I. Introduction to Anatomy and Physiology

Human anatomy and physiology is concerned with the structure and the function of the human body. It is the basic science from which all of the health related sciences are derived.

  1. Definitions.

1. Anatomy - The study of structure. It may be divided into several subdisciplines.

a. Gross anatomy - The study of structures which are visible to the unaided eye. It may be pursued by several different methods.

(1) Regional anatomy - Here all of the structures in a particular part of the body are studied together.

(2) Systemic anatomy - The study of anatomy by body systems, such as the anatomy of the nervous system.

(3) Surface anatomy - The study of internal body structures such as muscles and nerves by relating them to the overlying skin.

(4) Cross sectional anatomy - Here the body is sliced into sections much like a salami. All of the organs in a given sections are studied in relation to one another.

(5) Developmental anatomy - This is the study of the change in body structures from conception to birth. It is part of the science of development which is known as embryology.

b. Microscopic anatomy - The study of structures that require the use of a microscope to be visible. It is usually divided into:

(l) Cytology - The study of cells and cell structures.

(2) Histology - The study of tissues.

c. Ultrastructure or fine anatomy - Anatomy which requires the use of an electron microscope.

d. Non-invasive techniques - These methods permit the visualization of internal structure without actual dissection. They include the following techniques.

(l) X-rays - These take the form of high energy radiation which can penetrate the body and expose photographic film yielding a picture of the dense tissues through which they passed.

(2) Tomography - This is a special X-ray technique which utilizes a rotating X-ray tube and film which are coordinated by a computer. It is called computerized tomography (CT) scanning. It provides accurate cross sections of the body which permit the examination of tissues and organs, layer by layer.

(3) Magnetic resonance imaging (MRI) - This used to be called nuclear magnetic resonance (NMR). It utilizes a powerful magnetic field that interacts with the nuclei of atoms, primarily hydrogen, in the body. The images can be in color, two or three-dimensional, and resemble those produced by CAT scans. A major advantage is that radiation is not utilized and this technique can also provide information about the biochemistry of an organ or tissue. A new development called "functional MRI" now permits blood flow to the brain to be observed in real time. . Patients can be given mental tasks to perform and the flow to the active part of the brain observed as it happens. Functional MRI utilizes radio waves in addition to the magnetic field. Another new variation is the magentic resonance spectroscopy (MRS). This technique resonates atoms other than hydrogen and therefore permits additional information about biochemistry.

(4) Sonography (ultrasound) - This technique uses high frequency sound waves which are scattered and reflected by various tissues in the body. A computer can then reconstruct an image of the internal anatomy. The advantages of this techniques are that the equipment is cheap and easy to use. Sound waves are also harmless to body tissues. Disadvantages are largely due to the fact that sound waves cannot penetrate the body as well the other techniques and therefore the images are not as good.

(5) Positron Emissiion Tomography (PET) – This a technique for observing blood flow to to the brain using radioactive isotopes. It is best for observing metabolic processes. The most active brain cells absorb the radioactive isotope and causes emission of radiation which then is integrated by a computer showing blood flow in real time to the most active regions.

2. Physiology - This is the study of function. Physiology may be divided up by the levels considered, i.e., cellular physiology, or by the organs being studied, i.e. renal physiology.

Form (anatomy) and function (physiology) are always related. It is impossible to understand one without the other. This fundamental principle is also known as the complementarity of structure and function.

B. Characteristics of life - It is impossible to adequately define life. Rather we must describe life by listing a series of minimum requirements or characteristics which all living things possess. These are generally considered to be five in number and are as follows.

l. Responsiveness or excitability - This is the ability to respond to an external stimulus (a change in the environment).

2. Adaptability - Short term - This is the ability to alter physiological operations in order to compensate for environmental changes.

Long term – The ability of a species to change with time…evolution.

3. Metabolism - All living things execute a series of complex chemical reactions in which some molecules are broken down while others are built. The sum total of all of these reactions is termed metabolism.

4. Growth - The ability to increase in size from the inside to the outside.

5. Reproduction - The ability to produce copies which are similar to the original.

C. Levels of Organization - Organisms are extremely complex and difficult to analyze in their entirety. For this reason it is convenient to study them at various structural levels of organization. The higher levels of organization are made up of the lower levels, just as a house is built from bricks, but each brick is composed of grains of sand. The major levels of organization are as follows.

l. Subatomic - These are the fundamental particles that make up all matter, protons, electrons, and neutrons.

2. Atomic - Combinations of subatomic particles form structures known as atoms. There are 92 naturally occurring combinations or atoms which are known as the elements. These elements make up all of the universe, including living organisms.

3. Molecular - Various combinations of atoms can be held together by energy interactions known as chemical bonds. The structures formed are known as molecules.

4. Organelle - These are discrete structures made up of combinations of complex organic molecules. Organelles are components of cells and each is responsible for some cellular function.

5. Cell - The represent combinations of organelles and other molecules. They are considered to be the basic unit of life because the cell is the minimum amount of organized matter that can carry out the five characteristics of life.

6. Tissues - These are combinations of similar or related cells that work together to perform a common function. There are four fundamental tissue types found in the body.

7. Organs - These represent various combinations of the four types of tissues that work together to perform a common function.

8. Organ system - These are organ combinations that work together to perform a common function.

9. Organism - The combination of all the previous levels working together in an integrated fashion.

D. Homeostasis - This is usually defined as the ability of an organism to maintain a constant internal environment in spite of a changing external environment.

Life can only exist within a fairly narrow set of conditions (variables). These variables include levels of nutrients, electrolytes, water, temperatures, as well as others. In order to survive, living things must maintain a constancy in these essentials. The maintenance of these variables is though homeostatic control mechanisms. All control mechanisms have at least three components.

    1. Receptor – This is a sensor that monitors the variable and responds when the variable changes. It provides input to the control center.
    2. Control center – This determines the set point of a variable. It receives information from the receptor, analyzes it, and initiates action to restore the variable to the set point.
    3. Effector – This is the third component and is responsible for the carrying out the control centers instruction. It represents the output of the control center.

Most homeostatic control mechanisms work through negative feedback.

l. Negative feedback - This is where part of the output of a control system is fed back to the input of the system which it causes to move in the opposite direction of the output.

a. Physical example - thermostat controlled heat systems- Thermostats control temperature or the amount of heat. Every thermostat has a set point. When the temperature drops below this set point the thermostat turns on a heater (effector) which raises the temperature. When the temperature reaches the set point the heater is turned off. Note that it is the output of the system (heat) which controls the input (thermostat).

b. Biological example - hormone regulation - The hormone thyroxine is produced by the thyroid gland when it is stimulated by a second hormone known as TSH (thyroid stimulating hormone). TSH is the system input and thyroxine is the system output. When thyroxine falls below its set point concentration in the blood, TSH is released and thyroxine production goes up. When the set point concentration is released, thyroxine shuts down the cells producing TSH and consequently thyroxine production also ceases.


2. Positive feedback - In this case, part of the output feedback and increases the input. This leads very quickly to exagerated increases in output. Positive feedback normally disrupts homeostasis where constancy is the goal. However, there are a few functions in the body that must be completed quickly and positive feedback does play a role. The contraction of the uterus during the birth of a baby is one example and the immune response is another.



3. Homeostasis is a very important physiological concept and is the basis of all physiology. All of the organ systems function to maintain homeostasis for the cells. Each time you are introduced to a new organ or organ system you should ask yourself how it contributes to homeostasis. If you can answer that question then you know how the organ or system functions.

4. Loss of homeostasis results in disease. A knowledge of normal homeostasis is necessary for an understanding of disease.