Mechanisms of ion reabsorption
The primary ion that determines osmotic pressure and the reabsorption of water is sodium ($Na^+$).
The process of sodium reabsorption occurs in two stages:
- Passive influx: At the apical membrane of the cell (facing the tubular lumen), $Na^+$ enters passively along its concentration gradient. The potential of this membrane is 69 mV.
- Active efflux: At the basolateral membrane (facing the blood capillary), $Na^+$ is actively pumped out of the cell by the $Na^+,K^+$-ATPase. The potential of the basolateral membrane is 70 mV.
The total energy expenditure for sodium transport is low, as the overall potential difference between blood and urine is only 1 mV.
Other ions are reabsorbed through different mechanisms:
- Potassium ($K^+$) is actively reabsorbed at the apical membrane and enters the blood via diffusion.
- Mechanisms for $Ca^{2+}$, $Mg^{2+}$, $SO_4^{2-}$, and $HPO_4^{2-}$ are similar to the transport mechanisms of sodium, potassium, and chloride.
- Anions (e.g., $Cl^-$, $HCO_3^-$) return to the blood passively, following positively charged cations ($Na^+$, $K^+$).
Transport of organic substances and water
In the proximal convoluted tubules, essential organic compounds such as glucose, amino acids, vitamins, low-molecular-weight proteins, and trace elements are completely reabsorbed.
The transport of organic substances includes:
- Active influx: Substances cross the apical membrane via facilitated diffusion (co-transport) coupled with sodium ions. This process requires energy (macroergic phosphates).
- Passive efflux: Substances diffuse across the basolateral membrane into the blood along their concentration gradient.
Water is reabsorbed passively: it follows reabsorbed solutes (primarily sodium) down an osmotic gradient. As water is reabsorbed, the concentration of dissolved substances in the urine may increase, which also facilitates their passive reabsorption.
Threshold substances
There is a group of substances that are completely reabsorbed into the blood only up to a certain concentration, known as the renal threshold. If the concentration exceeds this threshold, transport systems become overwhelmed, and the substance begins to be excreted in the final urine.
A classic example is glucose:
- At normal blood concentrations (3.8–7.1 mmol/L), it is completely filtered and returned to the bloodstream.
- If the glucose level exceeds 7.1 mmol/L, a portion of it remains in the tubules.
- The appearance of glucose in the urine is called glucosuria.
Tubular secretion and pH regulation
Concurrently with reabsorption, tubular secretion takes place—the active transport of substances from the blood into the tubular lumen. The primary purpose of secretion is the removal of metabolic waste products (choline, ammonia, para-aminohippuric acid) and drug molecules that were not filtered in the glomeruli.
Processes in the epithelium of the proximal tubules are of particular importance:
- Ammoniagenesis: Cells take up glutamine and, using the enzyme glutaminase, break it down into glutamic acid and ammonia. Ammonia is secreted and excreted in the form of ammonium salts.
- Regulation of acid-base balance: The enzyme carbonic anhydrase breaks down carbonic acid. Bicarbonate ($HCO_3^-$) is reabsorbed into the blood to maintain alkaline reserve, while protons ($H^+$) are secreted into the urine, where they bind to sodium hydrogen phosphate ($Na_2HPO_4$) to form dihydrogen phosphate ($NaH_2PO_4$). This prevents acidosis and makes the final urine acidic.
Briefly: answers to topical questions
Which endogenous metabolic products are eliminated from the blood via tubular secretion?
Through tubular secretion, epithelial cells actively transport substances from the blood into the tubular lumen. The goal is the elimination of high-molecular-weight metabolic products that did not pass through the glomerular filter.
Substances and ions that are secreted into the urine include:
- Ammonia — eliminated via secretion and excreted in the urine as ammonium salts.
- Hydrogen ions ($H^+$) — secreted into the urine; their removal prevents body acidification and contributes to the acidic reaction of the final urine.
- Potassium ions ($K^+$) — can be secreted from the blood into the urine; in case of $K^+$ overload in the blood, passive secretion occurs in the distal tubules.
- Choline.
- Organic acids and bases.
Which hormones regulate tubular reabsorption of sodium and water in the distal nephron?
Tubular reabsorption of sodium and water in the distal nephron is regulated by aldosterone, vasopressin, and natriuretic peptide.
- Aldosterone — activates $Na^+,K^+$-ATPase, enhancing facultative reabsorption of $Na^+$ and passively following water.
- Natriuretic peptide — inhibits $Na^+,K^+$-ATPase, which decreases sodium and water reabsorption, thereby increasing diuresis.
- Vasopressin (ADH) — stimulates the adenylate cyclase system of distal tubule and collecting duct cells, increasing the number of aquaporin-2 channels on the apical membrane, which ensures facultative water reabsorption.
In which part of the nephron does facultative water reabsorption occur?
Facultative (regulated) water reabsorption occurs in the distal convoluted tubules and collecting ducts of the kidneys.
- In the distal convoluted tubules, up to 10% of water is reabsorbed. This process is variable and hormonally regulated depending on the body's needs.
- In the collecting ducts, about 4% of water undergoes facultative reabsorption. Here, under the influence of antidiuretic hormone (vasopressin), final urine concentration takes place by increasing wall permeability (activating aquaporins) and allowing water to move down an osmotic gradient.
Class and exam questions
In which part of the nephron is glucose reabsorbed?
Normally, glucose is completely reabsorbed in the proximal convoluted tubules.
What is the significance of hydrogen ion secretion?
Secretion of $H^+$ ions removes excess acids from the body, preventing blood acidification (acidosis), and ensures the acidic reaction of urine.
How is water reabsorbed in the renal tubules?
Water is reabsorbed passively. It follows actively reabsorbed solutes (primarily sodium ions) down an osmotic gradient.
What are threshold substances?
These are substances (e.g., glucose) that are completely reabsorbed only up to a specific blood concentration level. Exceeding this threshold causes them to appear in the urine.
What else to study on this topic
- Mechanism of $Na^+,K^+$-ATPase action
- Functional characteristics of carbonic anhydrase
- Urine buffer systems and the role of sodium hydrogen phosphate
- Physiological basis of ammoniagenesis
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Tubular reabsorption" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Excretion" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.