Anatomy, Arteries

Structure and Function

The arteries throughout the body are composed of 3 different layers (see Image. Artery Layers). The innermost layer of the artery, known as the intima, consists of a single layer of endothelial cells overlying a smooth muscle layer; the remainder comprises smooth muscle and elastin. The tunica intima forms a tube through which oxygen-rich blood flows to the appropriate perfusion site. See Image. Structure of an Artery Wall. In this way, there is no leakage from the artery, and nutrient-rich blood can reach the appropriate area before it delivers oxygen and other nutrients. The second layer is known as the media or the middle layer. This media layer consists of smooth muscle that can dilate or constrict, thereby adjusting the pressures experienced by the arterial walls during systolic pumping. As the muscle contracts, the walls experience increased pressure from the left ventricle; similarly, as the vessels dilate, the observed pressure decreases.[6] The last layer is the outermost, known as the adventitia.

The adventitia is crucial for connecting arteries to other tissues in the body, including the vascular nerves that control the smooth muscle in the arteries. In this way, arteries do not move freely throughout the body; instead, they are anchored in place to maintain a consistent and effective cardiovascular system. Usually, arteries are under the greatest pressure because they receive blood from the left ventricle, the heart's most powerful chamber. However, the pulmonary artery is different for 2 reasons. Not only does it move unoxygenated blood from the heart to the lungs, but it also operates at a lower pressure because the right ventricle exerts less force than the left. The pulmonary artery delivers blood to the lungs, whereas the remaining arteries deliver blood to specific areas of the body. Blood from the left ventricle is pumped through the aorta, the largest artery. The aorta then splits into 4 sections: the ascending aorta, the aortic arch, which curves over the heart, the descending thoracic aorta, and the abdominal aorta. The ascending aorta arises from the heart and further branches into the carotid artery, which supplies the brain with blood.[7]

The aortic arch is similar to the ascending aorta and carries blood up to the back and neck. The descending aorta brings blood from the heart to smaller vessels in the chest or ribs. Finally, the abdominal aorta supplies blood to the iliac arteries, which provide blood flow to many organs in the abdominal region.[8]

Embryology

During development, arteries continually remodel to maintain homeostasis in changing environments. The heart and arteries develop from the mesoderm germ layer as it is composed of smooth muscle. After the mesoderm differentiates into segments of the circulatory system through specific growth factor signaling, arteries begin to form, and circulation, powered by the heart, initiates. At the beginning of development, the arteries form from the pharyngeal arches. Each arch gives rise to a specific arterial segment during the 9-month pregnancy.

During pregnancy, the fetus shares a circulatory system with the mother, resulting in unique blood flow patterns. The uterine arteries carry blood from the mother to the placenta, where it perfuses the placenta and then flows to the fetus, supplying it with oxygen. Fetal hearts also have a unique blood flow as blood does not become oxygenated in its lungs but instead by the mother. A fetus has a unique connection between the aorta and the pulmonary artery called the ductus arteriosus.[9] This connection allows blood to flow past the fetal lungs, which are not functional, while the fetus is in the amniotic sac.

Muscles

In all of these examples, arteries are comprised of smooth muscle. They are nonstriated muscles that surround the vessels, providing support and integrity to the entire artery. This smooth muscle responds to various signals and innervation to constrict or dilate, thereby maintaining a stable blood pressure, including via epinephrine and angiotensin II. The sympathetic nervous system can release high levels of epinephrine, which activates alpha-adrenergic receptors, thereby causing arterial constriction. This increase in pressure can aid in perfusion during both trauma and hormonal imbalance.[10]

Additionally, the kidney can indirectly affect blood pressure through its release of renin. Renin catalyzes the formation of angiotensin II, a potent vasoconstrictor.[11] Angiotensin II interacts directly with smooth muscle in arterial walls, causing vasoconstriction when blood pressure falls below normal. Additionally, when blood pressure decreases due to bleeding or peripheral arterial dilation, as in shock, the arteries interact with the renin-angiotensin-aldosterone system to compensate for the decrease.

As arteries age, they harden and become less compliant, creating a systemic imbalance that can lead to secondary hypertension through reduced renal glomerular filtration rate. Therefore, through the endocrine system, hormones are released into the bloodstream, which then signal the vascular nerves to alter their tone to maintain homeostasis.

Surgical Considerations

The different surgical considerations with regard to arterial anatomy are:

1. Aneurysms - May need surgical ligation, stenting, or clipping based on the location
2. Arteriovenous fistula - May need separation of the vessels
3. Ligation of an arterial bleeder - May need a trans-fixation suture for major arteries
4. Thrombo-embolism - May need open/interventional radiology-assisted evacuation

Clinical Significance

There are many additional ways in which arteries help maintain balance, including through vasodilators such as histamine and serotonin. This, however, can become dangerous if balance is not maintained. An angiogram, or the insertion of a catheter and contrast agent into an artery, can be used to monitor blood flow and identify bleeding or blockages. However, environmental factors also influence arterial structure. For example, during anaphylaxis shock, massive amounts of histamine are released into the body, causing the arteries to dilate to an unsafe amount. This reduces pressure for the perfusion of nutrient-rich blood. In an attempt to compensate for this loss, the body enters a state of compensated shock, as indicated by increased heart rate. This does not work forever, and without dilation, the patient soon goes into a state of shock. In addition to being balanced, many medical issues may arise, further complicating the situation.

One of the most common medical conditions affecting arteries is atherosclerosis, in which arteries accumulate plaque, impairing normal cardiovascular function. This accumulation of plaque can occur anywhere, allowing a blockage to develop. A myocardial infarction occurs if the blood supply to the heart is cut off.[12] Similarly, if blood flow to the brain is blocked, this is known as a stroke. A lack of blood supply to any area of the body can cause permanent severe damage or even death. Ischemia of any vital organ is severe and may present in a wide variety of forms. For example, if the blood supply to the retina is cut off, one may develop amaurosis fugax or blindness.[13] Trans fats and high cholesterol have been shown to adversely affect the arteries, leading to atherosclerosis. Therefore, it is crucial to maintain a healthy lifestyle through diet and exercise. Individuals who maintain a healthy lifestyle yet have elevated LDL levels should also be screened for genetic causes, such as familial hypercholesterolemia.

Individuals with a history of stroke or transient ischemic attack (TIA) should be evaluated with a carotid ultrasound to assess the level of stenosis. Vascular surgery may be performed to further dilate the arterial lumen, depending on the degree of stenosis or obstruction in these major vessels. This process is often achieved by balloon angioplasty and stenting of the artery via subclavian artery access. This procedure is minimally invasive and referred to as TCAR. If TCAR is unsuccessful, an endarterectomy may be performed to open the artery and remove any atherosclerotic plaque buildup directly. The cardiovascular system is essential for keeping all other systems in the body. Arteries, in particular, are vital in supplying nutrients, including oxygen, to the rest of the body. It is crucial to recognize the role of arteries in the cardiovascular system to prevent future medical complications.