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The Na +/ K + ATPase pumps in the basal membrane create an electrochemical gradient, allowing reabsorption of Cl by Na + /Cl symporters in the apical membrane. At the same time that Na + is actively pumped from the basal side of the cell into the interstitial fluid, Cl follows the Na + from the lumen into the interstitial fluid by a paracellular route between cells through leaky tight junctions    . These are found between cells of the ascending loop, where they allow certain solutes to move according to their concentration gradient. Most of the K + that enters the cell via symporters returns to the lumen (down its concentration gradient) through leaky channels in the apical membrane. Note the environment now created in the interstitial space: With the “back door exiting” K + , there is one Na + and two Cl ions left in the interstitium surrounding the ascending loop. Therefore, in comparison to the lumen of the loop, the interstitial space is now a negatively charged environment. This negative charge attracts cations (Na + , K + , Ca ++ , and Mg ++ ) from the lumen via a paracellular route to the interstitial space and vasa recta.

Countercurrent multiplier system

The structure of the loop of Henle and associated vasa recta create a countercurrent multiplier system    ( [link] ). The countercurrent term comes from the fact that the descending and ascending loops are next to each other and their fluid flows in opposite directions (countercurrent). The multiplier term is due to the action of solute pumps that increase (multiply) the concentrations of urea and Na + deep in the medulla.

Countercurrent multiplier system

The left panel of this image shows the location of the loop of Henle. The right panel shows the interstitial osmolality and the exchange of sodium and chloride ions, as well as water and urea.

As discussed above, the ascending loop has many Na + pumps that actively pump Na + out of the forming urine into the interstitial spaces. In addition, collecting ducts have urea pumps that actively pump urea into the interstitial spaces. This results in the recovery of Na + to the circulation via the vasa recta and creates a high osmolar environment in the depths of the medulla.

Ammonia (NH 3 ) is a toxic byproduct of protein metabolism. It is formed as amino acids are deaminated by liver hepatocytes. That means that the amine group, NH 2 , is removed from amino acids as they are broken down. Most of the resulting ammonia is converted into urea by liver hepatocytes. Urea is not only less toxic but is utilized to aid in the recovery of water by the loop of Henle and collecting ducts. At the same time that water is freely diffusing out of the descending loop through aquaporin channels into the interstitial spaces of the medulla, urea freely diffuses into the lumen of the descending loop as it descends deeper into the medulla, much of it to be reabsorbed from the forming urine when it reaches the collecting duct. Thus, the movement of Na + and urea into the interstitial spaces by these mechanisms creates the hyperosmotic environment of the medulla. The net result of this countercurrent multiplier system is to recover both water and Na + in the circulation.

The amino acid glutamine can be deaminated by the kidney. As NH 2 from the amino acid is converted into NH 3 and pumped into the lumen of the PCT, Na + and HCO 3 are excreted into the interstitial fluid of the renal pyramid via a symport mechanism. When this process occurs in the cells of the PCT, the added benefit is a net loss of a hydrogen ion (complexed to ammonia to form the weak acid NH 4 + ) in the urine and a gain of a bicarbonate ion (HCO 3 ) in the blood. Ammonia and bicarbonate are exchanged in a one-to-one ratio. This exchange is yet another means by which the body can buffer and excrete acid. The presence of aquaporin channels in the descending loop allows prodigious quantities of water to leave the loop and enter the hyperosmolar interstitium of the pyramid, where it is returned to the circulation by the vasa recta. As the loop turns to become the ascending loop, there is an absence of aquaporin channels, so water cannot leave the loop. However, in the basal membrane of cells of the thick ascending loop, ATPase pumps actively remove Na + from the cell. A Na + /K + /2Cl symporter in the apical membrane passively allows these ions to enter the cell cytoplasm from the lumen of the loop down a concentration gradient created by the pump. This mechanism works to dilute the fluid of the ascending loop ultimately to approximately 50–100 mOsmol/L.

Questions & Answers

what is hypoxia
Akas Reply
I guess it's low supply the oxygen to the tissues
famuyiwa
yup
Natalie
A condition in which tissues (especially the blood) are deprived of an adequate supply of oxygen
Panthera
hanifa pia uko hapa
Panthera
where is present Glenoid Cavity ?
A- Reply
what is the muscular tissue
Md Reply
muscular tissue is a type of tissue that provide to help in cotraction to aur body.
A-
What's the difference in epithelial, connective, muscular and muscle tissue
Gifty
and it's similarities
Gifty
what is limb bone
Akshu Reply
this are bone attaching or joining to the axial bone.axial bone including skull,vertebrate and ribcage
Eliasi
how many bones make up the skull?
Matthew
22 bones
Husna
22bones
Bhanu
where is present Glenoid cavity ?
A-
how many bone in skull
Md
Explain the stages of mitosis and cell division
Bella Reply
systems of human body
Udezue Reply
define lymphatic system And give the composition of lymphatic fluid
sakshi Reply
the network of vessels through which lymphatic drains From the tissue into blood.lymph contain variety of substance like salts, glucose, proteins and fatsand water, white blood cells
Bhanu
yeah
Hassan
what is lymphatic system
Adie Reply
the network of vessels through which lymph drains from tissue into the blood
Bhanu
to describe the boundaries of four cavity
Pius Reply
homeostatic variables such as body temperature fluctuates within a normal range around the set point, or ideal, for a given homeostatic condition. for example, 98.6°F is a set point for body temperature. The response of the effector determines whether or not the homeostatic variable remains in the n
Chidinma Reply
why rbc is biconcave?
Sudhakar Reply
to carry oxygen easily
anwaar
What part of the brain controls the body temp
Ridwan
hypothalamus
JAYESH
what are epithelial tissues
Sachibu Reply
epithelial tissue that cover overall parts of the body and it's free from blood and nerves
Bhanu
Epithelial tissues are composed of cells laid out in sheets with strong cell-to-cell attachments.
Duah
Epithelial tissues perform a variety of functions that include; protection, secretion, filtration, diffusion, absorption, etc.
Duah
what control the flow of the blood ?
Donkor Reply
the pumping action of the heart
Holly
what is bony promises on the human body
Kelly Reply
what is the bony promises on human body
Kelly
what are bony prominences on human body
Kelly
support of the body
Bhanu
what are the characteristics of blood
yeboah Reply
they are red in colour
Tawoi
why blood is red in color?
Sudhakar
blood is red because it contains hemoglobin
Abena

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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