4.3 Filtration, Reabsorption, and Secretion
Unit D
Module 8 ~ Lesson 4
Tubular Reabsorption
Filtrate from the glomerulus moves into the Bowman's capsule. From there, the filtrate moves along the proximal tubule where some of the filtrate is reabsorbed from the proximal tubule through active and passive transport mechanisms.
Reabsorption is the movement of substances out of the renal tubules back into the blood capillaries located around the tubules. Glucose is actively transported out of the tubules and into the capillary net that surrounds the tubule. In ordinary cases, all the glucose that seeps out through the glomeruli into the tubules is reabsorbed back into the blood. If too much glucose is present, the tubules reach the limit of their ability to pass the sugar back into the bloodstream, and the tubules retain some of it. It is then carried along in the urine, often providing a doctor with her first clue that a patient has diabetes mellitus.
The descending loop of Henle reabsorbs mostly water and some ions. The medulla region of the kidney is a salty environment. The cells of the descending loop are permeable to water and only slightly permeable to ions. Therefore, mostly water leaves the filtrate to return to the blood. The concentration of sodium ions increases towards the bottom of the loop.
The ascending loop of Henle becomes impermeable to water and slightly permeable to solutes. Sodium ions diffuse from the filtrate to nearby blood vessels. The top of the ascending loop becomes thick walled and ions are transported via active transport. This reduces the concentration of the filtrate and replenishes the salty environment of the medulla.
The distal tubule reabsorbs sodium ions, chloride ions, and water depending on the needs of the body. As a person increases the amount of salt taken into the body, the kidneys decrease the amount of sodium reabsorption back into the blood. That is, more sodium is retained in the tubules. Therefore, the amount of salt excreted in the urine increases. The less the salt intake, the greater the amount of sodium reabsorbed back into the blood, and the amount of salt excreted in the urine decreases.
Secretion
Secretion is the process by which substances move into the distal and collecting tubules from blood in the capillaries around these tubules. Secretion is reabsorption in reverse. These substances are secreted through either an active transport mechanism or as a result of diffusion across the membrane.
Substances secreted are hydrogen ions (H+), potassium ions (K+), urea, and certain drugs. Potassium (K+) and hydrogen (H+) ions are actively transported into the distal tubule from the capillaries. Potassium concentration must be maintained as it is required to properly transmit nerve impulses. Hydrogen ions need to be balanced in order to maintain proper blood pH levels. Too many hydrogen ions in the blood can create an acidic environment, which inhibits cellular function.
Excretion
Once the filtrate has moved through the entire proximal tubule (loop of Henle), it enters the collecting duct. The collecting duct extends into the salty environment of the medulla. This causes water to move across the tubule membrane by osmosis.
If a person is dehydrated, the collecting duct becomes more permeable to water, and more water is reabsorbed in order to conserve water. This process is controlled by hormones. Finally, the filtrate, which is about 1% of the original volume, is now urine and is ready to be moved by the ureters to the bladder.
Read
Read "Tubular Reabsorption: Recovery of Substances in the Proximal Tubule", "Focusing on the Loop of Henle in the Proximal Tubule", "Tubular Reabsorption and Secretion in the Distal Tubule", and "Reabsorption from the Collecting Duct" on pages 312 to 315 of your textbook.