Secretion and Reabsorption

individuals with a low GFR. It is estimated by measuring the creatinine molecule, which is a

protein from muscle cell metabolism that is minimally secreted by the nephron.

SECRETION AND REABSORPTION

About 180 liters of filtrate get through the glomerulus per day—much of it needing

reabsorption back into the bloodstream. As you have learned, this occurs in the PCT, the loop

of Henle, DCT, and collecting ducts. Different parts of that system reabsorb and secrete things

in various ways. Much of this is passive reabsorption along concentration gradients; however,

the reabsorption of water is highly regulated by the kidneys (and the body as a whole).

Aldosterone, antidiuretic hormone, and renin (indirectly) will affect the water recovery. Most of

water recovery happens in the nephron with only ten percent reaching the collecting ducts.

This is where ADH kicks in to recover all, some, or none of the water, depending on how much

water is needed by the body.

Solutes like glucose, amino acids, oligopeptides, vitamins, and lactate get reabsorbed in the PCT

for the most part so they aren’t lost in the urine. Creatinine is secreted in the PCT, while urea is

secreted and partially reabsorbed in the PCT (it is later secreted and reabsorbed in other parts

of the kidney). Sodium and chloride are mostly reabsorbed throughout the nephron.

Bicarbonate is reabsorbed at about 80-90 percent in the PCT, while hydrogen ions are secreted.

Potassium can be reabsorbed in the PCT and loop of Henle but is secreted under the regulation

of aldosterone in the collecting ducts. Calcium, magnesium, and phosphate are all reabsorbed

throughout the nephron.

The mechanisms necessary for the reabsorption or secretion of these solutes include diffusion,

active transport, facilitated diffusion, osmosis, and secondary active transport. These have

already been discussed in previous chapters. Some of these mechanisms need ATP energy,

while others are completely passively transported across the cell membranes.

The initial filtrate is similar to blood except for the lack of cells and large proteins in the filtrate.

There is continual modification after that to make the final urine end product. This modification

starts in the PCT. Some substances are secreted in the PCT, while others are reabsorbed.

Ultimately these are returned to the circulation by the peritubular and vasa recta capillaries.

The high pressure of the glomerular capillaries happens because the efferent arterioles are

contracted and the afferent arterioles are more relaxed. Sodium is pumped out of the PCT and

back into the peritubular capillary—with water necessarily following. This is called obligatory

water reabsorption.

The PCT is where most of the substances cross back into the bloodstream. Some, such as

glucose and amino acids, use symport mechanisms (which is secondary transport) tied to the

transportation of sodium. The PCT has simple cuboidal cells that sit against a basement

membrane. Substances that cross these cells via a sodium symport mechanism include calcium,

chloride, glucose, amino acids, and phosphorus. All of these reabsorption processes ultimately

involve ATP energy and the active transport of sodium.

About two-thirds of the total amount of sodium, potassium, and water entering the nephron

get reabsorbed in the PCT. Other substances, like amino acids, glucose, and vitamins get 100

percent absorbed (unless the glucose level is extremely high). Glucose is bound along with

sodium and is transported along with sodium in the PCT. Much of the chloride, calcium,

magnesium, and phosphorus get reabsorbed in the PCT.

The reabsorption of bicarbonate will be covered more extensively in the chapter on acid-base

balance. Its absorption is tied to the acid-base balance in the body, involving an enzyme called

carbonic anhydrase. Bicarbonate forms carbonic acid (CO2 and water) in the lumen of the PCT.

The CO2 gets transported back through the basement membrane, where it gets enzymatically

converted back into bicarbonate ions into the interstitial space outside of the PCT. At the same

time, hydrogen ions get secreted into the PCT lumen.

As mentioned, there are thick and thin segments of the loop of Henle. They participate in the

recovery of more sodium and water in the filtrate. When these are recovered, they enter the

vasa recta to get back into the systemic circulation. The thin segments of the loop of Henle have

aquaporin channel proteins to allow water to flow unrestricted, accounting for about 15

percent of the water taken up by the nephron. Small amounts of urea, sodium, and other ions

are recovered.