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The first sacral vertebra through the fifth sacral vertebra.

There are 5 sacral vertebral bones. They are represented by the symbols S1 through S5.

The sacral vertebrae are situated in the spinal column just below the lumbar vertebrae and right above the coccyx, which is the lowest segment of the vertebral column. The sacral vertebrae are normally found fused together to form the sacrum.

SA node
The SA node (SA stands for sinoatrial) is one of the major elements in the cardiac conduction system, the system that controls the heart rate. This stunningly designed system generates electrical impulses and conducts them throughout the muscle of the heart, stimulating the heart to contract and pump blood.

The SA node is the heart's natural pacemaker. The SA node consists of a cluster of cells that are situated in the upper part of the wall of the right atrium (the right upper chamber of the heart). The electrical impulses are generated there. The SA node is also called the sinus node.

The electrical signal generated by the SA node moves from cell to cell down through the heart until it reaches the atrioventricular node (AV node), a cluster of cells situated in the center of the heart between the atria and ventricles. The AV node serves as a gate that slows the electrical current before the signal is permitted to pass down through to the ventricles. This delay ensures that the atria have a chance to fully contract before the ventricles are stimulated. After passing the AV node, the electrical current travels to the ventricles along special fibers embedded in the walls of the lower part of the heart.

The autonomic nervous system, the same part of the nervous system as controls the blood pressure, controls the firing of the SA node to trigger the start of the cardiac cycle. The autonomic nervous system can transmit a message quickly to the SA node so it in turn can increase the heart rate to twice normal within only 3 to 5 seconds. This quick response is important during exercise when the heart has to increase its beating speed to keep up with the body's increased demand for oxygen.

Sabin vaccine
The oral polio vaccine. The first vaccine against poliomyelitis was introduced by Dr. Jonas Salk in 1955, was given by injection and required 4 "shots." The oral form of the vaccine, subsequently developed by Dr. Albert Sabin, is in standard use today since it is easier to administer and is more effective than the Salk vaccine. The Salk vaccine is now exclusively of historic interest. Although Albert Sabin's vaccine prevailed, Salk's name is better known, in part because of the attendant publicity he received and the Salk Institute in LaJolla, California (founded by the National Foundation/ March of Dimes).

Sac, pericardial
The pericardial sac is a conical sac of fibrous tissue which surrounds the heart and the roots of the great blood vessels.

The pericardium has outer and inner coats. The outer coat is tough and thickened, loosely cloaks the heart, and is attached to the central part of the diaphragm and the back of the sternum (breastbone). The inner coat is double with one layer closely adherent to the heart while the other lines the inner surface of the outer coat with the intervening space being filled with fluid.

This small amount of fluid, the pericardial fluid, acts as a lubricant to allow normal heart movement within the chest.

The word "pericardium" means around the heart. The outer layer of the pericardium is called the parietal pericardium. The inner part of the pericardium that closely envelops the heart is called the visceral pericardium or epicardium.

From the Latin “sacculus” meaning a small pouch. As for example the alveolar saccules (little air pouches) within the lungs .

Saccular aneurysm
An aneurysm that resembles a small sack. A berry aneurysm is typically saccular. An aneurysm is a localized widening (dilatation) of an artery, vein, or the heart. At the area of an aneurysm, there is typically a bulge and the wall is weakened and may rupture. The word “aneurysm” comes from the Greek “aneurysma” meaning “a widening.”

Sacral agenesis
Failure of formation of all or part of the sacrum (the lowest section of the spine).

Currarino syndrome is a condition characterized by the combination of:

Partial absence of the sacrum (the lowest portion of spine),
Anorectal (anal and rectal) abnormalities, and
An abnormal mass in front of the sacrum (due to a meningocoele or teratoma).
The malformations in Currarino syndrome are all in tissues that have their embryological origin in the tail bud and may reflect disturbances in its formation during early embryonic life.

A mutation (change) in a gene called the HLXB9 homeobox gene has been identified as responsible for autosomal dominant Currarino syndrome, also known as hereditary sacral agenesis.

Sacral vertebrae
There are 5 sacral vertebral bones. They are represented by the symbols S1 through S5 and are situated between the lumbar vertebrae and the coccyx (the lowest segment of the vertebral column). The sacral vertebrae are normally fused to form the sacrum.

The large, heavy bone at the base of the spine, which is made up of fused sacral vertebrae. The sacrum is located in the vertebral column, between the lumbar vertebrae and the coccyx. It is roughly triangular in shape, and makes up the back wall of the pelvis. The female sacrum is wider and less curved than the male.

See also pelvis, vertebral column, sacral vertebrae.

SAD (seasonal affective disorder)
Depression that tends to occur (and recur) as the days grow shorter in the fall and winter. It is believed that affected persons react adversely to the decreasing amounts of light and the colder temperatures as the fall and winter progress.

Seasonal affective disorder has not been recognized very long as a medical condition. The term first appeared in print in 1985. Seasonal affective disorder is also sometimes called winter depression or the hibernation reaction.