General Human Physiology for the Sleep Technologist
Regina Patrick
LEARNING OBJECTIVES
On completion of this chapter, the reader should be able to:
1. Have a basic understanding of human anatomy and physiology.
2. Understand the unique functions of the endocrine system, the renal system, the digestive system, the immune system, the genitourinary system, the musculoskeletal system, metabolism, and thermoregulation in humans.
3. Understand how dysfunction in these systems can impact sleep.
4. Understand why certain sleep disorders are associated with dysfunction in one or more of these systems.
KEY TERMS
Acid-base balance
Digestive system
Electrolyte
Endocrine system
Genitourinary system
Immune system
Metabolism
Musculoskeletal system
pH
Renal system
Thermoregulation
The brain is the primary organ that controls sleep and wake. However, the function of many other body systems can impact sleep. This chapter will discuss the basic functions of the endocrine system, the renal system, the digestive system, the immune system, the genitourinary system, the musculoskeletal system, metabolism, and thermoregulation. This chapter will also discuss sleep disorders that are associated with dysfunctions in these systems.
THE ENDOCRINE SYSTEM
The endocrine system consists of several glands that secrete hormones that affect a different organ (i.e., target organ). The glands release their hormones directly into the bloodstream or into lymph (i.e., a transparent yellow liquid that flows through lymphatic vessels). Once the hormone travels to a target organ, it may affect the metabolism or function of the target organ. The hypothalamus, pituitary, pineal gland, thyroid, parathyroid glands, adrenal glands, gonads, and pancreas are a few of the organs that make up the endocrine system (Fig. 10-1).
Overview of Endocrine Hormone Classification
Three types of hormones produced in the endocrine system are steroid hormones, protein hormones (also called “peptide hormones”), and amine hormones. Steroid hormones are synthesized from cholesterol and secreted by the adrenal cortex, testes, and ovaries. Examples of steroid hormones are cortisol, aldosterone, testosterone, and estrogen. Protein hormones consist of amino acids that are bound together with peptide bonds. In a peptide bond, the amine group (-NH2) of one amino acid binds with the carboxyl group (-COOH) of another amino acid. Most hormones in the body are protein hormones. Insulin and calcitonin are examples of protein hormones. Amine hormones contain amino acids, but the acids do not form peptide bonds. Melatonin, thyroxine, and epinephrine are examples of amine hormones.
The release of many hormones is controlled by a negative feedback system. In such a system, when one substance reaches a certain level, it inhibits the release of another. For example, when the blood glucose level is high, the release of insulin inhibits the release of glucagon; when the blood glucose level is low, the release of glucagon inhibits the release of insulin. A negative feedback loop helps maintain proper levels of other substances (in this case, glucose) and proper functioning of the body. See Table 10-1 for a synopsis of the endocrine hormones and their main functions.
Table 10-1 The Source and Main Action of Various Endocrine Hormones | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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 Figure 10-1 The endocrine system is comprised of glands with a primary function of hormone secretion. (Reprinted with permission from Cohen, B. J. (2012). Memmler’s structure and function of the human body. Philadelphia, PA: Lippincott Williams & Wilkins/Wolters Kluwer. Copyright © Lippincott Williams & Wilkins/Wolters Kluwer.) |
ENDOCRINE ORGANS
Hypothalamus
The hypothalamus is a structure that lies at the base of the brain. The optic tract and optic chiasm (the point at which fibers of the left and right optic nerves cross to the opposite side of the brain) form part of the hypothalamus. The hypothalamus also contains various nuclei (i.e., groups of cells having a specialized function) that are involved in water balance, maintaining body temperature (i.e., thermoregulation), sleep, and food intake. Extending by a stalk from the hypothalamus is the pituitary, a gland that contains an anterior lobe and a posterior lobe. The anterior hypothalamus (i.e., front portion of the hypothalamus) produces various hormones that are transported into the anterior pituitary by way of a special blood capillary network; on arriving to the anterior pituitary, the posterior hypothalamic hormones stimulate the release of or inhibit the production and release of anterior pituitary hormones. In the posterior hypothalamus (i.e., the back portion of the hypothalamus), the axons of neurosecretory cells (i.e., neurons or nerve-like cells that secrete substances that act on another structure) extend from the posterior hypothalamus into the posterior pituitary. These neurosecretory cells synthesize hormones in the posterior hypothalamus; the hormones are then transported to the terminals of the axons in the posterior pituitary where they are stored and then released when necessary.
Pituitary Gland
The pituitary gland, which is located below the hypothalamus, extends from the hypothalamus by a stalk and consists of two distinct parts: a front portion (i.e., the anterior pituitary) and a rear portion (i.e., the posterior pituitary). The anterior pituitary receives various releasing and inhibiting hormones from the hypothalamus. As their name implies, these hormones cause the anterior pituitary to release hormones (e.g., the growth hormone-releasing hormone stimulates the anterior pituitary to release growth hormone) or inhibit their production and release of hormones (e.g., somatostatin inhibits the anterior pituitary’s production of growth hormone). The posterior pituitary gland acts as a reservoir for various hypothalamic hormones such as antidiuretic hormone and releases them into the bloodstream
when needed. The anterior pituitary histologically consists of the endocrine tissue, whereas the posterior pituitary histologically has the structure of nerve tissue.
when needed. The anterior pituitary histologically consists of the endocrine tissue, whereas the posterior pituitary histologically has the structure of nerve tissue.
Pineal Gland
The pineal gland is a small, flattened, cone-shaped gland in the upper rear brainstem. It receives sensory information from the optic nerves such as the level of light. It produces several hormones, the most important of which is the sleep-promoting hormone melatonin.
Thyroid
The thyroid is a butterfly-shaped gland that lies in front of the lower portion of the throat, just above the trachea. It secretes and stores various thyroid hormones (e.g., thyroxine [T4], triiodothyronine [T3], and calcitonin). Thyroxine and triiodothyronine are involved in regulating a person’s metabolic rate, and calcitonin is involved in maintaining proper calcium levels in the body.
Parathyroid Glands
The parathyroid glands are four small glands on the posterior surface of the thyroid. The parathyroid glands produce parathyroid hormone (also called “parathormone”), which has a role in the regulation of calcium and phosphate levels in the body.
Adrenal Gland
The adrenal gland is a small organ that lies on the top of the kidney. It consists of an inner core (i.e., the medulla) and an outer portion (i.e., cortex) that covers the medulla. The medulla secretes epinephrine and norepinephrine, which are released to prepare the body for the “flight or fight” response in times of stress. The adrenal cortex has three layers, each of which secretes different hormones. The cortex’s outermost layer secretes mineralocorticoids. These chemicals maintain the level of mineral salts (e.g., sodium chloride) in the body. The main mineralocorticoid produced by the adrenal cortex is aldosterone. The middle layer of the cortex produces glucocorticoids. These chemicals are involved in maintaining the blood glucose level and maintaining blood pressure. The main glucocorticoid produced by the adrenal cortex is cortisol. The innermost level of the cortex secretes small amounts of sex hormones (e.g., testosterone and estrogen).
Gonads
The gonads are organs that produce the reproductive cells. The gonads in men are the testes, which produce spermatozoa; the gonads in women are the ovaries, which produce ova (i.e., egg cells). The testes produce the sex hormone testosterone, and the ovaries produce estrogen. These hormones are involved in reproductive behavior, fertility, and the development of secondary sex characteristics (e.g., facial hair in men).
Pancreas
The pancreas is an oblong gland situated behind the stomach. On the surface of the pancreas, microscopic groups of specialized cells are scattered like islands among the pancreatic cells. These cells are called the “islets (or islands) of Langerhans.” Two types of cells exist within each islet of Langerhans—alpha and beta cells. The alpha cells secrete the hormone glucagon, which converts the liver starch glycogen into glucose when blood glucose levels are low; the beta cells secrete insulin, which reduces the blood glucose level when glucose levels are too high. Insulin and glucagon are antagonistic in their actions.
Endocrine Function and Sleep Disorders
The secretion of endocrine hormones increases and decreases with circadian rhythmicity. For example, the production of melatonin normally increases in the evening (thereby promoting sleep) and decreases in the morning (thereby promoting wakefulness). Factors such as night shift work, blindness, or dysfunction of certain hypothalamic cells can disrupt the rhythmic production of endocrine hormones. This disruption can lead to sleep and wake cycles that occur at undesired times (e.g., advanced or delayed sleep phase) or lead to the destruction of the cyclicity of a person’s sleep and wake cycles (e.g., free-running rhythm). As a result, a person may struggle with sleep problems such as insomnia or excessive daytime sleepiness (EDS).
The production of melatonin is influenced by a person’s exposure to light. In the normal eye, light stimulates the retina, which relays signals through the optic nerves. The signals ultimately reach the pineal gland, which produces melatonin. The greater the amount of light, the greater the number of signals reaching the pineal gland and the less the amount of melatonin it produces. As dark progresses through the evening, melatonin production increases and ultimately induces sleep. Night shift work, working frequently changing shifts, and blindness can alter a person’s exposure to light and disrupt the production of melatonin and lead to sleep problems.
Night Shift Work/Frequently Changing Shifts
A person who works third shift typically works all night in a lighted setting and drives home in the bright light of morning. Both of these factors reduce the production of melatonin. Once the person arrives home, he or she may have difficulty going to sleep at a desired time
because of the reduced melatonin level. Once a person falls asleep, the delayed sleep onset can shorten the person’s sleep time and result in struggles with sleepiness when the person is awake during a work shift.
because of the reduced melatonin level. Once a person falls asleep, the delayed sleep onset can shorten the person’s sleep time and result in struggles with sleepiness when the person is awake during a work shift.
It is possible for a shift worker to shift when the greatest amount of melatonin production occurs. Reducing daytime exposure to light can help shift the greatest melatonin production to daytime hours and thereby promote sleep during the day. For example, after working a shift, the worker could drive home with dark sunglasses on and use room-darkening shades in the bedroom. However, the ability to reset and maintain a consistent rhythm of melatonin production may not be possible if shift changes are frequent (e.g., every week).
Blindness
Some blind people—especially people who have no light perception—have a free-running circadian rhythm. In a free-running rhythm, a person’s sleep-wake phases move forward every day, rather than remaining stable at the same time every day. For example, the person may fall asleep at 8:00 p.m. one night, fall asleep the next night at 9:00 p.m., fall asleep the following night at 10:00 p.m., and so forth. When trying to follow a normal sleep-wake schedule, people with a free-running rhythm will experience a period of excessive sleepiness, a period in which they can sleep “normally,” and a period of wakefulness (i.e., insomnia) because their sleep-wake phases do not occur at the same time every day, and, therefore, flow into and out of phase with the societal norm.
Dysfunction of Hypothalamic Cells
Scientists have recently discovered that hypothalamic cells that produce the wake-promoting excitatory neuropeptide orexin (also called “hypocretin”) are inexplicably and irreversibly destroyed in some patients with narcolepsy. The primary symptom of narcolepsy is EDS. This symptom may be accompanied by cataplexy (i.e., the sudden, temporary loss of skeletal muscle tone), hypnagogic or hypnopompic hallucinations (i.e., realistic dream imagery occurring with sleep onset or on awakening, respectively), and paralysis on awakening from sleep or on going to sleep (i.e., sleep paralysis). The latter three symptoms may be the result of the muscle atonia and dream imagery features of rapid eye movement (REM) sleep intruding into wake. In recent years, scientists have noted another symptom in people with narcolepsy: sleep disrupted by frequent arousals that occur for no apparent reason such as apnea. The arousals may reflect a dysfunction in the neural control of sleep and wake.
Orexin-producing neurons project from the hypothalamus into brainstem areas such as the locus ceruleus, raphe nuclei, and reticular formation. These areas are involved in various aspects of REM sleep and wakefulness. Therefore, the destruction of orexin-producing hypothalamic cells may be involved in the improper manifestation of REM sleep features in wake, sleep disruption, and sleepiness in narcolepsy.
THE RENAL SYSTEM
The renal system consists of the kidneys, ureters, and urinary bladder (Fig. 10-2). The urinary system maintains blood volume, removes waste, regulates blood pressure, and is involved in maintaining pH balance in the blood.
The kidney is a bean-shaped organ that is approximately the size of a fist. The ureter is a fibromuscular tubular structure that descends from the kidney to the urinary bladder. It transports urine (an amber fluid that is a waste product) from the kidney to the urinary bladder. The urinary bladder is a musculomembranous sac in the front portion of the pelvis. It receives urine and voids it to the exterior of the body through the urethra (a membranous canal).
Each kidney contains millions of nephrons (Fig. 10-3), which are the structural component of the kidney. Each nephron contains a glomerulus (a balllike cluster of capillaries), a proximal convoluted tubule and distal convoluted tubule, and a collecting tubule. Afferent arterioles (i.e., microscopic arteries) bring blood from the renal artery into the glomerulus. A hollow-walled structure, called Bowman’s capsule, surrounds each glomerulus. (To use an analogy, imagine a fist that is pushed into a balloon, so that the balloon surrounds the fist; the fist represents the glomerulus and the balloon, Bowman’s capsule.) Some amount of
blood plasma exits from the glomerular capillaries and enters the hollow wall of the surrounding Bowman’s capsule. The remaining blood flows out of the glomerulus through the efferent arteriole. The efferent arteriole branches to form a capillary network, called the “peritubular capillaries,” that surrounds a nephron’s tubules. The peritubular capillaries drain blood into veins that ultimately feed into the renal vein. The renal vein takes blood away from the kidney.
blood plasma exits from the glomerular capillaries and enters the hollow wall of the surrounding Bowman’s capsule. The remaining blood flows out of the glomerulus through the efferent arteriole. The efferent arteriole branches to form a capillary network, called the “peritubular capillaries,” that surrounds a nephron’s tubules. The peritubular capillaries drain blood into veins that ultimately feed into the renal vein. The renal vein takes blood away from the kidney.
 Figure 10-2 The renal system. (National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland.) |
 Figure 10-3 A nephron and its blood supply. The nephron regulates the proportion of water, waste, and other materials in urine according to the body’s constantly changing needs. A nephron consists of a glomerular capsule, convoluted tubules, the nephron loop (loop of Henle), and a collecting duct. Blood filtration occurs through the glomerulus in the glomerular capsule. Materials that enter the nephron can be returned to the blood through the surrounding peritubular capillaries. (Reprinted with permission from Knight, L. (2013). Medical terminology: An illustrated guide Canadian edition (2nd ed., Figure 13.3). Philadelphia, PA: Lippincott Williams & Wilkins/Wolters Kluwer. Copyright © Lippincott Williams & Wilkins/Wolters Kluwer.) |
The blood plasma in the hollow wall of Bowman’s capsule empties into the proximal convoluted tubule. This fluid is called the “glomerular filtrate.” Once the glomerular filtrate is in the proximal convoluted tubule, ions such as sodium and chloride, glucose, and amino acids exit from the glomerular filtrate and enter the surrounding perivascular capillaries, and thereby reenter the bloodstream. This process is called “reabsorption.” Through reabsorption, most nutrients and approximately 65% of salt and water in the glomerular filtrate in the proximal tubule are returned to the blood. Electrolytes, which are positively or negatively charged molecules or atoms such as sodium (Na+), potassium (K+), bicarbonate (HCO3–), and calcium (Ca2+), are also reabsorbed from the glomerular filtrate. Some nitrogen-containing compounds (e.g., urea), ions, salts, and water are not reabsorbed from the glomerular filtrate and are excreted as waste.
The glomerular filtrate next passes through the loop of Henle, which has a descending arm and an ascending arm. The glomerular filtrate becomes increasingly concentrated as it flows down the descending arm and then up the ascending arm of the loop of Henle. The glomerular filtrate then travels to the distal convoluted tubule. In the distal convoluted tubule, salt and water continue to be reabsorbed into the bloodstream. As the filtrate passes through the loop of Henle, chemical compounds such as uric acid, creatinine, ammonia, hydrogen ions, and drugs (e.g., penicillin) exit from the perivascular capillaries and enter the glomerular filtrate. In this way, these compounds are removed from the blood. By the time the glomerular filtrate passes through the distal convoluted tubule, it is urine. The urine drips into the collecting duct and descends through the collecting duct to the renal pelvis, which joins the ureter. The ureter transports the urine into the urinary bladder.
Renal Control of Electrolyte Balance
The kidneys maintain the balance of electrolytes in the body. Electrolytes are ingested with foods and are lost through perspiration and feces. Some important electrolytes are Na+, K+, Ca2+, HCO3–, phosphate (PO43-), and chloride (Cl–). The amount of electrolytes taken into the body must equal the amount lost by the body. Losing a greater amount of electrolytes than what is consumed can set the stage for heart arrhythmias, muscle cramps, and other problems.
The steroid hormone aldosterone, which is produced and released by the adrenal gland, regulates sodium and potassium levels in the body. Aldosterone is released when the blood sodium level is low. It acts on the tubules to increase sodium reabsorption into the blood. When the blood potassium level is high, aldosterone causes potassium ions to be excreted into the glomerular filtrate and exit the body through urine. Potassium is usually excreted in the form of potassium salts such as potassium chloride (KCl).
Other hormones can impact the kidney’s regulation of electrolytes. For example, the distal tubules are sensitive to the actions of parathyroid hormone (i.e., parathormone) and calcitonin. Parathyroid hormone causes
calcium to be reabsorbed from the distal tubules into the blood and thereby raises the blood calcium level. Calcitonin decreases the reabsorption of calcium, phosphate, and sodium ions from the distal tubules and thereby allows these electrolytes to be excreted in the urine.
calcium to be reabsorbed from the distal tubules into the blood and thereby raises the blood calcium level. Calcitonin decreases the reabsorption of calcium, phosphate, and sodium ions from the distal tubules and thereby allows these electrolytes to be excreted in the urine.
Atrial natriuretic peptide has the opposite action of aldosterone. Atrial natriuretic peptide is secreted from the heart’s atrial wall in response to increased intravascular volume or stretching of the atria beyond the normal dimension. It stimulates kidney tubules to excrete sodium into the urine (i.e., natriuresis), which leads to more water entering the urine. Natriuresis reduces fluid volume in body tissues and blood pressure.
Renal Control of Acid-Base Balance
The kidneys also regulate the acid-base balance (i.e., pH, the concentration of hydrogen ions [H+]) of the body fluids. An improper acid-base balance can result in problems such as sleep-disordered breathing, excessive sleepiness, coma, and death.
In a solution, acids release hydrogen ions (thereby increasing the hydrogen concentration of the solution) and bases remove hydrogen ions (thereby decreasing the hydrogen concentration of the solution). The concentration of hydrogen ions is expressed by pH. A fluid with a pH of 7 is neutral, a fluid with a pH less than 7 is acidic, and a fluid with a pH greater than 7 is basic (i.e., alkaline).
The pH of many body fluids is normally maintained within a narrow range. For example, the normal pH of arterial blood ranges between 7.35 and 7.45. A person experiences acidosis (i.e., too acidic) if the arterial blood pH is less than 7.35 and experiences alkalosis (i.e., too alkaline) if the arterial blood pH is greater than 7.45. The narrow pH range in the blood and the other body fluids is maintained by various buffers.
A buffer is a solution containing two or more chemical compounds that prevent drastic changes in pH when an acid or a base enters a system (e.g., the bloodstream). A buffer usually contains a weak acid and the salt of that acid. In the salt of an acid, some of the hydrogen atoms in the acid have been replaced by other molecules. An example of a buffer is carbonic acid (H2CO3) and its salt sodium bicarbonate (NaHCO3).
A buffer helps the kidneys regulate the acid-base balance. For example, lactic acid, a waste product of muscle metabolism, easily dissociates in solution. In a solution such as blood, lactic acid dissociates into lactate (C3H5O3–) and hydrogen (H+) ions. The negatively charged lactate ions interact with the sodium ions from the sodium bicarbonate molecules to form sodium lactate. The remaining hydrogen ions from the lactic acid bind with the bicarbonate ions from the sodium bicarbonate molecule to form carbonic acid. Carbonic acid is a weak acid and therefore does not increase the acidity of the fluid drastically. In this way, the pH is buffered against the greater increase in acidity that lactic acid would have caused.
When the blood is too acidic, hydrogen ions are transported into the lumen of the tubules, enter the glomerular filtrate, and are ultimately excreted in urine. The bicarbonate ions are transported out of the glomerular filtrate and into the perivascular vessels surrounding the nephrons. These actions reduce the acidity of the blood.
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