Defining Anatomical Orientations - Human Anatomy and Physiology

Terms used to describe the orientation of a structure commonly have a word that means the opposite so they can be related to one another. Proximal means that the structure is closer to a point of attachment, while lateral refers to a structure being away from the middle of the body. The opposing term for proximal is distal, meaning away from the point of attachment, and the opposing term for lateral is medial, meaning toward the center line.

Superior indicates above, while inferior indicates below. Anterior refers to the front, while posterior refers to the back. Superficial indicates that a structure is near the surface, while deep indicates that it is under other structures.

Normal blood pH is about 7.4 in most tissues (it is a bit lower in veins since they carry waste products, which are acidic). To get back to the physiological set point of pH = 7.4, we want to remove the acid from the blood. The major blood buffer system is shown in the following equation:

As we know, carbon dioxide is one of the major byproducts of respiration, and is considered waste for our bodies. Combined with water and catalyzed by carbonic anhydrase, it is converted into carbonic acid. Carbonic acid is a weak acid and will partially dissociate into hydrogen ions and bicarbonate ions. Thus, overall, carbon dioxide and water yields acid (hydrogen ions). As a result, excess carbon dioxide in the blood will lower the pH.

In order to increase the pH, we must stop this equation from proceeding in the forward direction; thus, (remember Le Chatelier's principle) we must remove carbon dioxide from the left side. This will push the reaction in the reverse direction, quenching hydrogen ions (acid) and removing them from the blood, increasing blood pH back to normal.

Since we want to get rid of excess carbon dioxide, we breathe faster. Oxygen does not have any effect on blood pH. Furthermore, the atmospheric oxygen level (21%) is plenty for our bodies to utilize, as when we exhale there is about 15% oxygen left over, meaning we only use about 25% of the oxygen we breathe (this is why CPR works!).

Pronation is the act of rotating the wrist so that the palm is facing downward. The opposite effect is rotating the wrist so that the palm faces upward. This action is called supination.

Eversion is the action of rotating the foot such that the medial region is oriented downward. Dorsiflexion is the action of decreasing the angle between the foot and the leg. Circumduction refers to the pattern of circular motion caused by multiple muscles at a joint, such as rotating the index finger in small circles.

The body is divided into two primary cavities, which are then subdivided into several smaller cavities. The primary cavities are the dorsal cavity and the ventral cavity. The dorsal cavity is subdivided into the vertebral cavity, which runs through the vertebrae and holds the spinal cord, and the cranial cavity. The cranial cavity is located within the skull and contains the brain. Since the cranial cavity is not given as an answer option, the best choice is the dorsal cavity.

The ventral cavity is more complex and contains more subcavities. The main divisions are the thoracic cavity, abdominal cavity, and pelvic cavity. The diaphragm separates the thoracic and abdominal cavities.

There are not technical structures known as the anterior cavity or skull cavity.

A coronal section runs from right to left across the body, dividing it into an anterior (ventral) portion and posterior (dorsal) portion. When a coronal section is used to examine the brain, the lateral ventricle and thalamus are generally very prominent.

If the body were separated into inferior and superior sections, for example head and body, then a transverse slice must have been used. A transverse slice is the only view to section the body through the horizontal plane.

A sagittal slice runs from anterior to posterior, dividing the body into left and right regions. A perfectly centered sagittal slice is called a medial sagittal section.

A transverse slice is the only view to section the body through the horizontal plane. The result is a superior region (above the cut) and an inferior region (below the cut). A transverse section of the brain is usually used to show the lentiform nucleus (consisting of the globus pallidus and putamen) and the caudate.

A coronal section runs from right to left across the body, dividing it into an anterior (ventral) portion and posterior (dorsal) portion.

A sagittal slice runs from anterior to posterior, dividing the body into left and right regions. A perfectly centered sagittal slice is called a medial sagittal section.

"Proximal" is a way to describe locations in the extremities and appendages. A more proximal structure is located closer to the trunk, generally indicating closer to the shoulder or hip. In contrast, distal indicates a location that is farther from the trunk and closer to the end of the appendage. For example, there are three bones in each finger known as the phalanges. These bones are named according to orientation: proximal phalanx, intermediate phalanx, and distal phalanx. All of these, of course, are located distal to the metacarpals (wrist).

The terms "medial" and "lateral" are used to describe relative location toward the midline. A medial structure is closer to the midline (think nose and navel), while a lateral structure is farther from the midline. The term used to describe one structure that is next to another is "adjacent."

Anterior and posterior are opposites, with anterior referring to the front of the body and posterior referring to the back. Ventral is used interchangeably with anterior, and dorsal is used interchangeably with posterior. Essentially, anterior and ventral both indicate "front," while posterior and dorsal both indicate "back."

Sternal is used to describe the sternum and is often used as a medial reference point in thoracic biology or medicine. Superficial means closer to the skin, whereas deep indicates that a structure is farther from the skin. Superior indicates "upward" and is frequently a synonym with cranial (toward the head). In contrast, inferior indicates "downward" and is often a synonym with caudal (toward the coccyx).

Lateral is a general directional that can be applied to objects farther from the midline of the body. The midline is defined as the medial sagittal plane, generally including the navel and nose. Structures such as the shoulders, ears, and hips are considered lateral. When used in relative terms, we can say that the lungs are lateral to the heart and the ovaries are lateral to the uterus. In contrast, the opposite term of lateral is medial; the heart is medial to the lungs and the uterus is medial to the ovaries.

The latissimus dorsi muscles are located on both sides of the spine, and their name is directionally descriptive. They have no bearing on directional terms, unless used as reference points. Anatomical structures attached to the midline could be described as lateral, but only if they deviate farther left or right from the midline. A structure can be attached to the midline, but run completely vertical in the medial sagittal plane.

A coronal plane is any vertical plane that divides the body into ventral (anterior) and dorsal (posterior) segments. A coronal plane will cross the body from one side (right) to the other (left).

A sagittal plane is a vertical plane that divides the body into right and left, and will cross the body from front (anterior) to back (posterior).

A transverse plane sections the body into superior and inferior regions and will cross the body in the horizontal plane.

In anatomic position, radial deviation of the wrist results in your hand moving laterally away from the midline of the body.

Ulnar deviation of the wrist will cause the hand to move medially.

The anterior-posterior directions in humans refers to the stomach and backside, respectively. Another term to describe this same plane is the ventral and dorsal directions. Dorsal and posterior can be considered synonymous.

Ventral is the opposite of dorsal and refers to the anterior or front of the body. Caudal refers to the lower region of the body and is somewhat synonymous with inferior. Superior is the opposite of inferior, and refers to the upper portion of the body. Distal refers to the region farthest from the attachment of an extremity; this is different from lateral, which refers to the region farthest from the midline.

In the anatomical position, the limbs of the human body are oriented in a superior/inferior direction. Distal regions will be associated with inferior since the limbs are free hanging, whereas proximal will be associated with superior. Superior refers to the region highest on the body, while proximal refers to the region closest to the point of limb attachment. These two terms refer to the same area of the arm in anatomical position, while distal and inferior would also correlate.

The head is superior (closer to the head) to the bellybutton. The hands are appendicular (distant from the core), not the bellybutton. The kidneys are deep, while the skin is superficial (at the surface). The face is anterior (front of body) while the back of neck is posterior (back of body).

Anatomical terms are an easy way to talk about certain parts of the body without confusion. If the top of the head is the superior most portion of the body and the feet are the inferior most part of the body, then the femur is inferior to the hip, but superior to the knee joint.

The midsagittal plane or the median plane divides the body vertically into two symmetrical halves. It runs from the top of the head straight down the face, between the eyes, down the nose and through the naval. The horizontal or transverse plane divides the body into superior and inferior halves. The frontal plane divides the body in half between the anterior and posterior sides. It is sometimes called the coronal plane.

The terms inferior and superior refer to lower and upper parts of the body. For instance the knee is inferior to the hip, while the elbow is superior to the knee and the hip. The most inferior part of the body is the sole of the foot or plantar region.

Adduction is movement of a body part toward the body's midline. So if a person has their arms straight out at the shoulders and brings them down to their sides, it is adduction. For fingers and toes, adduction brings the digits toward the center of the hand or foot. For example, if a person has their fingers spread wide apart, bringing them together would be adduction.

Anatomical position is the position with the body erect with arms at the sides and the palms forward. Distal is defined as situated away from the center of the body or from the point of attachment. Proximal describes a structure situated nearer to the center of the body or the point of attachment. Superior describes a structure situated nearer the vertex of the head in relation to the specific reference point; it is the opposite of inferior.