Regulatory Functions of the Kidney
The kidney is involved in the fluid, electrolyte, and acidbase balance of the blood.
Fluid and Electrolyte Balance
The average adult male is about 60% water by weight. The average adult female is only about 50% water by weight because females generally have more subcutaneous adipose tissue, which contains less water. About two-thirds of this water is inside the cells (called intracellular fluid), and the rest is largely distributed in the plasma, tissue fluid, and lymph (called extracellular fluid). Water is also present in such fluids as cerebrospinal fluid and synovial fluid, these fluids are referred to as "other" fluids.
Maintenance of the water content in all body fluids requires that the body is in fluid balance: The total water intake should equal the total water loss. Consider water enterance in the body; for example, metabolic water is created by chemical reactions in cells, food contains water, and a person dri n ks water. The osmolarity (ability to bring about osmosis) of the blood is constantly monitored within the hypothalamus, which determines whether or not a person is thirsty and takes a drink of water. Water is lost from the body in a variety of ways-for example, from sweating, feces formation, evaporation from the lungs, and urine formation.
Hormonal Control
Aldosterone, antidiuretic hormone (ADH), and atrial natriuretic hormone (ANH) work together to control blood volume and pressure.
Aldosterone Aldosterone, a hormone secreted by the adrenal cortex, primarily maintains sodium (Na +) and potassium (K+) balance. The adrenal cortex is the outer portion of the adrenal glands, which lie atop the kidneys.
Blood volume is constantly monitored by the afferent arteriole cells within the juxtaglomerular apparatus. When blood volume and therefore blood pressure is not sufficient to promote glomerular filtration, afferent arteriole cells secrete the enzyme renin. Renin changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin 1. Later, angiotensin I is converted to angiotensin II in the lungs by angiotensin-converting enzyme. Angiotensin II, a powerful vasoconstrictor, stimulates the adrenal cortex to release aldosterone. When aldosterone is released, sodium (N a +) is reabsorbed into the blood at the distal convoluted tubules and collecting ducts of the nephron. The increase in Na + in blood causes water to be reabsorbed, leading to an increase in blood volume and blood pressure.
The renin-angiotensin-aldosterone system seems to be always active in some people who have hypertension. Diuretics are drugs that have been developed to counteract hypertension. They inhibit the reabsorption of Na + so that less water is reabsorbed from the nephron.
Antidiuretic Hormone Antidiuretic hormone (ADH) is released by the posterior lobe of the pituitary when the solutes in blood become more concentrated, due to a lack of water intake once Na + has been reabsorbed. To understand the action of this hormone, consider its name. Diuresis means increased amount of urine, and antidiuresis means decreased amount of urine. When ADH is present, more water is reabsorbed from the distal convoluted tubules and collecting ducts, and the amount of urine decreases. In practical terms, if an individual does not drink much water on a certain day, the posterior lobe of the pituitary releases ADH, causing more water to be reabsorbed and the blood volume to be maintained at a normal level, resulting in less urine formation. On the other hand, if an individual drinks a large amount of water and does not perspire much, the posterior lobe of the pituitary does not release ADH, causing more water to be excreted and the blood volume to be maintained at a normal level, resulting in a greater amount of urine formation.
Drinking alcohol causes diuresis because alcohol inhibits ADH secretion. The dehydration that follows is believed to contribute to the symptoms of a hangover.
Atrial Natriuretic Hormone
The actions of aldosterone and ADH are opposed by atrial natriuretic hormone (ANH). Tl1is hormone is released by cardiac cells when the atria of the heart are stretched, due to increased blood volume. ANH inhibits renin secretion by the juxtaglomerular apparatus and aldosterone secretion by the adrenal cortex. Its effect, therefore, is to cause the excretion of Na+ -that is, natriuresis. When N a + is excreted, so is water, and therefore, blood volume and blood pressure decrease.
Blood volume and blood pressure are raised vvhen aldosterone and ADH are secreted. The actions of aldosterone and ADH are opposed by ANH. and in this VIIay, normal blood volume and pressure are maintained.
Electrolytes
The osmolarity of body fluids, including plasma, is dependent upon the concentration of substances-particularly electrolytes-within the fluids. Electrolytes are compounds and molecules that are able to ionize and, thus, carry an electrical current. The most common electrolytes in the plasma are sodium (Na+), potassium (K+), and bicarbonate (HC03 -). Na+ and K+ are termed cations because they are positively charged, and HC03 - is termed an anion because it is negatively charged. The kidneys control blood composition by regulating electrolyte excretion.
Sodium The movement of Na+ across an axon membrane, you will recall, is necessary to the formation of a nerve impulse and muscle contraction. The concentration of Na + in the blood is also the best indicator of the blood's osmolarity.
Potassium The movement of K+ across an axon membrane is also necessary to the formation of a nerve impulse and muscle contraction. Abnormally low K+ concentrations in the blood, as might occur if diuretics are abused, can lead to cardiac arrest.
Bicarbonate Ion HC03 - is the form in which carbon dioxide is carried in the blood. The bicarbonate ion has a very important function in that it helps maintain the pH of the blood, as is discussed in the paragraphs that follow.
Other Ions The plasma contains many other ions. For example, calcium ions (Ca2+) and phosphate ions (HPO4 2-) are important to bone formation and cellular metabolism. Their absorption from the intestine and excretion by the kidneys is regulated by hormones, which was discussed in chapter 10.
The kidneys monitor blood composition by regulating excretion of sodium. potassium. bicarbonate. and other ions.
Acid-Base Balance
The hydrogen concentration [H+] of body fluids is important because proteins such as cellular enzymes function properly only when the pH is maintained at about 7.4., acids decrease the pH of solutions, and bases increase the pH of solutions.
The pH of the blood stays near 7.4 because the blood is buffered. A buffer is a chemical or combination of chemicals that can take up excess hydrogen (H+) or excess hydroxide (OH-). One of the most important buffers in the blood is carbonic acid (H2C03) and the bicarbonate ion (HC03 -):
H2C03 ----> H+ + HC03 -
Ifthe pH of the blood rises (less acidity), carbonic acid dissociates to release H+ If the pH of the blood decreases (more acidity), the bicarbonate ion combines with H+ to give carbonic acid. Proteins also help buffer the blood because they are charged in such a way that they can combine with either H+ or OIr.
The kidneys are the final adjusters of pH, contributing to homeostasis by maintaining the blood pH level within a narrow range. The entire nephron takes part in this process. The excretion of hydrogen ions (H+) and ammonium (NH4 +), together with the reabsorption of sodium (Na+) and bicarbonate ions (HC03 -), is adjusted to keep the pH within normal bounds. If blood is acidic, hydrogen ions are excreted in combination with ammonium, while sodium and bicarbonate ions are reabsorbed. This restores the pH because NaHC03 is a base. If blood is basic, fewer hydrogen ions are excreted, and fewer sodium and bicarbonate ions are reabsorbed.
The kidneys contribute to homeostasis by making adjustments in the excretion of hydrogen ions and ammonium, and in the reabsorption of sodium and bicarbonate ions. to maintain the blood pH level vvithin a narrow range.
The kidney is involved in the fluid, electrolyte, and acidbase balance of the blood.
Fluid and Electrolyte Balance
The average adult male is about 60% water by weight. The average adult female is only about 50% water by weight because females generally have more subcutaneous adipose tissue, which contains less water. About two-thirds of this water is inside the cells (called intracellular fluid), and the rest is largely distributed in the plasma, tissue fluid, and lymph (called extracellular fluid). Water is also present in such fluids as cerebrospinal fluid and synovial fluid, these fluids are referred to as "other" fluids.
Maintenance of the water content in all body fluids requires that the body is in fluid balance: The total water intake should equal the total water loss. Consider water enterance in the body; for example, metabolic water is created by chemical reactions in cells, food contains water, and a person dri n ks water. The osmolarity (ability to bring about osmosis) of the blood is constantly monitored within the hypothalamus, which determines whether or not a person is thirsty and takes a drink of water. Water is lost from the body in a variety of ways-for example, from sweating, feces formation, evaporation from the lungs, and urine formation.
Hormonal Control
Aldosterone, antidiuretic hormone (ADH), and atrial natriuretic hormone (ANH) work together to control blood volume and pressure.
Aldosterone Aldosterone, a hormone secreted by the adrenal cortex, primarily maintains sodium (Na +) and potassium (K+) balance. The adrenal cortex is the outer portion of the adrenal glands, which lie atop the kidneys.
Blood volume is constantly monitored by the afferent arteriole cells within the juxtaglomerular apparatus. When blood volume and therefore blood pressure is not sufficient to promote glomerular filtration, afferent arteriole cells secrete the enzyme renin. Renin changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin 1. Later, angiotensin I is converted to angiotensin II in the lungs by angiotensin-converting enzyme. Angiotensin II, a powerful vasoconstrictor, stimulates the adrenal cortex to release aldosterone. When aldosterone is released, sodium (N a +) is reabsorbed into the blood at the distal convoluted tubules and collecting ducts of the nephron. The increase in Na + in blood causes water to be reabsorbed, leading to an increase in blood volume and blood pressure.
The renin-angiotensin-aldosterone system seems to be always active in some people who have hypertension. Diuretics are drugs that have been developed to counteract hypertension. They inhibit the reabsorption of Na + so that less water is reabsorbed from the nephron.
Antidiuretic Hormone Antidiuretic hormone (ADH) is released by the posterior lobe of the pituitary when the solutes in blood become more concentrated, due to a lack of water intake once Na + has been reabsorbed. To understand the action of this hormone, consider its name. Diuresis means increased amount of urine, and antidiuresis means decreased amount of urine. When ADH is present, more water is reabsorbed from the distal convoluted tubules and collecting ducts, and the amount of urine decreases. In practical terms, if an individual does not drink much water on a certain day, the posterior lobe of the pituitary releases ADH, causing more water to be reabsorbed and the blood volume to be maintained at a normal level, resulting in less urine formation. On the other hand, if an individual drinks a large amount of water and does not perspire much, the posterior lobe of the pituitary does not release ADH, causing more water to be excreted and the blood volume to be maintained at a normal level, resulting in a greater amount of urine formation.
Drinking alcohol causes diuresis because alcohol inhibits ADH secretion. The dehydration that follows is believed to contribute to the symptoms of a hangover.
Atrial Natriuretic Hormone
The actions of aldosterone and ADH are opposed by atrial natriuretic hormone (ANH). Tl1is hormone is released by cardiac cells when the atria of the heart are stretched, due to increased blood volume. ANH inhibits renin secretion by the juxtaglomerular apparatus and aldosterone secretion by the adrenal cortex. Its effect, therefore, is to cause the excretion of Na+ -that is, natriuresis. When N a + is excreted, so is water, and therefore, blood volume and blood pressure decrease.
Blood volume and blood pressure are raised vvhen aldosterone and ADH are secreted. The actions of aldosterone and ADH are opposed by ANH. and in this VIIay, normal blood volume and pressure are maintained.
Electrolytes
The osmolarity of body fluids, including plasma, is dependent upon the concentration of substances-particularly electrolytes-within the fluids. Electrolytes are compounds and molecules that are able to ionize and, thus, carry an electrical current. The most common electrolytes in the plasma are sodium (Na+), potassium (K+), and bicarbonate (HC03 -). Na+ and K+ are termed cations because they are positively charged, and HC03 - is termed an anion because it is negatively charged. The kidneys control blood composition by regulating electrolyte excretion.
Sodium The movement of Na+ across an axon membrane, you will recall, is necessary to the formation of a nerve impulse and muscle contraction. The concentration of Na + in the blood is also the best indicator of the blood's osmolarity.
Potassium The movement of K+ across an axon membrane is also necessary to the formation of a nerve impulse and muscle contraction. Abnormally low K+ concentrations in the blood, as might occur if diuretics are abused, can lead to cardiac arrest.
Bicarbonate Ion HC03 - is the form in which carbon dioxide is carried in the blood. The bicarbonate ion has a very important function in that it helps maintain the pH of the blood, as is discussed in the paragraphs that follow.
Other Ions The plasma contains many other ions. For example, calcium ions (Ca2+) and phosphate ions (HPO4 2-) are important to bone formation and cellular metabolism. Their absorption from the intestine and excretion by the kidneys is regulated by hormones, which was discussed in chapter 10.
The kidneys monitor blood composition by regulating excretion of sodium. potassium. bicarbonate. and other ions.
Acid-Base Balance
The hydrogen concentration [H+] of body fluids is important because proteins such as cellular enzymes function properly only when the pH is maintained at about 7.4., acids decrease the pH of solutions, and bases increase the pH of solutions.
The pH of the blood stays near 7.4 because the blood is buffered. A buffer is a chemical or combination of chemicals that can take up excess hydrogen (H+) or excess hydroxide (OH-). One of the most important buffers in the blood is carbonic acid (H2C03) and the bicarbonate ion (HC03 -):
H2C03 ----> H+ + HC03 -
Ifthe pH of the blood rises (less acidity), carbonic acid dissociates to release H+ If the pH of the blood decreases (more acidity), the bicarbonate ion combines with H+ to give carbonic acid. Proteins also help buffer the blood because they are charged in such a way that they can combine with either H+ or OIr.
The kidneys are the final adjusters of pH, contributing to homeostasis by maintaining the blood pH level within a narrow range. The entire nephron takes part in this process. The excretion of hydrogen ions (H+) and ammonium (NH4 +), together with the reabsorption of sodium (Na+) and bicarbonate ions (HC03 -), is adjusted to keep the pH within normal bounds. If blood is acidic, hydrogen ions are excreted in combination with ammonium, while sodium and bicarbonate ions are reabsorbed. This restores the pH because NaHC03 is a base. If blood is basic, fewer hydrogen ions are excreted, and fewer sodium and bicarbonate ions are reabsorbed.
The kidneys contribute to homeostasis by making adjustments in the excretion of hydrogen ions and ammonium, and in the reabsorption of sodium and bicarbonate ions. to maintain the blood pH level vvithin a narrow range.
