Surface projections of the organs of the trunk

Situs solitus is the medical term referring to the normal position of thoracic and abdominal organs. Anatomically, this means that the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...

is on the left with the pulmonary

atrium Atrium may refer to: Anatomy * Atrium (heart), an anatomical structure of the heart * Atrium, the genital structure next to the genital aperture in the reproductive system of gastropods * Atrium of the ventricular system of the brain * Pulmona ...

on the right and the systemic atrium on the left along with the cardiac apex. Right-sided

organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...

are the

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...

, the

gall bladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath the liver, although ...

and a trilobed lung as well as the

inferior vena cava The inferior vena cava is a large vein that carries the deoxygenated blood from the lower and middle body into the right atrium of the heart. It is formed by the joining of the right and the left common iliac veins, usually at the level of th ...

, while left-sided organs are the

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

, single

spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .

, a bilobed lung, and the

aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...

Variants Variant may refer to: In arts and entertainment * ''Variant'' (magazine), a former British cultural magazine * Variant cover, an issue of comic books with varying cover art * ''Variant'' (novel), a novel by Robison Wells * "The Variant", 2021 e ...

on the normal picture are relatively uncommon. Complete reversal of all organs is known as

situs inversus Situs inversus (also called situs transversus or oppositus) is a congenital condition in which the major visceral organs are reversed or mirrored from their normal positions. The normal arrangement of internal organs is known as situs solitus. Al ...

, while reversal of some organs but not others is called

situs ambiguus Situs ambiguus is a rare congenital defect in which the major visceral organs are distributed abnormally within the chest and abdomen. Clinically heterotaxy spectrum generally refers to any defect of Left-right asymmetry and arrangement of the vi ...

or heterotaxy. Isolated reversal of the heart with normally-patterned viscera otherwise is termed

dextrocardia Dextrocardia (from Latin ''dextro'', meaning "right hand side," and Greek ''kardia'', meaning "heart") is a rare congenital condition in which the apex of the heart is located on the right side of the body, rather than the more typical placement t ...

Development

Although much of humans' external anatomy is bilaterally symmetric, many internal structures must be signaled to develop asymmetrically in order to form the normal situs solitus orientation. This induced asymmetry is thought to be achieved by a number of morphological factors that guide development. For instance, different levels of

NODAL Nodal homolog is a secretory protein that in humans is encoded by the ''NODAL'' gene which is located on chromosome 10q22.1. It belongs to the transforming growth factor beta superfamily (TGF-β superfamily). Like many other members of this superf ...

proteins on opposite sides of the embryo have been linked to proper bending of the embryonic heart, giving it its asymmetrical form. As for the question as to how these development-guiding proteins themselves become distributed asymmetrically throughout the embryo, a certain type of embryonic node cell has been identified with cilia known as nodal cilia that move only in a counter-clockwise orientation, thus potentially producing a sort of 'current' in the embryo that would asymmetrically distribute these molecules.

Development

References

See also