Read an Excerpt
The Merck Manual of Medical Information
By Mark H. Beers Pocket Books
Copyright © 2004 Mark H. Beers
All right reserved. ISBN: 0743477340
Excerpt
Chapter One: Anatomy
Biology includes the study of the anatomy and physiology of living organisms. Anatomy is the study of structure, and physiology is the study of function.
Because the structure of living organisms is complex, anatomy is organized by levels, from the smallest components of cells to the largest organs and their relationships to other organs. Gross anatomy is the study of the body's organs as seen with the naked eye during visual inspection and dissection. Cellular anatomy is the study of cells and their components, which require special instruments such as microscopes and special techniques for observation.
Cells
Often thought of as the smallest unit of living organisms, a cell is made up of many even smaller parts, each with its own function. Human cells vary in size, but all are quite small. The largest, a fertilized egg, is too small to be seen with the naked eye.
Human cells have a membrane that holds the contents together. However, this membrane is not just a sac. It has receptors that identify the cell to other cells. The receptors also react to substances produced in the body and to drugs taken into the body, selectively allowing these substances or drugs to enter and leavethe cell. Reactions that take place at the receptors often alter and control a cell's functions.
Within the cell membrane are two major compartments, the cytoplasm and the nucleus. The cytoplasm contains structures that consume and transform energy and carry out the cell's functions; the nucleus contains the cell's genetic material and the structures that control cell division and reproduction.
The body is composed of many different types of cells, each with its own structure and function. Some, such as white blood cells, move freely, unattached to other cells. Others, such as muscle cells, are firmly attached one to another. Some cells, such as skin cells, divide and reproduce quickly; nerve cells, on the other hand, don't reproduce at all. Some cells, especially glandular cells, have as their primary function the production of complex substances, such as a hormone or an enzyme. For example, cells in the breast produce milk, those in the pancreas produce insulin, those in the lining of the lungs produce mucus, and those in the month produce saliva. Other cells have primary functions that are not related to the production of substances - for example, cells in the muscles and heart contract. Nerve cells conduct electrical impulses, allowing communication between the central nervous system (brain and spinal cord) and the rest of the body.
Tissues and Organs
Related cells joined together are collectively referred to as a tissue. The cells in a tissue are not identical, but they work together to accomplish specific functions. A sample of tissue removed for examination under a microscope (biopsy) contains many types of cells, even though a doctor may be interested in only one specific type.
Connective tissue is the tough, often fibrous tissue that binds the body's structures together and provides support. It is present in almost every organ, forming a large part of skin, tendons, and muscles. The characteristics of connective tissue and the types of cells it contains vary, depending on where it's found in the body.
The body's functions are conducted by organs. Each organ is a recognizable structure that performs specific functions - for example, the heart, lungs, liver, eyes, and stomach. An organ is made of several types of tissue and therefore several types of cells. For example, the heart contains muscle tissue that contracts to pump blood, fibrous tissue that makes up the heart valves, and special cells that maintain the rate and rhythm of heartbeats. The eye contains muscle cells that open and close the pupil, clear cells that make up the lens and cornea, cells that produce the fluid within the eye, cells that sense light, and nerve cells that conduct impulses to the brain. Even an organ as apparently simple as the gallbladder contains different types of cells, such as those that form a lining resistant to the irritative effects of bile, muscle cells that contract to expel bile, and cells that form the fibrous outer wall holding the sac together.
Organ Systems
Although an organ has a specific function, organs also function as part of a group, called an organ system. The organ system is the organizational unit by which medicine is studied, diseases are generally categorized, and treatments are planned. This book is, in large part. organized around the concept of the organ system.
An example of an organ system is the cardiovascular system, which includes the heart (cardio) and blood vessels (vascular). The cardiovascular system is responsible for pumping and circulating the blood. The digestive system, extending from the mouth to the anus, is responsible for receiving and digesting food and excreting waste. This system includes not only the stomach, small intestine, and large intestine, which move food, but also associated organs such as the pancreas, liver, and gallbladder, which produce digestive enzymes, remove toxins, and store substances necessary for digestion. The musculoskeletal system includes the bones, muscles, ligaments, tendons, and joints that support and move the body.
Of course, organ systems do not function in isolation. For example, after a large meal is eaten, the digestive system needs more blood to perform its functions. Therefore, it enlists the aid of the cardiovascular and nervous systems. Blood vessels of the digestive system widen to transport more blood. Nerve impulses are sent to the brain, notifying it of the increased work. The digestive system even directly stimulates the heart through nerve impulses and chemicals released into the bloodstream. The heart responds by pumping more blood; the brain responds by perceiving less hunger, more fullness, and less interest in vigorous activity.
Communication between organs and organ systems is vital. Communication allows the body to adjust the function of each organ according to the needs of the whole body. The heart must know when the body is resting so that it can slow down and when organs need more blood so that it can speed up. The kidneys must know when the body has too much fluid so that they can excrete more urine and when the body is dehydrated so that they can conserve water.
Through communication, the body keeps itself in balance - a concept called homeostasis. Through homeostasis, organs neither underproduce nor overproduce, and each organ facilitates the functions of every other organ.
Communication to maintain homeostasis can occur through the nervous system or through chemical stimulation. The autonomic nervous system, in large part, controls the complex communication network that regulates bodily functions. This part of the nervous system functions without a person's thinking about it and without much noticeable indication that it is working. Chemicals used to communicate are called transmitters. Transmitters that are produced by one organ and travel to other organs through the bloodstream are called hormones. Transmitters that conduct messages between parts of the nervous system are called neurotransmitters.
One of the best known transmitters is the hormone epinephrine (adrenaline). When a person is suddenly stressed or frightened, the brain instantly sends a message to the adrenal glands, which quickly release epinephrine. Within moments, this chemical has the entire body on alert, a response sometimes called preparation for fight or flight. The heart beats more rapidly and powerfully, the eyes dilate to allow more light in, breathing quickens, and the activity of the digestive system decreases to allow more blood to go to the muscles. The effect is rapid and intense.
Other chemical communications are less dramatic but equally effective. For example, when the body becomes dehydrated and needs more water, the volume of blood circulating through the cardiovascular system decreases. This decreased blood volume is perceived by receptors in the arteries in the neck. They respond by sending impulses through nerves to the pituitary gland, at the base of the brain, which then produces antidiuretic hormone. This hormone signals the kidneys to produce less urine and retain more water. Simultaneously, the brain senses thirst, stimulating a person to drink.
The body also has a group of organs - the endocrine system - whose primary function is to produce hormones that regulate the function of other organs. For example, the thyroid gland produces thyroid hormone, which controls the metabolic rate (the speed at which the body's chemical functions proceed); the pancreas produces insulin, which controls the use of sugar; and the adrenal glands produce epinephrine, which stimulates many organs to prepare the body for stress.
Barriers on the Outside and the Inside
As strange as it may seem, defining what's outside and what's inside the body isn't always easy because the body has many surfaces. The skin, which is actually an organ system, is one obvious surface, forming a barrier that prevents many harmful substances from entering the body. Although covered by a thin layer of skin, the ear canal is usually thought of as inside the body because it penetrates deep into the head. The digestive system is a long tube that begins at the mouth, winds through the body, and exits at the anus. Is food that's partially absorbed as it passes through this tube inside or outside of the body? Nutrients and fluid aren't really inside the body until they are absorbed into the bloodstream.
Air passes through the nose and throat into the windpipe (trachea), then into the extensive, branching airways of the lungs (bronchi). At what point does this passageway stop being outside and become inside the body? Oxygen in the lungs isn't useful to the body until it enters the bloodstream. To enter the bloodstream, oxygen must cross through a thin layer of cells lining the lungs. This layer acts as a barrier to viruses and bacteria, such as those that cause tuberculosis, which may be carried into the lungs with air. Unless these organisms penetrate the cells or enter the bloodstream, they don't cause disease. Because the lungs have many protective mechanisms, such as antibodies to fight infection and cilia to sweep debris out of the airways, most infectious organisms never cause disease.
Body surfaces not only separate the outside from the inside, but also keep structures and substances in their proper place so that they can function properly. For example, internal organs don't float in a pool of blood; blood is normally confined to blood vessels. If blood leaks out of the vessels into other parts of the body (hemorrhage), it not only fails to bring oxygen and nutrients to tissues but also can cause severe harm. For example, a very small hemorrhage into the brain destroys brain tissue because there is no room for expansion within the confines of the skull. On the other hand, a similar amount of blood leaking into the abdomen doesn't destroy tissue.
Saliva, so important in the mouth, can cause severe damage if inhaled into the lungs. The hydrochloric acid produced by the stomach rarely causes harm there, However, the acid can burn and damage the esophagus if it flows backward and can damage other organs if it leaks through the stomach wall. Feces, the undigested part of food expelled through the anus, can cause life-threatening infections if it leaks through the intestinal wall into the abdominal cavity.
Anatomy and Disease
The human body is remarkably well designed. Most of its organs have a great deal of extra capacity or reserve: They can still function adequately even though damaged. For example, more than two thirds of the liver must be destroyed before serious consequences occur, and a person can survive after an entire lung is surgically removed as long as the other lung is functioning normally. Other organs can tolerate little damage before they malfunction. For example, if a stroke destroys a small amount of vital brain tissue, a person may be unable to speak, move a limb, or maintain balance. A heart attack, which destroys heart tissue, may slightly impair the heart's ability to pump blood, or it may result in death.
Disease affects anatomy, and changes in anatomy can cause disease. Abnormal growths, such as cancer, can directly destroy normal tissue or produce pressure that ultimately destroys it. If the blood supply to a tissue is blocked or cut off, the tissue dies (infarction), as in a heart attack (myocardial infarction) or stroke (cerebral infarction).
Because of the relationship between disease and anatomy, methods of seeing into the body have become a mainstay of the diagnosis and treatment of disease. The first breakthrough came with x-rays, which enabled doctors to see into the body and examine organs without surgery. Another major advance was computed tomography (CT), in which x-rays are linked with computers. A CT scan produces detailed, two-dimensional images of the body's interior.
Other methods of producing images of internal structures include ultrasound scanning, which uses sound waves; magnetic resonance imaging (MRI), which uses the movement of atoms in a magnetic field; and radionuclide imaging, which uses radioactive chemicals injected into the body. These are noninvasive ways to see into the body, in contrast to surgery, which is an invasive procedure.
Anatomy in This Book
Because anatomy is so important to medicine, almost every section of this book begins with a description of the anatomy of an organ system. Illustrations throughout the book focus on the part of the anatomy being discussed.
Copyright © 1997 by Merck & Co., Inc.
Continues...
Excerpted from The Merck Manual of Medical Information by Mark H. Beers Copyright © 2004 by Mark H. Beers. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Loading...