Organization of the Human Body
This chapter is an overview of the human body. It introduces you to characteristics that humans have in common with other organisms and the way the human body is organized. This chapter also introduces you to a set of terms used to describe regions and parts of the body.
What is Anatomy? Anatomy is the study of structure. For example, if I told you the heart is a hollow, cone-shaped organ with an average size of 14 centimeters long and 9 centimeters wide, I have just told you the structure of the heart. In other words, I have described the ANATOMY of the heart! What is the anatomy of your hand?
What is Physiology? Physiology is the study of function. For example, if I told you the heart pumps blood into blood vessels for the transportation of nutrients throughout the body, I have just described the Physiology of the heart.
Why are Anatomy and Physiology commonly studied together? Because they are ALWAYS related. In other words, structure is always related to function! For example, the anatomy of the heart (a hollow, muscular organ) allows it to do its function (pump blood into tubular blood vessels). If the heart was not hollow, could blood flow into it? If the heart was not muscular, could it pump blood? How is the structure of the hand related to the function of the hand?
The following is a list of characteristics that humans share with other organisms. An organism is defined as a living thing.
Movement - refers to the ability of an organism to travel from one point to another; also refers to the motion of internal parts. What internal parts of the human body are constantly moving?
Responsiveness - refers to the ability of an organism to sense the internal and external changes and to react to those changes. For example, if you are outside in a short sleeved shirt on a winter day, you have the ability to sense coldness and respond by going to a heated place.
Growth - refers to an increase in size.
Reproduction - refers to the ability of an organism to make new offspring; also refers to the ability of cells to reproduce to repair injured tissue.
Respiration - refers to the ability to obtain oxygen for the release of energy from food and to remove gaseous wastes such as carbon dioxide.
Digestion - refers to the ability to break down food into simpler forms that can be absorbed and used by body cells.
Absorption - refers to the movement of substances across membranes; for example in the digestive tract, broken down food substances move across membranes of the intestines to enter the blood stream.
Circulation - refers to the movement of substances in body fluids.
Assimilation - refers to the ability of an organism to change absorbed substances into different substances. For example, body cells assimilate amino acids absorbed from the digestive tract into proteins.
Excretion - refers to the ability to remove waste products.
The maintenance of life for humans is dependent on several environmental factors as well the ability to maintain homeostasis (stable internal conditions).
Humans depend on the following environmental factors to maintain life.
Water - the most abundant compound in the body; it is required for chemical reactions to take place, the transportation of substances in the body and the regulation of body temperature.
Foods - provide the body with nutrients that are used to make energy, to regulate chemical reactions and to build news cells and tissues.
Oxygen - gas that is used to release energy from nutrients.
Heat - form of energy used to drive chemical reactions.
Atmospheric pressure- the application of pressure the atmospheric air exerts on the outside of the body; it is important for breathing.
Hydrostatic pressure - the application of pressure caused by a fluid. The pumping action of the heart creates a hydrostatic pressure called blood pressure.
Homeostasis is defined as the maintenance of stable internal conditions. What conditions in the body must remain stable? How about body temperature, blood pressure, the concentration of various chemicals in the blood? Can you think of any more examples?
Let's look at body temperature. The average human body temperature is maintained at 37 degrees Celcius. When a person's body temperature rises above 37 degrees Celcius, we say the person is out of homeostasis because the internal condition of the body has changed.
What does to the body do to bring a person's body temperature back to 37 degrees Celcius? Sweating is a common response to an increase in body temperature. Sweating is a process that acts to cool the body. Therefore, we say sweating is a homeostatic mechanism that returns the body's temperature back to normal.
What is the homeostatic mechanism that returns the body's temperature to 37 degrees Celcius if the body temperature falls below normal?
What is a homeostatic mechanism used to control blood pressure? If you do not know this, you can look it up in your book!
The structure of the body can be divided into different levels of organization. The simplest unit of all substances is called the atom. Atoms join together to form molecules. Molecules join together to form macromolecules. Macromolecules join together to form organelles. Organelles combine to form cells. Cells combine to form tissues. Tissues combine to form organs. Organs combine to form organ systems. Organ systems combine to form organisms!
The four most common atoms in the human body are carbon, hydrogen, oxygen and nitrogen. Water is an example of a molecule. It is formed when two hydrogen atoms bond to an oxygen atom. Proteins and carbohydrates are examples of macromolecules. You will learn more macromolecules in Chapter 2. Mitochondria and lysosomes are examples of organelles. You will learn more organelles in Chapter 3. Leukocytes (white blood cells) and erythrocytes (red blood cells) are examples of cells. Blood is an example of a tissue. The heart and blood vessels are examples of organs. The cardiovascular system is an example of an organ system.
Click here to see the levels of organization in the human body.
The axial portion of the body includes the head, neck and trunk. The axial portion of the body includes the following cavities:
Dorsal Cavity - the dorsal cavity is divided into the cranial cavity and the vertebral canal. The cranial cavity houses the brain while the vertebral canal houses the spinal cord.
Ventral Cavity - the ventral cavity is divided into the thoracic cavity and the abdominopelvic cavity. The thoracic cavity and the abdominopelvic cavity are separated from each other by the muscle called the diaphragm. The lungs, heart, esophagus and trachea are found in the thoracic cavity. The middle of the thoracic cavity is called the mediastinum. The mediastinum contains the heart, esophagus and trachea. The sides of the thoracic cavity contain the lungs. We say the lungs are in the pleural cavity of the thoracic cavity. The abdominopelvic cavity is divided into a superior abdominal cavity and an inferior pelvic cavity. Most of the organs of digestion are found in the abdominal cavity while the bladder and internal reproductive organs are located in the pelvic cavity.
Click here to see a lateral view of the major body cavities
Click here to see a coronal view of the major body cavities
Small cavities in the head include the following:
oral cavity - contains the teeth and tongue
nasal cavity - space behind the nose
orbital cavities - contains the eyes
middle ear cavities - contains the middle ear bones
Click here to see the cavities within the head
In the thoracic cavity you find the following membranes:
The parietal pleura - this membrane lines the thoracic cavity
The visceral pleura- this membrane covers the lungs
The parietal pericardium- this membrane surrounds the visceral pericardium
The visceral pericardium - this membrane covers the heart
In the abdominopelvic cavity you find the following membranes:
The parietal peritoneum- this membrane lines the abdominopelvic cavity
The visceral peritoneum - this membrane covers abdominopelvic organs
Integumentary System - contains the skin, hair and nails. It functions to protect the body, produce vitamin D, and to regulate body temperature
Skeletal System - contains bones, cartilage and ligaments. It functions to support the body, protect organs and to make new blood cells. It also serves as an attachment site for muscles.
Muscular System - contains muscles. It functions to produce body movement and heat.
Nervous System - contains the brain, spinal cord and nerves. It is a controlling system of the body. It stimulates muscles to contract and glands to secrete. It also interprets sensory information.
Endocrine system - contains the pituitary gland, pancreas, thyroid, and other endocrine glands. Secretes hormones which help to control chemical reactions in body cells.
Cardiovascular System - contains the heart and blood vessels. It pumps blood for the transportation of oxygen, nutrients, hormones and wastes throughout the body.
Lymphatic System - contains the lymph nodes, lymphatic vessels, thymus and spleen. It aids in defending the body against diseases. It also carries fat molecules away from the digestive organs and transport lymph fluid to the bloodstream.
Digestive System - contains the pharynx, esophagus, stomach, intestines, liver, gallbladder and pancreas. It breaks down food into substances that can be absorbed by cells of the body. It also eliminates solid waste products that cannot be absorbed in the form of feces.
Respiratory System - contains the lungs, trachea, bronchi, nose, pharynx, and larynx. It carries out breathing and the exchange of gases between the air and blood.
Urinary System - contains the kidneys, ureters, bladder and urethra. It filters wastes products from the blood stream, forms urine and carries out urination.
Reproductive System - male reproductive system contains the penis, scrotum, testes, prostate gland and seminal vesicles. The female reproductive system contains the uterus, uterine tubes, vagina, clitoris and vulva. The reproductive systems produce new offspring. The female reproductive system also nourishes and supports the developing offspring.
Click here to view a table that summarizes the organ systems.
Anatomical terms are used to describe the location of body parts, imaginary planes used to section the body, and various body regions. In order to correctly use these terms, one must assume the body is in the Anatomical Position. In the Anatomical Position, a body is standing upright, facing forward with the arms at the sides and the palms of the hands facing forward.
Superior and Inferior - Superior means above or closer to the head. Inferior means below or closer to the feet. For example, the thoracic cavity is superior to the abdominal cavity.
Anterior (Ventral) and Posterior (Dorsal) - Anterior means towards the front of the body. Posterior means towards the back of the body. For example, the nose is anterior to the ears.
Medial and Lateral - Medial means closer to the midline of the body. The midline of the body is the line dividing the body into equal left and right halves. Lateral means farther away from the midline of the body. For example, the ears are lateral to the nose or you could say the nose is medial to the ear.
Proximal and Distal - Proximal means closer to a point of attachment or closer to the trunk of the body. Distal means farther away from a point of attachment or farther away from the trunk of the body. For example, the knee is proximal to the toes. The fingers are distal to the elbow.
Superficial and Deep - Superficial means closer to the surface of the body. Deep means more internal or farther away from the surface of the body. For example, skin is superficial to muscles. Bones are deep to skin.
Sometimes in order to study internal body parts, the body has to be cut into sections. It is useful to use the following terms to describe how the body was cut into sections.
Sagittal - A sagittal plane is a lengthwise cut that divides the body into left and right portions. A midsagittal plane runs lengthwise down the midline of the body and divides it into equal left and right halves. For example, if you chop someone's arm off at the shoulder, you have used a sagittal plane to create a sagittal section.
Transverse - A transverse plane is a cut that divides the body into superior and inferior portions. For example, if you chop someone's head off, you have used a transverse plane to create a transverse section.
Coronal or Frontal - A coronal plane is a cut that divides the body into anterior and posterior portions. For example, if you chop the tip of someone's nose off, you have used a coronal plane to create a coronal section.
Click here to view the way these planes divide the body
The following terms are used to describe various regions of the body:
Abdominal - area between the thorax and pelvis.
Acromial - shoulder region.
Antebrachial - forearm region.
Antecubital - area in front of the elbow.
Axillary - armpit area.
Brachial - upper arm area.
Buccal - cheek area.
Carpal - wrist area.
Celiac - abdominal area.
Cephalic - head area.
Cervical - neck area.
Costal - rib area.
Coxal - hip area.
Crural - lower leg area.
Cubital - elbow area.
Digital - finger area.
Dorsal - back area.
Femoral - thigh area.
Frontal - forehead area.
Genital - area of external reproductive organs.
Gluteal - buttock area.
Inguinal - the depressed area where the thigh meets the abdomen.
Lumbar - area of the lower back.
Mammary - breast area.
Mental - chin area.
Nasal - nose area.
Occipital - lower, posterior area of the head.
Oral - mouth area.
Orbital - eye area.
Otic - ear area.
Palmar - palm of the hand area.
Patellar - knee area.
Pectoral - chest area.
Pedal - foot area.
Pelvic - pelvis area.
Perineal - area between the anus and genital area.
Plantar - sole of the foot area.
Sacral - region of the back between the hip bones.
Sternal - anterior, middle region of the thorax.
Tarsal - instep of the foot area.
Umbilical - navel area.
Vertebral - posterior, middle area of the back along the spine.
Click here to see anterior views of the body with various anatomical regions labeled.
Click here to see a posterior view of the body with various anatomical regions labeled.
The abdominal area is further divided up into these nine regions:
Epigastric region - the upper middle portion.
Left and right hypochondriac regions - each one is lateral to the epigastric region.
Umbilical region - the middle portion.
Left and right lumbar regions - each one is lateral to the umbilical region.
Hypogastric region - the lower middle portion.
Left and right iliac regions - each one is lateral to the hypogastric regions.
Click here to see the nine abdominal regions