10.10: Circulatory Pathways

The circulatory pathways form a connected network that ensures continuous blood flow through the pulmonary, systemic, and coronary circuits to deliver oxygen and nutrients throughout the body.

By the end of this section, you will be able to:

• Describe the overall organization of the pulmonary and systemic circuits and trace the general pathway of blood through each.
• Identify the major divisions of the aorta and explain how the names of arteries and veins change as they pass anatomical landmarks.
• Explain what an anastomosis is and describe the function of the Circle of Willis as an example of an arterial anastomosis.

Organization of the Circulatory Pathways

As you explore the vessels of the pulmonary and systemic circuits, you will notice that many arteries and veins share the same names. Those with identical names typically run side by side and are found on both the right and left sides of the body. 

Some vessels, however, are unpaired or differ in branching patterns. For example, there are right and left femoral arteries and veins, but only one aorta in the midline of the body. Likewise, certain superficial veins, such as the great saphenous vein of the leg, have no arterial counterpart.

Another detail to keep in mind is that vessel names often change as they pass anatomical landmarks — much like a road that changes names at a city boundary. For instance, the left subclavian artery becomes the axillary artery as it enters the armpit (axilla) and then the brachial artery as it travels down the upper arm (brachium).

Finally, you will come across anastomoses, which are connections where two blood vessels that had separated join together again. These are especially common in veins, providing alternate routes for blood flow if one pathway becomes blocked. Some important arterial anastomoses also occur in the brain, ensuring continuous blood supply to this vital organ. A well-known example of an arterial anastomosis is the Circle of Willis, a ring-shaped network of arteries located at the base of the brain. It connects the major arteries supplying the brain — the internal carotid arteries (from the front) and the basilar artery (from the back) — forming a circular pathway. This arrangement allows blood to be rerouted if one part of the brain’s blood supply becomes narrowed or blocked, helping to maintain consistent delivery of oxygen and nutrients.

In short, the Circle of Willis acts like a “traffic roundabout” for cerebral circulation, ensuring that blood can still reach all areas of the brain even if one route is compromised.

Figure \(\PageIndex{1}\): Circle of Willis and Associated Arteries. This circular network of arteries located at the base of the brain connects the internal carotid, the middle cerebral, and the basilar arteries. This arterial anastomosis provides multiple routes for blood to reach brain tissue, helping to maintain cerebral perfusion even if one vessel becomes blocked. The internal carotid arteries supply the anterior portion of the circle, while the basilar artery contributes posteriorly. Branches such as the middle cerebral arteries distribute blood to the cerebral hemispheres.

 

Overview of Systemic Circulation

Blood relatively high in oxygen concentration is returned from the pulmonary circuit to the left atrium via the four pulmonary veins. From the left atrium, blood moves into the left ventricle, which pumps blood into the aorta. The aorta and its branches — the systemic arteries — send blood to virtually every organ of the body.

Figure \(\PageIndex{2}\): Parts of Aorta in Relation to Heart and Diaphragm. The ascending aorta, aortic arch, descending (thoracic) and abdominal aorta are shown.  (Image Credit: "OLI - Drawing Parts of aorta in relation to heart and diaphragm - English labels" by Open Learning Initiative, license: CC BY-NC-SA)

 

The Aorta

The aorta is the largest artery in the body. It arises from the left ventricle and curves downward through the chest and abdomen before dividing into the two common iliac arteries at about the level of the fourth lumbar vertebra. The aorta has three main regions: the ascending aorta, the aortic arch, and the descending aorta, which passes through the diaphragm and continues as the abdominal aorta. The aortic valve at its base prevents blood from flowing backward into the left ventricle during relaxation.

The ascending aorta extends upward for about 5 cm from the heart, then it curves to the left, forming the aortic arch. The arch then turns downward and becomes the descending aorta, which travels along the vertebral column. As it passes through the diaphragm at the aortic hiatus, it transitions into the abdominal aorta, which ultimately splits into the right and left common iliac arteries. Arteries branching from the aorta deliver oxygenated blood to nearly every organ and tissue of the body.

As you read about the circular pathways of blood flow, you may notice that some large arteries are referred to as trunks. The term trunk indicates a major vessel that branches into several smaller arteries. For example, the celiac trunk gives rise to the left gastric, common hepatic, and splenic arteries.

To help visualize the organization of the circulatory system, imagine taking a “Voyage of Discovery” like Lewis and Clark’s 1804–1806 expedition. Picture yourself traveling by boat through a vast network of rivers and streams, charting each branch as it flows into smaller and smaller tributaries. In much the same way, you can trace the pathways of blood through the body’s arterial and venous branches.

For this course, we will focus on only a few of the major routes of blood flow and name only the most important arteries and veins. Interestingy, circulation patterns can vary slightly from person to person. For example, a common variation in circulation patterns is found in the Circle of Willis. In many people, one or more of the communicating arteries may be smaller than usual or even absent. Despite these differences, blood flow to the brain is usually unaffected because the circle provides alternate routes for circulation if one part becomes narrowed or blocked.

Another frequent variation occurs in the branches of the aortic arch. While the typical pattern includes three branches — the brachiocephalic trunk, left common carotid artery, and left subclavian artery — some individuals have only two, because the left common carotid shares a common origin with the brachiocephalic trunk. These differences are normal and usually have no effect on health.

Table \(\PageIndex{1}\): Components of the Aorta

Vessel	Description
Aorta	Largest artery in the body, originating from the left ventricle and descending to the abdominal region, where it bifurcates into the common iliac arteries at the level of the fourth lumbar vertebra; arteries originating from the aorta distribute blood to virtually all tissues of the body
Ascending aorta	Initial portion of the aorta, rising superiorly from the left ventricle for a distance of approximately 5 cm
Aortic arch	Graceful arc to the left that connects the ascending aorta to the descending aorta; ends at the intervertebral disk between the fourth and fifth thoracic vertebrae
Descending aorta	Portion of the aorta that continues inferiorly past the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta
Thoracic aorta	Portion of the descending aorta superior to the aortic hiatus
Abdominal aorta	Portion of the aorta inferior to the aortic hiatus and superior to the common iliac arteries

Aortic Arch Branches

There are three major branches of the aortic arch: the brachiocephalic artery, the left common carotid artery, and the left subclavian (literally “under the clavicle”) artery. As you would expect based upon proximity to the heart, each of these vessels is classified as an elastic artery.

The brachiocephalic artery (or trunk) is located only on the right side of the body; there is no corresponding artery on the left. The brachiocephalic artery branches into the right subclavian artery and the right common carotid artery. The left subclavian and left common carotid arteries arise independently from the aortic arch but otherwise follow a similar pattern and distribution to the corresponding arteries on the right side (see Figure \(\PageIndex{6}\)).  Each subclavian artery supplies blood to the arms, chest, shoulders, back, and central nervous system. 

Figure \(\PageIndex{3}\): Branches of the Aortic Arch. The aortic arch gives rise to three large branches: the brachiocephalic artery (or trunk), the left common carotid artery, and the left subclavian artery. (Image Credit: "Aortic Arch and Main Branches" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of "Palmer - Drawing Aortic arch - English labels" by Beckie Palmer, © StatPearls Publishing LLC, license: CC BY.)

 

Major Systemic Blood Vessels

Figure \(\PageIndex{4}\): Large Arteries and Veins.  The largest arteries and vessels in the body are depicted here.  Branches of these vessels are showing in later images in this section.  (Image Credit: "C_M3_26.jpg" by Cenveo is licensed under CC BY 3.0, image from the OLI Repository, modifications by Jennifer Lange.)

Pulmonary Circulation

Blood returning from the systemic circuit enters the right atrium through the superior and inferior venae cavae and the coronary sinus, which drains blood from the heart muscle itself. This blood is low in oxygen and high in carbon dioxide, since oxygen has been used by body tissues and carbon dioxide has been picked up as a waste product.

From the right atrium, blood flows into the right ventricle, which pumps it into the pulmonary circuit — the pathway to and from the lungs.

The pulmonary trunk carries blood away from the right ventricle. At its base is the pulmonary semilunar valve, which prevents blood from flowing backward during relaxation. The trunk quickly divides into the right and left pulmonary arteries, which carry blood to each lung. Within the lungs, these arteries branch into smaller arterioles and pulmonary capillaries that surround the alveoli, where gas exchange occurs: carbon dioxide leaves the blood and oxygen enters.

The newly oxygenated blood then travels through pulmonary venules and larger pulmonary veins—two from each lung—that return it to the left atrium. The pulmonary circuit ends here, ready for the oxygen-rich blood to enter the systemic circuit.

Figure \(\PageIndex{5}\): Pulmonary Circuit. Blood exiting from the right ventricle flows into the pulmonary trunk, which bifurcates into the two pulmonary arteries. These vessels branch to supply blood to the pulmonary capillaries, where gas exchange occurs within the lung alveoli. Blood returns via the pulmonary veins to the left atrium. (Image credit: "Pulmonary Circuit" by OpenStax is licensed under CC BY 3.0)

 

Table \(\PageIndex{1}\): Pulmonary Arteries and Veins

Vessel	Description
Pulmonary trunk	Single large vessel exiting the right ventricle that divides to form the right and left pulmonary arteries
Pulmonary arteries	Left and right vessels that form from the pulmonary trunk and lead to smaller arterioles and eventually to the pulmonary capillaries
Pulmonary veins	Two sets of paired vessels — one pair on each side—that are formed from the small venules, leading away from the pulmonary capillaries to flow into the left atrium

Coronary Circulation

The coronary arteries and veins encircle the heart like a crown, supplying the heart muscle with its own blood supply. The first branches of the ascending aorta are the right and left coronary arteries, which originate just above the aortic semilunar valve. These vessels deliver oxygen-rich blood to the myocardium so it can keep contracting effectively.

The left anterior descending (LAD) artery is the most commonly blocked vessel during a heart attack, followed by the right coronary artery (RCA) and the left circumflex artery (LCX). The LAD is especially critical because it supplies blood to a large portion of the heart’s main pumping chamber (the left ventricle). When this artery becomes blocked, the damage can be severe and sometimes fatal, which is why it is often referred to as the “widowmaker.”

Figure \(\PageIndex{6}\): Aorta and Coronary vessels.  Image on the left: The first two arteries to branch off the the aorta are the right and left coronary arteries.  (Image Credit: "Aorta and Coronary Arteries" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of "Palmer - Drawing Aortic arch - English labels" by Beckie Palmer, © StatPearls Publishing LLC, license: CC BY.) Image on the right: Major vessels include the right and left coronary arteries, the anterior and posterior interventricular arteries, and the circumflex artery. https://commons.wikimedia.org/wiki/F...d_arteries.svg

The left coronary artery is short.  It passes posterior to the pulmonary trunk and, upon emergence, splits into the anterior interventricular artery and the circumflex artery.  The anterior interventricular artery travels toward the inferior margin of the heart in the anterior interventricular sulcus.  The circumflex artery travels in the left atrioventricular sulcus to the posterior side of the heart.  Along the way it gives off the left marginal artery to the lateral side of the left ventricle.  The great cardiac vein travels first with the anterior interventricular artery and then with the circumflex artery.  On the posterior surface it joins with other branches and becomes the coronary sinus.

 

Blood Collection

Venipuncture is the process of inserting a needle into a vein to draw blood or start an IV. It may sound routine, but it is one of the most valuable diagnostic tools in medicine — a quick snapshot of what is happening inside the body. Performed by trained professionals such as nurses or phlebotomists, venipuncture allows clinicians to assess how well the blood carries oxygen, how the immune system is responding, and whether the body’s clotting mechanisms are functioning properly. It can also reveal important details about metabolism, hydration, and kidney health, making this simple procedure a key step in understanding a patient’s overall condition.

     

Figure \(\PageIndex{7}\): Venipuncture for Blood Collection. A healthcare professional performs venipuncture — the process of inserting a sterile needle into a vein, typically in the antecubital fossa (inner elbow)—to collect a blood sample for laboratory testing. The sample tubes fill as venous pressure drives blood into the collection system.   Image credit: "Venipuncture" by Infusion Nurses Society, CC BY-SA 4.0 via Wikimedia Commons.

The most frequently used vein is the median cubital vein, located in the antecubital fossa (the bend of the elbow). This vein is preferred because it is typically large, close to the skin’s surface, and less likely to move during puncture.

 

Check out these videos on Arteries, Veins, and Their Names.