Nephrons

Nephrons are the smallest functional unit of the kidneys. The nephron using 4 different mechanisms to convert blood into urine:

  1. Filtration

  2. Reabsorption

  3. Secretion

  4. Excretion

Filtration

The blood entering the kidneys from the renal artery flows into each nephron and enters the glomerulus (small clusters of blood vessels). From the glomerulus, the blood pressure drops and blood flows into Bowman’s capsule.

Because red blood cells are too large to pass into Bowman’s capsule, only water, ions, and amino acids pass into it. This mixture is called the filtrate and as it travels through the rest of the nephron, it gets further modified.

Reabsorption and Secretion

The filtrate leaves Bowman’s capsule and enters the proximal convoluted tubule.

Proximal Convoluted Tubule

Here, nutrients such as amino acids, glucose, and vitamins are reabsorbed. They are transported out of the nephron and into the interstitial space where they are reabsorbed by the capillaries that run next to the proximal convoluted tubule.

Reabsorption also occurs in the opposite direction where waste products like ammonia and urea are transported into the filtrate to be excreted.

Loop of Henle:

  1. Descending Limb

    • This portion is only permeable to water

    • This portion descends from the cortex into the medulla, here the concentration of salts increases and as such water further diffuses out of the filtrate.

  2. Ascending Limb

    • This portion is only permeable to salts

    • Here, salts leave the filtrate resulting in the high salt concentration of the medulla.

    • The filtrate gets diluted as it travels up this portion

Distal convoluted Tubule:

This portion is similar to the proximal convoluted tubule where waste products like ammonia and urea and transported into the filtrate for excretion. However, calcium and sodium are reabsorbed.

Collecting Ducts:

While not a part of the nephron, the filtrate empties into collecting ducts at the end of the distal convoluted tubule.

Filtrates from the other nephrons combine in the collecting ducts and move back into the medulla. Due to the high salt content of the medulla, even more water leaves the collecting ducts, thereby further concentrating the filtrate.

Excretion

Urine eventually leaves the collecting ducts through the renal papillae. The renal papillae empty into minor calyces, a few of which merge to form a major calyx. The major calyces drain into the renal pelvis and finally travel through the ureter to reach the urinary bladder.