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Biology for AP® Courses

32.5 Hormonal Control of Osmoregulatory Functions

Biology for AP® Courses32.5 Hormonal Control of Osmoregulatory Functions
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Learning Objectives

In this section, you will explore the following questions:

  • How do hormonal cures help the kidneys synchronize the osmotic needs of the body?
  • How do hormones and other chemical messengers including epinephrine, norepinephrine, rennin-angiotensin, aldosterone, antidiuretic hormone, and atrial natriuretic peptide help regulate waste elimination, maintain correct osmolarity, and perform other osmoregulatory functions?

Connection for AP® Courses

As we learned in an earlier section, the excretory system works with the circulatory and endocrine systems to maintain osmotic balance, eliminate wastes, and maintain blood pressure. For AP®, you do not need to memorize the list of hormones that control osmoregulatory functions or their specific function(s). However, information in this section applies to concepts previously explored.

The kidneys synchronize with hormonal cues. As you recall from our study of the endocrine system, hormones are small messenger molecules that travel in the bloodstream to affect a target cell. Different regions of the nephron have specialized cells with receptors to respond to chemical messengers and hormones. Table 32.1 summarizes the hormones that control the osmoregulatory functions. For example, the flight/flight hormones epinephrine and norepinephrine, released by the adrenal medulla and nervous subsystem, respectively, halt kidney function temporarily when the body is under extreme stress and much of the body’s energy is used to combat imminent danger. Another example is the rennin-angiotensin-aldosterone system that increases blood pressure and volume primarily by constricting blood vessels. Another hormone, antidiuretic hormone (ADH) increases membrane permeability to water in the collecting ducts of the nephron by adding aquaporins, causing more water to be reabsorbed. You’ve experienced the effects of ADH when it’s hot outside and you’re running around the athletic field; since you’re losing water by sweating and breathing hard, ADH prevents you from losing more water in urine and risking dehydration.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.D Cells communicate by generating, transmitting and receiving chemical signals.
Essential Knowledge 3.D.2 Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 3.34 The student is able to construct explanations of cell communication through cell-to-cell direct contact or through chemical signaling.
Essential Knowledge 3.D.2 Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 3.35 The student is able to create representation(s) that depict how cell-to-cell communication occurs by direct contact or from a distance through chemical signaling.
Hormones That Affect Osmoregulation
Hormone Where produced Function
Epinephrine and Norepinephrine Adrenal medulla Can decrease kidney function temporarily by vasoconstriction
Renin Kidney nephrons Increases blood pressure by acting on angiotensinogen
Angiotensin Liver Angiotensin II affects multiple processes and increases blood pressure
Aldosterone Adrenal cortex Prevents loss of sodium and water
Anti-diuretic hormone (vasopressin) Hypothalamus (stored in the posterior pituitary) Prevents water loss
Atrial natriuretic peptide Heart atrium Decreases blood pressure by acting as a vasodilator and increasing glomerular filtration rate; decreases sodium reabsorption in kidneys
Table 32.1

Epinephrine and Norepinephrine

Epinephrine and norepinephrine are released by the adrenal medulla and nervous system respectively. They are the flight/fight hormones that are released when the body is under extreme stress. During stress, much of the body’s energy is used to combat imminent danger. Kidney function is halted temporarily by epinephrine and norepinephrine. These hormones function by acting directly on the smooth muscles of blood vessels to constrict them. Once the afferent arterioles are constricted, blood flow into the nephrons stops. These hormones go one step further and trigger the renin-angiotensin-aldosterone system.

Renin-Angiotensin-Aldosterone

The renin-angiotensin-aldosterone system, illustrated in Figure 32.16 proceeds through several steps to produce angiotensin II, which acts to stabilize blood pressure and volume. Renin (secreted by a part of the juxtaglomerular complex) is produced by the granular cells of the afferent and efferent arterioles. Thus, the kidneys control blood pressure and volume directly. Renin acts on angiotensinogen, which is made in the liver and converts it to angiotensin I. Angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II. Angiotensin II raises blood pressure by constricting blood vessels. It also triggers the release of the mineralocorticoid aldosterone from the adrenal cortex, which in turn stimulates the renal tubules to reabsorb more sodium. Angiotensin II also triggers the release of anti-diuretic hormone (ADH) from the hypothalamus, leading to water retention in the kidneys. It acts directly on the nephrons and decreases glomerular filtration rate. Medically, blood pressure can be controlled by drugs that inhibit ACE (called ACE inhibitors).

The renin-angiotensin-aldosterone pathway involves four hormones: renin, which is made in the kidney, angiotensin, which is made in the liver, aldosterone, which is made in the adrenal glands, and A D H, which is made in the hypothalamus and secreted by the posterior pituitary. The adrenal glands are located on top of the kidneys, and the hypothalamus and pituitary are in the brain. The pathway begins when renin converts angiotensinogen into angiotensin I. An enzyme called A C E then converts angiotensin I into angiotensin I I. Angiotensin I I has several direct effects. These include arterial constriction, which increases blood pressure, decreasing the glomerular filtration rate, which results in water retention, and increasing thirst. Angiotensin I I also triggers the release of two other hormones, aldosterone and A D H. Aldosterone causes nephron distal tubules to reabsorb more sodium and water, which increases blood volume. A D H mediates the insertion of aquaporins into the nephridial collecting ducts. As a result, more water is reabsorbed by the blood. A D H also causes arteries to constrict. The hormone A N P is antagonistic to the angiotensin pathway. A N P decreases blood pressure and volume by increasing the glomerulus filtration rate, decreasing reabsorption of sodium ions by the nephron, and by inhibiting the release of renin from the kidney and aldosterone from the adrenal gland.
Figure 32.16 The renin-angiotensin-aldosterone system increases blood pressure and volume. The hormone ANP has antagonistic effects. (credit: modification of work by Mikael Häggström)

Mineralocorticoids

Mineralocorticoids are hormones synthesized by the adrenal cortex that affect osmotic balance. Aldosterone is a mineralocorticoid that regulates sodium levels in the blood. Almost all of the sodium in the blood is reclaimed by the renal tubules under the influence of aldosterone. Because sodium is always reabsorbed by active transport and water follows sodium to maintain osmotic balance, aldosterone manages not only sodium levels but also the water levels in body fluids. In contrast, the aldosterone also stimulates potassium secretion concurrently with sodium reabsorption. In contrast, absence of aldosterone means that no sodium gets reabsorbed in the renal tubules and all of it gets excreted in the urine. In addition, the daily dietary potassium load is not secreted and the retention of K+ can cause a dangerous increase in plasma K+ concentration. Patients who have Addison's disease have a failing adrenal cortex and cannot produce aldosterone. They lose sodium in their urine constantly, and if the supply is not replenished, the consequences can be fatal.

Antidiurectic Hormone

As previously discussed, antidiuretic hormone or ADH (also called vasopressin), as the name suggests, helps the body conserve water when body fluid volume, especially that of blood, is low. It is formed by the hypothalamus and is stored and released from the posterior pituitary. It acts by inserting aquaporins in the collecting ducts and promotes reabsorption of water. ADH also acts as a vasoconstrictor and increases blood pressure during hemorrhaging.

Atrial Natriuretic Peptide Hormone

The atrial natriuretic peptide (ANP) lowers blood pressure by acting as a vasodilator. It is released by cells in the atrium of the heart in response to high blood pressure and in patients with sleep apnea. ANP affects salt release, and because water passively follows salt to maintain osmotic balance, it also has a diuretic effect. ANP also prevents sodium reabsorption by the renal tubules, decreasing water reabsorption (thus acting as a diuretic) and lowering blood pressure. Its actions suppress the actions of aldosterone, ADH, and renin.

Science Practice Connection for AP® Courses

Think About It

Create a diagram to show an example of a hormone and how the hormone works in regulating an osmoregulatory process such as maintaining blood pressure and blood volume and altering kidney function to reduce the amount of water eliminated in urine.

Teacher Support

The question is an application of AP® Learning Objective 3.35 and Science Practice 1.1 because students are creating a diagram to depict how cell communication can occur via hormonal signaling, altering body physiology.

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