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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 are the three many components of the lymphatic system?
Milica Reply
anatomical terms and use them appropriatly in the language of anatomy of anterior body landmarks
Teody Reply
what is human anatomy?
rascal Reply
lts stady structured human body's
Sa
what is the study of how the body functions?
Bright
what is abdomipelvic cavity?
david Reply
where can we find the short bones
Chidi Reply
Carpal bones are examples of short bones
Dara
what is blood supply
Chidi
on upper limb and lower limb
Juma
carpal bones
Priya
during pregnancy which would more increase size the mothers abdominal or pelvic cavity?
Nurmalyn Reply
pelvic cavity I think
Priya
What is anatomical position
Nwoye
define the main directional terms of the body
cris Reply
during physical exercise respiratory rate increace two student are discussing the mechanisms involved. student A claim they are positive feedback and student B claim negative feedback do you agree with student A or B and why
cris
what is the physiology of circulation
Chidi
please I mean the physiology of criculation
Chidi
blood flow refers to the movement of blood through the vessels from arteries to the capillaries and then to the veins
Laraib
during pregnancy, which would more size the mother's abdominal or pelvic cavity? explain
cris Reply
list and define the three plane of devision of the body
cris
complete the following statements using correct directional terms for human being. 1. the navel is________to the nose 2. the heart is______to the breastbone(sternum) 3 the ankle is______to the knee 4 the ear is______to the eyes.
cris
1. superior 2. posterior 3. superior 4. lateral
Mnm
anterior fuerior
Chidi
name the system of the body and its function
cris Reply
11 system are human body 1.integumentary system 2. skeletal system 3. muscular system 4. nervous system 5. endocrine system 6. cardiovascular system 7. lymphatic system 8. respiratory system 9. digestive system 10. urinary system 11. reproductive system male and female.
Vineeta
during pregnancy, which would more size the mother's abdominal or pelvic cavity? explain
cris
how the body maintain hormeostasis in terms of bloodglucose level
cris Reply
It releases hormones from the pancreas insulin and glucagon
TONY
why human blood pressure high
amin Reply
fear, anxiety, sickness
Inemesit
why in mothers womb the foetus head is in anus direction?
Kick
As it seems the position downside n if we did such position thn soon we got vomiting then how foetus stay in downward position long time?
Kick
What is red blood cell
HANNAH Reply
A type of blood cell that is made in the bone marrow and found in the blood. Red blood cells contain a protein called hemoglobin, which carries oxygen from the lungs to all parts of the body. Checking the number of red blood cells in the blood is usually part of a complete blood cell (CBC) test. It
Noor
red blood cell are the most numerous blood cells.they comprise about 99% of all blood cells red blood cells are non nucleated it has red colour due to present to hemoglobin.
Vineeta
Thanks for the answers
HANNAH
how will you promote quality of life in ptb patient using the 14 basic needs and 21 nursing problems?
rOx Reply
coronary circulation ?
Juri Reply
Coronary circulation is the circulation of blood in the blood vessels that supply the heart muscle (myocardium). Coronary arteries supply oxygenated blood to the heart muscle, and cardiac veins drain away the blood once it has been deoxygenated. Because the rest of the body.
Riyaz
coronary circulation ,is flow of blood that supplies the heart tissue itself is the coronary circulation. the functional blood supply of the heart,is the shortest circulation in tha body.
Vineeta
what about the easy way to understand action potential
Bright
event of cardiac cycle
Juri Reply
hii
Chandan
hi 😷be safe your self
John
atrialsystole, ventricular systole,complete cardiac diastole
Priya
intrisic process which done by automic nerve
Juma

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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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