Lesson 1, Topic 1
In Progress

Physiology of the Endocrine System

April 11, 2024

Learning Objective: Examine the physiology of the endocrine system.
      The physiology of the endocrine system involves hormone regulation, target cells, hormone action, and prostaglandins. These topics are discussed in depth in the following sections.

Mechanisms of Hormone Regulation
Learning Objective: Describe the mechanisms of hormone regulation.
The goal of hormone regulation is to maintain homeostasis. Nervous system stimulation, endocrine control, and feedback systems regulate hormone secretion. The following examples demonstrate these three mechanisms:
            • Nervous system regulation: During a stressful event, the adrenal medulla releases adrenaline (epinephrine) in response to stimulation from the sympathetic nervous system.

FIGURE 24.4  Negative feedback loop. The secretion of most hormones is regulated by negative feedback mechanisms that tend to reverse any deviations from normal. In this example, an increase in blood glucose triggers the secretion of insulin. Because insulin promotes glucose uptake by cells, the blood glucose level is restored to its lower, normal level. From Patton KT, Thibodeau GA: The human body in health and disease, ed 7, St. Louis, 2018, Elsevier.

            • Endocrine control regulation: TSH from the anterior pituitary stimulates the thyroid to secrete T3 and T4. (A hormone from one gland stimulates another gland to secrete a hormone.)
            • Feedback system regulation: A negative feedback loop system example: If the calcium blood level falls below normal, the parathyroid glands are stimulated to release PTH. PTH increases blood calcium levels by stimulating the absorption of calcium from the intestines or by chemically breaking down bone to release stored calcium into the blood. The change in the blood calcium level is detected by the parathyroid gland, which then stops production of PTH. (An imbalance activates the endocrine gland, which acts to correct the imbalance by stopping the hormone secretion process.)

Target Cells
Learning Objective: Describe the role of target cells with hormones.
      Each hormone released into the bloodstream has specific target cells for action. The target cells have receptors that attract only certain hormones. The cell membrane only lets selected hormones pass into the cell and affect cellular action.

Hormone Action
Learning Objective: Describe the role of nonsteroid and steroid hormones.
      There are two categories of hormones: nonsteroid hormones and steroid hormones. All hormones are messengers, but how they deliver their message is where they differ. Both types of hormones maintain homeostasis.
      Nonsteroid hormones are made up of protein or amino acids. This type of hormone attaches to a target cell membrane. Another molecule takes the message from the nonsteroid hormone and carries it to the target cell nucleus or organelle, which puts the message into action in the cell.
      Steroid hormones are small lipid-soluble (fat-soluble) molecules that attach to a target cell membrane and then pass directly into the target cell. Once inside the target cell, steroid hormones travel to and enter the nucleus. They bind to a receptor site, which creates a hormone–receptor site complex. This complex communicates its message with deoxyribonucleic acid (DNA) in the nucleus, and the DNA tells the cell how to put the hormone’s message into action.

Learning Objective: Discuss the role of prostaglandins.
      Prostaglandins (PGs), also known as tissue hormones, are substances found in many body tissues. PGs are produced in tissues and diffuse only a short distance to affect cells in their local area. They help regulate processes such as respiration, blood pressure, digestive system secretions, and reproductive functions. They are powerful molecules that are made locally and act locally.

Life Span Changes
Changes in hormone levels vary with age. Some increase and others decrease. The following hormones decrease with age:
            • Estrogen: In females, the declining level leads to menopause.
            • Testosterone: In males, levels gradually decrease.
            • Growth hormone.
            • Melatonin: Older adults may experience a loss of the normal sleep/wake cycles.
      Cortisol, insulin, and thyroid hormone usually remain unchanged or slightly decrease with age. Norepinephrine, epinephrine, parathyroid hormone, follicle-stimulating hormone, and luteinizing hormone may increase with age.

Common Signs and Symptoms of Endocrine Conditions
Many diseases affect the endocrine system. Most of the pathology of the endocrine system is the result of either hyper- (excessive) or hypo- (deficient) hormonal secretion. Here are the common signs and symptoms of endocrine conditions:
            • Exophthalmia, a noticeable protrusion of the eyeball
            • Glucosuria, the presence of glucose in the urine
            • Goiter, the swelling of the neck and visible enlargement of the thyroid gland
            • Hirsutism, excessive facial or body hair growth in women
            • Hypocalcemia, a low blood calcium level
            • Hypoglycemia, a low blood glucose (sugar) level
            • Ketoacidosis, the presence of ketones in the blood that cause metabolic acidosis (a pH imbalance due to too much acid)
            • Ketonuria, the presence of ketones in the urine
            • Polydipsia, excessive thirst
            • Polyphagia, excessive eating
            • Polyuria, excessive urine volume