Describe how the renal tubules reabsorb useful solutes indigenous the glomerular filtrate and return them to the blood.Describe just how the nephron regulates water excretion.Explain the role of aldosterone and also of atrial natriuretic element in sodium and also water balance.Describe the system that maintains the medullary osmotic gradient.

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The procedure of producing urine wake up in three stages: filtration, reabsorption, and secretion. The physiologic goal is to modify the composition of the blood plasma and, in act so, eliminate only rubbish in the kind of urine. In the last section, we debated filtrate formation. Now, we will certainly examine exactly how most nutrients are selectively returned into the blood, and how the composition of to pee is regulated.

Reabsorption

With up to 180 liters every day passing through the nephrons that the kidney, that is quite noticeable that most of the fluid and also its materials must be reabsorbed. Reabsorption wake up in the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and also to a lesser degree, the collecting ducts.

Various sections of the nephron differ in their capacity to reabsorb water and details solutes. While lot of the reabsorption and secretion happen passively based on concentration gradients, the amount of water that is reabsorbed or shed is strict regulated. Many water is recovered in the proximal convoluted tubule, loop of Henle, and also distal convoluted tubule. About 10 percent (about 18 L) will the collecting ducts. Antidiuretic hormone and aldosterone are responsible for regulating exactly how much water is maintained in urine. The collecting ducts, under the influence of antidiuretic hormone, can recover almost every one of the water passing v them, in situations of dehydration, or nearly none of the water, in instances of over-hydration.


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Figure 1. Places of Secretion and Reabsorption in the Nephron. Arrows pointing away from the tubule show substances that are returning to the blood. Arrows pointing in the direction of the tubule indicate additional substances being eliminated from the blood and also moved into the filtrate.


Table 1. Substances Secreted or Reabsorbed in the Nephron and Their LocationsSubstanceProximal convoluted tubuleLoop that HenleDistal convoluted tubuleCollecting ducts
GlucoseAlmost 100 percent reabsorbed; an additional active transport through Na+
Oligopeptides, proteins, amino acidsAlmost 100 percent reabsorbed; symport v Na+
VitaminsReabsorbed
LactateReabsorbed
CreatinineSecreted
Urea50 percent reabsorbed by diffusion; additionally secretedSecretion, diffusion in descending limbReabsorption in medullary collecting ducts; diffusion
Sodium65 percent actively reabsorbed25 percent reabsorbed in thick ascending limb; energetic transport5 percent reabsorbed; active5 percent reabsorbed, created by aldosterone; active
ChlorideReabsorbed, symport v Na+, diffusionReabsorbed in thin and thick ascending limb; diffusion in ascending limbReabsorbed; diffusionReabsorbed; symport
Water67 percent reabsorbed osmotically through solutes15 percent reabsorbed in to decrease limb; osmosis8 percent reabsorbed if antidiuretic hormone; osmosisVariable quantities reabsorbed, regulated by antidiuretic hormone, osmosis
Bicarbonate80–90 percent symport reabsorption through Na+Reabsorbed, symport v 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 through Na+
Some drugsSecretedSecreted; activeSecreted; active
Potassium65 percent reabsorbed; diffusion20 percent reabsorbed in thick ascending limb; symportSecreted; activeSecretion managed by aldosterone; active
CalciumReabsorbed; diffusionReabsorbed in special ascending limb; diffusionReabsorbed if parathyroid hormone present; active
MagnesiumReabsorbed; diffusionReabsorbed in special ascending limb; diffusionReabsorbed
Phosphate85 percent reabsorbed, inhibited by parathyroid hormone, diffusionReabsorbed; diffusion

Mechanisms of Recovery

Mechanisms whereby substances move across membranes for reabsorption or secretion include basic diffusion, facilitated diffusion, active transport, an additional active transport, and also osmosis.

Simple diffusion moves a problem from a higher to a lower concentration under its concentration gradient. It calls for no energy and only demands to it is in soluble.

Facilitated diffusion is similar to straightforward diffusion in that it move a substance under its concentration gradient. The distinction is the it requires certain membrane transporters or channel proteins because that movement. The activity of glucose and, in particular situations, Na+ ions, is an example of helped with diffusion. In some situations of helped with diffusion, two various substances re-superstructure the very same channel protein port; these mechanisms are described by the state symport and antiport. Symport mechanisms relocate two or more substances in the very same direction in ~ the very same time, conversely, antiport mechanisms move two or much more substances in the contrary directions throughout the cell membrane.

Active transport is once a membrane transporter uses energy, normally the energy uncovered in a phosphate shortcut of ATP, to move a substance throughout a membrane from a low to a high concentration. The membrane transporteris very specific and must have actually an as necessary shaped binding pocket because that the substance to be transported. An instance would be the energetic transport of Na+ out of a cell and also K+ into a cell by the Na+/K+ pump. Both ions are moved in opposite directions from a reduced to a higher concentration.

Both symport and antiport may utilize concentration gradients maintained by ATP pumps. This is a mechanism defined by the hatchet secondary energetic transport. Because that example, a Na+ ATPase pump ~ above the basilar membrane that a cell might constantly pump Na+ the end of a cell, preserving a strong electrochemical gradient. On the opposite (apical) surface, a Na+/glucose symport protein channel assists both Na+ and glucose right into the cell as Na+ moves down the concentration gradient produced by the basilar Na+ ATPase pumps. The glucose molecule climate diffuses throughout the basal membrane by promoted diffusion into the interstitial an are and from there into peritubular capillaries.

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Most that the Ca2+, Na+, glucose, and also amino acids need to be reabsorbed by the nephron to keep homeostatic plasma concentrations. Various other substances, such together urea, K+, ammonia (NH3), creatinine, and some drugs room secreted into the filtrate as waste products. Acid–base balance is maintained through actions of the lungs and also kidneys: The lungs rid the human body of H+, vice versa, the kidneys secrete or reabsorb H+ and HCO3– . In the situation of urea, around 50 percent is passively reabsorbed by the proximal convoluted tubule. More is recovered through in the collecting ducts as needed. Antidiuretic hormone induces the insertion that urea transporters and also aquaporin channel proteins.

Table 2. Building material Filtered and Reabsorbed through the Kidney per 24 HoursSubstanceAmount filtered (grams)Amount reabsorbed (grams)Amount in pee (grams)
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 also Secretion in the Proximal Convoluted Tubule