What is the Difference Between Tubular Secretion and Reabsorption

The main difference between tubular secretion and reabsorption is that tubular secretion is the addition of selected materials from the blood into the ultrafiltrate, whereas reabsorption is the passage of selected materials from the ultrafiltrate into the blood. 

Tubular secretion and reabsorption are two physiological processes that occur in the nephron. They are important for the production of urine from the glomerular filtrate. 

Key Areas Covered

1. What is Tubular Secretion
– Definition, Process, Importance
2. What is Reabsorption
– Definition, Process, Importance
3. Similarities Between Tubular Secretion and Reabsorption
– Outline of Common Features
4. Difference Between Tubular Secretion and Reabsorption
– Comparison of Key Differences

Key Terms 

Reabsorption, Tubular Secretion

What is Tubular Secretion

Tubular secretion is the passage of materials from the peritubular capillaries into the renal tubular lumen. Following the tubular secretion, the substances remaining in the collecting duct are known as urine. The secreted substances include creatinine, hippuric acid, H⁺, K⁺, NH₃, waste products, and drugs. The H⁺ and NH₄⁺ ions secreted into the filtrate in the renal tubular lumen are responsible for the pH of the urine. Typically, the pH of urine is 6.0. Significantly, tubular secretion occurs in the opposite direction of the reabsorption. The processes involved in tubular secretion include passive diffusion and active transport.

Figure 1: Renal Physiology

Furthermore, tubular secretion occurs at the nephron’s proximal and distal convoluted tubules. There, K⁺ is secreted by the sodium-potassium pump, while co-transport and active transport are responsible for the secretion of H⁺.

What is Reabsorption

Reabsorption is the movement of materials from the renal tubular lumen into the blood. It occurs throughout the nephron; proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct. 70% of the filtrate is reabsorbed into the blood while passing through renal tubules and ducts. The substances that undergo reabsorption include glucose, amino acids, Na⁺, K⁺, HCO₃^–, Cl^–, and water. Also, it is an energy-consuming process, and molecular pumps are involved.

Figure 2: Material Transport in Nephron

Moreover, most of the water and glucose undergo reabsorption in the PCT. Approximately 65% of sodium ions are reabsorbed into the cells of PCT by symporters. Along with Na⁺ ions, symporters can reabsorb other molecules like glucose, amino acids, lactic acid, and HCO₃– ions. Also, 25% of the Na+ ions in the filtrate undergo reabsorption by the loop of Henle along with the remaining water. Water undergoes reabsorption in the descending limb of the loop of Henle, while Na⁺ and Cl^– ions undergo reabsorption in its ascending limb.

Additionally, the water reabsorption in the DCT depends on the blood’s anti-diuretic hormone (ADH) level. More ADH in the blood allows more water to be reabsorbed. 8% of the remaining Na⁺ ions in the filtrate undergo reabsorption in the DCT. Further, only if aldosterone is present, only 2% of Na⁺ ions undergo reabsorption from the remaining filtration at the collecting duct.

Similarities Between Tubular Secretion and Reabsorption

• Tubular secretion and reabsorption are two physiological processes that occur in the nephron.
• They are important for the production of urine from the glomerular filtrate.
• They allow the passage of different molecules between the blood and the glomerular filtrate. 

Difference Between Tubular Secretion and Reabsorption

Definition

Tubular secretion refers to the transfer of materials from peritubular capillaries to the renal tubular lumen, while reabsorption refers to the process by which the nephron removes water and solutes from the tubular fluid (pre-urine). 

Importance

Tubular secretion is the addition of selected materials from the blood into the ultrafiltrate, while reabsorption is the passage of selected materials from the ultrafiltrate into the blood. 

Effect on the Volume of the Ultrafiltrate

Tubular secretion increases the volume of the ultrafiltrate, while reabsorption decreases the volume of the ultrafiltrate. 

Occurrence

Tubular secretion occurs in the proximal and distal convoluted tubules, while reabsorption occurs throughout the urinary tubules.

Materials

Tubular secretions include creatinine, hippuric acid, H⁺, K⁺, NH₃, and foreign substances, while reabsorption materials include glucose, amino acids, Na⁺, K⁺, HCO₃-, Cl^–, and water.

Conclusion

In brief, tubular secretion and reabsorption are two processes that occur in the nephron. Tubular secretion is the addition of selected materials from the blood into ultrafiltrates, such as creatinine, hippuric acid, H⁺, K⁺, NH₃, and foreign substances. It occurs in the proximal and distal convoluted tubules. Also, it increases the volume of the ultrafiltrate. In comparison, reabsorption is the passage of selected materials from the filtrate into the blood, such as glucose, amino acids, Na⁺, K⁺, HCO₃-, Cl^–, and water. However, it occurs throughout the urinary tubules and decreases the volume of the ultrafiltrate. Therefore, the main difference between tubular secretion and resorption is their importance. 

References:

1. (2023, January 17). 24.3E: Tubular Secretion. Medicine LibreTexts. 
2. Reabsorption. Reabsorption – an overview | ScienceDirect Topics. 

Image Courtesy:

1. “Physiology of Nephron” By Madhero88 – Own work (CC-BY 3.0) via Commons Wikimedia
2. “Kidney nephron molar transport diagram” By M.Komorniczak – Own Work (CC-BY SA 3.0) via Commons Wikimedia