WEEK 4: TUBULAR STRUCTURE AND FUNCTION Flashcards
What is tubular reabsorption?
It is a process by which essential materials that were filtered are returned to the blood in peritubular capillaries from the tubular lumen.
State the percentages of how much each of the following substance is reabsorbed(R) and how much is filtered(F).
WATER
SODIUM
GLUCOSE
UREA
PHENOL
WATER
R:99%
F:1%
SODIUM
*R: 99.5
*F:0.5%
GLUCOSE
R:100%
F:0%
UREA
R:50
F:50%
PHENOL
R:0%
F:100%
Describe the transepithelial transport. The 5 barriers a substance must go through to be reabsorbed.
1.It must leave the tubular fluid by crossing the luminal membrane of the tubular cell.
2.It must pass through the cytosol from one side of the tubular cell to the other.
3.It must cross the basolateral membrane of the tubular cell to enter the interstitial fluid.
4.It must diffuse through the interstitial fluid
5.It must penetrate the capillary wall to enter the blood plasma.
1.he luminal cell membrane, the cytosol, the basolateral cell membrane, the interstitial fluid, the capillary wall
Differentiate between passive and active reabsorption.
PASSIVE REABSORPTION: All steps of transepithelial transport of a substance from the tubular lumen to the plasma are passive, that is no energy is spent for the substance’s movement which occurs down the electrochemical or osmotic gradient.
ACTIVE REABSORPTION: Takes place if any of the 5 steps of transepithelial transport requires energy. The net movement of the substance is usually against the electrochemical or osmotic gradient. Substances actively reabsorbed are usually of particular importance to the body, such as glucose, amino acids and other organic nutrients as well as sodium ions and other electrolytes as a phosphate ion.
State the % of sodium reabsorbed at the following sites
*Proximal convulated tubule
*Loop of henle
*Distal convulated tubule and collecting duct
*67%
*25%
*8%
What is the only part of the nephron where glucose is reabsorbed?
Proximal convulated tubule
State all the substances reabsorbed from the Proximal convulated tubule.
A few of the substances that are transported with Na+ (symport mechanism) on the apical membrane include Cl–, Ca++, amino acids, glucose, and PO3−4 Sodium
What is countercurrent multiplier system?
Where does it happen?
Countercurrent multiplication in the kidneys is the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine. This mechanism prevents you from producing litres and litres of dilute urine every day, and is the reason why you don’t need to be continually drinking in order to stay hydrated.
*
Why is urea reabsorbed?
Urea, in addition to sodium contributes for maintaining the hyperosmolarity in the medulla, so to reabsorb water. …
The urea reabsorption enables the formation of a high-osmolar urea gradient in the renal medulla, which is important for the renal urine concentration.
Describe water reabsorption
*65% of water is reabsorbed at the proximal tubule via AQUAPORIN 1 channels that are always open.
*15% is reabsorbed by the obligatorily from the loop of Henle.
- 20% AQUAPORIN 2 channels are located in the distal and collecting tubule where they are regulated by ADH/ VASOPRESSIN.
Describe the role of vasopressin / ADH in water reabsorption.
ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water.
*This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.
1.Vasopressin binds with its receptor on the basolateral membrane of a principal cell in the distal or collecting tubule. ( V2)
2.This binding activates the cAMP second messenger pathway
3.cAMP increases the opposite luminal surface permeability to water by promoting the INSERTION OF VASOPRESSIN REGULATED AQP-2 water channels into the membrane.
This membrane is impermeable to water in the absence of vasopressin.
4.Water enters the tubular cell from the tubular lumen through the inserted water channels. (Going into the cell on the membrane)
5.Water exits the cell through different, always open water channels EITHER AQP-3 or AQP-4 permanently positioned at the basolateral border, and then enters the blood, in this way being reabsorbed.
What is the difference between obligatory and facultative water reabsorption?
Water reabsorption together with solutes in tubular fluid is called obligatory water reabsorption. Reabsorption of the final water, facultative reabsorption, is based on need and occurs in the collecting ducts and is regulated by ADH.
Where does obligatory water reabsorption occur?
PCT and descending limb of nephron loop.
Describe the percentage and reabsorption of sodium ions along the nephron
*67% absorbed at the PROXIMAL TUBULE, ACTIVELY: Plays a role in the absorption of glucose, amino acids, chloride ions, water and urea.
*25% reabsorbed actively at the ASCENDING LOOP helps to establish the medullary interstitial vertical osmotic gradient, which is important in the kidney’s ability to produce urine of varying concentrations and volumes, depending on the body’s needs.
*8% actively reabsorbed from the distal and collecting tubules under the control of ALDOSTERONE .
-Important in the regulation of ECF volume and long-term control of BP: linked to Potassium ion secretion and hydrogen ion secretion.
What stimulates the reabsorption of calcium?
The most important regulator is PTH, which stimulates calcium absorption. PTH is a polypeptide secreted from the parathyroid gland in response to a decrease in the plasma concentration of ionized calcium.
