The Physiology of the Small and Large Intestines
The digestive system processes the food you eat. Food travels via the esophagus into the stomach and then into the small and large intestines. The small intestine starts at the pylorus of the stomach and ends at the cecum of the large intestine. The main function of the small intestine is continued digestion and absorption of nutrients.

The small intestine has three segments: the
duodenum, the jejunum, and the ileum.
The duodenum
The duodenum is the first segment of the small intestine and is also the shortest (about 25 centimeters, or roughly 10 inches). It starts at the pylorus and ends at the duodenojejunal junction. It has four parts:
Superior part: Horizontal to and in front of the 1st lumbar vertebra.
Descending part: Runs inferiorly along the right borders of the 2nd and 3rd lumbar vertebrae. It’s close to the head of the pancreas.
Inferior part: Crosses to the left, in front of the inferior vena cava, close to the level of the 3rd lumbar vertebra.
Ascending part: Starting near the 3rd lumbar vertebra, this part runs upward along the left side of the aorta. It joins the jejunum at the duodenojejunal junction. It’s supported by the suspensory muscle of the duodenum, also called the ligament of Treitz.
Parasympathetic nerve supply to the duodenum comes from the vagus nerve, and sympathetic nerves in this area include the greater and lesser splanchnic nerves. Blood is supplied to the duodenum by the superior pancreaticoduodenal artery, a branch of the gastroduodenal artery, and the inferior pancreaticoduodenal artery, a branch of the superior mesenteric artery. Blood is drained by the corresponding veins into the hepatic portal system. Lymphatic vessels follow the arteries to the pancreaticoduodenal lymph nodes, pyloric lymph nodes, and the superior mesenteric lymph nodes. Lymph flows from those nodes to the celiac lymph nodes.
The jejunum and the ileum
The jejunum is the middle portion of the small intestine. It starts at the duodenojejunal junction and changes into the ileum, which is the third portion. The jejunum takes up about two-fifths of the length of the small intestine, but no clear line demarcates where it turns into the ileum. The ileum ends at the ileocecal junction. The ileum and jejunum are attached to the posterior abdominal wall by the mesentery.
Sympathetic and parasympathetic nerves are brought by the superior mesenteric plexus. Blood is brought to the jejunum and ileum by branches from the superior mesenteric artery. Blood is drained by the superior mesenteric vein. Lymph nodes that drain this area include the juxtaintestinal lymph nodes, mesenteric lymph nodes, and central nodes. Lacteals are specialized lymphatic vessels found in the small intestine that absorb fat from the foods you eat.
The large intestine
Most of the large intestine is located in the abdomen; the sigmoid colon and rectum are in the pelvic cavity. The abdominal portion of the large intestine includes the cecum and the ascending, transverse, and descending colon. The main function of the large intestine is to absorb water from fecal material before it’s eliminated from the body. The colon is also home to friendly bacteria that synthesize vitamin K and keep bad microbes in check.

The large intestine is much larger in diameter than the small intestine and has omental appendages attached to it.
The cecum
The cecum is a pouch of intestine that hangs below the ileocecal junction in the right lower quadrant of the abdomen. Folds of mucosal tissue form the ileocecal valve that covers the ileal orifice. The appendix extends from the posteromedial part of the cecum.
Sympathetic and parasympathetic nerves come from the superior mesenteric plexus. Blood supply to the cecum comes via the ileocolic artery, a branch of the superior mesenteric artery. The appendicular artery branches from the ileocolic artery. Lymphatic vessels pass to the ileocolic lymph nodes and the superior mesenteric lymph nodes.
The ascending colon
The ascending colon travels from the cecum upward on the right side of the abdominal cavity to the right colic flexure near the right side of the liver. This part of the colon is retroperitoneal.
Nervous supply is brought to the ascending colon by the superior mesenteric plexus. The ileocolic and right colic arteries supply blood. Blood is drained away by the ileocolic and right colic veins. Lymph is drained by the epicolic and paracolic lymph nodes, and then it travels to the ileocolic and right colic lymph nodes.
The transverse colon
The transverse colon crosses from the right side of the abdomen to the left, ending at the left colic flexure. The sympathetic nerves that serve the transverse colon come from the superior and inferior mesenteric plexuses; the parasympathetic nerves arise from the vagus nerves and the pelvic splanchnic nerves.
Blood is brought to the transverse colon primarily by the middle colic artery. The distal portion of the transverse colon is served by the left colic artery, a branch of the inferior mesenteric artery. Venous blood is removed by the superior mesenteric and inferior mesenteric veins. Lymph is drained into the colic lymph nodes and into the colic nodes.
The descending colon
The descending colon travels behind the peritoneum and downward from the left colic flexure to the left iliac fossa where it continues as the sigmoid colon. Sympathetic nerve supply comes from the lumbar splanchnic nerves, the inferior mesenteric plexus, and the periarterial plexuses that surround the inferior mesenteric artery. Parasympathetic nerve supply comes from the pelvic splanchnic nerve.
Blood is brought to the descending colon by the left colic and sigmoid arteries, branches of the inferior mesenteric artery. Blood is drained away by the inferior mesenteric vein. Lymph is drained into the epicolic and paracolic lymph nodes, which drain into the intermediate colic lymph nodes. From here the lymph drains into the inferior mesenteric lymph nodes.

Clinical Anatomy Glossary
abdomen
This region of the body starts along the bottom of the ribcage and extends to the hips. It’s visible from the front.

Clinical Anatomy Glossary
abdominopelvic splanchnic nerves
These sympathetic (fight-or-flight) nerves of the abdomen carry the presynaptic fibers to the abdomen and pelvis.

Clinical Anatomy Glossary
anastomose
To join together.

Clinical Anatomy Glossary
androgen hormones
Male sex hormones.

Clinical Anatomy Glossary
ankylosing spondylitis
An arthritic disease of the vertebral joints that results in hyperkyphosis and may restrict lung expansion if the disease spreads superiorly. It may lead to fusion of intervertebral joints and spinal column rigidity.

Clinical Anatomy Glossary
anterior
In clinical anatomy, closer to the front of the body. For instance, the abdominal muscles are anterior to the spine.

Clinical Anatomy Glossary
aponeurosis
The broad tendinous structure that attaches a muscle to another muscle.

Clinical Anatomy Glossary
articulate
In clinical anatomy, to form a joint.

Clinical Anatomy Glossary
ball-and-socket joint
A ball-shaped head that fits into a bony socket. The shoulder and hip are ball-and-socket joints. This type of joint allows for free movement in several directions, including flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction.

Clinical Anatomy Glossary
bilateral
On both the left and right sides of the body, such as the eyes, the kidneys, and the arms and legs.

Clinical Anatomy Glossary
bladder
A thin-walled, bag-like organ that can hold up to two cups of urine.

Clinical Anatomy Glossary
bony process
A projection of bone that sticks out from the arch found in vertebrae.

Clinical Anatomy Glossary
brainstem
Connects the cerebrum with the spinal cord.

Clinical Anatomy Glossary
bursae
Fluid-filled sacs that help tendons glide over the bones and other tendons.

Clinical Anatomy Glossary
bursitis
An inflammation of a bursa, which is usually the result of repetitive motion injuries.

Clinical Anatomy Glossary
calcaneus
The part of the heel you stand on.

Clinical Anatomy Glossary
cardiovascular disease
Diseases of the heart and blood vessels.

Clinical Anatomy Glossary
cartilaginous joints
The joint surfaces in cartilaginous joints are covered with hyaline cartilage and have fibrocartilaginous discs between them. Like fibrous joints, cartilaginous joints can be immoveable.

Clinical Anatomy Glossary
cerebrum
The largest part of the brain; made up of the left and right cerebral hemispheres.

Clinical Anatomy Glossary
cervical
The neck region; starts below the head, ends at the thorax, and is visible from the front and rear from below the head to the shoulders.

Clinical Anatomy Glossary
circumduction
To move in a circular motion; doing arm circles, is circumduction.

Clinical Anatomy Glossary
condyloid joints
Joints with an oval surface on one bone that articulates with an oval-shaped depression in another bone. The metacarpophalangeal joints in the fingers are examples of condyloid joints.

Clinical Anatomy Glossary
contralateral
On opposite sides of the body. The right ear is contralateral to the left ear.

Clinical Anatomy Glossary
corpus callosum
A band of nerve fibers that allows the sides of the brain to communicate with the other.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
dendrite
The receiving part of a neuron. The signal received at the dendrite is transmitted toward the cell body of the neuron in the form of an electrical impulse. The impulse is transmitted away from the cell body to another neuron, muscle, or gland by the axon.

Clinical Anatomy Glossary
dermis
The lower layer of the skin containing collagen and elastic fibers that give strength to the skin.

Clinical Anatomy Glossary
diencephalon
The central part of the brain, underneath the cerebrum.

Clinical Anatomy Glossary
distal
Farther from the trunk or from the point of origin. The elbow is distal to the shoulder.

Clinical Anatomy Glossary
dorsal
An anatomical region that runs from immediately below the neck down to the area below the waist. It doesn’t include the shoulders. It’s visible from the rear.

Clinical Anatomy Glossary
dorsiflex
To move your foot and toes up.

Clinical Anatomy Glossary
duodenum
The first segment of the small intestine.

Clinical Anatomy Glossary
dural venous sinuses
The spaces between the two layers of the dura matter that collect blood from veins on the surface of the brain.

Clinical Anatomy Glossary
elastic cartilage
Cartilage that contains elastic fibers in the matrix, so it’s more flexible than either hyaline or fibrocartilage; your ear has elastic cartilage.

Clinical Anatomy Glossary
endoneurium
The delicate layer that surrounds each individual nerve fiber in the peripheral nervous system.

Clinical Anatomy Glossary
epidermis
The tough layer outermost layer of skin. It gets its toughness from a protein called keratin.

Clinical Anatomy Glossary
epineurium
The thick layer of connective tissue that surrounds a bundle of fascicles in the peripheral nervous system.

Clinical Anatomy Glossary
esophagus
A muscular tube that extends from the pharynx to the stomach.

Clinical Anatomy Glossary
eversion
To move the bottom of your foot away from the midline of the body.

Clinical Anatomy Glossary
facet
In clinical anatomy, a smooth joint-forming surface.

Clinical Anatomy Glossary
fascia
A layer of fibrous connective tissue that covers muscle.

Clinical Anatomy Glossary
fibrocartilage
This cartilage has a larger number of collagen fibers and less matrix. It’s found in the discs in joint spaces including the temporomandibular joint, knee joint, and joints between the bodies of the vertebrae.

Clinical Anatomy Glossary
fibrous joints
The bones of a fibrous joint are connected by fibrous tissue. They range from being immovable (like joints between the bones of the skull) to being slightly moveable (joints between the tibia and fibula in the legs).

Clinical Anatomy Glossary

Clinical Anatomy Glossary
frontal planes
Imaginary vertical planes that clinical anatomy uses to define regions of the body. They pass through the body at right angles to the midsagittal plane, so they divide the body into front and back. Frontal (coronal) planes can divide the body at any point, so you need to use a reference point to know where exactly the plane passes.

Clinical Anatomy Glossary
ganglion
A collection of nerve-cell bodies similar to the nuclei of the central nervous system, except that these ganglia are only found in the peripheral nervous system.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
hila
The plural of hilum, and the hilum is the part of the lung where the bronchus and pulmonary artery enter and the pulmonary vein exits the lungs.

Clinical Anatomy Glossary
hinge joint
A joint that allows flexion and extension (bending and straightening) of joints like the elbow and the knee.

Clinical Anatomy Glossary
hyaline cartilage
This durable type of cartilage covers most of the bone surfaces in synovial joints. It’s also found in the nasal septum, rings of the trachea, and costal cartilages of the ribs, and it forms the epiphyseal plates of growing bones.

Clinical Anatomy Glossary
hyperkyphosis
An abnormal increase in the thoracic curvature.

Clinical Anatomy Glossary
hypertension
A condition in which blood pressure remains elevated over time. Having high blood pressure increases the risk of heart disease, stroke, and kidney disease.

Clinical Anatomy Glossary
inferior
Closer to the feet. The chin is inferior to the nose.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
intermuscular septa
The septa divide muscles into various groups.

Clinical Anatomy Glossary
inversion
To move the bottom of your foot toward the midline of your body.

Clinical Anatomy Glossary
investing fascia
This part of the fascia covers deeper structures, such as muscles and ligaments.

Clinical Anatomy Glossary
ipsilateral
On the same side of the body. For example, the right ear and the right eye are ipsilateral.

Clinical Anatomy Glossary
keratin
A major structural component of the outer layers of skin.

Clinical Anatomy Glossary
keratinocytes
These cells are constantly shed and replaced by cells from the lower layers of the epidermis. These cells have lost most of their internal structures and organelles.

Clinical Anatomy Glossary
kidneys
Organs that filter the blood and produce urine.

Clinical Anatomy Glossary
lateral
In clinical anatomy, away from the body’s midline. For example, the little toe is lateral to the big toe.

Clinical Anatomy Glossary
lateral epicondyle
A bony prominence located proximal to the lateral condyle.

Clinical Anatomy Glossary
lateral rotation
To move a body part around its long axis with the anterior surface moving away from the midline, like turning your whole lower extremity so your foot points out toward the side.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
manubrium
The superior part of the sternum.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
matrix
A structural component made of water, collagen fibers, and crystallized calcium salts.

Clinical Anatomy Glossary
medullary cavity
Open space inside the bone.

Clinical Anatomy Glossary
meninges
Coverings of the brain. They protect the brain by housing a fluid-filled space, and they function as a framework for blood vessels.

Clinical Anatomy Glossary
midsagittal plane
An imaginary vertical plane that clinical anatomy uses to divide the body into left and right halves.

Clinical Anatomy Glossary
multinucleated
Containing more than one nucleus.

Clinical Anatomy Glossary
myoblasts
Embryonic muscle cells.

Clinical Anatomy Glossary
myocardium
Muscle of the heart.

Clinical Anatomy Glossary
nasal mucosa
Membranous lining of the nasal cavity that secretes mucus.

Clinical Anatomy Glossary
neurocranium
The part of the cranium that holds the brain.

Clinical Anatomy Glossary
osteoblasts
Cells that build up bone.

Clinical Anatomy Glossary
osteoclasts
Cells that break down bone.

Clinical Anatomy Glossary
osteocytes
Cells that maintain bone and collagen fibers.

Clinical Anatomy Glossary
papillae
Small, rounded protuberances.

Clinical Anatomy Glossary
parasympathetic stimulation
Constricts the bronchi, dilates blood vessels, and increases glandular secretions.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
perineurium
Dense connective tissue surrounding a bundle of nerve fibers called a fascicle in the peripheral nervous system.

Clinical Anatomy Glossary
periorbita
A covering of fibrous connective tissue.

Clinical Anatomy Glossary
pharyngotympanic tube
The eustachian tube.

Clinical Anatomy Glossary
pivot joints
Joints that include a bone shaped like a pivot and a ring made of bone or ligament. Pivot joints, such as the atlantoaxial joint of the cervical spine, have rotational movements.

Clinical Anatomy Glossary
plane joints
Joints with flat articular surfaces that allow for a sliding motion, such as the acromioclavicular joint of the shoulder.

Clinical Anatomy Glossary
plantarflex
To point your toes down.

Clinical Anatomy Glossary
popliteal fossa
A diamond-shaped space posterior to the knee joint, bordered by several muscles, skin, the popliteal fascia, the femur, and the joint capsule.

Clinical Anatomy Glossary
posterior
Closer to the rear. The spine is posterior to the abdominal muscles.

Clinical Anatomy Glossary
pronation
A medial rotation of the forearm so that the palm faces posteriorly (toward the rear).

Clinical Anatomy Glossary
proximal
Closer to the trunk or closer to the point of origin. The shoulder is proximal to the elbow.

Clinical Anatomy Glossary
retinacula
Band-like structures that hold tendons in place while joints move.

Clinical Anatomy Glossary
retrusion
To move backward.

Clinical Anatomy Glossary
rugae
Gastric folds that appear when the stomach muscle tissue is contracted.

Clinical Anatomy Glossary
saddle joints
Joints that have the appearance of a saddle. The carpometacarpal joint at the base of the thumb is an example of a saddle joint.

Clinical Anatomy Glossary
sagittal planes
Imaginary vertical planes that are parallel to the midsagittal plane and divide the body into unequal left and right portions. There are many possible sagittal planes, so you should always give a reference point where the plane passes through.

Clinical Anatomy Glossary
scapula
The shoulder blade.

Clinical Anatomy Glossary
sciatic foramen
The opening in the posterior portion of the pelvis formed by the sciatic notch of the ischium and the sacrospinous and sacrotuberous ligament.

Clinical Anatomy Glossary
scoliosis
A rotation and lateral flexion of the spine that may twist and turn the thoracic cage.

Clinical Anatomy Glossary
sebaceous glands
Glands connected to the hair follicles. They produce sebum, which is an oily substance that helps keeps the hair flexible.

Clinical Anatomy Glossary
skeletal muscles
The muscles responsible for making the skeleton move. They’re voluntary muscles because you can control whether the muscles move.

Clinical Anatomy Glossary
stratum basale
Forms the deepest layer of the skin. The cells of this layer continuously divide and form new keratinocytes to replace the ones that are constantly shed. This layer also contains melanocytes, which are the cells that produce skin coloring.

Clinical Anatomy Glossary
stratum corneum
Dead, mature skin cells called keratinocytes. These cells are constantly shed and replaced by cells from the lower layers of the epidermis. These cells have lost most of their internal structures and organelles.

Clinical Anatomy Glossary
stratum granulosum
The area of the skin where keratin is formed. The cells in this layer also produce materials that prevent evaporation, which helps waterproof the skin.

Clinical Anatomy Glossary
stratum lucidum
Found in thicker skin; helps reduce friction between the stratum corneum and the stratum granulosum. It’s composed of dead, flattened cells.

Clinical Anatomy Glossary
stratum spinosum
Contains keratin-producing cells that were formed in the stratum basale.

Clinical Anatomy Glossary
subcutaneous layer
Area below the skin; underneath the cutaneous layer and is sometimes called the hypodermis or superficial fascia.

Clinical Anatomy Glossary
subserous fascia
The part of the fascia that lies between the body walls such as the thoracic wall and the membranes that line corresponding body cavities.

Clinical Anatomy Glossary
superficial
Closer to the surface. For instance, the skin is superficial to the muscles.

Clinical Anatomy Glossary
superior
Closer to the top of the head. For example, the nose is superior to the chin.

Clinical Anatomy Glossary
superior oblique muscle
The muscle that turns the eyeball inferiorly and laterally.

Clinical Anatomy Glossary
supination
Lateral rotation of the forearm so the palm faces anteriorly.

Clinical Anatomy Glossary
sympathetic stimulation
Causes bronchodilation and constricts the blood vessels.

Clinical Anatomy Glossary
synapse
The junction in the brain where nerve impulses pass.

Clinical Anatomy Glossary
synovial joints
A typical synovial joint includes bones covered in hyaline cartilage and a joint cavity lined with a synovial membrane and filled with synovial fluid. A durable fibrous joint capsule surrounds the joint. Some synovial joints also have fibrocartilaginous discs between the bones.

Clinical Anatomy Glossary
thorax
The section of the body that starts immediately below the neck, at the clavicles, and ends along the bottom of the ribcage. It’s visible from the front.

Clinical Anatomy Glossary
transverse planes
Imaginary horizontal planes that pass through the body at right angles to the midsagittal and the frontal planes. They divide the body into upper and lower portions. You need to have a reference point to know exactly where a transverse plane lies.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
tympanic membrane
The eardrum.

Clinical Anatomy Glossary

Clinical Anatomy Glossary
ureters
Two long tubes that lead to the urinary bladder.

Clinical Anatomy Glossary
ventral
Toward the abdomen.