Chapter 22. Osmotic Regulation and Excretion
22.5. Hormonal Control of Osmoregulatory Functions
charles-molnar and jane-gair
Learning Objectives
By the end of this section, you will be able to:
- Explain how hormonal cues help the kidneys synchronize the osmotic needs of the body
- Describe how hormones like epinephrine, norepinephrine, renin-angiotensin, aldosterone, anti-diuretic hormone, and atrial natriuretic peptide help regulate waste elimination, maintain correct osmolarity, and perform other osmoregulatory functions
While the kidneys operate to maintain osmotic balance and blood pressure in the body, they also act in concert with hormones. Hormones are small molecules that act as messengers within the body. Hormones are typically secreted from one cell and travel in the bloodstream to affect a target cell in another portion of the body. Different regions of the nephron bear specialized cells that have receptors to respond to chemical messengers and hormones. Table 22.1 summarizes the hormones that control the osmoregulatory functions.
| 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 |
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 22.15 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. Angiot