Thoracic Aorta and Arteries
The thoracic aorta is a special region of the descending aorta as it passes through the mediastinum of the thorax. Superiorly (upward), the thoracic aorta is continuous with the aortic arch and inferiorly (downward), it becomes the abdominal aorta as passes through the diaphragm.
The thoracic aorta supplies oxygenated blood to the pericardium, esophagus, bronchi, and lungs via visceral branches (i.e., vessels that supply blood to internal organs such as the lungs). Another set of branches, the parietal branches (i.e., vessels that supply blood to the walls of a body cavity such as the thorax or particular organ), supply oxygenated blood to the thoracic cavity.
Mediastinal branches of the thoracic aorta supply blood to the lymph nodes and surrounding tissue in the mediastinum. Short and small diameter phrenic branches from the lowest (most inferior) regions of the thoracic aorta supply blood to the diaphragm. A set of esophageal arteries arise from the front (ventral) side of the thoracic aorta.
As the thoracic aorta courses downward (inferiorly) it gives off nine sets of arteries, the posterior intercostal arteries, that supply blood to the nine lowest spaces between the ribs (intercostal spaces). Superior intercostal arteries, branches of the subclavian arteries supply blood to the uppermost (superior) two intercostal spaces. Below the level of the last rib, the arteries branching off the thoracic aorta are termed the subcostal arteries.
The posterior intercostal arteries themselves branch into several smaller branches that supply specialized regions of the intercostal spaces. Muscular branches run ventrally to supply blood to the intercostal and pectoral muscles. These arteries often fuse (anastomose) with branches of the axillary artery.
Regions of the thoracic nerves termed the lateral cutaneous branches receive blood from corresponding lateral cutaneous branches off the posterior intercostal arteries.
In lactating (milk-producing) females, the mammary branches of the posterior intercostals associated with breast tissue and mammary glands (at about the level of the second to fifth ribs) often dilate (expand in diameter) to allow an increased blood supply.
Dorsal branches of the posterior intercostal arteries lead to spinal branches that enter the vertebral canal through the intervertebral foramen to, along with spinal arteries, supply oxygenated blood to the thoracic vertebrae, spinal cord, and associated tissue.
The left and right subclavian arteries also supply oxygenated blood to the internal thoracic region (including the thoracic wall and internal thoracic region), parts of the upper arm (upper limb), neck, spinal cord, meninges, and brain. The internal thoracic arteries, branching from the subclavian arteries, supply blood to the diaphragm, mediastinal structures, and the anterior thoracic wall.
The bronchi receive blood from branches of the thoracic aorta, termed bronical arteries, that are often found to show considerable variations. Normally, there is one bronchial artery on the right side of the body and two bronchial arteries on the left. The right bronchial artery branches from the third posterior intercostal artery, while the left bronchial arteries split directly from the thoracic aorta.
As a region of the descending aorta, the thoracic aorta arises in the embryo from the dorsal aortae that are located on each side of the notochord. At about the end of the first month of development, these embryonic dorsal aortae fuse to form the descending aorta.
In some cases there are blockages (occlusions) of the thoracic aorta. Such occlusions are termed a coarctation of the aorta and force blood to flow to the lower parts of the body through collateral arteries located in the chest wall (thoracic wall). This diversion of blood through higher resistance vessels results in an initially greatly increased blood pressure in the upper part of the body that may be as great as twice the normal pressures. Blood pressures found in the lower part of the body past the diversion are lower than normal. In cases where there is a persistent, long lasting occlusion, a compensation mechanism termed long-term local blood flow regulation works to equalize the actual blood flow on both sides of the blockage. Although increased blood pressure (such as those found in the upper part of the blood) usually mean a short term (acute) corresponding increases in blood flow, over time, physiological compensating mechanisms change the vascularity (number and size of blood vessels) of tissues to return blood flow to near normal levels.
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