Composition and mechanism of urine formation-1

Physiology of Urine Formation

Nephrons and collecting ducts carry out three fundamental processes to produce urine:

• Glomerular filtration
• Tubular secretion.
• Selective reabsorption

Glomerular Filtration

Blood filters through a glomerulus' capillaries during glomerular filtration. Blood pressure drives plasma and other dissolved materials out of the glomeruli and into the Bowman's capsule during glomerular filtration. The term "glomerular filtrate" refers to the liquid that enters the capsular space. Only roughly 1 liter of urine will be produced from the 180 liters of filtrate that are produced by glomerular filtration over the course of a 24-hour period.

Specialized endothelial cells are used to build the capillary walls. A porous wall known as the filtration membrane is created by the glomerular capsule's podocytes and the glomerular capillaries' endothelial cells. It is a permeable barrier that enables water and solutes to flow freely but prevents plasma proteins, blood cells, and platelets from the blood from doing so. Filtration membrane consists of three layers:

• The fenestrated capillary (glomerular) endothelium, which has numerous apertures or windows.
• The glomerular capsule's visceral membrane, which is composed of podocytes and has filtration slits.
• The basement membrane that separates the podocytes from the endothelium.

This membrane permits molecules with a diameter smaller than 3 mm, including nitrogenous waste, glucose, amino acids, and water. It prevents the loss of all of the water in the glomerular blood to the renal tubules by keeping the blood cells and plasma proteins in the capillaries and thereby maintaining colloid osmotic pressure. The three pressures help with glomerular filtration:

• Capillary hydrostatic pressure: The force that the glomerulus' capillaries are applying. This ranges from 45 to 70 mmHg. It encourages filtration.
• Filtrate hydrostatic pressure: The filtrate inside the capsule's pressure. In the range of 5 to 15 mmHg. It is against filtration.
• Osmotic pressure: The glomerulus's fluid pressure. Aproximately 25 mmHg. It is against filtration.

So, net filtration pressure (NEP) = The difference between the capillary hydrostatic pressure and the filtrate hydrostatic pressure is 15 to 20 mmHg.

• Glomerular Filtration Rate (GFR): Glomerular filtration rate, or GFR, is the amount of fluid filtered out of the blood and into the glomerular capsule each minute. Adults with both kidneys have a normal GFR of 120–125 ml/min. Urea, uric acid, sodium, potassium, glucose, and water are all present in the filtrate. The elements that control GFR include;
  • Total available filtration surface,
  • Permeability of the filtration membrane,
  • Net filtration pressure

The GFR and net filtration pressure are strongly correlated, so any change in NEP will impact GFR. As a result, elevated arterial blood pressure in the kidneys raises GFR; however, severe blood loss, for instance, lowers mean arterial blood pressure and lowers glomerular blood hydrostatic pressure, which prevents the formation of filtrate.

Selective Reabsorption

It is the procedure used to reabsorb specific chemicals into the renal tubules. The second step in the production of urine is this. The filtrate that the glomerular capsule receives enters each of the renal tubule's three segments. By the process of selective reabsorption, several chemicals are restored to the blood when the filtrate moves through these segments. Nearly 99% of the filtered water is typically reabsorbed. The renal tubule segments allow for the reabsorption of nutrients like glucose, amino acids, and some ions like sodium (Na+), potassium (K+), calcium (Ca2+), chloride (CI), and bicarbonate (HCO3). The proximal tubule's unique structural and functional traits are related to its rapid reabsorption. The proximal tubule's epithelium features a brush-like border made up of numerous small microvilli. The surface area over which reabsorption occurs is expanded by microvilli. Other regions of the tubule, as depicted in the picture, are less well suited to reabsorption. Through the processes of osmosis, diffusion, and active transport, selective reabsorption occurs. Urea, creatinine, and uric acid are examples of nitrogenous wastes that are not reabsorbed. Several hormones, including antidiuretic hormone, aldosterone, parathormone, and calcitonin hormone, control the reabsorption process.

Reabsorption at the Different Regions of the Renal Tubule

• Proximal Convoluted Tubule (PCT)
  As the majority of solute and water reabsorption from filtered fluid takes place in the PCT, these cells are by far the most active "reabsorbers" in the body. Normally, the PCT allows for the complete resorption of amino acids, glucose, and lactate. Additionally, these tubules recover about 90% of the potassium, chloride, and bicarbonate.
• Loop of Henle
  About 20–30% of the filtered Na+, K+, Ca2+; 10–20% of the filtered HCO3, CT; and 15% of the filtered water are reabsorbable by the Henle loop.
• Distal Convoluted Tubule and Collecting Duct
  The distal tubule and collecting duct are comparatively sodium- and water-impermeable in the absence of regulatory hormones (antidiuretic hormone). The DCT is stimulated by parathyroid hormone to reabsorb calcium.

Tubular Secretion

The tubular secretion process also allows different chemicals to enter the renal tubule. Tubular secretion primarily cleans the blood of waste products, controls the amount of electrolytes (ions) in body fluids, and controls the pH of the blood and extracellular fluids. A variety of regular metabolism's waste products are eliminated by tubular secretion. such as creatinine, potassium, ammonium, hydrogen, and certain organic acids. They transfer from the filtrates into the blood of the peritubular capillaries. As a result, both filtered and secreted substances can be found in the urine that is eventually excreted. This process is regulated in part by the aldosterone hormone, as will be discussed later.

Composition of Urine

The food, activity, water intake, and other factors all affect the composition of urine. However, the key ingredients are water, urea, sulfates, chloride, potassium, sodium, creatinine, phosphate, and urea. Urine is abnormal if it contains proteins, sugars, casts (decomposed blood), or mineral calculi. The color of the urine is translucent (clear, not hazy). The body must eliminate at least 450ml of urine each day in order to keep the extracellular fluid's osmotic concentration at the right level for excreting wastes and for maintaining healthy kidney function. A healthy individual produces 1000–1500 ml of urine every day.

Regulation of Urine Formation

Certain hormones regulate the amount and content of urine:

• Parathyroid hormone
• Antidiuretic hormone
• The Renin-angiotensin - aldesterone mechanism

Parathyroid Hormone
Together with the thyroid hormone calcitonin, the parathyroid hormone (PTH) generated by the parathyroid gland controls the reabsorption of calcium and phosphate from the distal collecting tubules. PTH is released by the parathyroid glands when there is less Ca2+ in the blood. The early DCT cells are then stimulated by PTH to reabsorb additional Ca2+ from the circulation. Additionally, PTH prevents PCT from reabsorbing phosphate.

Antidiuretic Hormone
When osmoreceptors in the hypothalamus notice that the blood is getting too concentrated, the posterior pituitary gland releases the antidiuretic hormone (ADH or vasopressin). When blood pressure falls, as happens after a hemorrhage, ADH is also released. The distal convoluted tubules and collecting ducts' epithelial cells are impacted by ADH, which increases their water permeability. Less water is then excreted in the urine as a result of passive osmosis moving water out of the ducts. The hypothalamic receptors are not triggered and ADH is not released when the blood is overly diluted, which could happen after consuming significant amounts of fluid. The connection between the cells of the distal tubules and the collecting ducts remains tight and water-tight in the absence of ADH. As a result, more water is excreted in the urine and the tubule's water cannot be reabsorbed. ADH's on/off switching is a homeostatic mechanism that controls blood and urine volume.

Renin-Angiotension-Aldosterone System
Blood travels through afferent arterioles and juxtaglomerular cells before arriving at the glomerulus. These cells react quickly to variations in renal blood pressure. The juxtaglomerular cells release the renin enzyme into the blood when blood volume and blood pressure drop. The release of renin from juxtaglomerular cells is also directly stimulated by sympathetic activation. Angiotensinogen is a protein that is affected by renin. It is generally detected in plasma and is produced by liver cells. Angiotensinogen is transformed into angiotensin I by renin. Angiotension I is changed into angiotension II by an additional enzyme known as ACE. In numerous ways, angiotension II raises blood pressure. It immediately affects the arterioles' smooth muscle, resulting in overall vasoconstriction. Additionally, it causes the afferent arterioles to contract, which lowers the glomerular filtration rate. It also prompts the adrenal cortex to release the hormone aldosterone. In turn, aldosterone encourages the distal convoluted tubules to transfer potassium into the lumen and sodium back into the circulation. In the collecting ducts, sodium reabsorption caused by aldosterone is coupled to potassium ion secretion, meaning that as Na+ enters, K+ diffuses out into the lumen. As water follows sodium through osmosis, the volume of the blood rises. As a result, the blood pressure rises.