# 25.6 Tubular reabsorption  (Page 4/18)

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Reabsorption of Major Solutes by the PCT
Basal membrane Apical membrane
Active transport Symport with Na +
Na + (exchange for K + ) K +
Facilitated diffusion Cl
K + Ca ++
Cl Mg ++
Ca ++ HCO 3
HCO 3 ${\text{PO}}_{4}^{3-}$
${\text{PO}}_{4}^{3-}$ Amino acids
Amino acids Glucose
Glucose Fructose
Fructose Galactose
Galactose Lactate
Lactate Succinate
Succinate Citrate
Citrate Diffusion between nephron cells
K +
Ca ++
Mg ++

About 67 percent of the water, Na + , and K + entering the nephron is reabsorbed in the PCT and returned to the circulation. Almost 100 percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here. Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the PCT are saturated, so that their capacity to move glucose is exceeded (transport maximum, or T m ). In men, the maximum amount of glucose that can be recovered is about 375 mg/min, whereas in women, it is about 300 mg/min. This recovery rate translates to an arterial concentration of about 200 mg/dL. Though an exceptionally high sugar intake might cause sugar to appear briefly in the urine, the appearance of glycosuria    usually points to type I or II diabetes mellitus. The transport of glucose from the lumen of the PCT to the interstitial space is similar to the way it is absorbed by the small intestine. Both glucose and Na + bind simultaneously to the same symport proteins on the apical surface of the cell to be transported in the same direction, toward the interstitial space. Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it. Na + is then actively pumped out of the cell at the basal surface of the cell into the interstitial space. Glucose leaves the cell to enter the interstitial space by facilitated diffusion. The energy to move glucose comes from the Na + /K + ATPase that pumps Na + out of the cell on the basal surface. Fifty percent of Cl and variable quantities of Ca ++ , Mg ++ , and ${\text{HPO}}_{4}^{2-}$ are also recovered in the PCT.

Recovery of bicarbonate (HCO 3 ) is vital to the maintenance of acid–base balance, since it is a very powerful and fast-acting buffer. An important enzyme is used to catalyze this mechanism: carbonic anhydrase (CA). This same enzyme and reaction is used in red blood cells in the transportation of CO 2 , in the stomach to produce hydrochloric acid, and in the pancreas to produce HCO 3 to buffer acidic chyme from the stomach. In the kidney, most of the CA is located within the cell, but a small amount is bound to the brush border of the membrane on the apical surface of the cell. In the lumen of the PCT, HCO 3 combines with hydrogen ions to form carbonic acid (H 2 CO 3 ). This is enzymatically catalyzed into CO 2 and water, which diffuse across the apical membrane into the cell. Water can move osmotically across the lipid bilayer membrane due to the presence of aquaporin water channels. Inside the cell, the reverse reaction occurs to produce bicarbonate ions (HCO 3 ). These bicarbonate ions are cotransported with Na + across the basal membrane to the interstitial space around the PCT ( [link] ). At the same time this is occurring, a Na + /H + antiporter excretes H + into the lumen, while it recovers Na + . Note how the hydrogen ion is recycled so that bicarbonate can be recovered. Also, note that a Na + gradient is created by the Na + /K + pump.

brain behaviour relation
Act as mother of all gland
johnson
What is neurons?
a specialized cell transmitting nerve impulses; a nerve cell.
Shain
😊
Luqman
thanks
Luqman
why are cells small
what is the change if take normal water in our body
What are variations in physiology
pls let's talk about d difference between mitosis and meiosis
through remodeling and formation of new bones
Monica
what is blood pressure
HALLELUYAH
HALLELUYAH
sketch and label blood vessels
HALLELUYAH
veins is........
KING
draw the male reproductive system
hello am new here
Pednyin
how life
Jeremaih
join
Hajiahamdy
Explain how different foods can affect metabolism
what is Endocrine system?
which secrete hormones and other products direct into the blood
Cell is basic, structural and functional unit of life
The cell is the structural and functional unit of all living organisms, and is sometimes called the "building block of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell.
Br_
hi am new here..wish to join you in this conversation
Rachel
welcome Rachel am Brianito
hingi
hey
HALLELUYAH
can some one help
HALLELUYAH
what is the basic function of the lymphatic system
HALLELUYAH
The other main function is that of defense in the immune system. Lymph is very similar to blood plasma: it contains lymphocytes. It also contains waste products and cellular debris together with bacteria and proteins. Associated organs composed of lymphoid tissue are the sites of lymphocyte producti
the function of lymphatic system are 1fluid balance 2 lipid absorption and 3 defence
Sidra
destroyed microognism
Usman
lymphatic systems main function is to transport lymph
Colleen
may i know the meaning of infestation of parasite?
Aminiely
Which of the following accurately describe external respirations
different between anatomy and physiology
anatomy is the study of STRUCTURE of the body while physiology is the study of the function of the part of the body
Gborgbor
Anatomy deals with the structure and parts of the body while physiology is the function of the the body parts
Archie
Anatomy is the study which deals with anatomical position while physiology deals with the function
Luqman
not understanding what is a cell
its the fundamental unit of life or its the primary step in which two or more cell combine to form a tissue .
Sidra
Is the smallest structural and functional unit of life
Aliyu
cell is the basic you unit of life
Esther
what is osteomalàcia
what is hydroxyapitate
Ellen
Hydroxyapatite, also called hydroxylapatite, is a naturally occurring mineral form of calcium apatite with the formula Ca₅(PO₄)₃, but it is usually written Ca₁₀(PO₄)₆(OH)₂ to denote that the crystal unit cell comprises two entities. Hydroxyapatite is the hydroxyl endmember of the complex apatite gro
Br_
..up
Br_
what's the best way to memorize the terms and what it does
Kesa
osteomalacia is where inorganic or mineral which is calcium and phosphorus are removed from a bone....this will make the bone become flexible n in children is called ricket
Rashida