Regulation of Urine Formation: Overview, Process, Diagrams

Regulation of Urine Formation: Did you know a normal kidney filters 125 ml/minute, i.e., 180 litres per day while an average adult drinks only 2 litres of water a day? Strange, isn’t it? Well, when the kidney filters blood the filtrate is diluted. This diluted filtrate moves through tubules where water and electrolytes are reabsorbed.

The reabsorption makes the urine highly concentrated. And the final amount of urine formed per day in a normal person is 1.5 litres per day. The process of formation of urine is highly regulated and worth paying attention to. Read further to learn the process of regulation of urine formation.

Process of Urine Formation

Glomerular Filtration: Water and solutes smaller than proteins are driven into the renal tubule through the glomerular capsule’s capillary walls and pores.
Reabsorption: Water, glucose, amino acids, and other necessary ions are carried from the filtrate into tubule cells, where they are eventually absorbed into the capillary blood.
Tubular Secretion: Tubule cells extract creatinine and medicines from the peritubular blood and secrete them into the filtrate.

Fig: Urinary System

Urine formation is under hormonal and neural control.

Hormonal Control of Urine Formation

The kidneys are affected by hormones in addition to the chemical composition and osmolarity of blood and plasma. The Antidiuretic hormone is the most significant hormone that controls kidney function directly (ADH). Antidiuretic hormone (ADH) is a polypeptide hormone produced by the hypothalamus and transported to and released by the posterior pituitary gland. Hormonal control is also aided by Aldosterone and Renin, in addition to ADH.

Actions of these hormones are listed below:

Hormone	Source	Action
Antidiuretic hormone (ADH) or, vasopressin	hypothalamus	1. ADH actively monitors and manages the amount of water in the body. 2. It serves to manage blood pressure by acting on the kidneys and blood arteries. 3. It permits the water in the urine to be reabsorbed into a particular location of the kidney, reducing the quantity of water expelled via the urine and therefore saving body fluid content.
Renin	Kidney (Juxtaglomerular cells)	1. Renin interacts with a protein called angiotensinogen in the blood, causing angiotensin I to be released. 2. Angiotensin-converting enzyme cleaves angiotensin I, breaking off two amino acids from the 10-amino-acid chain to produce angiotensin II. The resulting angiotensin II octapeptide (formerly known as hypertensin or angiotonin) constricts arterioles through receptors, raising both systolic and diastolic blood pressure.
Aldosterone	Cortex of the adrenal glands	The hormone aldosterone is in charge of salt reabsorption from the urine. The increase in sodium reabsorption is accompanied by a rise in water reabsorption. Both of these things work together to lower blood pressure.
Angiotensin II	Liver	1. Angiotensin II is one of the most effective vasoconstrictors known; it is much more potent than norepinephrine in terms of weight. 2. It also enhances cortisol and aldosterone production by acting directly on the adrenal cortex. 3. It’s the most potent sodium-retention hormone. It operates in particular conditions, such as when blood pressure is low or when extracellular fluid is present. It works by restoring normal blood pressure and extracellular fluid by boosting salt and water reabsorption from renal tubules.

ADH Regulation

Changes in blood volume, bodily fluid volume, and ionic concentration activate osmoreceptors in the body. When the body loses too much fluid, these receptors are activated, causing the hypothalamus to produce ADH from the neurohypophysis. ADH prevents diuresis by facilitating water reabsorption from the later portions of tubules. To complete the feedback, an increase in bodily fluid volume can turn off the osmoreceptors and decrease ADH release. Vasoconstrictor effects on blood arteries can potentially impact renal function. This produces a rise in blood pressure. A rise in blood pressure can result in an increase in glomerular blood flow and, as a result, in an increase in GFR (glomerular filtration rate).

Renin-Angiotensin System

It is blood pressure and cardiovascular function regulator. A decrease in glomerular blood flow, glomerular blood pressure, or GFR can activate JG cells, causing them to produce renin, which transforms angiotensinogen in the blood to angiotensin I and then to angiotensin II. As a potent vasoconstrictor, angiotensin II raises glomerular blood pressure and hence GFR. Angiotensin II also stimulates the secretion of Aldosterone from the adrenal cortex. Aldosterone causes Na+ and water to be reabsorbed from the tubule’s distal portions. Blood pressure and GFR both rise as a result of this.

Fig: Renin-Angiotensin System

ANF Regulation

The release of Atrial Natriuretic Factor can be triggered by an increase in blood flow to the heart’s atria (ANF). ANF can lower blood pressure by causing vasodilation (dilation of blood vessels). As a result, the ANF mechanism functions as a check on the renin-angiotensin system.

Neural Control of Lower Urinary System

The sympathetic nervous system controls the bladder’s ability to store urine. The parasympathetic nervous system, on the other hand, is in charge of bladder contractions and urine flow. When the sympathetic nervous system is engaged, the bladder expands without raising detrusor resting pressure (accommodation), and the internal urinary sphincter is stimulated to remain securely closed. The parasympathetic nerves cause the detrusor to contract, which aids urine function. The sympathetic impact on the internal urethral sphincter is inhibited just before parasympathetic stimulation, allowing the internal urethral sphincter to relax and open.

Summary

Urine contains 95% water and 5% ions such as sodium, potassium, and calcium, as well as nitrogenous wastes such as creatinine, urea, and ammonia. Glomerular filtration, reabsorption, and secretion are the three main stages in the generation of urine. The kidneys are affected by hormones in addition to the chemical composition and osmolarity of blood and plasma. The Antidiuretic Hormone is the most significant hormone that controls kidney function directly (ADH). Hormonal control is also aided by aldosterone and Renin, in addition to ADH. Each hormone plays a specific and coordinated role in controlled and regulated urine formation. ADH prevents diuresis by facilitating water reabsorption from the later portions of tubules.

Renin-Angiotensin System is a blood pressure and cardiovascular function regulator. Angiotensin II also stimulates the secretion of Aldosterone from the adrenal cortex. Aldosterone causes Na+ and water to be reabsorbed from the tubule’s distal portions. Blood pressure and GFR both rise as a result of this. The release of Atrial Natriuretic Factor can be triggered by an increase in blood flow to the heart’s atria (ANF). ANF can lower blood pressure by causing vasodilation (dilation of blood vessels). The sympathetic nervous system controls the bladder’s ability to store urine. The parasympathetic nervous system, on the other hand, is in charge of bladder contractions and urine flow.

FAQs on Regulation of Urine Formation

Q.1. How is urine produced regulated?
Ans: Urine volume and concentration is regulated through the same processes that regulate blood volume. Antidiuretic hormone (ADH)—produced by the posterior pituitary gland —increases the amount of water reabsorbed in the distal convoluted tubule and collecting duct.

Q.2. How is urine osmolarity hormonally regulated?
Ans: The hypothalamus produces a polypeptide hormone known as antidiuretic hormone (ADH), which is transported to and released from the posterior pituitary gland. The principal action of ADH is to regulate the amount of water excreted by the kidneys.

Q.3. What is the main hormone responsible for water regulation?
Ans: Antidiuretic Hormone (ADH) is responsible for water regulation.

Q.4. What are the three hormones that regulate urine volume?
Ans: This function is specifically controlled by three key hormones: antidiuretic hormone (ADH), aldosterone, and atrial natriuretic peptide (ANP).

Q.5. Which nervous system controls urination?
Ans: The sympathetic nervous system regulates the process of urine storage in the bladder. In contrast, the parasympathetic nervous system controls bladder contractions and the passage of urine.

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