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MCAT Content / Excretory System / Roles In Homeostasis

Roles in homeostasis

Topic: Excretory System

The excretory system plays a role in homeostasis by regulating blood pressure, water levels, acids and bases and nitrogenous waste levels

Kidneys regulate the osmotic pressure of a mammal’s blood through filtration and purification in a process known as osmoregulation. All the blood in the human body is filtered many times a day by the kidneys.

The kidneys also play a role in blood pressure control, using hormones that regulate blood pressure and water balance. The kidneys secrete renin directly into circulation that through a chain reaction results in the formation of angiotensin II, a potent vasoactive peptide that causes blood vessels to constrict, resulting in increased blood pressure. Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood. This increases the volume of fluid in the body, which also increases blood pressure.

Acid-base homeostasis concerns the proper balance between acids and bases; it is also called body pH. The kidneys control pH by the excretion of excess acid or base. In response to acidosis, the tubular cells reabsorb more bicarbonate from the tubular fluid, and the collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to an increase of the NH3 buffer. In its responses to alkalosis, the kidneys may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells and lower the rates of glutamine metabolism and ammonium excretion.

Mammals, including humans, are the primary producers of urea. Because they secrete urea as the primary nitrogenous waste product, they are called ureotelic animals. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals. It is the main nitrogen-containing substance in the urine of mammals. Urea is a colorless, odorless solid, highly soluble in water, and practically non-toxic. Dissolved in water, it is neither acidic nor alkaline. Kidneys control the levels of urea in the blood. 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. They are not reabsorbed; instead, they are expelled from the body in urine.


Practice Questions

 

Khan Academy

 

MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Question 62

Biology Question Pack, Vol. 1 Question 80

Biology Question Pack, Vol 2. Passage 11 Question 71

Biology Question Pack, Vol 2. Passage 11 Question 73

Section Bank B/B Section Question 15

 

Key Points

• Kidneys regulate the osmotic pressure of a mammal’s blood through extensive filtration and purification, in a process known as osmoregulation.

• When blood volume is low, renin, excreted by the kidneys, stimulates the production of angiotensin I, which is converted into angiotensin II. This substance has many effects, including an increase in blood pressure due to its vasoconstrictive properties.

• The cells that excrete renin are called juxtaglomerular cells. When blood volume is low, juxtaglomerular cells in the kidneys secrete renin directly into circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I.

• Aldosterone secretion from the adrenal cortex is induced by angiotensin II and causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood, thereby increasing blood volume and blood pressure.

• Acid-base homeostasis concerns the proper balance between acids and bases; it is also called body pH. The kidneys control pH by the excretion of excess acid or base.

• In response to acidosis, the tubular cells reabsorb more bicarbonate from the tubular fluid, and the collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to an increase of the NH3 buffer.

• In its responses to alkalosis, the kidneys may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells and lower the rates of glutamine metabolism and ammonium excretion.

• Ureotelic animals, which includes mammals, produce urea as the main nitrogenous waste material.

• Kidneys filter blood urea out and do not reabsorb it


Key Terms

homeostasis: The ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium, such as the ability of warm-blooded animals to maintain constant body temperature.

aldosterone: A corticoid hormone that is secreted by the adrenal cortex that regulates the balance of sodium and potassium and thus the water-balance levels in the body.

pH: in chemistry, a measure of the activity of the hydrogen ion concentration

angiotensin II: a hormone that causes blood vessels to contract

renin: secreted by the kidneys causing a cascade that creates angiotensin II

acidosis: increased acid levels in the blood

alkalosis: increased alkali levels in the blood

ammoniagenesis: the creation of ammonia

urea: a nitrogen-containing substance in the urine of mammals



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