Tubular Secretion And Tubular Reabsorption: Definition And Difference

What Are Tubular Reabsorption & Tubular Secretion?

It is a vital process in the nephron, which ensures that waste products are excreted and important substances reabsorbed into the bloodstream to maintain the homeostasis of the body. These processes are very basic in balancing electrolytes, flushing out waste products, and thus regulating blood pH, all of which are vital for normal functioning.

Nephron – Structural Basis of Reabsorption & Secretion

Nephron is termed as the functional unit of the kidney. It is a very important structure for filtering blood, reabsorption of important substances back into the blood, and secretion of waste products. There are approximately a million nephrons in each kidney.

Components

It is made up of the following structures:

• Glomerulus: The type of capillary bed that is involved in the filtration of the blood

• Bowman's capsule: The structure surrounding the glomerulus and into which filtrate enters

• Proximal convoluted tubule: The nephron segment is involved in the reabsorption of nutrients, ions, and water.

• Loop of Henle: Involved in urine concentration and reabsorption of water and salts back.

• Distal convoluted tubule: Further fine-tunes the composition of the fluid that eventually became urine.

• Collecting duct: Actual final concentration of urine

Tubular Reabsorption

The process by which the nephron removes water and solutes from the tubular fluid and returns them to the blood. Such reabsorption is essential for the conservation of valuable substances—e.g., glucose—and also for the maintenance of fluid and electrolyte balance.

Transport Mechanisms

It uses both active and passive transport mechanisms to transport the substances from the tubular fluid and enter into the blood.

• Active reabsorption implies pumping reabsorbed substances through the membrane (against the gradient of their concentration)

• Passive transfer (reabsorption of a substance) delivers the energy of the transfer, which is powered by the gradient of diffusion.

Reabsorption at Different Parts of Nephron

• Proximal convoluted tubule: Reabsorbs a very high percentage of filtered water, glucose, amino acids, and ions.

• Loop of Henle: Reabsorbs water and salts till the urine has been concentrated.

• Distal convoluted tubule: Balances the ion and water concentration.

• Collecting duct: Under hormonal control, for example by ADH, which provides variability of the added absorption fraction of water by the needs of a body.

Substances Reabsorbed

A large portion of filtrate is reclaimed as useful substances are selectively reabsorbed back into the bloodstream. This process ensures the conservation of essential nutrients and maintains fluid and electrolyte balance.

• Water

• Glucose

• Amino acids

• Na⁺, Cl⁻, HCO₃⁻

• Urea (partial)

Tubular Secretion

Tubular secretion is defined as the process through which the nephron adds substances from the blood back into the tubular fluid. It serves to clear the blood and regulate its pH.

Mechanism

An active process where substances are taken out of the blood and added to the tubular fluid, mainly for excretion

Secretion at Different Parts of Nephron

• Proximal convoluted tubule: Hydrogen ions, ammonia, and some drugs.

• Distal convoluted tubule: Site of potassium and hydrogen ions-secretion

• Collecting duct: Secretes hydrogen and potassium ions, for regulating the pH of urine

Substances Secreted

The kidneys secrete certain ions and metabolic wastes into the tubular fluid to maintain acid–base balance and remove harmful substances.

• Hydrogen ions

• Potassium ions

• Creatinine

• Drugs

• Toxins

Hormonal Regulation of Reabsorption & Secretion

Hormones that regulate reabsorption and secretion are:

ADH (Antidiuretic Hormone)

• ADH acts mainly on the collecting ducts, increasing their permeability to water and allowing more water to be reabsorbed into the bloodstream.

• This helps conserve body water and concentrate the urine when the body is dehydrated.

Aldosterone

• Aldosterone acts on the distal convoluted tubule (DCT) and collecting duct to enhance sodium reabsorption while simultaneously increasing potassium secretion.

• This hormone helps regulate blood pressure, blood volume, and electrolyte balance.

Tubular Secretion and Reabsorption NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Tubular reabsorption

• Hormonal regulation of reabsorption and secretion

Practice Questions for NEET

Q1. Out of the following excretory materials, the ones which are not reabsorbed include

1. Glucose and amino acid

2. Sulphates and creatinine

3. Chlorides and phosphates

4. Sodium and potassium

Correct answer: 2) Sulphates and creatinine

Explanation:

In Tubular Reabsorption in PCT :

• Sulphates and creatinine are not reabsorbed.

• Sodium and potassium are reabsorbed by the primary active transport.

• Glucose and amino acids Sulphates and creatinine are not reabsorbed.

Hence, the correct answer is option 2) Sulphates and creatinine.

Q2. When the volume of body fluid falls below normal, ADH

1. Decreases permeability of distal convoluted tubule and collecting tubule

2. Increases permeability of distal convoluted tubule and collecting tubule

3. Has nothing to do with permeability of convoluted tubule

4. Decreases permeability of proximal convoluted tubule

Correct answer: 2) Increases permeability of distal convoluted tubule and collecting tubule

Explanation:

ADH can also affect kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR.

Hence, the correct answer is option 2) Increases permeability of distal convoluted tubule and collecting tubule.

Q3. Which one of the following statements is correct with respect to kidney function regulation?

1. When someone drinks a lot of water, ADH release is suppressed.

2. Exposure to cold temperature stimulates ADH release.

3. An increase in glomerular blood flow stimulates the formation of angiotensin II.

4. During summer when body loses a lot of water by evaporation, the release of ADH is suppressed

Correct answer: 1) When someone drinks a lot of water, ADH release is suppressed.

Explanation:

Antidiuretic hormone (ADH) plays a crucial role in regulating water balance in the body by facilitating water reabsorption from the distal parts of the nephron (like the collecting ducts), thus preventing excessive urine production (diuresis). When the body’s fluid volume increases, it can signal the osmoreceptors in the hypothalamus to suppress the release of ADH, completing a negative feedback loop. This reduction in ADH release decreases water reabsorption, allowing the excess water to be excreted as urine, helping to maintain homeostasis in the body's fluid levels.

Hence the correct answer is option 1) When someone drinks a lot of water, ADH release is suppressed.

Also Read:

• MCQs on Tubular Reabsorption

• Urine Formation and Osmoregulation

• Dialysis

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