Transport Processes:
- Tubule wall of simple epithelium serves as a barrier between the tubule and surrounding
interstitium
- Substances may move between tubule and interstitial fluid
- Paracellular transport: Materials pass in between the epithelial cells of the tubule
- Transcellular transport: Materials pass through the epithelial cells of the tubule
- Must cross two membranes
- Luminal membrane in contact with the tubular fluid
- Basolateral membrane rests on basement membrane
- Requires specific transport proteins to control the movement of substances
- Moving materials from interstitial fluid to blood in peritubular capillaries is facilitated by high
colloid pressure
- Lower H2O content
- Higher solute concentration
- Promotes bulk flow
- Two processes
- Tubular Reabsorption: Movement of materials from the tubule into the blood
- Tubular Secretion: Movement of materials from the blood into the tubule
Tubular Reabsorption:
- Highly selective process
- Tubules have high reabsorptive capacities for substances needed by the body
- Tubules have little to no reabsorptive capacity for things not needed
- Amount reabsorbed is amount needed to maintain composition & volume of internal fluid
environment
- Glomerular filtrate = plasma – plasma proteins
- End Result: concentration of materials not wanted/needed by the body
Transport Maximum:
- The maximum amount of a substance that can be reabsorbed (or secreted) across the tubule
epithelium in a given period of time
- Dependent on the number of the transport proteins in the membrane specific for that substance
Renal threshold:
- The maximum plasma concentration of a substance that can be carried in the blood without
eventually appearing in the urine
- The point where Tm is exceeded
Completely Reabsorbed Substances:
- Substances that are not normally found in urine
- Returned back to the blood by reabsorption
- Typically occurs in the proximal convoluted tubule
- Includes:
- Nutrients (glucose, amino acids, lactate)
- Each nutrient requires a specific transport protein
- Filtered plasma proteins: Small and medium-seized peptides
- Gathered by pinocytosis
- Digested in lysosomes
, - Returned to blood as amino acids
Reabsorption of Nutrients:
- Occurs via secondary active transport against the concentration gradient
- Lumen to tubule cell: Uses Na+ co-transporter to get into tubule cell
- Indirect energy from the Na+/K+ pump
- Tubule cell to IF & Blood
- Passive diffusion down gradient out of cell to ECF with help of a carrier
- Does not use energy
Regulated Reabsorption:
- Nephron varies the amount of reabsorption
- Regulates the amount that remains in blood
- Regulates the amount excreted
- Includes many ions (Na+, K+, HCO3-, a2+, etc.) and water
- Na+ plays a pivotal role in the reabsorption of many other substances
Na+ reabsorption:Unlike most solutes, Na+ reabsorption varies along the entire length of the tubule
- ~65% is reabsorbed in the proximal convoluted tubule
- Pivotal role in glucose, amino acid, water, Cl- & urea reabsorption
- ~25% is reabsorbed in the Loop of Henle
- Critical role in varying urine concentration and volume
- ~10% is reabsorbed in a regulated fashion in the distal convoluted tubule, collecting tubule, and
collecting duct
- Key role in regulating ECF volume
+
Na reabsorption in the PCT:
- Na+ movement from tubular lumen to luminal cells is always a passive step
- Na+ channels
- Requires Na+-K+ ATPase to move Na+ from tubular cells to interstitial space
- Maintains Na+ levels low in cell to promote passive movement of Na+ from tubular lumen
- Movement of Na+ out of interstitial space into the capillaries occurs by diffusion due to
concentration gradient
+
Na reabsorption in the DCT:
- Dependent on Na+ load
- Total amount of Na+ in the body
- Regulated by Aldosterone and ANP
- Aldosterone: Steroid hormone produced in adrenal cortex
- Stimulates synthesis of Na+ channels and Na+/K+ pumps
- Promotes Na+ movement out of lumen & into blood
- H2O follows Na+ due to osmotic considerations
- Atrial Natriuretic Peptide (ANP): Inhibits aldosterone release
- Promotes Na+ & H2O excretion via the urine
H2O Reabsorption:
- Dependent on osmotic considerations
- Pathways
- Aquaporins: In PCT, aquaporins are always open
- In DCT, aquaporins are dependent on vasopressin (ADH)
- Tubule wall of simple epithelium serves as a barrier between the tubule and surrounding
interstitium
- Substances may move between tubule and interstitial fluid
- Paracellular transport: Materials pass in between the epithelial cells of the tubule
- Transcellular transport: Materials pass through the epithelial cells of the tubule
- Must cross two membranes
- Luminal membrane in contact with the tubular fluid
- Basolateral membrane rests on basement membrane
- Requires specific transport proteins to control the movement of substances
- Moving materials from interstitial fluid to blood in peritubular capillaries is facilitated by high
colloid pressure
- Lower H2O content
- Higher solute concentration
- Promotes bulk flow
- Two processes
- Tubular Reabsorption: Movement of materials from the tubule into the blood
- Tubular Secretion: Movement of materials from the blood into the tubule
Tubular Reabsorption:
- Highly selective process
- Tubules have high reabsorptive capacities for substances needed by the body
- Tubules have little to no reabsorptive capacity for things not needed
- Amount reabsorbed is amount needed to maintain composition & volume of internal fluid
environment
- Glomerular filtrate = plasma – plasma proteins
- End Result: concentration of materials not wanted/needed by the body
Transport Maximum:
- The maximum amount of a substance that can be reabsorbed (or secreted) across the tubule
epithelium in a given period of time
- Dependent on the number of the transport proteins in the membrane specific for that substance
Renal threshold:
- The maximum plasma concentration of a substance that can be carried in the blood without
eventually appearing in the urine
- The point where Tm is exceeded
Completely Reabsorbed Substances:
- Substances that are not normally found in urine
- Returned back to the blood by reabsorption
- Typically occurs in the proximal convoluted tubule
- Includes:
- Nutrients (glucose, amino acids, lactate)
- Each nutrient requires a specific transport protein
- Filtered plasma proteins: Small and medium-seized peptides
- Gathered by pinocytosis
- Digested in lysosomes
, - Returned to blood as amino acids
Reabsorption of Nutrients:
- Occurs via secondary active transport against the concentration gradient
- Lumen to tubule cell: Uses Na+ co-transporter to get into tubule cell
- Indirect energy from the Na+/K+ pump
- Tubule cell to IF & Blood
- Passive diffusion down gradient out of cell to ECF with help of a carrier
- Does not use energy
Regulated Reabsorption:
- Nephron varies the amount of reabsorption
- Regulates the amount that remains in blood
- Regulates the amount excreted
- Includes many ions (Na+, K+, HCO3-, a2+, etc.) and water
- Na+ plays a pivotal role in the reabsorption of many other substances
Na+ reabsorption:Unlike most solutes, Na+ reabsorption varies along the entire length of the tubule
- ~65% is reabsorbed in the proximal convoluted tubule
- Pivotal role in glucose, amino acid, water, Cl- & urea reabsorption
- ~25% is reabsorbed in the Loop of Henle
- Critical role in varying urine concentration and volume
- ~10% is reabsorbed in a regulated fashion in the distal convoluted tubule, collecting tubule, and
collecting duct
- Key role in regulating ECF volume
+
Na reabsorption in the PCT:
- Na+ movement from tubular lumen to luminal cells is always a passive step
- Na+ channels
- Requires Na+-K+ ATPase to move Na+ from tubular cells to interstitial space
- Maintains Na+ levels low in cell to promote passive movement of Na+ from tubular lumen
- Movement of Na+ out of interstitial space into the capillaries occurs by diffusion due to
concentration gradient
+
Na reabsorption in the DCT:
- Dependent on Na+ load
- Total amount of Na+ in the body
- Regulated by Aldosterone and ANP
- Aldosterone: Steroid hormone produced in adrenal cortex
- Stimulates synthesis of Na+ channels and Na+/K+ pumps
- Promotes Na+ movement out of lumen & into blood
- H2O follows Na+ due to osmotic considerations
- Atrial Natriuretic Peptide (ANP): Inhibits aldosterone release
- Promotes Na+ & H2O excretion via the urine
H2O Reabsorption:
- Dependent on osmotic considerations
- Pathways
- Aquaporins: In PCT, aquaporins are always open
- In DCT, aquaporins are dependent on vasopressin (ADH)
