Stephen sandbox

The cardiovascular system comprises the heart, blood, and blood vessels, which are the tubes that carry blood. It moves oxygen, carbon dioxide, nutrients, hormones, and waste products throughout the body, ensuring that tissues receive what they need and that byproducts are removed. For massage therapy students, understanding this system provides insight into how circulation interacts with the beneficial effects of massage, the benign side effects of massage, and massage contraindications.

Structure and function of the heart

The heart pumps blood through the cardiovascular system. It is a muscular organ roughly the size of a fist, located in a space between the lungs and superior to the diaphragm: the mediastinum. Its internal structure is divided into four chambers that pump blood: two upper chamber called atria (singular: “atrium”) and two lower chambers called ventricles. Blood moves through these chambers in order:

1. The right atrium receives deoxygenated blood returning from the cells of the body and pumps it to the right ventricle.
2. After the right ventricle receives deoxygenated blood from the right atrium, it pumps it to the lungs for gas exchange.
3. The left atrium receives the recently oxygenated blood from the lungs and pumps it to the left ventricle.
4. After the left ventricle receives this oxygen-rich blood from the left atrium it then pumps the blood out to the tissues and cells of the rest of the body.

To keep blood flowing in one direction, the heart relies on a number of one-way valves. The atrioventricular (AV) valves prevent blood from being pushed back up into the atria from the ventricles. The right AV valve is called the tricuspid valve because it has three “cusps” (flaps). The left AV valve is called the bicuspid valve (“two flaps”), or mitral valve (because it it shaped like a religious figure’s hat called a “mitre”…these names were assigned during a time when the Catholic church was powerful even in scientific life). These AV valves are supported by connective tissue cords anchored to tiny mound-shaped muscles in the ventricles. This support prevents the valve cusps from being pushed back up into the atria, which would allow the backflow of blood. These connective tissue cords are called chordae tendineae and the muscles they are anchored to are called the papillary muscles. At the exits of the ventricles are the semilunar valves, which prevent backflow into the heart from the lungs (the pulmonary valve), and from the rest of the body via the aorta (the aortic valve).

Tissue layers of the heart

TK, including lumen

The heart is enclosed in the a protective sac that contains fluid to reduce friction as the heart beats: the pericardium (TK maybe move to tissues section).

How the heart beats

Students often incorrectly assume that each heartbeat is triggered by the brain. Though the brain can influence heart rate, the heart’s rhythmic contractions are generated by an intrinsic conduction system that includes specialized pace-making cells and the connections that carry their signals to the rest of the muscle cells in the heart. The sinoatrial (SA) node, located in the right atrium, serves as the heart’s natural pacemaker, generating impulses that initiate each heart beat (and effectively set the heart rate). These impulses first spread through the atria, causing them to contract. The SA node’s impulses also reach the atrioventricular (AV) node, which introduces a brief delay so the ventricles can fully fill before they contract. Once the ventricles have filled the AV node sends a “ventricles contract!” signal that moves through progressively finer pathways: starting in the thick bundle of His, which splits into the smaller right and left bundle branches, which each branch into even thinner Purkinje fibers that penetrate the ventricular muscle and deliver the AV node’s signal and trigger a coordinated ventricular contraction.

Each heartbeat – or cardiac cycle – includes two phases:

1. Contraction of the ventricles (“systole”), which propels blood away from the heart, to the lungs and other tissues of the body;
2. The relaxing and refilling of the all chambers of the heart (“diastole”). Late in diastole is when the atria contract to complete the filling of the ventricles.

There are two ways that you might already be familiar with your own cardiac cycle. First, you might recognize these words from when your blood pressure is assessed. Blood pressure is expressed as the higher systolic pressure over the lower diastolic pressure. Second, the familiar heart sounds, “lubb” and “dubb”, correspond to the closing of valves during the cardiac cycle: the first sound marks the closure of the AV valves at the start of ventricular contraction, and the second reflects the closing of semilunar valves as the ventricles relax.

Blood vessels connect heart to tissues

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 to restart the trip.

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.
3. Those elastic arteries eventually branch into smaller muscular 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 capilary beds is:

1. Capilaries 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 the heart and lungs

TK…didn’t name lungs…Pulmonary circulation starts in the right ventricle, sending blood through the pulmonary trunk to the lungs for gas exchange, then back through the pulmonary veins to the left atrium.

Circulation between the heart chambers and heart muscle

Coronary circulation serves the heart muscle itself through coronary arteries branching from the aorta. Blood drains from the myocardium through cardiac veins into the coronary sinus, which empties into the right atrium. Blockage in these vessels can lead to myocardial infarction.

Tissues layers of blood vessels

Most vessels share a similar structure of three layers, or tunics. The tunica interna is a smooth endothelial lining; the tunica media consists of smooth muscle and elastic fibers, which allow vessels to constrict or dilate; and the tunica externa provides connective tissue support.

TK more about capilaries

revised 2025-08-28 6:41:01 AM EDT