Difference Between Filtration and Reabsorption

The key difference between filtration and reabsorption is that filtration is the first step of urine formation in which blood is filtered via glomerulus of the nephron while reabsorption is the second step of urine formation in which necessary nutrients return back to the blood from the glomerular filtrate.

Metabolism, not only produces necessary products, but it also generates a lot of unwanted products inside our bodies. However, the excretion process operates quickly and removes these waste products from our body. The kidney is the main organ that carries out excretion. There is a pair of kidneys in humans.

Kidney has a good blood supply, and it regulates the composition of blood in a regular state. Therefore, kidneys are important in homeostasis. The basic structural and functional unit of the kidney is nephrons. Each kidney has about one million nephrons. Each nephron contains six main regions: renal corpuscle, proximal convoluted tubule, descending limb of the loop of Henle, ascending limb of the loop of Henle, distal convoluted tubule and collecting duct. Blood purification and urine formation mainly occur in the nephrons. There are three main steps of urine formation: ultrafiltration, selective reabsorption, and secretion.

CONTENTS

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

What is Filtration?

Filtration is the first step in urine formation. It takes place in the renal capsule under a pressure. The pressure comes from the blood pumping pressure. Blood enters the glomerulus at high pressure, direct from the heart. The glomerulus is a knot of capillaries in the renal capsule. The diameter of these capillaries is less than that of the renal arteriole. Therefore, as the blood enters the narrow capillaries, the pressure increases more inside the renal capsule.

Moreover, the diameter of the efferent arteriole is less than that of afferent arteriole diameter. Thus, this increases the blood pressure in the glomerulus. At this point, water and small molecules are squeezed out of the capillaries through the epithelium of the renal capsule into the interior of the capsule. We call this filtrate glomerular filtrate, and it has the composition of blood, but it doesn’t have large blood proteins, platelets, and other large molecules.

What is Reabsorption?

Filtration produces about 125 cm3 of glomerular filtrate per minute in humans and 1.5 dm3 of urine per day. Thus, a great deal of reabsorption must occur. Furthermore, the filtrate contains a lot of essential nutrients for the body. Hence, reabsorption is an essential step at this moment in order to uptake necessary nutrients and excrete unnecessary waste. Essential molecules are reabsorbed to the blood from the filtrate through selective reabsorption.

Moreover, this process takes place as the filtrate passes through various sections of nephrons. Some areas are specifically adapted to reabsorb certain elements only. Largest reabsorption takes place in the proximal convoluted tubule in which glucose, amino acids, ions, water vitamins, hormones, about 80% of NaCl are reabsorbed to the blood. The loop of Henle reabsorbs water and sodium chloride. Due to the reabsorption, filtrate becomes concentrated. Finally, it excretes from the body as urine.

What are the Similarities Between Filtration and Reabsorption?

• Filtration and reabsorption are two steps of urine formation.
• Both take place in the nephrons.

What is the Difference Between Filtration and Reabsorption?

Filtration is the first step of urine formation that takes place in the glomerulus of the nephron. Whereas, reabsorption is the second step that takes place in other parts of the nephron. So, this is the key difference between filtration and reabsorption. Blood filters via the glomerular capillaries to the renal capsule. When the glomerular filtrate travels along the proximal convoluted tubule, descending limb of the loop of Henle, ascending limb of the loop of Henle and distal convoluted tubule, necessary nutrients are reabsorbed to the blood. Moreover, another difference between filtration and reabsorption is that the filtration is not a very selective process, but reabsorption is highly selective.

The following infographic summarizes the difference between filtration and reabsorption.

Summary – Filtration vs Reabsorption

Filtration and reabsorption are two major steps in urine formation. Filtration occurs first and then reabsorption occurs. During filtration, blood filters inside the renal capsule and forms the glomerular filtrate. During reabsorption, necessary nutrients in the glomerular filtrate are reabsorbed back to the blood from the rest of the parts of the nephron. Unlike filtration, reabsorption is selective. Thus, this is a summary of the difference between filtration and reabsorption.

Reference:

1. “Nephron – Definition, Function and Structure.” Biology Dictionary, Biology Dictionary, 29 Apr. 2017, Available here.

Image Courtesy:

1. “Kidney nephron molar transport diagram” By Nephron-urine.svg: M•Komorniczak -talk-, polish wikipedist.Kidney_nephron_molar_transport_diagram.png: *Nephron-urine.svg: M•Komorniczak -talk-, polish wikipedist.derivative work: Juvo415 (talk)derivative work: Mcstrother (talk) – Nephron-urine.svgKidney_nephron_molar_transport_diagram.png (CC BY-SA 3.0) via Commons Wikimedia
2. “Kidney nephron molar transport diagram” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia