Urine Formation, Osmoregulation

Introduction

Normal human urine consists of 5 percent solid waste and 95 percent water. Urine formation helps modulate the acid-base balance in the body, including fluid volume and plasma composition. The process of urine formation happens in three basic mechanisms:

• Glomerular filtration
• Tubular reabsorption
• Tubular secretion

Osmoregulation, on the other hand, is the physiological process of maintaining a fixed concentration of certain molecules and ions in the fluid that surrounds the cells. The process of osmoregulation involves

• Multiple body-to-body signalling mechanisms
• A neural network
• Reflex and behavioural mechanisms

Urine Formation

The main purpose of urine formation is to eliminate waste from the human body. It is also responsible for regulating blood pressure and blood volume. That includes controlling the levels of electrolytes and metabolites. Urine formation also helps maintain the blood pH level. Urine is formed in the kidneys through a very simple process – filtration of blood. That, in turn, is done through three basic mechanisms.

Glomerular Filtration

To understand the first mechanism, we need to understand what the Glomerular Filtration Rate (GFR) is. It refers to the volume of liquid per unit time in all glomeruli. The physiological value of GFR is 120ml/min. We can convert the same into 180l/day. About 99% of the filtrate is reabsorbed during the tubular resorption. That leaves behind 1.5-2 l of urine per day. The oncotic pressure of plasma proteins and the ratio of hydrostatic pressure in the capillaries determine the movement of fluid through the filtration membrane. 

Such forces are also termed Starling’s forces. The main determinant of glomerular filtration is the renal blood flow in the human body. It happens in the glomerulus, where blood filtration takes place.

Tubular Reabsorption

The renal tubules present in the human body are supposed to do a significant task. They need to ensure smooth functioning of the tubular reabsorption of the primary urine. They absorb various constituents like ions, water, glucose, urea, and amino acids. This entire process is dependent on the extracellular fluid volume of the human body. The primary function of tubular reabsorption is to maintain fluid volume in the body under every normal condition.

Tubular Secretion

The transportation of the substances takes place through primary active transport, passive diffusion, and secondary active transport. Moreover, it uses paracellular and transcellular routes to transport substances within the body. The transportation of water always remains passive.

Osmoregulation 

As the name suggests, osmoregulation regulates the osmotic pressure of electrolytic balance in organisms and fluids. Its main function is to maintain the fluid balance across membranes within the human body. It mostly regulates the body fluids and their compositions. The process of osmoregulation includes the following steps:

• Multiple body-to-body signalling mechanisms

These mechanisms take place to report the status of total body fluids. They also report the distribution of fluids in the human body. That helps every part of the body interact through particular signals.

• A neural network

A neural network directly refers to the functioning of the human brain. This network is also called the visceral neuraxis. It receives and integrates any input related to the body fluids.

• Reflex and behavioural mechanisms 

The reflex mechanisms involve both automatic and endocrine processes. Moreover, the behavioural mechanisms involve thirst and sodium-appetite-related behaviours. The network of visceral neuraxis controls and activates both these mechanisms.

Conclusion

The processes of urine formation and osmoregulation are vital for the health and well-being of all human beings. The human body excretes wastes, such as creatinine, ammonia, urea, sodium, potassium, and calcium ions through urine excretion. This helps the body remain healthier and keep functioning normally. Similarly, the process of osmoregulation helps maintain a balance between water and minerals at the cellular level. Its main advantage is the maintenance of a proper balance of electrolytes in the body. Both the processes of urine formation and osmoregulation are interdependent. The absence of either of the processes can have dire consequences on our health. So, it is extremely necessary to understand these mechanisms in detail.