Difference Between Filtration and Reabsorption

Main Difference – Filtration vs Reabsorption

Filtration and reabsorption are two processes that occur in close proximity to the nephron of the kidney. Hence, they are two processes of the kidney. Along with secretion and excretion, filtration and reabsorption are involved in the formation of urine starting from the plasma. The main difference between filtration and reabsorption is that filtration is the movement of water and solutes across a cell membrane due to the hydrostatic pressure from the cardiovascular system while reabsorption is the movement of water and solutes back into the plasma from renal tubules.

Key Areas Covered

1. What is Filtration
      – Definition, Process, Features
2. What is Reabsorption
     – Definition, Process, Features
3. What are the similarities between Filtration and Reabsorption
     – Outline of Common Features
4. What is the difference between Filtration and Reabsorption
      – Comparison of Key Differences

Key Terms: Afferent Arteriole, Bowman’s Capsule, Collecting Duct, Distal Convoluted Tubule,  Efferent Arteriole, Filtration,  Glomerular Filtration, Loop of Henle, Proximal Convoluted Tubule, Reabsorption, Renal ultrafiltration, Tubular Reabsorption

What is Filtration

Filtration is the movement of water and solutes through the walls of glomerular capillaries and Bowman’s capsule of the nephron, under the pressure of the cardiovascular system. Filtration in the kidney can be considered as a passive process. In the kidney, the renal artery forms many afferent arterioles, which deliver blood to an individual nephron in the kidney. The blood leaves the nephron through the efferent arteriole. The diameter of the afferent arteriole is greater than that of the efferent arteriole. Therefore, the pressure of the blood inside the glomerulus increases, facilitating the filtration of most of the blood components into the Bowman’s capsule. The glomerular filtration is also called renal ultrafiltration.

The rate of filtration is 125 ml/min or 180 liters per day. Thus, the entire blood of a human is filtered by the kidney for 20 to 25 times per day. The filtrate comprises mainly water, glucose, small proteins (typically smaller than 30, 000 Daltons), ions like sodium, potassium, and chloride. Once filtered, the filtrate enters the Bowman’s capsule in order to flow into the proximal tubule through the lumen of the nephron. The anatomy and physiology of filtration in the glomerular capsule are shown in figure 1.

Figure 1: Filtration

What is Reabsorption

70% of the filtrate is reabsorbed into the blood while passing through renal tubules and ducts. This process is referred to as reabsorption or tubular reabsorption. It is a selective process in which only selected molecules are reabsorbed from the filtrate. Reabsorption is an energy-consuming process and molecular pumps are involved in the above-mentioned selective reabsorption. Reabsorption depends on the body’s need to reuptake molecules as well. Tubular reabsorption occurs in four distinct parts of the nephron: proximal convoluted tubule, loop of Henle, distal convoluted tubule, and the collecting duct. Reabsorption in different parts of the nephron is shown in figure 2.

Figure 2: Reabsorption

In the Proximal Convoluted Tubule (PCT)

Most of the water and glucose is reabsorbed in the PCT. Approximately 65% of sodium ions are reabsorbed into the cells of PCT by symporters. Along with the sodium ions, symporters reabsorb other molecules like glucose, amino acids, lactic acid, and bicarbonate ions.

In the Loop of Henle

25% of the sodium ions in the filtrate is reabsorbed by the loop of Henle along with the remaining water. Water is reabsorbed in the descending limb of the loop of Henle, while sodium and chloride ions are reabsorbed in its ascending limb.

In the Distal Convoluted Tubule (DCT)

The water reabsorption in the DCT depends on the anti-diuretic hormone (ADH) level in the blood. The more ADH in the blood allows more water to be reabsorbed. 8% of the remaining sodium ions in the filtrate is reabsorbed in the DCT. 

In the Collecting Duct

Only if aldosterone is present, only 2% of sodium ions is reabsorbed from the remaining filtration at the collecting duct. 

Similarities Between Filtration and Reabsorption

• Both filtration and reabsorption are two processes involved in the formation of urine from plasma.

• Both processes occur in the nephrons of the kidney.

Difference Between Filtration and Reabsorption

Definition

Filtration: Filtration is the movement of water and solutes across a cell membrane due to the hydrostatic pressure of the cardiovascular system.

Reabsorption: Reabsorption is the movement of water and solutes back into the plasma from renal tubules.

Governed by

Filtration: Filtration is governed by the hydrostatic pressure.

Reabsorption: Reabsorption is governed by osmotic pressure as well as oncotic pressure.

Active/Passive Process

Filtration: Filtration is a passive process.

Reabsorption: Reabsorption is an active process.

Selective/Physical

Filtration: Filtration is a physical process.

Reabsorption: Reabsorption is a selective process.

Correspondence

Filtration: Filtration is the initial event of the formation of urine.

Reabsorption: Reabsorption follows filtration.

Occurrence

Filtration: Filtration occurs at the Bowman’s capsule of the nephron.

Reabsorption: Reabsorption occurs at the PCT, loop of Henle, DCT, and collecting duct of the nephron.

Filtrate

Filtration: Filtration produces a diluted filtrate.

Reabsorption: The filtrate is concentrated by reabsorption.

Conclusion

Filtration and reabsorption are two subsequent processes occurring in the kidney nephrons during the formation of urine. The main difference between filtration and reabsorption is the function of the each process during the formation of urine. Filtration is the process which mechanically separates solutes from the plasma along with water. It occurs in the Bowman’s capsule. Most of the solutes are reuptaken during reabsorption at the subsequent parts of the nephron.

Reference:

1. “How Your Kidneys Work.” HowStuffWorks. N.p., 10 Jan. 2001. Web. Available here. 20 June 2017. 
2. “Blood Cleaning by the Kidneys.” Kidneys Filter Blood : Processes of Glomerular Filtration, Tubular Reabsorption and Tubular Secretion. N.p., n.d. Web. Available here. 20 June 2017. 

Image Courtesy:

1. “Anatomy and physiology of animals Filtration in the glomerulus capsule” – original uploader was Sunshineconnelly at English Wikibooks – Transferred from en.wikibooks to Commons by Adrignola using CommonsHelper (CC BY 3.0) via Commons Wikimedia
2. “Kidney nephron molar transport diagram” By (CC BY-SA 3.0) via Commons Wikimedia