Question

Define excretion. Explain mechanism of urine formation.

Answer 1

Hint: Excretion is the process of removing metabolic waste from the body. In humans, this function is performed by the kidneys, lungs, and skin. The regulatory activities of the kidneys produce urine as a by product. Urine formation involves three main steps.

Complete answer:
Let’s with the definition of excretion.
Excretion Excretion is defined as elimination of metabolic waste from the body of animals to regulate the body fluids and tissues composition.
The regulatory activities of the kidneys produce urine as a by product. Urine formation involves three main steps - glomerular filtration, tubular reabsorption, and tubular secretions.
Glomerular filtration (ultrafiltration): The glomerulus filters low molecular weight substances from the blood and holds high molecular weight substances, especially proteins. Therefore, protein is retained in the glomeruli and is usually not found in the urine. If protein is found in the urine, it means kidney damage or another disease affecting the glomerular membrane.
Tubular reabsorption - The primary urine formation rate is 120 ml / minute, while the urine flow rate into the bladder under the same conditions is 1-2 ml / minute. Hence, this suggests that about 99% of the glomerular filtrate is absorbed during its passage through the various segments of the renal tubule. Glucose. Therefore, even with normal blood sugar levels, glucose is practically absorbed in the tubules. The reabsorption capacity depends on the renal threshold of this substance.
Tubular secretion: Although most of the substances are absorbed by tubular cells, some substances are actively transported or excreted in the tubular lumen. The substances released from the tubular epithelium in humans are creatinine and potassium. The tubular epithelium also removes a number of foreign substances that are introduced into the body for therapeutic and diagnostic purposes.

Note:
Don’t get confused with the terms excretion and defecation. Defecation is the disposal of waste and undigested food through the anus which is commonly called faces.
Kidney function is regulated by three important hormones. These hormones are aldosterone (from the adrenal cortex), parathyroid hormone (from the parathyroid glands), and vasopressin (from the posterior lobe of the pituitary gland).Aldosterone restricts sodium ions excretion and stimulates the excretion of Potassium ions. Vasopressin, an antidiuretic hormone, is primarily responsible for water reabsorption. In the absence of this hormone, a large amount of very dilute urine will be excreted