Mechanism of Urine Formation and Release

While our body is digesting the food, it produces a variety of metabolic wastes. As an example, the digestion of protein results in the formation of amino acids, which breaks down even more to generate ammonium ions. Excess ammonium ions result in urea, which is harmful to human health and needs to be released from the body. There shouldn’t be excess  salt in the body. Millions of nephrons found in the kidneys carry out the mechanism of urine formation. Formation and Excretion of urine is an essential function and is a vital activity for the whole body. Several distinct processes are involved in this. Let’s  study the mechanisms of urine formation and release.

Urine formation and release

Our kidneys are very remarkable organs of Excretory system and they get more than a litre of blood per minute and excrete around 1.5 litres of urine each day while working together. This process comprises of three fundamental mechanisms:

1) Glomerular filtration.

2) Tubular reabsorption( selective reabsorption).

3) Tubular secretion.

Glomerular filtration 

The initial stage in the mechanism of urine formation is glomerular filtration. Essentially, it is the process by which your kidneys remove excess fluid and waste items from  blood and deposit them in urinary collecting tubules so that it can be discharged  from the body. 

The first stage in producing urine is to break your blood cells from the liquid portion of your blood (plasma), including all dissolved solutes. Every nephron in your kidneys has a small filter, known as a glomerulus, which continually filters your blood to remove waste products.

Approximately 1100–1200ml of blood is filtered out of the body by the kidneys per minute. The capillaries of the glomerulus are narrower than the arterioles of the glomerulus. As a result, this puts pressure on the glomerular capillaries, which causes them to constrict. Because of the high pressure in the capillaries, holes formed by podocytes in the capillaries filter the blood. In the lumen of Bowman’s capsule, a steady stream pumps the filtered blood. 

Large cells, such as red blood cells, white blood cells, plasma proteins, and other proteins, cannot move through these holes.

Ultrafiltration is the term used to describe the mechanism of the urine formation process that happens via glomerular capillaries in the Bowman’s capsule. Therefore, the filtrate that is produced is referred to as glomerular filtrate.

In this case, the glomerular filtrate is identical to blood plasma. Except for fat and protein, the only difference is that filtrate is free of these substances.

When blood passes through the glomerular capillaries and enters the arteriole, it forms an afferent arteriole. 

The glomerular filtration rate refers to the quantity of filtrate produced by both kidneys in one minute and is measured in millilitres per minute. It is around 125mL each minute or 180L per day on average.

Tubular reabsorption

 Comparison of the amount of filtrate produced per day(180litres/day) and that of the urine released (1.5 litres) indicate that about 99% of the filtrate is reabsorbed by the renal tubules. The process of removing solutes and water from the filtrate and returning to your bloodstream is known as tubular reabsorption. We use “reabsorption” to describe this process because it is the second time our bloodstream absorbs these nutrients. The first time is after a meal.

A passive or an active process of reabsorption occurs in the nephron. The selective permeability of the nephron in each portion of the kidney is highly variable, depending on the quantity and kind of material reabsorbed. In the peritubular capillaries, absorption occurs by a variety of processes, which include:

• Passive diffusion — Fluids move through kidney epithelial cells’ plasma membranes due to concentration gradients, known as “passive diffusion”

• Active transport is characterised by membrane-bound ATPase pumps (such as NA+/K+ ATPase pumps) with carrier proteins that transport substances across the plasma membranes of kidney epithelial cells by using ATP

• Cotransport — This process must take place for the reabsorption of water, water may follow other molecules actively carried, such as glucose and sodium ions in the nephron, especially the kidney

These procedures include:

• The materials passing across the luminal barrier and the basolateral membrane
• Two plasma membranes of the kidney epithelial cells entering the peritubular capillaries on the opposite side, and some compounds may also pass via tight junctions, microscopic gaps between the renal epithelial cells that allow some molecules to flow through

Tubular secretion

In filtering blood to make liquid waste in urine, tubular secretion is merely one of the numerous steps. When it comes to waste disposal and acid-base balance, the excretory system of many organisms plays a vital role.

When unfiltered chemicals are transferred from the peritubular capillary into the lumen of the tubule, we call it tubular secretion. It happens mainly in the PCT and DCT. In addition, secretion is often used to eliminate chemicals from the body that are too big to be filtered or present in good amounts in the blood (e.g., H+, K+).

Many compounds filtered by the kidney are transported between the various parts of the kidney via diffusion and osmotic gradients. Still, tubular secretion is transported through active transport between the tubules.

There are many distinct kinds of transport proteins in the membranes of the tubular cells that make up the transport epithelium. It takes energy in the form of ATP to keep these transporters running, which allows them to convey various chemicals into the tubular lumen. Multiple types of transporters are located in different parts of the tubule, and the function of those regions is partly determined by the transporters found in those locations.

The proximal tubule is responsible for the secretion of drugs and toxins, and H+ is also carried in both the proximal and distal tubule areas to maintain an optimal pH. The Na+ / H+ exchanger is an example of a transporter essential in acid base balance . K+ is carried at varying rates within the distal tubule. It depends on how much is being transported more than the body’s requirements. Eventually, these released compounds find their way into the urine and are excreted from the body through the urine.

Conclusion

Several steps are involved in urine formation, such as glomerular filtration, selective reabsorption, and tubular secretion. The nephron is the basic structural unit of the kidney and every nephron passes through all three phases of urine creation. First, it produces concentrated urine carried to the urinary bladder via the ureters and subsequently expelled from the body by the kidneys and other organs.