Describing the body: direction, cavities and levels of organization
Anatomy describes how the body looks and physiology how it works, and an adage holds that form follows function. A structure in the body is located by the anatomical directions and planes, the axial and appendicular portions, the body cavities and the abdominal quadrants and regions. The body is built up in levels of organization, from atoms and the biomolecules they form, through cells, tissues, organs and organ systems, to the whole organism, and every system but the reproductive system interacts to maintain homeostasis.
Form and function
Anatomy describes how the body and its components look, their form, and physiology describes how they work, their function. Knowledge of anatomy and physiology is key to understanding disease states, health, growth and development, aging, and drug actions. An adage is that form follows function: with continued study, students come to learn that structures look a certain way because they have a specific function, and the reverse.
Directions, planes and body cavities
The ability to identify and correctly describe body areas is particularly important in the health sciences, and anatomical directions describe the location of structures and organs. The directions for humans differ slightly from those for four-legged animals, insects and other animals.
The directions come in opposite pairs. Anterior, also ventral, means in front of or in the forward part, and its opposite, posterior, means situated behind or toward the rear. Superior, or cephalad, means above, higher or directed upwards, and its opposite, inferior or caudal, means situated below or directed downward. Medial means of or toward the middle, and lateral of or toward the side. Proximal means nearest to a point of reference, and distal farther from it, toward the end of a structure. Three planes name the sections: the sagittal plane is parallel to the long axis of the body, the coronal plane pertains to the crown of the head, and the transverse plane lies crosswise.
The body divides into an axial portion, the head, neck and trunk, and an appendicular portion, the appendages, meaning the arms and legs. For all human sophistication, the overall body plan is the same as an earthworm's: a tube within a tube. Vertebrates are characterized by a dorsal hollow nerve tube, and the digestive tract is a tube running from mouth to anus.
The organs are tightly packed within the body's cavities, and the body has two principal cavities, the dorsal and the ventral body cavity. The dorsal body cavity consists of the cranial cavity, which houses the brain, and the vertebral cavity, which houses the spinal cord. The ventral body cavity consists of the thoracic, abdominal and pelvic cavities. The thoracic cavity is subdivided into the pericardial cavity, which surrounds the heart, and the pleural cavity, which houses the lungs, and the diaphragm separates it from the abdominopelvic cavity. The abdominal and pelvic cavities have no definite dividing structure comparable to the diaphragm. Organs such as the stomach, liver and intestines are found in the abdominal cavity, while parts of the urinary and reproductive systems are usually found in the pelvic cavity.
For descriptive accuracy, especially when preparing a patient for surgery or charting, or to let a patient pinpoint pain, the abdominopelvic cavity can be divided further into four anatomical quadrants or nine anatomical regions. The four quadrants are the right upper, left upper, right lower and left lower. Right and left refer to the patient's right and left, not the observer's, which is generally a mirror image.
Levels of organization
The body is very organized, and cooperation and harmony are required among all its levels of organization. From lowest to highest, the levels are atoms, molecules, cells, tissues, organs, organ systems, and the organism.
Atoms are the building blocks of all matter, and atoms join to form molecules. Organic molecules produced by living cells are biomolecules, and their four important groups are carbohydrates, proteins, lipids and nucleic acids.
Carbohydrates contain carbon, hydrogen and oxygen in a 1:2:1 proportion. They range from simple to complex, from glucose, a sugar the body uses exclusively to produce energy, to glycogen, a storage form easily converted back to glucose. Glucose fuels the citric acid cycle, also called the Krebs cycle, which produces adenosine triphosphate (ATP), the energy currency of the cell. Most processes in the body, muscle contraction among them, require ATP.
Most of the body's weight is due to protein. Muscle, hair, skin, enzymes, growth factors and some hormones are proteins. Proteins form structures, as hair does, protect against disease as antibodies do, and help maintain homeostasis as hemoglobin, hormones and enzymes do.
Lipids can be fats, oils or waxes, and they are important energy sources for the body. They have a bad reputation, but cholesterol, a lipid, is the major structural component of cell membranes. A thin layer of fat under the skin protects blood vessels and nerves from mechanical damage and insulates against cold. Some hormones, such as the sex hormones estrogen and androgen, are cholesterol based.
Nucleic acids are important in directing the activities of the cell. Deoxyribonucleic acid (DNA) is built from nucleotides, each a deoxyribose five-carbon sugar, a phosphate group, and a nitrogenous base: adenine, cytosine, thymine or guanine. Ribonucleic acid (RNA) also comprises nucleotides, but ribose replaces deoxyribose and uracil replaces thymine.
All processes necessary for life occur in cells, the basic building blocks of all living organisms. Cells are organized into tissues, a tissue being a group of cells and their nonliving matrix, or glue, that performs a specialized function. There are four groups of tissue: epithelial, which covers and lines and also forms glands; connective, which includes blood, adipose tissue, bone, ligament and tendon; muscle, which is skeletal, cardiac or smooth; and nervous, meaning the brain and spinal cord. Tissues are organized into organs, discrete structures with discrete functions, and organs join to form systems. All the organ systems join to form the organism.
Homeostasis
Survival is the body's most important function, and it depends on maintaining or restoring homeostasis, the relative consistency of the internal environment. Because of homeostasis, the chemical composition, values and basic characteristics of blood and interstitial fluid remain constant within very narrow limits. Regulation of body temperature is a good example. When the body is very cold and its core temperature is in danger of falling below the standard normal of 98.6°F, muscles contract involuntarily and shivering begins, and the heat from muscle contraction raises core temperature to normal. When the body is very warm and its core temperature is in danger of rising above normal, sweating begins, and as sweat evaporates from the skin it removes heat and core temperature falls to normal. Every system in the body except the reproductive system interacts to maintain homeostasis.
Definitions
A body cavity is a hollow place or space in the body containing organs, such as the thoracic, abdominal or pelvic cavity. The ventral body cavity is the anterior cavity containing the thorax, abdomen and pelvis. The abdominopelvic cavity is its largest segment and contains most of the internal organs. The thoracic cavity is the portion of the ventral body cavity subdivided into the pleural and pericardial cavities. The pleural cavity is the portion that contains the lungs, and the pericardial cavity the portion that contains the heart. The cranial cavity is the portion of the dorsal body cavity that contains the brain, and the vertebral cavity the portion that contains the spinal cord. The diaphragm is a muscle that separates the thoracic and abdominal cavities.
Axial means of or pertaining to the axis of a structure or part, such as the long axis of a tooth. An anatomical quadrant is any of the four parts into which the transverse, coronal or sagittal planes are divided by rectangular coordinate axes lying in that plane. An anatomical region is any of the major subdivisions into which the body or one of its parts is divisible, and there are nine in the abdominopelvic cavity.
Anatomy is the science of the structure of the animal body and the relation of its parts, and it is largely based on dissection, from which it takes its name. Physiology is the science of the functions of the living organism and its parts and of the physical and chemical factors and processes involved. Homeostasis is the tendency toward stability of the internal environment of the organism, the maintenance of equilibrium between the organism and its environment.
An atom is any one of the ultimate particles of a molecule or of any matter, and the smallest particle of an element capable of entering into a chemical reaction. A molecule is a very small mass of matter, the smallest amount of a substance that can exist alone, an aggregation of atoms, and specifically a chemical combination of two or more atoms that form a specific chemical substance. A cell is the fundamental structural and functional unit of living organisms. An organelle is a small functional structure within a cell, such as a mitochondrion, Golgi body, lysosome or ribosome. A tissue is an aggregation of similarly specialized cells united in performing a particular function. An organ is a part of the body that performs a special function or functions. An organ system is a regularly interacting or interdependent group of body parts, each performing a special function or functions, forming a unified whole.
A carbohydrate is any of a class of organic compounds containing carbon, hydrogen and oxygen in specific proportions. A protein is any of a particular group of complex organic molecules composed of amino acids. A growth factor is a protein that binds to receptors to stimulate cellular proliferation or differentiation. A nucleic acid is a high-molecular-weight biomolecule composed of nucleotides. A nucleotide is a compound of a base, a sugar and a phosphate group, and the building block of DNA and RNA.
Cytoplasm is the protoplasm, the living substance, of a cell, excluding the nucleus. The endoplasmic reticulum is a network of tubules and vesicles in the cytoplasm. A membrane is a thin layer of tissue that covers a surface, lines a cavity, or divides a space or organ. A matrix is the fluid substance in which cells are embedded.