Arteries and veins are blood vessels that carry blood away from and towards the heart, respectively. They form two connected circuits: pulmonary circulation moves blood through the lungs for gas exchange, while systemic circulation delivers blood to the body's tissues and returns it to the heart.
What is the difference between arteries and veins?
The distinction depends on direction of flow, not oxygen content. An artery carries blood away from the heart; a vein carries blood towards it. Oxygenated blood is relatively rich in oxygen, whereas deoxygenated blood contains less oxygen but is not oxygen-free.
Within most tissues, arteries branch into arterioles, small arteries that regulate entry into capillary networks. Capillaries are microscopic vessels across whose thin walls substances pass between blood and surrounding tissues. Blood then enters venules, small collecting vessels that join to form veins.
Pulmonary circulation connects the right side of the heart to the left through the lungs. Systemic circulation connects the left side to the right through the rest of the body. These circuits are arranged in series: blood passes through one and then the other rather than choosing between them.
How does pulmonary circulation connect the right ventricle to the left atrium?
The right atrium is the heart chamber that receives blood returning from the body. An atrium is a receiving chamber; a ventricle is a chamber that pumps blood out of the heart. Pulmonary circulation begins at the right ventricle; the route below includes the preceding passage through the right atrium:
- Blood enters the right atrium and passes through the tricuspid valve into the right ventricle. This valve lies between the two chambers and prevents backward flow during ventricular contraction.
- The right ventricle pumps blood through the pulmonary valve into the pulmonary trunk. The pulmonary valve prevents blood from returning to the ventricle after ejection.
- The pulmonary trunk divides into right and left pulmonary arteries. Each carries blood towards a lung, where branches become progressively smaller arteries and arterioles.
- Blood passes through capillaries surrounding alveoli, the small air spaces where gas exchange occurs. Oxygen passes into the blood, while carbon dioxide passes from blood into alveolar air.
- Venules collect this blood and unite into larger veins. A pulmonary vein carries oxygenated blood into the left atrium.
Pulmonary arteries carry deoxygenated blood because they receive blood from the right ventricle before gas exchange. They are still arteries because their flow is away from the heart. Conversely, pulmonary venous blood has already passed through the lung's gas-exchange capillaries.
How does systemic circulation return blood to the right atrium?
Systemic circulation begins as blood leaves the left ventricle and ends when it returns to the right atrium. Its main route continues directly from the pulmonary pathway:
- Blood passes from the left atrium through the mitral valve into the left ventricle. The mitral valve prevents backward flow into the atrium during ventricular contraction.
- The left ventricle ejects blood through the aortic valve into the aorta, the main artery of systemic circulation. The aortic valve prevents return flow into the ventricle.
- Branches of the aorta distribute blood through arteries and arterioles to capillary networks in the body's tissues. Here, oxygen and nutrients pass towards tissues, while carbon dioxide and other substances enter the blood.
- Blood collects in venules and veins. The superior vena cava returns blood chiefly from the head, neck, upper limbs and chest; the inferior vena cava returns blood chiefly from regions below the diaphragm, the muscular partition between chest and abdomen.
- Both vessels empty into the right atrium, completing the route.
The heart muscle also receives a systemic arterial supply. A coronary artery arises from the beginning of the aorta and supplies the myocardium, the muscular wall of the heart. Much of its venous blood reaches the right atrium through the coronary sinus, a collecting venous channel.
The aorta, pulmonary trunk, pulmonary venous vessels and both venae cavae are the great vessels entering or leaving the heart. Their connections establish the direction of flow between the chambers and the two circuits.
How do the walls of blood vessels differ?
Most arteries and veins have three wall layers, called tunics. The tunica intima is the inner lining and includes endothelium, a single layer of flattened cells in contact with blood. The tunica media is the middle layer, containing smooth muscle, which contracts involuntarily, and elastic tissue, which stretches and recoils. The tunica adventitia is the outer supporting layer of connective tissue.
Capillaries do not have these three developed layers. Their walls consist chiefly of endothelium, usually supported by a basal lamina, a thin layer of extracellular material beneath the cells. In sinusoidal capillaries, this supporting layer may be discontinuous or absent. This short barrier supports exchange rather than the containment of high pressure.
| Vessel type | Main wall features | Main role |
|---|---|---|
| Artery | Developed middle layer; large arteries contain abundant elastic tissue, while many distributing arteries contain more smooth muscle | Carries blood away from the heart and distributes it to regions |
| Arteriole | Narrow channel with a small amount of smooth muscle around its lining | Regulates flow into capillary networks |
| Capillary | Endothelium usually supported by a basal lamina, without a developed muscular wall | Allows exchange between blood and tissues |
| Venule | Smallest venules have thin walls; larger venules acquire smooth muscle | Collects blood from capillaries |
| Vein | Generally thinner middle layer and a wider internal channel than a comparable artery; many have valves | Returns blood towards the heart |
The lumen is the internal channel through which blood flows. Venous valves are folds of the inner lining that help prevent reverse flow, particularly in the limbs; they are not present in every vein. Arterial elastic recoil helps maintain forward flow between ventricular contractions, while arteriolar smooth muscle adjusts the width of the lumen.
What is portal circulation?
A portal circulation carries blood through two successive exchange beds before returning it to the heart. In the hepatic portal system, blood from much of the digestive tract, as well as the spleen and pancreas, enters the hepatic portal vein. This vessel delivers blood to liver sinusoids, specialised vascular channels where substances are exchanged between blood and liver cells across the sinusoidal endothelium and the intervening perisinusoidal space. Blood then leaves through hepatic veins and enters the inferior vena cava. The liver also receives arterial blood through the hepatic artery, so its incoming supply is not solely portal blood.
What are anastomoses and end arteries?
An anastomosis is a connection between blood vessels. Arterial anastomoses join alternative routes into a region, while venous anastomoses connect drainage routes. Networks of arterial connections occur around many joints, allowing blood to reach tissues through more than one pathway.
An anatomical end artery has no arterial anastomosis with neighbouring arteries supplying the same territory. A standard example is the central retinal artery, which supplies the inner retina, the light-sensitive tissue at the back of the eye.
A functional end artery has connections, but these do not provide an adequate alternative supply if its main route is interrupted. Coronary arterial branches are commonly described in this way. The distinction concerns the availability of an effective alternative arterial route, not simply whether any small connection exists.
How can you follow both circulations as one route?
Follow chamber connections before considering smaller branches: right atrium, right ventricle, pulmonary vessels, left atrium, left ventricle, systemic vessels and back to the right atrium. At each stage, separate the vessel's direction from the oxygen content of its blood.
In the 3D model next to the article, rotate and zoom to change your viewpoint, then select and isolate a structure to examine its position. Hiding or fading a selected structure, switching layers or pulling parts apart can help you compare spatial relationships from different viewing angles.
The two circuits share a continuous blood pathway but serve different exchange sites. Pulmonary capillaries exchange gases with alveolar air; systemic capillaries exchange substances with body tissues. This distinction explains both the sequence of flow and the different oxygen content of their arteries and veins.