Difference Between Ultrafiltration and Selective Reabsorption

The key difference between ultrafiltration and selective reabsorption is that ultrafiltration is the process of filtering small molecules such as water, glucose, amino acids, sodium chloride and urea from blood to the glomerulus capsule due to the high hydrostatic pressure, while selective reabsorption is the process of reabsorbing certain important molecules from the glomerular filtrate back to the blood.

Nephron is the basic structural and functional unit of our kidney. It is a microscopic structure composed of several parts. There are millions of nephrons in one kidney. Nephrons filter toxins and wastes from our blood. Wastes and toxins excrete from our body as urine. Thus, the formation of urine takes place mainly in the nephrons. Urine production occurs via four mechanisms. Ultrafiltration and selective reabsorption are two of these main steps. So, this article will highlight the difference between ultrafiltration and selective reabsorption.

CONTENTS

1. Overview and Key Difference
2. What is Ultrafiltration 
3. What is Selective Reabsorption
4. Similarities Between Ultrafiltration and Selective Reabsorption
5. Side by Side Comparison – Ultrafiltration vs Selective Reabsorption in Tabular Form
6. Summary

What is Ultrafiltration?

Ultrafiltration is the first step in urine production. It takes place in the glomerulus of the nephron. It is the process of filtering blood in order to remove nitrogenous waste and excess fluid from our body as urine. Ultrafiltration occurs at the barrier between the blood and the filtrate in the glomerular capsule. The filtrate that results from the ultrafiltration is glomerular filtrate or ultrafiltrate. It consists of molecules such as water, salts, amino acids, glucose and urea filtered out from the blood.

Ultrafiltration takes place due to the pressure in the glomerular capillaries. Afferent arteriole delivers blood to the glomerulus. On the other hand, efferent arteriole carries blood away from the glomerulus.  The diameter of the efferent arteriole is smaller than the diameter of the afferent arteriole. Therefore, a pressure exists in the glomerulus, and it pushes small molecules of the blood through the small pores of the glomerular capillaries.

What is Selective Reabsorption?

Selective reabsorption is a major mechanism of urine production. It is the process of absorbing certain molecules from the glomerular filtrate back to the blood.

Selective reabsorption takes place in the proximal convoluted tubule. During this process, sodium (Na⁺) and chloride (Cl⁻) ions, glucose, amino acids and vitamins move back into the blood. Generally, selective reabsorption utilizes energy. Moreover, with the consumption of energy, these essential ions actively transport into the blood capillaries. Sodium-potassium pump is one ion channel involved in selective absorption.

What are the Similarities Between Ultrafiltration and Selective Reabsorption?

• Ultrafiltration and selective reabsorption are two major steps in urine production.
• Moreover, ultrafiltration produces glomerular filtrate, and selective reabsorption absorbs certain molecules from the glomerular filtrate.

What is the Difference Between Ultrafiltration and Selective Reabsorption?

Ultrafiltration is the process of filtering small molecules from blood to the glomerular filtrate at the glomerular capsule. On the other hand, selective reabsorption is the process of absorbing important substances from the ultrafiltrate back to the blood at the proximal convoluted tubule. So, this is the key difference between ultrafiltration and selective reabsorption.

Moreover, a further difference between ultrafiltration and selective reabsorption is that the ultrafiltration occurs under pressure while selective reabsorption occurs via an active transport with the use of energy.

Below is the summary of the difference between ultrafiltration and selective reabsorption.

Summary – Ultrafiltration vs Selective Reabsorption

Ultrafiltration is the first step of urine production in which small molecules filter through Bowman’s capsule to the filtrate from the blood. On the other hand, selective reabsorption is the second step of urine production in which important molecules reabsorb back to the blood capillaries from the ultrafiltrate. So, this is the key difference between ultrafiltration and selective reabsorption. Furthermore, ultrafiltration occurs in the glomerulus, while selective reabsorption occurs in the proximal convoluted tubule.

Reference:

1. “Renal Physiology: Glomerular Filtration.” Khan Academy, Khan Academy, Available here.

Image Courtesy:

1. “Physiology of Nephron” By Madhero88 – Own workReferenceshere (CC BY 3.0) via Commons Wikimedia
2. “2618 Nephron Secretion Reabsorption” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia