12 The Endocrine System: The Body’s Other Control System
Multimedia Asset Directory Slide 66 Hyperglycemia Animation Slide 67 Hypoglycemia Animation Slide 71 Pathology of Diabetes Video Slide 72 Insulin Video Slide 73 Hypothalmic Pituitary Axis Animation Slide 74 Monitoring Glucose Levels Video Slide 97 Aging and the Endocrine System Video Slide 98 Phlebotomy Video Slide 99 Dieticians Video
Introduction • The nervous system and the endocrine system are totally interconnected and always monitor each other’s activities. • The endocrine system also collects information and sends orders but it is a slower, more subtle control system. While it acts slowly, the effects last longer than those of the nervous system.
Learning Objectives • Discuss the functions of the various endocrine glands. • Explain mechanisms of control of hormone levels. • Describe the purpose and effects of hormones within the body. • Discuss the functions of the various endocrine glands. • Differentiate between hormonal and humoral control.
adrenal cortex (ad REE nal KOR teks) adrenal medulla (ad REE nal meh DULL lah) endocrine (EHN doh krin) epinephrine (EP ih NEFF rin) homeostasis (HOH mee oh STAY sis) hypothalamus (HIGH poh THAL ah mus) norepinephrine (nor EP ih NEFF rin) oxytocin (AHK see TOH sin) parathyroid gland (PAIR ah THIGH royd) Pronunciation Guide Click on the megaphone icon before each item to hear the pronunciation.
pineal gland (PIN ee al) pituitary (pih TOO ih TAIR ee) prolactin (proh LAK tin) testes (TESS teez) thymus (THIGH mus) Pronunciation Guide Click on the megaphone icon before each item to hear the pronunciation.
The Endocrine Organs • The endocrine system is a series of organs and glands in your body that secrete chemical messengers into your blood stream. • Exocrine glands, like sweat glands, secrete out of the body, but are not part of the endocrine system that secrete into the body. • Many glands, like the hypothalamus and pancreas, have multiple functions.
Hormones • The chemical messengers released by endocrine glands are called hormones. • Hormones are released into the blood stream and travel all over your body, some affecting millions of cells simultaneously. Their effects last for minutes or even hours or possibly days. • Many hormones are secreted all the time, with the amount secreted changing as needed.
How Hormones Work • Hormones work by binding to receptors on target cells. They bind to not only sites outside the cell, like neurotransmitters, but also to sites inside the cell. • If hormones bind to the outside of the cell, they can have several different effects, either changing cellular permeability or sending the target cell a message that changes enzyme activity inside the cell.
Steroids • One special class of hormones, called steroids, is particularly powerful because steroids can bind to sites inside cells. • Steroids are lipid molecules which can pass easily through the target cell membrane, allowing them to interact directly with the cell’s DNA to change cell activity.
Steroids • They are carefully regulated because only small amounts are needed to perform their task.
Control of Endocrine Activity • The amount of hormone secreted changes based on situational demands. • Many endocrine organs secrete hormones continuously.
Homeostasis and Negative Feedback • Many of the chemical and physical characteristics of your body have a standard level, or set point, that is the ideal level for that particular value. Blood pressure, blood oxygen, heart rate, and blood sugar are examples. • Your control systems (endocrine and nervous system) work to keep levels at or near ideal.
Homeostasis and Negative Feedback • There is a way for your body to measure the variable, a place where the ideal level is stored, and a way for your body to fix levels that are not near ideal. For example, the hypothalamus stores the ideal set-point for temperature.
Figure 12-2 Homeostasis is analogous to regulation of temperature via a thermostat.
Homeostasis and Negative Feedback • If any of your body’s dozens of characteristics become seriously abnormal, your control systems work to bring them back to set point via a process called negative feedback. • Negative feedback counteracts a change. Thus, as blood pressure rises, your body works to bring it down to normal, the set point. If blood pressure falls, your body works to raise it back up to normal.
Homeostasis and Negative Feedback • Hormones work the same way. If hormone levels rise, negative feedback will turn off the endocrine organ that is secreting the hormone.
Figure 12-3 Homeostasis and negative feedback as related to control of body temperature.
Positive Feedback • Positive feedback increases the magnitude of a change. • The flow of sodium into a neuron during depolarization is a real-life example we have already visited. The more depolarized a neuron becomes, the more sodium flows in, so it becomes more depolarized, so more flows in, etc. This kind of process is also known as a vicious cycle. Positive feedback is not a way to regulate the body, since positive feedback increases a change away from set point.
Clinical Application: Childbirth and Positive Feedback • Positive feedback is harmful if the vicious cycle cannot be broken, but is sometimes necessary for a process to run to completion. A good example is the continued contraction of the uterus during childbirth. When a baby is ready to be born, a signal tells the hypothalamus to release oxytocin from the posterior pituitary, increasing the intensity of contractions.
Clinical Application: Childbirth and Positive Feedback • As the uterus contracts, the pressure inside the uterus increases the signal to the hypothalamus and more oxytocin is released, causing the uterus to contract harder. This cycle of ever-increasing uterine contractions, due to ever-increasing release of oxytocin from the hypothalamus, continues until the pressure inside the uterus decreases when the baby is born.
Neural Control • Some hormones are directly controlled by the nervous system. For example, the adrenal glands receive signals from the sympathetic nervous system. When the sympathetic nervous system is active, it sends signals to the adrenal glands to release epinephrine and norepinephrine as hormones, prolonging the effects of sympathetic activity.
Hormonal Control • Other hormones are part of a hierarchy of hormonal control, where one gland is controlled by the release of hormones from another gland up the chain, which is controlled by another gland’s release of hormones up the chain. Orders are sent from one organ to another, like a relay race. • Negative feedback controls the flow of orders via hormones from one part of the chain to the other.
Humoral Control • Some endocrine organs directly monitor the body’s internal environment by monitoring body fluids, such as blood, and then respond accordingly. • Humoral is the term that pertains to body fluids or substances and therefore this is called humoral control. For example, the pancreas secretes insulin in response to rising blood sugar levels.
From the Streets:Endocrine Emergencies • Symptoms of endocrine emergencies can usually be assigned to one of two categories: • Symptoms of underproduction • Hyposecretion • Symptoms of overproduction • Hypersecretion
Endocrine Glands:The Hypothalamus • The hypothalamus, located in the diencephalon, is an important link between the two control systems. • The hypothalamus controls much of your physiology, including hunger, thirst, fluid balance, and body temperature to name a small number of its functions.
Endocrine Glands:The Hypothalamus • The hypothalamus also acts as a “commander in chief” for the other glands in the endocrine system since it controls the pituitary gland, and thus, most other glands in the endocrine system.
The Pituitary • The pituitary, also a part of the diencephalon, has been commonly known as the “master gland” because of its role in controlling other endocrine glands. • The pituitary gland really acts only under orders from the hypothalamus. • If the hypothalamus is the “commander in chief,” the pituitary is a high-ranking general. • The pituitary is split into two segments, the posterior pituitary and the anterior pituitary.
The Posterior Pituitary • The posterior pituitary is an extension of the hypothalamus. Hypothalamic neurons, specialized to secrete hormones instead of neurotransmitters, extend their axons through a stalk in the posterior pituitary. • These neurons secrete two hormones, antidiuretic hormone (ADH) and oxytocin. While they are secreted by the pituitary, they are made by the hypothalamus.
Antidiuretic Hormone • The name suggests exactly what it does, it decreases urination, which decreases fluid loss, increasing body fluid volume. • ADH is secreted when the hypothalamus senses decreased blood volume or increased blood osmolarity (more solids suspended in blood).
Antidiuretic Hormone • ADH circulates through the blood stream and targets the kidneys specifically, causing them to absorb more water. • ADH is very important in long-term control of blood pressure, especially during dehydration.
Oxytocin • Oxytocin is important in maintaining uterine contractions during labor and is involved in milk production in nursing mothers. • Oxytocin’s function in males is unknown.
The Anterior Pituitary • The anterior pituitary makes and secretes a number of hormones under hormonal control of the hypothalamus. • The hypothalamus secretes a hormone that controls hormone secretion by the anterior pituitary, which usually controls the secretion of hormones by another endocrine gland (growth hormone is an exception). • The hormone levels are controlled by negative feedback to both the pituitary and the hypothalamus.
Clinical Application:Stature Disorders • Stature disorders are those that result in well below normal (dwarfism) or well above normal (gigantism or giantism) height. Some are caused by abnormalities in skeletal development or nutritional deficiencies, but growth hormone (GH) problems are often implicated.
Clinical Application:Stature Disorders • If GH secretion is insufficient during childhood, children don’t grow to normal height. If GH deficiency is diagnosed before closure of the growth zones of the long bones it can be treated with GH supplements, otherwise these children grow to be undersized adults.
Clinical Application:Stature Disorders • Oversecretion of GH during childhood results in very, very tall people. Robert Wadlow, one of the tallest men to ever live, according to The Guinness Book of World Records, was more than 8 feet tall. People with gigantism have many health problems, including a skeleton that can’t support them. Surgery and medication are the only treatments.
Clinical Application:Stature Disorders • If the oversecretion of GH starts after the bones have stopped growing the person doesn’t get taller but the tissue of the hands, feet, face, and many internal organs will continue to grow out of control causing pain and organ dysfunction. Most oversecretion is due to noncancerous pituitary tumors.
From the Streets:Vasopressin • Antidiuretic hormone (ADH), also called vasopressin, is one of two hormones secreted by the posterior pituitary. • ADH causes increased reabsorption of water in the kidneys and vasoconstriction of peripheral vessels. • Both mechanisms increase blood pressure.
From the Streets:Vasopressin • Vasopressin may be used in certain types of cardiac arrest as an alternative to epinephrine.
Figure 12-7 The hypothalamus, anterior and posterior pituitary glands, and their targets and associated hormones.
The Thyroid Gland • The thyroid gland is located in the anterior portion of the neck and is butterfly-shaped. • The thyroid gland secretes the hormones triiodothyronine (T3) and thyroxine (T4) under orders from the pituitary. The third hormone is calcitonin, involved in calcium storage. • T3 and T4 contain iodine and control cell metabolism and growth.