Question

Brush border is a characteristic of
(a) Proximal convoluted tubule
(b) Neck of the nephron
(c) Collecting tube
(d) All of the above

Answer 1

Hint: Nephrons, in kidneys, are the primary structures for blood filtration. A typical structure of the nephron consists of a renal corpuscle or malpighian corpuscle with renal tubules.Brush border is a characteristic of a portion of renal tubule which is heavily involved in reabsorption.

Complete answer:
Nephrons are small, highly coiled tubular structures that contain a ‘renal corpuscle’ from where the blood plasma is filtered and a ‘renal tubule’ into which the filtrate enters and passes through. Renal tubules have four differentiated regions namely the proximal tubule (PCT), the loop of Henle, and the distal tubule (DCT). Many nephron distal convoluted tubules open into a straight tube called a collecting duct.
The proximal convoluted tubule is the anterior-most portion of the renal tubules or the first tubule in which the filtrate enters from the bowman’s capsule. It is lined by cuboidal epithelium, the cells of which bear microvilli or finger- like processes at their apical portion. This gives a brush-border appearance of the proximal convoluted tubule. The basal portion of these cells bears numerous mitochondria to perform the active reabsorption of salts.

So, the correct answer is ‘Proximal convoluted tubule.’

Additional Information:
- A renal corpuscle consists of a small cup- like structure bounded by two layers of epithelium known as Glomerular or Bowman’s capsule. This capsule surrounds the glomerulus or capillary network which is a labyrinth of fine capillaries. Blood plasma is filtered in the Bowman’s capsule and the filtered fluid then enters the renal tubules of the nephron
- The filtrate passes into the proximal convoluted tubules where selective active reabsorption takes place.
- It is followed by a U-shaped segment that descends into the medulla and creates a salt gradient. The primary role of Henle's loop is the recovery of urine water and sodium chloride.
- Distal convoluted tubule is a folded structure that is connected to the Henle loop where further selective reabsorption takes place. Its terminal part is straight and is referred to as a junctional tubule or connecting tubule.

Note:
- The amount of filtrate formed per minute in the renal corpuscles of both kidneys is referred to as the glomerular filtration rate or GFR. In males, it amounts to 125 mL/min while in females, it is 105 mL/min.
- On average, the daily volume of glomerular filtrate is 150 liters in females and 180 litres in males. But more than $99\%$ of this filtrate is returned to the bloodstream via tubular reabsorption, so only 1- 2 litres of urine is excreted on an average by an adult.