Regulation of Kidney Function - Important Topic for NEET

The kidneys' function is efficiently monitored and regulated by a hormonal feedback control mechanism involving the hypothalamus, juxtaglomerular apparatus, and, to some extent, the heart. Changes in blood volume, body fluid volume, and ionic concentration activate hypothalamic osmoreceptors. When there is an excessive loss of fluid from the body or an increase in blood pressure, the hypothalamic osmoreceptors respond by stimulating the neurohypophysis to secrete an antidiuretic hormone (ADH) or vasopressin (positive feedback).

The average kidney is about the size of a cell phone and weighs about 4-6 ounces. Despite accounting for only 0.5% of the body's weight on average, the kidney receives more blood than all other organs except the liver. Nephrons are the kidney's filtering units, and each kidney has between 1 and 2 million nephrons.

In this article, we will look into the different roles of these hormones which regulate the functioning of kidneys and also look into the structure and the function of this amazing organ.

Regulation of Kidney Function

Urine is described as a liquid or semi-solid solution of generated waste by physiological functions which also contains hazardous chemicals created by excretory organs during fluid circulation. Waste is often expelled from the human body in the form of urine. In the excretion process, our kidneys play an important role. It is made in the nephron, which is the kidney's structural and functional unit.

The kidney's major purpose is to produce urine and purify the blood. The kidney filters waste and other compounds that are no longer needed by the body. All the activities and functions in the kidney are regulated by the body by releasing some hormones and autoregulation. In autoregulation, the kidney regulates its function with the help of its own structures and cells.

Different stages such as filtration, reabsorption, and secretion during urine formation are regulated. The pituitary gland is stimulated by changes in urine flow and bodily fluids to release hormones that control the pituitary gland's activity, including vasopressin (ADH), renin, and aldosterone. 

Kidney Structure

The kidney is a part of the excretory system and helps in urine formation and excretion from the body. Some facts are given below about the kidney:

• The kidneys are two bean-shaped organs.  

• It is divided into two areas: a dark outer area (the cortex) and a lighter inner area (the medulla). 

• There are 10 to 18 renal pyramids in the medulla, which are triangular structures with a striped appearance. The renal tubules and related blood arteries give them this appearance.

Structure of Kidney and Nephron

• Waste is often expelled from the human body in the form of urine.

• Urine is made in the nephron, which is the basic structural unit of a kidney. 

• Nephrons are important as they are the functional and structural units in the kidney. The nephrons, which filter the whole volume of blood in the body 60 times a day, are the backbone of the urinary system.

• Nephron has a glomerulus, Bowman’s capsule, and renal tubules. Bowman's capsule is the first segment of the renal tube, which surrounds the glomerulus and receives the filtrate from the glomerular capillaries. The glomerulus is a capillary network in the kidney that performs as a filtration site.

Function of Kidney and Its Structure

• The structures of the kidney have different functions in urine formation and regulation.

1. Renal Capsule: It covers the outer surface of the kidney and helps in the ultrafiltration of the urine. 

2. Renal Medulla: It is the innermost part of the kidney that consists of nephrons and helps in urine formation. 

3. Medullary Pyramid: It carries urine from the collecting ducts to the calyx.

4. Major and Minor Calyx: These carry the urine to the renal pelvis of the kidney. 

5. Renal Pelvis: It collects urine from the kidney and passes it to the ureter.

• The primary function of the kidneys is to maintain equilibrium. They make the body's internal environment stable and comfortable by controlling fluid levels and balancing electrolytes and other elements.

• It produces a variety of hormones and enzymes, including erythropoietin, which is a hormone that is produced in response to hypoxia and Renin regulates angiotensin and aldosterone, which regulates blood pressure. 

Hormones That Regulate the Kidney Function

Renin, angiotensin II, prostaglandins, bradykinin, ADH, aldosterone, ANP, vitamin D, PTH, and erythropoietin are the hormones involved in the hormonal regulation of the kidneys.

Juxtaglomerular (JG) Apparatus 

• The granular cells (juxtaglomerular cells) in the JGA's glomerular arterioles (especially the afferent) are specialised, smooth muscle cells in the wall (juxtaglomerular apparatus). These JG cells are responsible for the production of renin.

• Renin levels are increased by the renal sympathetic nerves through a direct action on granular cells (mediated by beta1-adrenergic receptors) and a decrease in flow to the macula densa. 

• Increased renal sympathetic nerve activity causes a drop in GFR and an increase in proximal sodium ions and water reabsorption, both of which reduce the amount of fluid supply to the macula densa. This further helps in renin production at a high level.

The Renin-Angiotensin-Aldosterone System 

• The precursor angiotensinogen is synthesised in the liver. 

• The stimulation of the kidney increases the production of renin, an enzyme that converts angiotensinogen to angiotensin I by a cleavage mechanism that reduces the polypeptide from 453 to 10 amino acids. 

• Angiotensin is an enzyme in the endothelium that converts angiotensin I to angiotensin II. 

• Angiotensin II stimulates vasoconstriction in the cardiovascular system, stimulates aldosterone production in the adrenal glands, and increases sodium reabsorption in the kidneys, which enhances both salt and water reabsorption. 

• The vasoconstrictor actions along with salt and water retention result in an overall effect of blood pressure elevation. Angiotensin II functions as a vasoconstrictor in the arterial wall and induces Aldosterone synthesis in the adrenal glands. It also promotes salt reabsorption within the kidneys. It acts on the adrenal cortex to enhance aldosterone secretion, as well as the kidneys themselves, due to increased salt and water accumulation.
  

Hypothalamus

Antidiuretic Hormone(ADH)

• The hypothalamus produces ADH in neurosecretory cells, which then carry it down their axons to nerve terminals in the posterior pituitary gland, where it is stored.

• Blood plasma levels are monitored by osmoreceptor cells in the hypothalamus. 

• If the concentration is too high, signals are sent to ADH neurosecretory cells, causing ADH to be produced from the posterior pituitary nerve terminals. 

• ADH produces a decrease in blood plasma concentration by increasing water uptake in the kidney collecting duct.

• When osmoreceptors detect a decline in blood plasma levels, the neurosecretory cells are not triggered to release ADH, and the blood ADH level drops, resulting in dilute urine.

Atrial Natriuretic Peptide (ANP)

• It is a 28-amino-acid peptide hormone found in granules in atrial myocytes, particularly in the right atrium. 

• Increased blood flow in the atria of the heart causes ANP to be released. The renal tubules, particularly in the collecting duct, are then inhibited from reabsorbing salt and water. Reduced sodium ion and water levels enhance urine output, allowing blood volume to return to normal.

• It reduces aldosterone secretion from the adrenal cortex as well as renin release, lowering angiotensin levels.

Regulation of Kidney Function Flow Chart

Regulation of Kidney Function Flowchart

Conclusion

Most people are aware that one of the kidneys' primary functions is to remove waste products and excess fluid from the body. The urine removes these waste products and excess fluid. Urine production involves highly complex excretion and re-absorption steps. This process is required to keep the body's chemical balance stable. The kidneys are responsible for the critical regulation of the body's salt, potassium, and acid content.

In addition, the kidneys produce hormones that influence the function of other organs. A hormone produced by the kidneys, for example, stimulates red blood cell production. Other hormones produced by the kidneys aid in blood pressure regulation and calcium metabolism. Our kidney's function is monitored and regulated by feedback mechanisms involving the hypothalamus, juxtaglomerular apparatus (JGA), and the heart.