Angiotensin 2
- exerts other effects to recify original stimuli
- increased antidiuretic hormone
- increased thirst
- increased arteriolar vasoconstriction
Atrial Natriuretic Peptide and Reabsorption
- inhibits Na reabsorption
- secreted by atria in response to stretch of Na retention, expansion to ECF vol and increased arterial pressure
- release promotes natrieurtic, diuretic and hypotensive effects to help correct stimulus
Passive Reabsorption of Urea at End of Proximal Tubule
- as water reabsorbed, urea concentration within tubular fluid increases
- concentration gradient created for urea to passively be reabsorbed
- 50% of filtered ura is passively reabsorved in prox tubule
Tubular Secretion
- transfer of substances from peritubular capillaries into the tubular lumen
- involved transepithelial transport
- kidney tubules can selectively add substances to stubstances already filtered
Hydrogen Ion Secretion
- regulated acid-base balance
- secreted in prox, distal and colelcting tubules
- amt secreted depends on acidity of body
Potassium Ion Secretion
- keeps plasma K concentration at proper level
- maintain normal membrane excitability in muscles and nerves
- secreted only in distal and collecting tubules under control of aldosterone
Dual Control of Aldosterone Secretion by K and Na
- decreased Na = Increase ang 2 = increacse aldosterone
- increase K = increase aldosterone
- Increase TAubular K secretion = Increase urinary K excretion
- increase tubular Na reabsorption = decrease urinary Na excretion
Importance of Regulating Plasma K Concentration
- K is used in membrane electrical activity of excitable tissues
- both increases and decreases in plasma concentration
- can alter intracellular - extracellular K gradient, can change resting MP