The endocrine system comprises the glands and other tissues that produce hormones, which are released from glandular tissue directly into the circulatory system (Figure 6-19). The endocrine system functions with the nervous systems to
Nasal cavity Pharynx Larynx Trachea
Primary bronchi
Secondary bronchi
Tertiary bronchi Bronchiole
Diaphragm pulmonaryFrom
artery
Topulmonary veins Terminal bronchiole Alveolar duct
Alveolar sacs
Alveoli Capillaries
Right lung
Left lung
Upper respiratory tract
Lower respiratory tract
FIGURE 6-18 Structure of the lungs. The bronchial tree branches until it terminates in alveoli, where gas exchange occurs.
tightly regulate body function, maintaining ho- meostasis. Because of the fine degree of control required, endocrine glands are themselves regu- lated by other endocrine glands, in multiple con- trol systems known as feedback loops. Many of these feedback loops involve the brain as well, especially the small portion of the brain known as the hypothalamus.
Hypothalamus
Pineal gland Pituitary
(Hypophysis) gland
Thyroid gland Parathyroid glands
Thymus gland
Adrenal (Suprarenal) glands
Pancreas
Ovaries Testes
FIGURE 6-19 The glands of the endocrine system.
BOX 6-7 Disorders of the Respiratory System Infection
• Pneumonia: Infection of the lower airway; can be viral, fungal, bacterial, or mycobacterial
• Strep throat: Infection of the throat by Streptococcus bacteria
• Tuberculosis: Infection by Mycobacterium tuberculosis
• Upper respiratory infection (URI): Infection of the nose, throat, or larynx; can be viral, fungal, or bacterial Inflammation
• Bronchitis: Inflammation of the bronchi as a result of irritation or infection
• Emphysema: Chronic inflammation of alveoli and bronchioles; can be caused by smoking
• Pleurisy: Inflammation of the pleural membrane Obstruction
• Asthma: Obstructed bronchi resulting from airway constriction
• Chronic obstructive pulmonary disease (COPD): Caused by emphysema
• Pulmonary edema: Fluid in the lungs Insufficient Ventilation
• Amyotrophic lateral sclerosis (ALS; also called Lou Gehrig’s disease)
• Muscular dystrophy
• Polio
• Spinal cord injury Developmental Disorder
• Infant respiratory distress syndrome (IRDS): Collapsed alveoli; common in premature infants
Neoplastic Disease
• Lung cancer
Test Disorder or Purpose
Arterial blood gases (ABGs): pH (acidity), Po2 (oxygen), Pco2
(carbon dioxide)
Most respiratory disorders;
assess lung function Cold agglutinins (test for
antibodies that react with red blood cells at cold temperatures)
Atypical pneumonia
Electrolytes Impaired gas exchange
Microbiologic tests
• Cultures
• Throat swabs
• Bronchial washings
Microbial infection—
pneumonia or pharyngitis Purified protein derivative (PPD) Skin test for tuberculosis Respiratory syncytial virus (RSV) Viral pneumonia
TABLE 6-7 Common Laboratory Tests for Respiratory Disorders
Hormones
Hormones are chemical substances released into the circulation by one group of cells that affect the function of other cells. Hormones exert their effects by binding to receptors at the target cell. Receptors may be on the surface of the target cell, in its cyto- plasm, or in its nucleus. There are three principal types of hormones: steroid hormones (including testosterone and progesterone), amino acid deriva- tives (including thyroxine and epinephrine, which is also called adrenaline), and peptides (including in- sulin). Each type has its own mechanisms of action and duration of response. For instance, epinephrine works quickly to produce a short-lived response, whereas steroid hormones, such as testosterone, work slowly to produce long-lasting effects.
Endocrine Glands Pituitary Gland
The pituitary gland is located at the base of the brain, just outside the cranium. It is involved in virtually every feedback loop in the endocrine system, sending hormones out and receiving hor- monal messages from every other endocrine gland, including the thyroid, the adrenals, and the ovaries.
For this reason, it is often called the master gland of the endocrine system. The hypothalamus secretes a variety of hormones that directly influence pituitary function.
The pituitary has two lobes: the anterior and the posterior. The anterior pituitary secretes thyroid- stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH), which act on the ovaries and testes. In addition, the anterior pituitary secretes growth hormone (GH), which regulates the rate of growth throughout the body;
melanocyte-stimulating hormone (MSH), which increases melanin pigment production in skin cells;
and prolactin, which increases milk production in the mammary glands.
As noted earlier, the posterior pituitary produces ADH, which regulates water reabsorption by the kidney. It also produces oxytocin, which stimulates smooth muscle contraction in the uterus during labor and in the mammary glands during nursing.
Thyroid Gland
The thyroid gland wraps around the trachea, just below the larynx. The thyroid secretes thyroxine—
also called T4, because it contains four atoms of iodine—and small amounts of triiodothyronine, or
T3. These two hormones regulate the basal meta- bolic rate, or energy consumption rate, of virtually every cell in the body. The thyroid also secretes calcitonin, which lowers calcium levels in body fluids.
Parathyroid Gland
The parathyroid gland is located behind the thyroid.
It secretes parathyroid hormone (PTH), also called parathormone, which regulates the amount of calcium and phosphorus in the circulation.
Thymus Gland
The thymus sits behind the sternum and plays a key role in the early development of the immune system. Among other functions, it secretes thymo- sin, which develops and maintains immunity. The thymus shrinks throughout life, and its function in adults, if any, is not clear.
Pancreas
The pancreas is located behind the stomach. It has two distinct functions. As discussed earlier, it pro- duces digestive enzymes that are secreted into the small intestine. As an endocrine gland, it produces two hormones that regulate the level of the sugar glucose in the blood: insulin and glucagon.
Insulin promotes the uptake of glucose by the body’s cells, thereby lowering blood sugar levels. It also promotes the conversion of glucose into glyco- gen, a storage carbohydrate, in the liver and in skeletal muscles. Glucagon promotes the break- down of glycogen back into glucose and increases its release into the blood, thereby elevating blood sugar levels. The antagonistic effects of these two hormones allow tight regulation of blood glucose levels.
Adrenal Glands
There are two adrenal glands, one located above each kidney (the word adrenal means “above the kidney”). Each adrenal gland is composed of two parts:
• The inner adrenal medulla secretes the cate- cholamines epinephrine and norepinephrine under the direct control of the autonomic ner- vous system. These hormones have the familiar effects of increasing the heart rate, increasing blood flow to the skeletal muscles, and produc- ing a subjective feeling of heightened awareness and anticipation. They are a crucial part of the fight-or-flight response.
• The outer adrenal cortex secretes steroid hor- mones. These include the glucocorticoids, which influence glucose metabolism; the miner- alocorticoids, which control electrolyte balance;
and androgens, or male sex hormones.
The principal glucocorticoids are cortisol, corti- sone, and corticosterone, which decrease glucose consumption and promote fat usage. They also reduce inflammation, accounting for their use on skin rashes and other irritations. Aldosterone is the principal mineralocorticoid, helping regulate sodium and water balance in the kidneys.
Gonads
The gonads are the ovaries and testes. The testes produce testosterone, the principal male sex hor- mone. Testosterone promotes sperm maturation, increases protein synthesis in skeletal muscle, and causes the development of male secondary sex characteristics, including facial hair and a deep voice. The ovaries produce estrogens and proges- terone, whose functions include regulation of the menstrual cycle, development of the uterus, and development of the mammary glands and other female secondary sex characteristics.
Disorders of the Endocrine System Disorders affecting the endocrine system most often involve either hypersecretion or hyposecre- tion (Box 6-8). Hypersecretion is most often caused by a tumor of the glandular tissue or excess admin- istration (as with insulin); hyposecretion may result from genetic disease, autoimmunity, or nutritional
deficiency. Diseases of the endocrine system are treated by an endocrinologist. Common laboratory tests ordered for these disorders are listed in Table 6-8.