Filtration vs Reabsorption
Lot of unwanted products are generating inside the bodies while the metabolism takes place. Excretion is the removal of these unwanted products, so that they won’t be harmful to the body. Urea is the nitrogenous waste product of humans and other land living mammals. Kidneys are the main organs which are responsible for this function. There is a pair of kidneys in humans situated towards the back of the lower part of the abdominal cavity. Kidney has a good blood supply, and it regulates the composition of blood in a regular state. Therefore, kidneys are important in homeostasis. The kidneys receive blood from the aorta via the renal arteries, and the blood is returned to posterior vena cava through renal veins. The waste products are brought to the kidneys via blood. The basic structural and functional unit of the kidney is nephrons. Each kidney has about one million nephrons. Each nephron is composed of six main regions as follows.
1. Renal corpuscle
2. Proximal convoluted tubule
3. Descending limb of the loop of Henle
4. Ascending limb of the loop of Henle
5. Distal convoluted tubule
6. Collecting duct
Urine production is done is three steps. They are Ultrafiltration, selective reabsorption, and secretion.
What is Filtration?
The first step in the formation of urine is filtration. This is taking place in the renal capsule. This filtration takes places under pressure. This pressure comes from the blood pumping pressure. Blood enters the glomerulus at high pressure direct from the heart.
Glomerulus is a knot of capillaries in the renal capsule. The diameter of these capillaries is less than that of the renal arteriole; therefore, as the blood enters the narrow capillaries, pressure increases more. Also, the diameter of efferent arteriole is less than the afferent arteriole diameter which again increases the blood pressure in the glomerulus. At this point, water and small molecules are squeezed out of the capillaries through the epithelium of the renal capsule and goes into the interior of the capsule. This filtrate is known as the glomerular filtrate, and it has the composition of blood except the large blood proteins, platelets and other large molecules.
What is Reabsorption?
Filtration produces about 125 cm3 of glomerular filtrate per minute in humans, but per day, only 1.5 dm3 of urine is produced. So a great deal of reabsorption must occur. Further, the filtrate contains a lot of essential nutrients for the body. These cannot be excreted, but only the unnecessary waste should be removed. So through a selective reabsorption, the essential molecules are reabsorbed to the blood from the filtrate.
This process takes place as the filtrate passes through various sections of nephrons. Some areas are specifically adapted to reabsorb certain elements only. Largest reabsorption takes place in the proximal convoluted tubule. Glucose, amino acids, ions, water vitamins, hormones, about 80% of NaCl are reabsorbed here. In the loop of Henle, water and sodium chloride are reabsorbed. After the reabsorption, the filtrate gets concentrated, and urine is produced.
What is the difference between Filtration and Reabsorption?
- Filtration takes place initially and then the reabsorption.
- Filtration occurs in the renal capsule whereas reabsorption takes place in the other parts of the nephrons.
- After filtration, a diluted filtrate is produced, and after reabsorption, it gets concentrated.
- Reabsorption occurs selectively whereas, in the filtration, most of the molecules are filtered (not very selective).