The heart and blood are major parts of the circulatory system. All components contribute to oxygen and nutrient delivery throughout your body.

Combined with the cardiovascular system, the circulatory system helps fight off disease, helps the body maintain a normal body temperature, and provides the right chemical balance for the body to achieve homeostasis, or a state of stability among all its systems.

The circulatory system consists of four major components:

  • heart
  • arteries
  • veins
  • blood

About the size of two adult hands held together, the heart rests near the center of the chest. Thanks to consistent pumping, the heart keeps the circulatory system working at all times.

Heart wall

There are three layers of the heart wall. The epicardium is the heart wall’s outer layer, the myocardium is the middle — and muscular — layer, and the endocardium is the heart’s innermost layer.


The heart has four chambers: the right and left atria, and the right and left ventricles. Together, they make up the heart’s internal cavity.

The four chambers play an important role in circulation. The atria receive blood from the veins, while the ventricles push blood out of the heart. Because the ventricles have to be much stronger to perform this pumping activity, their myocardial layers are thicker than those of the atria.

Arteries carry blood away from the heart.

The artery walls have three layers: tunica intima (inner), tunica media (middle), and tunica externa (outer).

The middle layer is usually the thickest. It’s made up of smooth muscle that changes the size of the artery to regulate blood flow.

There are three main types of arteries. They get smaller and smaller the further they are from the heart.

Elastic arteries

The aorta and pulmonary arteries are the elastic arteries. They receive blood directly from the heart and need to be elastic to accommodate the surge and contraction as blood pushes through with each heartbeat.

The aorta is the body’s most important artery.

Pulmonary arteries take deoxygenated blood from the right ventricle to the lungs. They’re the only arteries that carry deoxygenated blood.

Muscular arteries

The muscular arteries move blood from the elastic arteries through the body. They’re made of smooth muscle, which can expand and contract as blood flows. The femoral and coronary arteries are two examples of muscular arteries.


The smallest arteries are the arterioles, which move blood from the muscular arteries to the capillaries. The capillaries connect the arteries, which take blood from the heart, and the veins, which take blood to the heart.

The number of capillaries in a body system depends on the amount of material exchange. Skeletal muscle, the liver, and the kidney all have a large number of capillaries because their body systems need a lot of oxygen and nutrients. The cornea of the eye is one area that has no capillaries.

The blood moves back to the heart through veins.

The blood travels from the capillaries into the venules, which are the smallest veins. As the blood moves closer to the heart, the veins get larger and larger.

Like the arteries, veins have walls made up of layers called the tunica intima, tunica media, and tunica externa. There are some important differences between the arteries and veins:

  • In veins, the walls have less smooth muscle and connective tissue.
  • The walls of veins are thinner than artery walls.
  • Veins have less pressure and can hold more blood than arteries.

At any time, about 70 percent of the body’s total blood supply is in the veins.


The veins include valves, small pieces of tissue which keep blood flowing in the right direction.

The valves in the medium and large veins keep the blood flowing towards the heart. In the arms and legs, these valves make sure gravity doesn’t pull blood in the wrong direction.

There are four valves in the heart.

The tricuspid valve separates the right atrium from the right ventricle. The mitral valve, or bicuspid valve, separates the left atrium from the left ventricle.

The remaining two valves are the semilunar valves. The pulmonic valve, or pulmonary valve, separates the pulmonary artery from the right ventricle. The aortic valve separates the aorta and the left ventricle.

Blood is the transport medium of nearly everything within the body. It moves hormones, nutrients, oxygen, antibodies, and other important things needed to keep the body healthy.

The blood has four main components.


About 55 percent of the blood is plasma. Plasma is what makes blood liquid.

Plasma moves blood cells through the body by way of the circulatory system. It also carries hormones, nutrients, antibodies, and waste products.

Plasma is made up of:

  • water
  • salts
  • sugar
  • fat
  • protein

Red blood cells

Red blood cells, also called erythrocytes, make up about 40 to 45 percent of the blood’s volume. These cells have no nucleus, which means they can easily change shape as they move through the body’s arteries and veins.

Red blood cells contain a protein called hemoglobin. It carries oxygen from the lungs to the rest of the body and returns carbon dioxide to the lungs, where it’s exhaled.

White blood cells

White blood cells, also called leukocytes, make up just 1 percent of the blood. They protect the body from infection.

There are five major types of white blood cells.

Most white blood cells are neutrophils, which live for less than 1 day. Neutrophils are the body’s immediate response team.

Types of lymphocytes include B lymphocytes (B cells) and T lymphocytes (T cells). B lymphocytes make antibodies, while T lymphocytes regulate other immune cells and target infected cells and tumors.

The other major types are basophils, eosinophils, and monocytes.


Platelets, also called thrombocytes, are cell fragments.

Platelets are essential for blood clotting. They stick to an injured blood vessel lining to provide the basis for a clot. This stops bleeding and promotes healing.

Oxygen enters the bloodstream through tiny membranes in the lungs that absorb oxygen as it’s inhaled. As the body uses oxygen and processes nutrients, it creates carbon dioxide, which your lungs expel as you exhale.

The circulatory system works thanks to constant pressure from the heart and valves throughout the body. This pressure ensures that veins carry blood to the heart and arteries transport it away from the heart. (Hint: To remember which one does which, remember that that “artery” and “away” both begin with the letter A.)

There are three different types of circulation that occur regularly in the body:

  • Pulmonary circulation. This part of the cycle carries oxygen-depleted blood away from the heart, to the lungs, and back to the heart.
  • Systemic circulation. This is the part that carries oxygenated blood away from the heart and to other parts of the body.
  • Coronary circulation. This type of circulation provides the heart with oxygenated blood so it can function properly.
Did you know?

Arteries and veins may be classified as pulmonary, systemic, or coronary.

  • Pulmonary arteries take blood with low levels of oxygen from the right ventricle to the lungs.
  • Pulmonary veins move oxygen-rich blood from the lungs to the heart’s left atrium.
  • Systemic arteries take oxygen-rich blood from the left ventricle to the body’s tissues.
  • Systemic veins move blood with low levels of oxygen from the body’s tissues to the heart’s right atrium.
  • Coronary arteries take oxygen-rich blood from the aorta to the heart muscle.
  • Coronary veins move blood with low levels of oxygen from the heart’s myocardium (middle muscular layer) to its right atrium.

There are many symptoms of poor circulation, including:

The symptoms depend on the type of circulatory condition. As an example, peripheral arterial disease can cause leg and foot problems like:

  • leg cramping while walking or resting
  • cold feet or legs
  • change of leg color
  • change in toenail color or thickness
  • loss of hair on the legs and feet
  • ulcers (sores) that don’t heal on the legs and feet

There are several conditions that can affect the heart and circulatory system, including:

  • Peripheral arterial disease. In peripheral arterial disease, blood flow in the arteries of the legs is restricted. This is usually due to buildup of plaque in the arteries.
  • Arteriosclerosis. In arteriosclerosis, plaque buildup in the blood vessels becomes calcified and hard. The arteries are less flexible, leading to higher blood pressure, stroke, heart damage, and kidney damage.
  • Heart attack. During a heart attack, a blockage of blood flow to the heart muscle leads to death of heart muscle tissue. It’s also known as a myocardial infarction.
  • Angina. In angina, the heart muscle isn’t getting enough blood. This leads to crushing chest pain, fatigue, nausea, and shortness of breath.
  • Mitral valve conditions. In mitral valve prolapse, mitral valve stenosis, or mitral valve regurgitation, problems with the mitral valve cause oxygenated blood in the heart to flow backward, or blood flow to be slowed down or constricted.
  • Arrhythmias or dysrhythmias. These terms are both used to describe a heart rate that’s abnormal.
  • Ischemia. In ischemia, there’s not enough blood flow in the heart, and muscles don’t get enough oxygen.
  • Aortic disease. This group of conditions affects the aorta. One example is aortic aneurysm, where the aorta is weak and bulges out.