Porth's Essentials of Pathophysiology, 4e

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Disorders of Fluid, Electrolyte, and Acid–Base Balance

C h a p t e r 8

the effective circulating volume , which can be described as that portion of the ECF that fills the vascular com- partment and is “effectively” perfusing the tissues. 1,2 The effective circulating volume is monitored by sen- sors that are located both in the vascular system and the kidney. While thirst and ADH are the main regulators of water intake and output, the sympathetic nervous system and the renin-angiotensin-aldosterone sys- tem function in the regulation of sodium balance by the kidneys (see Chapter 18). The sympathetic ner- vous system responds to changes in arterial pressure and blood volume by adjusting the glomerular fil- tration rate and the rate at which sodium is filtered from the blood. Sympathetic activity also regulates renal reabsorption of sodium and renin release. The renin-angiotensin-aldosterone system exerts its action through angiotensin II and aldosterone. Angiotensin II acts directly on the renal tubules to increase sodium reabsorption. It also acts to constrict renal blood ves- sels, thereby decreasing the glomerular filtration rate and slowing renal blood flow so that less sodium is filtered and more is reabsorbed. Angiotensin II is also a powerful regulator of aldosterone , a hormone secreted by the adrenal cortex. Aldosterone acts to increase sodium reabsorption by the kidneys, while increasing potassium elimination. Thirst and Disorders of Thirst Like appetite and eating, thirst and drinking are two separate entities. 1,2,8,9 Thirst is the conscious sensation of the need to obtain and drink fluids high in water content. Drinking water or other fluids often occurs as the result of habit or for reasons other than those related to thirst. Most people drink without being thirsty, and water is consumed before it is needed. As a result, thirst is basically an emergency response. It usually occurs only when the need for water has not been anticipated. Thirst is controlled by the thirst center in the hypo- thalamus. There are two stimuli for true thirst based on water need: (1) cellular dehydration caused by an increase in ECF osmolality, and (2) a decrease in the effective circulating volume, which may or may not be associated with a decrease in serum osmolality. Sensory neurons, called osmoreceptors , which are located in or near the thirst center in the hypothalamus, respond to changes in ECF osmolality by swelling or shrinking (Fig. 8-7). Thirst normally develops when there is as little as a 1% to 2% change in serum osmolality. 9 Stretch receptors in the vascular system that monitor the effective circulating volume also aid in the regulation of thirst. Thus, thirst is one of the earliest symptoms of hemorrhage and is often present before other signs of blood loss appear. A third stimulus, the production of angiotensin II by the renin-angiotensin mechanism in the kidney, func- tions in the production of nonosmotic thirst. Angiotensin II increases in response to low blood volume and low blood pressure. This system is considered a backup

Paraventricular nucleus

Osmoreceptors

Supraoptic nucleus

Hypothalamo- hypophysial tract

Anterior lobe Posterior lobe

Pituitary gland

Capillary plexus

A

Blood volume

Serum osmolality

Secretion of ADH

Thirst

Reabsorption of water by the kidney

Water ingestion

Extracellular water volume

B

Feedback

system for thirst should other systems fail. Because it is a backup system, it probably does not contribute to the regulation of normal thirst. However, elevated levels of angiotensin II may lead to thirst in conditions such as congestive heart failure and chronic kidney disease, in which a decrease in renal blood flow leads to increased renin levels. Hypodipsia. Hypodipsia represents a decrease in the ability to sense thirst. Water deficit is commonly associated with lesions in the area of the hypo- thalamus (e.g., head trauma, meningiomas, occult hydrocephalus, subarachnoid hemorrhage). There is also evidence that thirst is decreased and water intake reduced in elderly persons, despite higher FIGURE 8-7. (A) Sagittal section through the pituitary and anterior hypothalamus. Antidiuretic hormone (ADH) is formed primarily in the supraoptic nucleus and to a lesser extent in the paraventricular nucleus of the hypothalamus. It is then transported down the hypothalamohypophysial tract and stored in secretory granules in the posterior pituitary, where it can be released into the blood. (B) Pathways for regulation of extracellular water volume by thirst and antidiuretic hormone. ADH, antidiuretic hormone.