Question

What is meant by excretion? Explain the process OR describe the mechanism of urine formation.

Answer 1

There are many organ systems in the human body. Excretory system is one of them. Excretory system comprises a pair of kidneys, a pair of ureter and urethra.

Complete answer:
Excretion:
Excretion is a metabolic process in which waste compounds left after digestion is eliminated from our body. These compounds carry molecules that are not digested by the body as they were not required or the body was unable to digest them as they did not go through the breakdown process with the other molecules. Excretion process is a vital process of our body as it helps in maintaining acid-base balance of the body and balance between water and inorganic ions in the body. This process is also important as it helps in maintaining homeostasis in our body. Waste compounds are excreted in the form of urine and stool.

Urine formation:
Compounds like water and other substances are filtered out by glomerulus from the bloodstream. Glomerulus is present in the kidney and is a cup like structure and when water and other substances pass through its membrane it filters out the unwanted substances and urine formation begins. The filtration membrane blocks cells and large particles to enter through it and water and small solutes pass through it. This filtrate passes through a duct called renal tubule and water and other essential solutes are reabsorbed through this tube wall. Waste solutes and unwanted water remaining forms secretion and combine with filtrates and become urine. Then, urine collects into the collecting duct from the nephron tubule and passes through the renal pelvis and then into the ureter and then finally down to the bladder. Urine contains about 95% water and 5% waste products. Various nitrogenous compounds like uric acid and ammonia are also eliminated through urine.

Note: Counter current means flow in the opposite direction. It is a mechanism for water conservation. Mammals have the ability to produce concentrated urine. The Loop of Henle and Vasa Recta play an important role in this.