The functional unit of the kidney is the nephron. There are approximately 1 million nephrons in each kidney, and they are responsible for filtering the blood and producing urine. Each nephron consists of a glomerulus, which is a tuft of capillaries that filter the blood, and a series of tubules that transport and modify the filtrate to form urine. The filtrate initially contains water, electrolytes, glucose, amino acids, and waste products such as urea and creatinine.

Anatomy of The Nephron

The nephron is composed of several distinct regions, each with a specific function in the process of urine formation. These regions include the renal corpuscle, the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct.

  • The Renal Corpuscle: This is the initial filtering unit of the nephron, consisting of a glomerulus and a surrounding Bowman’s capsule. Blood enters the glomerulus through the afferent arteriole and is filtered through a specialized capillary network known as the glomerular basement membrane. This process results in the formation of a protein-free, cell-free fluid called filtrate, which is collected in Bowman’s capsule. From the renal corpuscle, the filtrate enters the proximal convoluted tubule.
  • The Proximal Convoluted Tubule (PCT): This is a region responsible for the reabsorption of most of the filtered water, glucose, amino acids, and ions. This region is highly convoluted, providing a large surface area for the exchange of solutes and water between the tubule and the surrounding capillaries. After leaving the proximal convoluted tubule, the filtrate enters the loop of Henle.
  • The Loop of Henle: This region divided into a descending and an ascending limb. The descending limb is permeable to water, allowing it to be reabsorbed into the surrounding capillaries, while the ascending limb is impermeable to water but actively reabsorbs ions, such as sodium and chloride.
  • The Distal Convoluted Tubule (DCT): This region follows the loop of Henle and is responsible for further regulation of electrolyte balance, particularly the reabsorption of sodium and the secretion of potassium and hydrogen ions. This region also plays a role in regulating acid-base balance in the body.

Finally, the collecting duct collects urine from multiple nephrons and transports it to the renal pelvis for elimination from the body.

Physiology of The Nephron

The nephron is the functional unit of the kidney, responsible for filtering blood and producing urine. The physiology of the nephron involves several complex processes, including glomerular filtration, tubular reabsorption, and tubular secretion.

Glomerular filtration is the first step in urine formation, which occurs in the renal corpuscle. Blood flows into the glomerulus, a tuft of capillaries with small pores called fenestrae. The pressure of the blood forces plasma, electrolytes, and small molecules like glucose and amino acids to pass through these fenestrae and enter the Bowman’s capsule, a structure that surrounds the glomerulus. This process is driven by the hydrostatic pressure of the blood and the size of the molecules.

The filtrate that enters the Bowman’s capsule is similar in composition to plasma, but it does not contain proteins or blood cells. This filtrate then moves into the proximal convoluted tubule (PCT), where most of the reabsorption occurs. The PCT has microvilli, which increase its surface area for reabsorption.

In the PCT, glucose, amino acids, bicarbonate, and most of the electrolytes (such as Na+, K+, Ca2+, and Cl-) are reabsorbed into the blood through active transport or facilitated diffusion. Water follows the solutes passively due to the osmotic gradient created by the reabsorption of solutes. About 65% of the filtered water is reabsorbed in the PCT.

The descending loop of Henle is permeable to water, but not to electrolytes. As the filtrate moves down the descending limb, water is reabsorbed by osmosis due to the high osmolarity of the medullary interstitium. The ascending limb of the loop of Henle is impermeable to water but actively reabsorbs Na+, K+, and Cl- ions into the interstitium, creating a concentration gradient in the medulla.

The distal convoluted tubule (DCT) and the collecting duct are responsible for fine-tuning the urine composition. In the DCT, aldosterone promotes the reabsorption of Na+ ions and the secretion of K+ ions. In the presence of antidiuretic hormone (ADH), the collecting duct becomes permeable to water, which is reabsorbed to concentrate urine. The final urine is then drained into the renal pelvis and the ureter, which leads to the bladder.

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