Anatomy is the study of structure. Physiology is the study of function. There is a unity of form and function (physiology cannot be divorced from anatomy).
Cell theory and the theory of natural selection are the two theories that have the broadest implications for understanding the human body. All structure and function result from the activity of cells. The human body is a product of evolution.
There is a hierarchy of complexity to human structure. The organism is composed of organ systems, organ systems are composed of organs, organs are composed of tissues, tissues are composed of cells, cells are composed partly of organelles, organelles are composed of molecules, and molecules are composed of atoms.
- The organism is a single, complete individual.
- An organ system is a group of organs with a unique collective function.
- An organ is a structure composed of two or more tissue types that work together to carry out a particular function.
- A tissue is a mass of similar cells and cell products that form a discrete region of an organ and perform a specific function.
- Cells are the smallest units of an organism that carry out all the basic functions of life.
- Organelles are microscopic structures in a cell that carry out its individual funtions.
Reductionism is the theory that a large, complex system such as the human body can be understood by studying its simpler components. Holism is the complementary theory that there are emergent properties of the whole organism that cannot be predicted from the properties of its separate parts.
Life is a collection or properties that help to distinguish living from nonliving things:
- Organization.
- Cellular composition.
- Metabolism.
- Responsiveness and movement.
- Homeostasis.
- Development.
- Reproduction.
- Evolution.
Homeostasis is the body’s ability to maintain relatively stable internal conditions. Physiology is largely a group of mechanisms for maintaining homeostasis. Negative feedback is the process in which the body senses a change and activates mechanisms that negate or reverse it. Positive feedback is a self-amplifying cycle in which a physiological change leads to even greater change in the same direction. Matter and energy tend to flow down gradients. A physiological gradient is a difference in chemical concentration, electrical charge, physical pressure, temperature, or other variable between one point and another.
Atlas A. General Orientation to Human Anatomy
In describing the body, anatomists assume that it is in anatomical position – that of a person standing upright with the feet flat on the floor, arms at the sides, and the palms and face directed forward.
Many views of the body are based on sections or planes. Section implies an actual cut or slice to reveal internal anatomy, whereas plane implies an imaginary flat surface passing through the body. A sagittal plane passes vertically through the body and divides it into right and left. A frontal (coronal) plane passes vertically through the body and divides it into front and back. A transverse plane passes across the body perpendicular to its long axis and divides it into upper and lower.
The location of one structure relative to another are described by directional terms:
- ventral (toward the front or belly) and dorsal (toward the back or spine)
- anterior (toward the ventral side) and posterior (toward the dorsal side)
- cephalic (toward the head or superior end), rostral (toward the forehead or nose), and caudal (toward the tail or inferior end)
- superior (above) and inferior (below)
- medial (toward the median plane) and lateral (away from the median plane)
- proximal (closer to the point of attachment or origin) and distal (farther from the point of attachment or origin)
- ipsilateral (on the same side of the body) and contralateral (on the opposite sides of the body) [right and left]
- superficial (closer to the body surface) and deep (farther from the body surface)
The body is divided into two main regions. The axial region consists of the head, neck, and trunk; the trunk is further divided into the thoracic region and the abdominal region. The appendicular region consists of the upper and lower limbs. The body wall encloses multiple body cavities – the cranial cavity, the vertebral canal, the thoracic cavity, and the abdominopelvic cavity. Each cavity is lined with a membrane and contains internal organs called viscera.
The human body has 11 organ systems. The organ systems can be classified by their principal functions:
- Systems of protection, support and movement
- Integumentary system
- Principal organs: skin, hair, nails, cutaneous glands
- Principal functions: protection; water retention; thermoregulation; vitamin D synthesis; cutaneous sensation; nonverbal communication
- Skeletal system
- Principal organs: bones, cartilage, ligaments
- Principal functions: support; movement; protective enclosure of viscera; blood formation; mineral storage; electrolyte and acid-base balance
- Muscular system
- Principal organs: skeletal muscles
- Principal functions: movement; stability; communication; control of body openings; heat production
- Integumentary system
- Systems of internal communication and control
- Nervous system
- Principal organs: brain, spinal cord, nerves, ganglia
- Principal functions: rapid internal communication; coordination; motor control and sensation
- Endocrine system
- Principal organs: pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas, testes, ovaries
- Principal functions: hormone production; internal chemical communication and coordination
- Nervous system
- Systems of fluid transport
- Circulatory system
- Principal organs: heart, blood vessels
- Principal functions: distribution of nutrients, oxygen, wastes, hormones, electrolytes, heat, immune cells, and antibodies; fluid, electrolyte, and acid-base balance
- Lymphatic system
- Principal organs: lymph nodes, lymphatic vessels, thymus, spleen, tonsils
- Principal functions: recovery of excess tissue fluid; detection of pathogens; production of immune cells; defense against disease
- Circulatory system
- Systems of intake and output
- Respiratory system
- Principal organs: nose, pharynx, larynx, trachea, bronchi, lungs
- Principal functions: absorption of oxygen; discharge of carbon dioxide; acid-base balance; speech
- Urinary system
- Principal organs: kidneys, ureters, urinary bladder, urethra
- Principal functions: elimination of wastes; regulation of blood volume and pressure; stimulation of red blood cell formation; control of fluid, electrolyte, and acid-base balance; detoxification
- Digestive system
- Principal organs: teeth, tongue, salivary glands, esophagus, stomach, small and large intestines, liver, gallbladder, pancreas
- Principal functions: nutrient breakdown and absorption. Liver functions include metabolism of carbohydrates, lipids, proteins, vitamins, and minerals; synthesis of plasma proteins; disposal of drugs, toxins, and hormones; and cleansing of blood
- Respiratory system
- Systems of reproduction
- Male reproductive system
- Principal organs: testes, epididymides, spermatic ducts, seminal vesicles, prostate, bulbourethral glands, penis
- Principal functions: production and delivery of sperm; secretion of sex hormones
- Female reproductive system
- Principal organs: ovaries, uterine tubes, uterus, vagina, mammary glands
- Principal functions: production of eggs; site of fertilization and fetal development; fetal nourishment; birth; lactation; secretion of sex hormones
- Male reproductive system
—August 2020
Lance Jones 