Filtration, reabsorption, and secretion take place in the process of producing urine.The physiologic goal is to change the composition of the blood plasma, which, in turn, allows only waste to be eliminated as urine.I discussed filtrate formation in the last section.We will now examine how nutrients are selectively returned to the blood and how urine is regulated.

Reabsorption

In a day, up to 180 liters of fluid pass through nephrons of the kidney, such a volume must be reabsorbed.The proximal convoluted tubule, loop of Henle, distal convoluted tubule, and to a lesser extent, collecting ducts undergo reabsorption.

Nephrons reabsorb specific solutes and water differently depending on the portion of the tissue.The majority of reabsorption and secretion occurs passively due to concentration gradients, but the amount of water reabsorbed and lost is closely regulated.It is usually recovered in the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.Around 10 percent (about 18 L) reaches the collecting ducts.Antidiuretic hormone and aldosterone regulate how much water is retained in urine.As a result of antidiuretic hormone, the collecting ducts can recover almost all of the water that passes through them, in the case of dehydration, or almost none of the water if the person is overhydrated.


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1. Figure.Nephron secretion sites and reabsorption sites.There are arrows pointing away from the tubules, indicating substances that return to the blood.An arrow pointing to a tubule indicates that additional substances are being removed from the blood and moved into the filtrate.


GlucoseAlmost 100 percent reabsorbed; secondary active transport with Na+
Oligopeptides, proteins, amino acidsAlmost 100 percent reabsorbed; symport with Na+
VitaminsReabsorbed
LactateReabsorbed
CreatinineSecreted
Urea50 percent reabsorbed by diffusion; also secretedSecretion, diffusion in descending limbReabsorption in medullary collecting ducts; diffusion
Sodium65 percent actively reabsorbed25 percent reabsorbed in thick ascending limb; active transport5 percent reabsorbed; active5 percent reabsorbed, stimulated by aldosterone; active
ChlorideReabsorbed, symport with Na+, diffusionReabsorbed in thin and thick ascending limb; diffusion in ascending limbReabsorbed; diffusionReabsorbed; symport
Water67 percent reabsorbed osmotically with solutes15 percent reabsorbed in descending limb; osmosis8 percent reabsorbed if antidiuretic hormone; osmosisVariable amounts reabsorbed, controlled by antidiuretic hormone, osmosis
Bicarbonate80–90 percent symport reabsorption with Na+Reabsorbed, symport with Na+ and antiport with Cl–; in ascending limbReabsorbed antiport with Cl–
H+Secreted; diffusionSecreted; activeSecreted; active
NH4+Secreted; diffusionSecreted; diffusionSecreted; diffusion
HCO3–Reabsorbed; diffusionReabsorbed; diffusion in ascending limbReabsorbed; diffusionReabsorbed; antiport with Na+
Some drugsSecretedSecreted; activeSecreted; active
Potassium65 percent reabsorbed; diffusion20 percent reabsorbed in thick ascending limb; symportSecreted; activeSecretion controlled by aldosterone; active
CalciumReabsorbed; diffusionReabsorbed in thick ascending limb; diffusionReabsorbed if parathyroid hormone present; active
MagnesiumReabsorbed; diffusionReabsorbed in thick ascending limb; diffusionReabsorbed
Phosphate85 percent reabsorbed, inhibited by parathyroid hormone, diffusionReabsorbed; diffusion

Mechanisms of Recovery

In order for substances to move across membranes, they need to go through simple diffusion, facilitated diffusion, active transport, secondary active transport, and osmosis.

Substances diffuse up their concentration gradients from a higher to a lower concentration.No energy is required and the substance only needs to be soluble.

The process of facilitated diffusion is similar to simple diffusion in that it moves substances down concentration gradients.In contrast, it requires specific membrane transporters and channel proteins to move.It is important to note that facilitated diffusion occurs when glucose is transported or, in some circumstances, Na+ ions are moved.It is possible for two substances to share the same channel protein port in case of facilitated diffusion; these are known as symports and antiports.Across the cell membrane, symport mechanisms move two or more substances in the same direction, while antiport mechanisms move two or more substances in opposite directions.

.A membrane transporter must contain an appropriately shaped binding pocket to allow a substance to be transported.Na+ and K+ are transported actively by the Na+/K+ pump out and into the cell, respectively.A lower ion moves in the opposite direction to a higher ion as it is concentrated.

Symport and antiport can utilize concentration gradients maintained by ATP pumps.We call this a secondary active transport mechanism.Na+ ATPase pumps located on the cell's basilar membrane may keep Na+ constantly flowing out of the cell, maintaining a strong electrochemical gradient. .As a result, glucose diffuses from the basal membrane into the interstitial space, where it is subsequently absorbed in peritubular capillaries.

To maintain equilibrium, the nephron must reabsorb Ca2+, Na+, glucose, and amino acids.In addition, waste products like urea, K+, ammonia (NH3), creatinine, and some drugs are secreted into the filtrate.Both the lungs and kidneys regulate the acid-base balance of the body: The lungs remove H+ from the body, while the kidneys secrete or reabsorb H+ and HCO3–.About 50 percent of urea is passively reabsorbed by the proximal convoluted tubule.←.It triggers the insertion of urea transporters and aquaporin channel proteins.

Water180 L179 L1 L
Proteins10–2010–200
Chlorine6306255
Sodium5405373
Bicarbonate300299.70.3
Glucose1801800
Urea532825
Potassium28244
Uric acid8.57.70.8
Creatinine1.401.4

Reabsorption and Secretion in the Proximal Convoluted Tubule