Formation of Urine: Glomerular Filtration & Beyond - Essay Sample

Introduction

Kidney plays a vital role in the body and helps in getting rid of the excess metabolic wastes and toxins. In case there is an interruption in the urine formation process, toxins accumulate in the body leading to serious body harm or even the death of an individual. The urine is formed through four major processes, namely: Glomerular filtration, tubular reabsorption, tubular secretion, and water conservation.

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Glomerular filtration: This is the initial stage of the urine formation process. In this stage, water and other impurities in the human body (glucose, amino acids, and vitamins) are transported from the glomerulus to the Bowman's capsule via the capillaries. This is a controlled process because in case the rate of filtration is too high, reabsorption of fundamental water and solutes takes place. This can cause dehydration and other complications in the body. If the rate of filtration in the body is lower than the expected rate, wastes that could be otherwise be eliminated are reabsorbed (Alpern & Hebert 2007). The process under which glomerular filtration takes place is known as renal ultrafiltration. Hydrostatic pressure that exists in the glomerulus pushes filtrate out of the blood vessels into the slits located in the nephron. Osmotic pressure then works against the greater force of hydrostatic pressure. The pressure difference between the two then controls the effective pressure that controls the filtration process. These are the factors that determine the filtration rate in the kidney.

Tubular reabsorption: After renal ultrafiltration, tabular reabsorption follows. In this stage, water and other fundamental substances are reclaimed into the body for use. The fluid that passes through filtration is similar to the blood plasma and contains proteins. In the fluid passes through the bladder and out of the body, there is a significant volume of extracellular fluids that would be lost. Fortunately, the nephrons in the kidney help absorb back the fluid into the circulatory system. Besides reabsorption, the nephrons in the kidney help secrete toxic substances from the blood into the filtrate for later release out of the body (Windelspecht 2004). These processes complete the transformation of the glomerular filtrate to urine.

Tabular reabsorption is also known as selective absorption because it is the second time that the substances have been absorbed; the first time being during the digestion process. The nephrons are designed to maintain body fluid homeostasis. This involves stabilizing fluid body volumes and maintaining conducive levels of salt and minerals in the body. The nephrons are segmented into five segments which are responsible for reabsorption of different substances which are used for body functioning.

Tubular secretion: This is the third stage of urine formation and involves the removal of chemical substances from the filtrate fluid and secreted into the tubular fluid. The role of the entire process is to remove wastes and control the pH level. The process occurs in all parts of the nephron, from the convoluted tubule to the collecting duct which is at the end of the nephron. Tubular secretion is the opposite of reabsorption and is caused by active transport of fluids and passive diffusion (Suzuki, 2004). Only a few substances are secreted, which are mostly waste products. The leftover substance is urine, which is left in the collecting duct after reabsorption and secretion.

Water conservation: This is the final stage of urine formation in the kidney. This process involves the preservation of excess water from the body. If the human body loses too much water, dehydration often follows. Water is reabsorbed into the human body through osmosis, and the volume of urine excreted is based on the volume of fluid in the body. The higher the volume of water, the more the urine excreted from the body. Countercurrent flow involves the ascending and descending loops of Henle which leads to the formation of urine in opposing direction to form a concentration gradient when joint with variable permeability and sodium pumping (Alpern & Hebert 2007).

Role of ADH in the Process of Osmoregulation

Antidiuretic hormone (ADH) is secreted in the neurohypophysis, which is a posterior pituitary gland. It is also known as Vasopressin. The human body controls the amount of water in the body, hormones, and electrolyte concentration through osmoregulation process. The hormones play a fundamental part in signaling molecules that control many body functions. ADH helps in controlling the volume of urine formation in the body. It helps increase the reabsorption of water from the urine back to the renal system to prevent dehydration. This increases the amount of water in the body and reduces the amount of urine excreted out of the body. For this reason, it can be stated that ADH helps conserve water in the kidney and returns water into the body for re-circulation.

When blood is highly concentrated with waste and other toxins from the body, ADH hormones help regulate the level of concentration. Osmo-receptor, located in the Hypothalamus, identifies the increase in concentration, which then triggers the posterior pituitary gland to secrete ADH. The ADH is then transported to the kidney through the blood as chemical messengers. Water reabsorption is regulated by increasing absorptivity of the distal convoluted tubule and collecting ducts, opening the water channels. The water channels have protein content and is synthesized by the Golgi complex. The ADH are then attached to cell surface receptors, and this triggers the collecting ducts to bring water channel to the surface membrane of the nephrons. Through the water channels, water in channeled from the glomerular filtrate into the blood capillary to reduce the volume of urine and make it hypertonic. The process is called anti-diuresis. When there is a high volume of water intake, the process is repeated. ADH is secreted as the distal convoluted tubule, and collecting duct becomes impermeable to hypotonic urine. This is known as diuresis.

Conclusion

In a nutshell, the function of the ADH is simple. It increases the reabsorption of water into the body. For example, a person takes the recommended volume of water daily then suddenly changes the routine to less volume of water intake. After the second week, there will be a change in the urine smell and color. When he goes back to the normal water intake volume, the smell and color of the excreted urine become less intense. This is the result of the ADH hormone in the body. A decrease in the amount of water intake reduces water volume in the blood. To live a healthy life, blood and other body cells need to be surrounded by a sufficient amount of water. Too less or too much water in the body is unhealthy. There is a built-in system that controls the entire process. When blood passes through the hypothalamus, a signal is sent to the brain that there is water imbalance in the body. In the event of less volume of water, the hypothalamus sends signals of dehydration to the pituitary glands triggering the release of ADH to facilitate reabsorption of water from the urine and back to the bloodstream.

References

Alpern, R. J., & Hebert, S. C. (2007). Seldin and Giebisch's The Kidney: Physiology & Pathophysiology 1-2. Burlington: Elsevier Science.

Suzuki, H., (2004). Kidney and blood pressure regulation: 8 tables. Basel: Karger.

Windelspecht, M., (2004). Human body systems. Westport, Conn: Greenwood.