Describe the structure of a nephron.

A nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains approximately one million nephrons, which work together to filter waste, regulate fluid and electrolyte balance, and maintain homeostasis. The nephron is made up of several key structures:

1. Renal Corpuscle:

This is the part of the nephron responsible for the initial filtration of blood. It consists of two components:

  • Glomerulus: A network of tiny, porous capillaries that receive blood from the afferent arteriole. As blood flows through the glomerulus, water, salts, glucose, and waste products are filtered out.
  • Bowman’s Capsule (Glomerular Capsule): A cup-shaped structure surrounding the glomerulus that collects the filtrate (fluid that has been filtered from the blood). The space between the glomerulus and the Bowman’s capsule is where the filtration takes place, with substances moving into the capsule to be processed further.

2. Proximal Convoluted Tubule (PCT):

  • This is the first segment of the renal tubule that follows the Bowman’s capsule. It is highly coiled and lined with microvilli to increase surface area for reabsorption.
  • In the PCT, essential substances like glucose, amino acids, water, and ions (such as sodium and potassium) are reabsorbed into the bloodstream. Some waste products, such as urea, remain in the filtrate.

3. Loop of Henle:

  • This U-shaped portion of the nephron extends into the medulla of the kidney and has two limbs:
    • Descending Limb: This part is permeable to water but not to salts. As filtrate moves down the descending limb, water is reabsorbed, concentrating the urine.
    • Ascending Limb: This part is impermeable to water but actively pumps out salts like sodium and chloride into the surrounding tissue. This helps create a concentration gradient in the kidney medulla, which plays a key role in water reabsorption.

4. Distal Convoluted Tubule (DCT):

  • This segment is more tightly coiled and located further from the renal corpuscle. Here, further regulation of electrolytes occurs, such as the reabsorption of sodium and calcium, and the secretion of potassium and hydrogen ions into the filtrate.
  • The DCT plays an important role in maintaining acid-base balance and fine-tuning the composition of urine.

5. Collecting Duct:

  • Several distal convoluted tubules from different nephrons merge to form collecting ducts. These ducts travel through the kidney’s medulla and empty into the renal pelvis.
  • In the collecting duct, the final adjustments to urine are made. Water is reabsorbed under the influence of antidiuretic hormone (ADH), and the concentration of urine is further regulated.
  • The collecting ducts ultimately carry urine to the renal pelvis, where it will move to the ureter for elimination from the body.

Blood Supply to the Nephron:

  • The nephron’s function depends on a complex network of blood vessels, particularly the afferent arteriole, which brings blood to the glomerulus, and the efferent arteriole, which carries filtered blood away.
  • The efferent arteriole leads into a network of capillaries called the peritubular capillaries (surrounding the renal tubule), where exchange of substances like water and electrolytes occurs between the blood and the filtrate.

Summary of Filtration and Reabsorption:

  1. Filtration: Blood is filtered in the glomerulus, with water, salts, glucose, and waste products entering the Bowman’s capsule.
  2. Reabsorption: In the proximal convoluted tubule and the Loop of Henle, essential substances are reabsorbed into the blood.
  3. Secretion: Waste products like potassium and hydrogen ions are actively secreted into the filtrate in the distal convoluted tubule.
  4. Final Adjustment: The collecting duct fine-tunes the urine’s composition and concentration, under the influence of hormones.

This entire process allows the kidneys to filter blood, maintain fluid and electrolyte balance, and excrete waste products efficiently.

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