The transport processes for tubular resorption and its characteristics along with renal threshold. The resorption of nutrients, sodium, potassium, water, calcium, and phosphate. The importance of vasopressin, bicarbonate, and hydrogen for blood and urine pH. Describing the processes of nitrogenous ...
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)
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