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Renal Physiology Part 1: Functions of Kidney, Vitamin D Activation, RAAS Pathway, Renal Circulation

Functions of the Kidney:

The kidneys are vital organs responsible for a wide range of functions that are essential for maintaining homeostasis in the body. These functions include:

Regulation of fluid and electrolyte balance: The kidneys regulate the amount of water and electrolytes in the body by filtering excess fluid and electrolytes from the blood and excreting them in the urine.

Regulation of acidbase balance: The kidneys help maintain the pH balance of the blood by excreting excess acids or bases.

Removal of metabolic waste products: The kidneys filter out waste products such as urea, creatinine, and uric acid from the blood and excrete them in the urine.

Regulation of blood pressure: The kidneys produce hormones that regulate blood pressure, such as renin and angiotensin II.

Production of erythropoietin: The kidneys produce erythropoietin, a hormone that stimulates the production of red blood cells in the bone marrow.

Activation of vitamin D: The kidneys are involved in the conversion of vitamin D to its active form, which is necessary for calcium absorption in the gut.

Vitamin D Activation:

Vitamin D is a fatsoluble vitamin that plays a crucial role in bone health and calcium metabolism. Vitamin D is activated in the kidneys through a series of steps:

First, vitamin D is synthesized in the skin when it is exposed to sunlight.

The liver converts vitamin D into a precursor molecule called 25hydroxyvitamin D (25(OH)D).

25(OH)D is transported to the kidneys, where it is converted to its active form, 1,25dihydroxyvitamin D (1,25(OH)2D), by the enzyme 1alphahydroxylase.

1,25(OH)2D then binds to vitamin D receptors in the intestine, promoting the absorption of calcium and phosphate from the diet.

RAAS Pathway:

The reninangiotensinaldosterone system (RAAS) is a complex pathway involved in the regulation of blood pressure and fluid balance in the body. The RAAS pathway is activated in response to a decrease in blood pressure or blood volume:

The enzyme renin is released from the juxtaglomerular cells in the kidneys in response to low blood pressure or low blood volume.

Renin catalyzes the conversion of angiotensinogen, a protein produced by the liver, into angiotensin I.

Angiotensin I is then converted to angiotensin II by the enzyme angiotensinconverting enzyme (ACE), which is primarily found in the lungs.

Angiotensin II is a potent vasoconstrictor, meaning it causes the blood vessels to narrow, which increases blood pressure.

Angiotensin II also stimulates the release of aldosterone from the adrenal glands, which promotes sodium retention and water reabsorption in the kidneys, further increasing blood pressure and restoring fluid balance.

Renal Circulation:

The kidneys receive about 20% of the cardiac output, which makes them one of the most highly perfused organs in the body. The renal circulation consists of two main components:

The renal artery brings oxygenated blood to the kidneys.

The renal vein carries deoxygenated blood away from the kidneys.

Renal circulation is essential for the proper functioning of the kidneys, which play a critical role in regulating fluid and electrolyte balance, blood pressure, and acidbase balance in the body. The physiology of renal circulation involves several complex mechanisms that ensure proper blood flow and filtration in the kidneys.

The renal artery supplies oxygenated blood to the kidneys, which is then filtered by the glomeruli. The glomeruli are specialized capillaries that remove waste products, excess electrolytes, and water from the blood, which is then collected by the renal tubules. The filtered blood exits the glomeruli via the efferent arterioles, which branch off into the peritubular capillaries and vasa recta in the renal cortex and medulla, respectively.

The peritubular capillaries and vasa recta reabsorb useful substances, such as glucose and amino acids, from the renal tubules back into the blood, while also secreting certain waste products and ions into the tubules for excretion in the urine.


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