Introduction to the Human Body

Human anatomy studies the structures of the body. Some body structures are so small that they can only be seen and examined with a microscope. Others are large enough to be viewed, handled, measured, and weighed.

The word anatomy comes from a Greek root meaning “to cut apart.” Early study of the human body began with observing the outside of the body and the injuries seen in soldiers and other patients. Later, physicians were allowed to dissect dead bodies to learn more. Dissection means...

The human body is organized in levels that build from simple parts to a complete living being. These levels increase in complexity from subatomic particles, atoms, and molecules to organelles, cells, tissues, organs, organ systems, and the organism.

At the chemical level, scientists study the smallest building blocks of matter. Matter is made of elements, which are pure substances such as hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of an element is an atom, and...

Human life depends on metabolism, movement, development, growth, and reproduction. These functions help the body stay alive, maintain itself, and continue the species. Together, they show how the body uses energy and changes over time.

Metabolism is the body’s use of energy and food molecules. It helps the body make fuel for movement, support body functions, and build and maintain body structures. Metabolism includes two kinds of reactions: anabolism and catabolism.

Anabolism is the process...

Human survival depends on air, water, food, a narrow temperature range, and a narrow range of atmospheric pressure. The Earth and its atmosphere provide these conditions, but the body can only function well when they stay within limits.

Oxygen is one of the most important needs. Atmospheric air is only about 20 percent oxygen, but that oxygen supports the chemical reactions that keep the body alive, including the reactions that make ATP. Brain cells are especially sensitive to a lack of oxygen...

Homeostasis keeps the body’s internal conditions stable. The body does this by holding each physiological condition near a set point. A set point is the value around which the normal range changes, and the normal range is the small group of values that is healthiest and most stable. Human body temperature is a common example. Its set point is about 37°C, or 98.6°F, and it can shift a little above or below that point while still staying in range.

Negative feedback helps maintain homeostasis.

Negative and positive feedback help keep body conditions stable. These feedback loops control many body processes and support homeostasis, which is the maintenance of a steady internal environment.

Negative feedback reduces a change and brings a system back toward its normal state. Positive feedback increases a change and pushes a process further in the same direction. Together, they show how the body can fine-tune responses when conditions shift.

These control systems are important in many...

Homeostasis keeps the body’s internal conditions stable so cells, tissues, organs, and organ systems can work properly. The body uses control systems to regulate temperature, blood pressure, pH balance, and fluid balance. These controls depend on feedback loops, which can be positive or negative.

Homeostatic imbalance happens when those feedback loops fail and the body can no longer maintain a stable internal environment. Injury, disease, environmental changes, genetic mutations, and aging can...

Anatomical terms help describe the human body clearly in biology and medicine. Health care professionals and anatomists use this standard language to avoid confusion and to make descriptions more precise. Most anatomical words come from Greek and Latin roots, and these roots usually stay the same because the languages are rarely used in everyday conversation.

Many anatomical words are built from a root plus a prefix or suffix. The root often names an organ, tissue, or condition. The prefix or...

Body positions used in anatomy give a shared way to describe where body parts are and how they face. These standard references help people talk about anatomical structures with the same meaning. The anatomical position is the main reference point for body orientation.

In anatomical position, the body stands upright and faces forward. The feet are parallel and flat on the floor. The arms rest at the sides with the palms facing forward, and the head and eyes point forward.

The most common...

Anatomy regions and body landmark terms help identify specific parts of the human body. These regional terms divide the body into areas with structures that support similar functions. They also make it easier to describe the exact area affected by an injury or disease.

The human body is organized into two major regions: the axial region and the appendicular region. The axial region includes the body from the head to the abdomen and forms the central body axis. The appendicular region includes...

Anatomical direction and body position terms help describe where one body structure is located in relation to another. These terms use comparisons, such as one tissue being inferior to another or a tumor being superficial to a deeper structure. They are based on the standard anatomical reference position, which uses the body in an upright pose.

Anterior and posterior are a common pair of directional terms. Anterior, also called ventral, means toward the front of the body. Posterior, also...

Anatomy sections and body planes are imaginary flat surfaces used to divide the body for study. They help show the orientation, relationships, and spatial layout of anatomical structures. These reference planes are also useful for describing organs and body parts in a standard way.

The sagittal plane divides the body or an organ vertically into right and left sides. When that cut goes through the middle and makes equal halves, it is called the midsagittal or median plane. When it makes unequal...

Body joint motion terms describe how bones and muscles move at different joints. These standard names make it easier to talk about range of motion in the body. They are used to describe actions at the shoulder, hip, elbow, knee, wrist, fingers, toes, and spine.

Flexion and extension are movements in the sagittal plane, also called the anterior-posterior plane. Flexion is the forward bending motion of the head or vertebral column. Extension is the backward movement or straightening of a joint,...

The abdominal cavity is divided into quadrants and regions to help locate organs and describe pain clearly. Clinicians and anatomists use imaginary lines to mark the abdominopelvic cavity. This system makes it easier to communicate about a patient’s symptoms or a suspicious mass.

The simpler method divides the area into four quadrants. One horizontal line and one vertical line cross at the umbilicus, or navel. These quadrants are the right upper quadrant, right lower quadrant, left upper...