Lesson

Anatomy is the study of the structure and shape of the body. Physiology is the study of how the body and its parts work and function. As we begin our study of the human body this week, we will learn the lingo (some might call it jargon) of anatomy. This will provide us with the information we need to navigate the complex structures of the body in the coming weeks. We will also review basic chemistry—the foundation of physiology. The proper functioning of our bodies requires millions of chemical reactions, 24 hours a day, 7 days a week.

Over the course of our studies, we will examine the human body from the simplest level, atoms interacting with one another, to cells, to tissues, to organs, and to organ systems, which come together to make up a fully functioning human body. As we begin, let's consider: What is necessary for life? What is necessary for survival? Make a list. Did you get all of them?

Consider these life functions, which we, as a body, carry out: the functions each organ in our system are made possible by a series of chemical reactions in our cells. So not only do we get to investigate the structures and functions of the human body, we also get to work in a little chemistry.

We might remember a study of chemistry from our early or secondary education years—matter, energy, atoms, isotopes, and elements. How is this relevant to anatomy and physiology? Our own cells use chemistry every day just to function. The study of chemical reactions as it pertains to living things is referred to as biochemistry.

At the heart of chemistry is the atom, so let's begin with that. An atom is defined as the smallest particle of an element that still contains all the properties of that element and can still enter into a chemical reaction with other elements. Each atom is comprised of a compact nucleus containing positively charged protons and neutrally charged neutrons, with negatively charged electrons in orbit around the nucleus. The number of these subatomic particles define the nature of that particular atom—say, a hydrogen atom with its single electron in orbit around a single proton.

Atoms that are alike combine to form elements; there are 92 naturally occurring elements, and each is represented by a chemical symbol. Atoms have the capacity to enter into chemical reactions with other atoms; in other words, when one atom interacts with another, their subatomic particles can rearrange, separate, or combine. There are several ways in which elements can combine or bond. One type of bond is an ionic bond, which is formed when one atom gains electrons, whereas the other atom loses electrons from its outermost orbit or shell. Another type of bond is the covalent bond, where rather than exchange electrons, the electrons are shared between their outermost shells. When two or more elements combine, they form a compound. The smallest unit of a compound, which can still retain all the properties of that compound and remain stable, is called a molecule.

There are two types of compounds, the second of which is especially relevant to our studies. The first are compounds that are inorganic; they are made of molecules that do not contain the element carbon (C), the element common to all living things, plants, and animals alike. Those molecules that do contain carbon are referred to as organic compounds. Organic compounds always contain carbon. There are four groups of organic compounds: carbohydrates, proteins, lipids, and nucleic acids.

Carbohydrates are compounds consisting of carbon, hydrogen, and oxygen, and there are three groups of these. They are monosaccharides, disaccharides, and polysaccharides. Lipids contain less oxygen than hydrogen; some examples of lipids are fats, phospholipids, and steroids. Proteins are among the most diverse and essential organic compounds found in all living things. The smallest unit of which a protein is comprised of is the amino acid. Specialized proteins, which help to control chemical reactions within a cell, are called enzymes.

Nucleic acids are the largest known organic molecules. They are made from thousands of repeating subunits known as nucleotides, arranged in a type of code. There are two major types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material of cells located in the nucleus of the cell, which determines the nature and function of that cell. RNA is structurally similar to DNA and exists in two forms: messenger RNA (mRNA) and transfer RNA (tRNA). The function of these two nucleic acids will be covered in more detail in upcoming weeks.

Your body is an amazingly complex machine. Trillions of cells in the body are in a state of nearly constant activity, going about the business of life. To keep your body in good working order, internal conditions must be maintained within a fairly narrow range despite constantly changing external conditions. The word homeostasis (homeo = the same; stasis = standing still) describes the body's ability to monitor and adjust internal conditions to remain within this narrow range. All organ systems of the body actively participate in maintaining homeostasis. The nervous and endocrine systems, which relay information to other organ systems, play an especially critical role.

These homeostatic control mechanisms feed back to influence the initial change in the internal environment by either shutting off or reducing the initial stimuli (negative feedback) or increasing the initial stimuli (positive feedback).

Negative Feedback Mechanisms

Because the body seeks to maintain a consistent internal environment, most homeostatic control mechanisms operate by reducing the stimulus and returning the variable to the original condition. These are negative feedback systems. These systems work like the heating system in your home. The variable is the temperature of your home, a thermometer acts as the receptor, the thermostat acts as the control center, and the furnace is the effector. Let's say the temperature is set at 68 degrees. If the temperature drops below 68, the thermometer detects the change and reports it to the thermostat. The thermostat then sends a signal to trigger the furnace to turn on. Once the temperature has reached 68, the temperature that is detected by the thermometer is sent to the thermostat. The thermostat then sends a signal to trigger the furnace to turn off, thus maintaining a constant temperature. So the initial stimulus, a drop in temperature, was reversed by the control mechanism. Note that although the response was an increase in temperature, this is an example of negative feedback. Negative feedback is used to regulate heart rate, blood pressure, and blood levels of glucose and oxygen, among many other examples in the body.

Positive Feedback Systems

Positive feedback systems result in an increase in the original stimulus, pushing a variable farther from its original value. Because the goal of homeostasis is generally to maintain stable internal conditions, positive feedback systems are rare and tend to control infrequent events. For example, positive feedback is used to intensify uterine contractions when a woman goes into labor.

In the coming weeks, we will cover each organ system in detail. As we embark on this journey, you will find it useful to familiarize yourself with anatomical terminology. These terms refer to location, position, and direction, usually with respect to another structure, and thus allow health professionals to accurately report the location of any injury in terms that all health professionals will understand.

Our bodies can further be divided into planes. This becomes helpful when studying a structure, such as an organ, from several different aspects or views.

Finally, we refer to the location of certain bodily structures, such as the heart or stomach, by defining our body cavities with respect to the structures they contain. There are seven body cavities: the dorsal cavity, the cranial cavity, the spinal cavity, the thoracic cavity, the abdominopelvic cavity, the abdominal cavity, and finally, the pelvic cavity.