Urine Formation
Excretion is a biological process, which plays a vital role in eliminating toxins and other waste products from the body. The kidneys filter unwanted substances from the blood and produce urine to excrete them. Constituents of normal human urine include 95 per cent water and 5 per cent solid wastes. It is produced in the nephron, which is the structural and functional unit of the kidney.
There are three main steps of urine formation: glomerular filtration, reabsorption, and secretion. These processes ensure that only waste and excess water are removed from the body. Urine is about 95% water and 5% waste products. Nitrogenous wastes excreted in urine include urea, creatinine, ammonia, and uric acid. Ions such as sodium, potassium, hydrogen, and calcium are also excreted.
1. The Glomerular Filtration
Each kidney contains over 1 million tiny structures called nephrons. Each nephron has a glomerulus, the site of blood filtration. The glomerulus is a network of capillaries surrounded by a cup-like structure, the glomerular capsule (or Bowman’s capsule). As blood flows through the glomerulus, blood pressure pushes water and solute
Glomerular filtration occurs in the glomerulus where blood is filtered. This process occurs across the three layers- the epithelium of Bowman’s capsule, the endothelium of glomerular blood vessels, and a membrane between these two layers.
Blood is filtered in such a way that all the constituents of the plasma reach the Bowman’s capsule, except proteins. Therefore, this process is known as ultrafiltration.
2. Reabsorption
Around 99 per cent of the filtrate obtained is reabsorbed by the renal tubules. This is known as reabsorption. This is achieved by active and passive transport.
The glomerulus filters water and small solutes out of the bloodstream. The resulting filtrate contains waste, but also other substances the body needs: essential ions, glucose, amino acids, and smaller proteins. When the filtrate exits the glomerulus, it flows into a duct in the nephron called the renal tubule. As it moves, the needed substances and some water are reabsorbed through the tube wall into adjacent capillaries. This reabsorption of vital nutrients from the filtrate is the second step in urine creation.
3. Secretion
The next step in urine formation is tubular secretion. Here, tubular cells secrete substances like hydrogen ions, potassium ions, etc into the filtrate. Through this process, the ionic, acid-base and the balance of other body fluids are maintained. The secreted ions combine with the filtrate and form urine. The urine passes out of the nephron tubule into a collecting duct and passes out of the kidney through the renal pelvis, into the ureter, and down to the bladder.
Osmoregulation
Osmoregulation is the process of regulating body fluids and their compositions. It maintains the osmotic pressure of the blood and helps in homeostasis. This is why it is recommended to consume more water about 2-3 litres, which helps in the proper functioning of the kidneys. For example, we consume lots of water during summers, but still, we urinate fewer times in summers than in winters and the concentration of the urine is also more. The reason is that we lose lots of water from the body in summer through sweating. Thus, to maintain the fluid balance in the body the kidneys reabsorb more water.
Key Points
- Urine is formed in three main steps- glomerular filtration, reabsorption and secretion.
- It comprises 95 % water and 5% wastes such as ions of sodium, potassium and calcium, and nitrogenous wastes such as creatinine, urea and ammonia.
- Osmoregulation is the process of maintaining homeostasis of the body.
- It facilitates the diffusion of solutes and water across the semi-permeable membrane thereby maintaining osmotic balance.
- The kidney regulates the osmotic pressure of blood through filtration and purification by a process known as osmoregulation.
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