Arteries are the blood vessels that carry blood away from the heart. Every time the heart contracts, it pushes blood into a branching network of arteries that reaches every organ, muscle, and tissue in your body. The largest is the aorta, which exits the left side of the heart and immediately begins dividing into smaller and smaller branches. But the full picture is more interesting than a one-word answer, because arteries come in strikingly different types, behave differently depending on their size and location, and face a unique set of problems as they age.
How the Arterial Tree Is Organized
Think of the arterial system as a tree. The aorta is the trunk, roughly the diameter of a garden hose where it leaves the heart. It arches upward, sends branches to the head and arms, then curves down through the chest and abdomen, splitting along the way into progressively smaller arteries. These feed into muscular arteries that supply specific organs and limbs, which in turn branch into tiny arterioles just upstream of the capillaries where oxygen and nutrients actually get delivered to cells.
Arterioles sit at the very end of the arterial side and, together with the smallest arteries, generate most of the resistance to blood flow in your body. That resistance is what keeps your blood pressure in a functional range and determines how much blood each tissue receives at any given moment.1PubMed Central. The dynamic structure of arterioles Resistance arteries and arterioles in the peripheral circulation are responsible for controlling blood pressure, distributing flow within tissues, and regulating the pressure that reaches the capillaries.2PubMed Central. Myogenic Tone in Peripheral Resistance Arteries and Arterioles: The Pressure Is On!
Elastic Arteries Versus Muscular Arteries
Not all arteries are built the same way. The largest arteries near the heart, like the aorta, are classified as elastic arteries. Their walls are loaded with stretchy elastin fibers that allow them to expand when the heart pumps and then snap back between beats, smoothing out the pulsating flow into something more continuous. This recoil effect, sometimes called the Windkessel function, acts like a secondary pump: the artery wall stores energy during each heartbeat and releases it between beats, pushing blood forward even while the heart is relaxing.
Muscular arteries are farther from the heart and have walls dominated by smooth muscle cells rather than elastin sheets. They are stiffer in the circumferential direction and serve a different purpose: routing blood to specific areas and adjusting flow by constricting or relaxing. A comparison between the thoracic aorta and a muscular artery in the leg found that the aorta had roughly 36% lower circumferential stiffness and about eight times more elastic energy available for pulsation, while the muscular artery maintained higher longitudinal compliance.3PubMed Central. Mechanical, structural, and physiologic differences in human elastic and muscular arteries of different ages: Comparison of the descending thoracic aorta to the superficial femoral artery In short, elastic arteries are designed to absorb the shock of each heartbeat, while muscular arteries are designed to direct traffic.
What Artery Walls Are Made Of
Regardless of type, artery walls share a three-layer design. The innermost layer, the tunica intima, includes a sheet of endothelial cells lining the inside of the vessel. This lining is not just a passive barrier; it actively regulates blood clotting, inflammation, and how tightly the vessel contracts. Beneath the endothelium sits a subendothelial layer and an internal elastic membrane. The middle layer, the tunica media, is made up of smooth muscle cells surrounded by proteins they produce, and under normal conditions this layer lacks its own blood supply. The outermost layer, the tunica adventitia, is a mix of connective tissue, immune cells, tiny blood vessels that feed the artery wall itself, and nerve fibers.4PubMed Central. Pathogenesis of atherosclerosis in the tunica intima, media, and adventitia of coronary arteries: An updated review
The relative thickness of these layers changes depending on the artery. In large elastic arteries, the media is dominated by elastin. In muscular arteries, the media is thicker with smooth muscle. In arterioles, the wall is thin but the muscle-to-lumen ratio is high, which is what gives them their ability to dramatically change diameter and control local blood flow.
The Pulmonary Arteries Are the Exception That Proves the Rule
When people first learn that arteries carry blood away from the heart, they often assume that means arteries always carry oxygen-rich blood. That is mostly true, but the pulmonary arteries are a major exception. These vessels leave the right side of the heart and carry oxygen-depleted blood to the lungs, where it picks up fresh oxygen before returning to the left side of the heart through pulmonary veins. So the definition of an artery is based on direction of flow (away from the heart), not on oxygen content.
The pulmonary arteries operate under much lower pressure than the aorta and its branches, because the lungs are close to the heart and do not need a forceful push. Their walls are thinner and more compliant for the same reason. This is why pulmonary hypertension, a condition where pressure in these arteries climbs too high, can cause serious problems: the right side of the heart is not built to pump against high resistance the way the left side is.
Fetal Circulation and the Ductus Arteriosus
Before birth, the arterial layout is different in one important respect. A fetus does not breathe through its lungs, so there is little reason to send the full volume of blood through the pulmonary circulation. Instead, a short vessel called the ductus arteriosus connects the pulmonary artery directly to the aorta, allowing most of the blood to bypass the lungs entirely. This small vessel has outsized functional importance for maintaining normal fetal circulation.5PubMed Central. The fetal ductus arteriosus and its abnormalities–a review
Within hours to days after birth, the ductus arteriosus normally closes on its own as the newborn begins breathing and pulmonary blood flow increases. When it fails to close, the condition is called patent ductus arteriosus, and depending on its size, it can range from a harmless murmur to a defect that requires medical intervention. This is one of the more common congenital heart-related issues.
How Coronary Arteries Feed the Heart Itself
The heart is a muscle, and like any muscle, it needs its own blood supply. The coronary arteries branch off the aorta almost immediately after it exits the heart and wrap around the surface of the heart, diving inward to supply the thick cardiac muscle. What makes coronary arteries unusual is their relationship with the heartbeat itself. During systole, when the heart muscle contracts forcefully, it squeezes the coronary vessels embedded within it, which could in theory choke off their own blood supply.
In practice, most coronary filling happens during diastole, when the heart is relaxed between beats. Research measuring flow in the circumflex artery found that systolic contraction does not meaningfully limit diastolic coronary flow at heart rates below about 160 beats per minute.6PubMed. Systole has little effect on diastolic coronary artery blood flow This means that under normal conditions, the squeezing effect of each heartbeat is a non-issue, though at very high heart rates the window for diastolic filling narrows enough that it could become relevant.
The Circle of Willis and Brain Blood Supply
The brain receives blood through four major arteries: two internal carotid arteries at the front and two vertebral arteries at the back. These converge at the base of the brain into a ring-shaped arrangement called the circle of Willis. In theory, this ring provides backup routes so that if one feeding artery becomes blocked, blood can still reach every part of the brain through the remaining connections.
In reality, the circle of Willis is highly variable from person to person. Autopsy studies comparing functional and nonfunctional collateral arteries in this ring found that the functional ones had a median diameter of about 1.1 mm, while the nonfunctional ones were only about 0.5 mm.7Stroke / AHA/ASA Journals. Collateral configuration of the circle of Willis: transcranial color-coded duplex ultrasonography and comparison with postmortem anatomy Many people have incomplete circles, with one or more connecting segments too small to carry meaningful flow. This anatomical variation helps explain why some people tolerate a blocked carotid artery with few symptoms while others suffer a major stroke from the same type of blockage.
What Goes Wrong With Arteries
Because arteries carry blood under high pressure and are exposed to constant mechanical stress, they are vulnerable to several serious conditions. The most common is atherosclerosis, which involves a slow buildup of fatty plaques inside the artery wall. The process begins with dysfunction of the endothelial lining in areas where blood flow is disturbed, such as bends and branch points. When the endothelium stops functioning properly, it triggers changes in clotting, inflammation, and the chemical balance within the vessel wall.8PubMed Central. Endothelial Cell Dysfunction and the Pathobiology of Atherosclerosis
Over time, immune cells infiltrate the artery wall, lipids accumulate, and what starts as a thickened patch of intima gradually develops into a plaque with a core of dead cells and debris covered by a fibrous cap. As the plaque matures, that cap can thin and eventually rupture, triggering a blood clot that suddenly blocks the artery. This is the mechanism behind most heart attacks and many strokes.9PubMed. Pathophysiology of atherosclerosis plaque progression The process affects all three layers of the artery wall and involves chronic inflammation throughout.4PubMed Central. Pathogenesis of atherosclerosis in the tunica intima, media, and adventitia of coronary arteries: An updated review
Aneurysms and the Role of Elastin
While atherosclerosis narrows arteries, aneurysms involve the opposite problem: the artery wall weakens and balloons outward. Abdominal aortic aneurysms are the best-studied example and are characterized by a progressive widening of the aorta in the abdomen. The central event in most aneurysms is the breakdown of elastin, the protein that gives artery walls their ability to stretch and recoil. In human aneurysm tissue, researchers consistently find reduced elastin content, impaired cross-linking between elastin fibers, and extensive fiber fragmentation.10PubMed Central. Elastin in the Pathogenesis of Abdominal Aortic Aneurysm
Multiple destructive processes converge to make this happen. Enzymes called matrix metalloproteinases chew through the structural proteins of the wall, chronic inflammation draws immune cells that further damage the tissue, smooth muscle cells die off or change their behavior, and oxidative stress compounds the damage.11PubMed. Aortic Wall Degeneration in Aortic Aneurysms. Pathological and Molecular Insights Interestingly, mild aerobic exercise has been shown in animal models to reduce the activity of these destructive enzymes and slow elastin breakdown, suggesting that moderate physical activity may help protect the aortic wall.12PubMed. Mild aerobic exercise blocks elastin fiber fragmentation and aortic dilatation in a mouse model of Marfan syndrome associated aortic aneurysm
Arterial Stiffness and What It Means for Your Health
As you age, the elastic arteries near your heart gradually stiffen. Elastin fibers fray, collagen accumulates, and the vessel wall becomes less able to stretch and recoil with each heartbeat. The aorta normally remodels over a lifetime in an attempt to preserve its shock-absorbing Windkessel function, while muscular arteries farther downstream follow a different aging trajectory.3PubMed Central. Mechanical, structural, and physiologic differences in human elastic and muscular arteries of different ages: Comparison of the descending thoracic aorta to the superficial femoral artery When the aorta stiffens beyond a certain point, it loses its ability to buffer the pulsatile flow from the heart, which sends harder pressure waves into the organs downstream.
High blood pressure accelerates this process. Hypertension drives remodeling of the arterial wall through both mechanical stress and inflammatory signals, leading to changes in the composition and organization of the wall’s structural proteins. In larger arteries, this results in stiffening; in smaller resistance arteries, it leads to thickening that increases vascular resistance further.13PubMed Central. Mechanisms of Vascular Remodeling in Hypertension Both large and small arteries undergo structural and functional changes that are considered a hallmark of hypertensive disease.14PubMed Central. Vascular Smooth Muscle Remodeling in Conductive and Resistance Arteries in Hypertension
Arterial stiffness is now considered an established biomarker for cardiovascular risk. The standard way to measure it clinically is pulse wave velocity, which tracks how fast the pressure wave from each heartbeat travels along the aorta. A stiffer aorta transmits the wave faster. Carotid-femoral pulse wave velocity is regarded as the gold standard measurement because it directly reflects aortic stiffness and has the strongest track record for predicting cardiovascular outcomes.15PubMed. Expert consensus document on the measurement of aortic stiffness in daily practice using carotid-femoral pulse wave velocity Updated recommendations continue to refine how these measurements are validated for clinical use.16PubMed Central. 2024 Recommendations for Validation of Noninvasive Arterial Pulse Wave Velocity Measurement Devices
How Giraffe Arteries Handle Extreme Pressure
If you want to appreciate what arteries are capable of, giraffes are a fascinating case study. To push blood up their roughly two-meter necks to the brain, giraffes maintain a mean arterial blood pressure of about 200 mmHg, roughly double the pressure in a healthy human.17PubMed. The Remarkable Cardiovascular System of Giraffes That kind of pressure would wreck a human cardiovascular system, but giraffes have evolved a suite of arterial adaptations to handle it.
In their legs, where gravity creates additional pressure on top of what the heart generates, the arteries undergo dramatic structural changes. Just below the elbow, foreleg arteries abruptly thicken and narrow, creating a built-in pressure drop. Small arteries in the legs have a much higher wall-to-lumen ratio and more smooth muscle cells per unit length than arteries in the neck, allowing them to generate extraordinary contractile tension. Leg arteries contracted against pressures of about 500 mmHg in testing, compared to about 320 mmHg for neck arteries. On top of that, the tissue surrounding leg blood vessels has low compliance, meaning it resists swelling and helps contain the pressure.18PubMed. Protection against high intravascular pressure in giraffe legs
The brain end of the system has its own protections. Small cerebral arteries in giraffes display strong myogenic responses, meaning they automatically constrict when pressure rises, particularly at around 100 mmHg, while extracranial arteries respond at much higher pressures in the 200 to 250 mmHg range.19PubMed Central. Hemodynamics and Drinking in the Giraffe This tiered system of pressure regulation prevents the delicate capillaries in the brain and legs from being exposed to the full force of the giraffe’s powerful heartbeat.
Vascular Grafts and the Problem of Compliance Mismatch
When a section of artery becomes too diseased to function, surgeons can replace it with a synthetic graft or a vessel harvested from elsewhere in the body. One persistent challenge with synthetic grafts is that they do not stretch and recoil the way a natural artery does. This mismatch in elastic properties between the graft and the native artery wall causes turbulence and abnormal stress at the connection points, which can lead to complications including thickening of the vessel lining and eventual graft failure.20Journal of Biomechanics. Evaluation of a novel compliance-matching aortic graft in a swine model
Researchers are actively working on grafts designed to match the compliance of a healthy aorta, with the idea that if the graft moves like a real artery, the body will tolerate it better. Early animal studies are testing novel graft materials that aim to replicate the stretch-and-recoil behavior of elastic arteries, though no commercially available graft perfectly mimics native arterial mechanics yet. The problem illustrates how much the physical properties of arteries matter: it is not enough for a replacement vessel to simply hold blood. It needs to move with each heartbeat, or the body treats it as a foreign obstacle.