Question

The maximum amount of electrolytes and water ( \(70-80\) per cent) from the glomerular filtrate is reabsorbed in which part of the nephron? (a) Ascending limb of loop of Henle (b) Distal convoluted tubule (c) Proximal convoluted tubule (d) Descending limb of loop of Henle

Short answer

The maximum reabsorption of electrolytes and water from the glomerular filtrate occurs in the proximal convoluted tubule.

Step-by-step solution

Understand the Function of the Nephron and Its Regions

The nephron is the functional unit of the kidney responsible for the filtration and reabsorption of electrolytes and water. Different regions of the nephron have specialized roles in this process.

Identify the Region of Maximum Reabsorption

The proximal convoluted tubule is responsible for the majority of reabsorption. Approximately 70-80% of the electrolytes and water from the glomerular filtrate is reabsorbed in this region.

Choose the Correct Answer

Given that the proximal convoluted tubule is the site where the maximum amount of electrolytes and water is reabsorbed, the correct answer is (c) Proximal convoluted tubule.

Key concepts

Proximal Convoluted Tubule

The proximal convoluted tubule (PCT) is the first segment of the nephron the filtrate enters after processing by the glomerulus. It plays a crucial role in the body’s capacity to manage electrolytes, water balance, and pH. Within this twisting passageway, a significant proportion of the initial filtrate volume is reabsorbed, approximately 70-80%. This includes vital substances such as glucose, amino acids, and a large fraction of the filtered bicarbonate.

In addition to solute reabsorption, the PCT also reabsorbs water, which helps in concentrating urine. This is made possible by the presence of a permeable lining in the PCT that allows water to follow the reabsorbed solutes, a mechanism known as 'solvent drag'.

Importance of Transporters

Membrane proteins and transporters facilitate the movement of molecules from the filtrate back into the blood. These include sodium-potassium pumps, which create a concentration gradient allowing for secondary active transport of other substances.

Glomerular Filtration

Glomerular filtration is the first step in the formation of urine within the renal nephron. It occurs when blood pressure forces plasma, electrolytes, and small molecules through the filtration barrier within the glomerulus. This process results in the creation of glomerular filtrate, a liquid that closely resembles plasma but excludes larger proteins and cells.

Regulated by various factors including blood pressure, glomerular filtration rate (GFR) is an essential concept for understanding kidney function. A healthy GFR ensures that the kidneys are effectively filtering waste while retaining the substances needed for homeostasis.

The Filtration Barrier

The glomerulus’s three-layered filtration barrier comprises endothelial cells, the glomerular basement membrane, and podocytes. These layers work together to prevent the passage of cellular elements and maintain the selective permeability necessary for precise filtration.

Electrolytes and Water Reabsorption

Reabsorption of electrolytes and water in the nephron is a selective and active process vital for fluid and electrolyte balance. After filtration, the proximal convoluted tubule reabsorbs the bulk of these substances, but the process continues along the nephron’s course.

Electrolytes like sodium, potassium, and chloride are reabsorbed via various transport mechanisms, including active transport and co-transport. Alongside electrolytes, water follows through osmosis, driven by the osmotic gradient created by solute reabsorption. This is especially prominent in the proximal tubule, where most reabsorption occurs.

Regulation of Reabsorption

Hormones such as aldosterone and antidiuretic hormone (ADH) play significant roles in later parts of the nephron, fine-tuning the reabsorption of water and certain electrolytes to maintain homeostasis.