Nephrons take a simple filtrate of the blood and modify it into urine. Many changes take place in the different parts of the nephron before urine is created for disposal. The term forming urine will be used hereafter to describe the filtrate as it is modified into true urine. The principle task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three principle functions—filtration, reabsorption, and secretion. They also have additional secondary functions that exert control in three areas: blood pressure (via production of renin), red blood cell production (via the hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, the active form of vitamin D).

Urine is a waste byproduct formed from excess water and metabolic waste molecules during the process of renal system filtration. The primary function of the renal system is to regulate blood volume and plasma osmolarity, and waste removal via urine is essentially a convenient way that the body performs many functions using one process.

Steps involved in urine formation are  Glomerular filtration,  Tubular reabsorption   and Tubular secretion

Figure 1:Steps involved in urine formation.

Glomerular Filtration

During filtration, blood enters the afferent arteriole and flows into the glomerulus where filterable blood components, such as water and nitrogenous waste, will move towards the inside of the glomerulus, and nonfilterable components, such as cells and serum albumins, proteins  will exit via the efferent arteriole. These filterable components accumulate in the glomerulus to form the glomerular filtrate.

Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole. The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. The “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys. GFR is regulated by multiple mechanisms and is an important indicator of kidney function.

Tubular Reabsorption and Secretion

1. Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed, and this occurs either by passive or active transport. Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones. Sodium (Na ) is the most abundant ion and most of it is reabsorbed by active transport and then transported to the peritubular capillaries. Because Na is actively transported out of the tubule, water follows it to even out the osmotic pressure. Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries. However, every solute has a transport maximum and the excess is not reabsorbed

Figure 2: Countercurrent exchange in Loop of Henle

In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration and tends to absorb water from the renal tubule and concentrate the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, as illustrated in Figure 2, it acts as a countercurrent multiplier. The vasa recta around it acts as the countercurrent exchanger.

The loop of Henle (seen in Figure 2) acts as a countercurrent multiplier that uses energy to create concentration gradients. The descending limb is water permeable. Water flows from the filtrate to the interstitial fluid, so osmolality inside the limb increases as it descends into the renal medulla. At the bottom, the osmolality is higher inside the loop than in the interstitial fluid. Thus, as filtrate enters the ascending limb, Na  and Cl⁻ ions exit through ion channels present in the cell membrane. Further up, Na  is actively transported out of the filtrate and Cl⁻ follows. Osmolarity is given in units of milliosmoles per liter (mOsm/L)

Each part of the nephron seen in this figure performs a different function in filtering waste and maintaining homeostatic balance.

1. The glomerulus forces small solutes out of the blood by pressure.
2. The proximal convoluted tubule reabsorbs ions, water, and nutrients from the filtrate into the interstitial fluid (tubular reabsorption), and actively transports toxins and drugs from the interstitial fluid into the filtrate tubular secretion. The proximal convoluted tubule also adjusts blood pH by selectively secreting ammonia (NH₃) into the filtrate, where it reacts with H  to form NH₄ . The more acidic the filtrate, the more ammonia is secreted.
3. The descending loop of Henle is lined with cells containing aquaporins that  from the filtrate into the interstitial fluid.
4. In the thin part of the ascending loop of Henle, Na  and Cl⁻ ions diffuse into the interstitial fluid. In the thick part, these same ions are actively transported into the interstitial fluid. Because salt but not water is lost, the filtrate becomes more dilute as it travels up the limb.
5. In the distal convoluted tubule, K  and H  ions are selectively secreted into the filtrate, while Na , Cl⁻, and HCO₃⁻ ions are reabsorbed to maintain pH and electrolyte balance in the blood.
6. The collecting duct reabsorbs solutes and water from the filtrate, forming dilute urine. (credit: modification of work by NIDDK)