Blood vessels

Revised 2025-08-29 8:59:43 PM EDT

Blood vessels connect heart chambers to all of body’s cells and tissues, including the lungs and the heart muscle itself. Blood travels through a closed loop of interconnected blood vessels in order to exchange nutrients and wastes at the level of capillaries – our smallest blood vessels – and then return to the heart chambers to restart the trip.

Circulation between heart chambers and the body’s tissues

Oxygen-rich blood leaving the heart to go to the body’s tissues is often called arterial blood because it will first flow through arteries before it reaches the tissues it is destined for. The flow of oxygen-rich blood as it leaves the heart filled with oxygen and nutrients is:

1. The left ventricle contracts, expelling its oxygen-rich blood into our largest artery: the aorta. (The aorta is an elastic artery: it stretches during systole and recoils during diastole.)
2. The aorta branches into smaller elastic arteries (also called conduction arteries).
3. Those elastic arteries eventually branch into smaller muscular arteries (also called distribution arteries) that are capable of either constricting to deliver less blood to a region of the body (“vasoconstriction”), or dilating to deliver more blood to a region of the body (“vasodilation”).
4. Those muscular arteries eventually branch into even smaller vessels called “arterioles”, which have special valves that control the flow of blood into capillaries (“precapillary sphincters”).

Once blood is in the capillaries the capillaries act like a transfer station: oxygen and nutrients diffuse from the blood to the cells that surround the capillary bed, and waste products from the surrounding cells diffuse into the capillaries. (“Capillary bed” is just a way of describing the dense network of capillaries that are found in most tissues.)

The blood leaving the capillaries to head back to the heart is then of course oxygen-poor (“deoxygenated”) and is called “venous blood” because it will flow through veins on the way back to the heart. The flow of venous blood as it leaves the capillary beds is:

1. Capillaries merge into larger venules, which receive deoxygenated blood from the capillaries.
2. These venules merge in order to form small veins, which are larger than the capillaries feeding them.
3. Small veins merge and form large veins.
4. Large veins empty into our two largest veins: the inferior vena cava (receiving deoxygenated blood that came from tissues more or less superior to the diaphragm) and the superior vena cava (receiving deoxygenated blood that came from tissues more or less inferior to the diaphragm).
5. The superior and inferior vena cava dump the deoxygenated blood into the right atrium.

Capillaries are the exchange sites for gases and nutrients. They have only a thin endothelial layer, allowing diffusion between blood and tissue fluid. Fluid that escapes from capillaries into interstitial spaces is eventually returned to circulation by the lymphatic system.

On the return trip, veins carry blood back to the heart at lower pressure. They contain one-way valves that prevent backflow, especially in the limbs. Two key mechanisms assist venous return: the skeletal muscle pump, in which muscle contractions compress veins and push blood forward, and the respiratory pump, where breathing movements alter pressure in the thoracic cavity to draw blood toward the heart.

The circulatory route described above is called “systemic circulation”: the left ventricle pumps blood into the aorta for distribution throughout the body, returning via the superior and inferior vena cava and the coronary sinus to the right atrium.

Circulation between heart chambers and lungs

Bringing oxygen to the body’s cells is one of the primary functions of blood. In order to do so the blood needs to pick up inhaled oxygen from the lungs and also deliver waste carbon dioxide to the lungs for exhaling. To accomplish this there is a dedicated route for blood to move back and forth between the heart and lungs. It starts with the right ventricle expelling deoxygenated blood through a single large blood vessel – the pulmonary trunk – which then splits into the left and right pulmonary arteries. Each pulmonary artery delivers deoxygenated blood to its lung for gas exchange. Freshly oxygenated blood then leaves the lungs and travels to the left atrium of the heart through the pulmonary veins. This circulatory route between the heart and lungs is called pulmonary circulation.

Sidenote

Naming arteries and veins

By definition, arterial blood flow (the aorta and other arteries, as well as smaller arterioles) is blood moving away from the heart, while venous blood flow (venules, veins, the superior and inferior vena cava) is blood moving toward the heart.

Initially this can feel confusing because the terms “artery” and “vein” describe the direction of blood flow, not whether the blood is oxygen-rich or oxygen-poor. Throughout most of the body (systemic circulation and coronary circulation) arteries happen to be carrying oxygenated blood and veins deoxygenated blood. However the exception to that pattern is that the pulmonary veins carry oxygenated blood back toward the left atrium.

Circulation between heart chambers and heart muscle

It would be convenient if heart muscle (the myocardium) received oxygen and nutrients directly from the blood already in the heart’s chambers, but it does not. Instead there are blood vessels dedicated to delivering oxygenated blood to the heart muscle and removing waste products. This circulatory route is called coronary circulation.

Sidenote

Where "coronary" comes from

The term coronary comes from the Latin word corona, meaning “crown,” which reflects the way these vessels encircle the heart like a crown on a head.

Coronary circulation begins with the right and left coronary arteries, which branch off from the base of the aorta just after it leaves the left ventricle. These arteries and their smaller branches penetrate the myocardium, delivering oxygen and nutrients to the heart muscle cells. After exchanging gases and nutrients, the deoxygenated blood is collected by cardiac veins, which drain into the coronary sinus and ultimately empty into the right atrium.

Because the myocardium is so metabolically active, coronary circulation is critical: even brief interruptions can cause chest pain (angina) or tissue death (myocardial infarction).

Tissues layers of blood vessels

Most vessels share a common structure of three layers, or tunics:

Definitions

Tunica interna

The tunica interna is the deepest layer. It is a smooth endothelial lining of the space that blood travels through (the lumen).

Tunica media

The tunica media is the middle layer. It consists of smooth muscle and elastic fibers, which allow vessels to constrict or dilate.

Tunica externa

The tunica externa is the most superficial layer. It provides connective tissue support.

Capillaries, however, are an exception to this pattern: they are composed of only a single layer of endothelial cells resting on a thin basement membrane. This extremely thin construction minimizes the barrier to diffusion of plasma, which is what makes capillaries the primary sites of exchange between blood and tissues. Depending on their type, capillaries may have tight junctions, small pores (fenestrations), or even large gaps (sinusoids) in their walls, which determine how easily different substances can pass through.

Notable blood vessels

The MBLEx authors may expect you to be familiar a number of blood vessels because their accessibility requires that we consider them to be endangerment sites for which we are cautious about pressure:

Definitions

Carotid arteries

Located in the neck, alongside the trachea and sternocleidomastoid; major suppliers of blood to the brain, vulnerable to compression.

Jugular veins

Superficial and deep veins of the neck that return blood from the head; easily compressed and sensitive to pressure.

Brachial artery

Found in the upper arm between the biceps and triceps; commonly used for blood pressure measurement, vulnerable at the medial arm.

Radial and ulnar arteries

At the wrist, supplying blood to the hand; radial pulse is easily palpated at the lateral wrist.

Femoral artery

Located in the groin region; a major supplier of blood to the lower limb and highly vulnerable to deep pressure.

Popliteal artery

Behind the knee; relatively superficial and important for lower leg circulation.

Abdominal aorta

Runs along the midline of the abdomen; deep but sensitive to excessive abdominal pressure.

Subclavian artery and vein

Beneath the clavicle; carry blood to and from the upper limb, vulnerable near the thoracic outlet.

Temporal artery

Just in front of the ear and across the temple; superficial and sensitive to pressure.

Pulse points

The MBLEx authors may also require you to match the name of arterial pulse points with a description of their location.

The higher pressure of arterial blood often allows us feel the rhythm and intensity of blood flow by palpating arterial pulses. During bodywork you might notice change in intensity of a pulse point as a way of assessing whether more blood is flowing through an area following a series of massage strokes. Be able to match the names of these pulse points with descriptions of where to palpate them:

Definitions

Abdominal aorta

Above the umbilicus (belly button) and slightly left of midline.

Brachial artery

On the medial upper arm, between biceps and triceps, near the elbow crease.

Common carotid artery

Medial to the sternocleidomastoid, at the level of the hyoid bone.

Dorsalis pedis artery

Between the first and second metatarsals on the dorsum of the foot.

Facial artery

At the inferior mandible, along the anterior edge of the masseter.

Femoral artery

Inferior to the inguinal ligament, midway between the ASIS and the pubic symphysis.

Popliteal artery

In the posterior knee’s popliteal fossa.

Posterior tibial artery

Inferior and posterior to the medial malleolus (inside of the ankle).

Radial artery

On the thumb side of the anterior wrist, just proximal to the base of the thumb.

Superficial temporal artery

Anterior to the ear, along the zygomatic arch.

Ulnar artery

On the anterior wrist, proximal to the pisiform bone (pinky side of the wrist).