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Category: Medical

Topic: Basic Anatomy and Physiology

Level: EMT

Next Unit: The Respiratory System

14 minute read

The Circulatory System

The circulatory system serves two vital roles in the body; the delivery of oxygen and nutrients to cells and the removal of waste and de-oxygenated blood from the tissues. Oxygen delivery is tightly regulated via control of the rate of blood flow and regulation gas exchange at the level of the lungs and bodily tissues. The removal of waste is facilitated by the return of blood through the venous system back to the right side of the heart.

The heart is the pump of the circulatory system and is about the size of a fist. The heart is composed of 4 chambers and has two sets of coronary arteries. Specialized muscles called cardiac muscle (or the myocardium) make up the bulk of the heart and have a special architecture that interconnects them and allows them to pump in an efficient pattern.

This pattern makes each side of the heart a two-stage pump for blood.

  1. The first two chambers on each side are known as the atria, they serve as low-pressure reservoirs that fill during the period known as diastole (heart muscle relaxation). At the beginning of systole (contraction), the atria gently contract and guide blood into the ventricles.
  2. The much stronger ventricle on each side then contracts and forces blood into the lungs and circulation.

This simple pattern of pumping is made possible by a complex set of nerves, valves, and septal structures.

  • NERVES: the heart has a "pacemaker" called the Sino-atrial (SA) node that sets the heart rate, The Atrio-Ventricular (AV) node delays conduction to the ventricles and ensures orderly and--more importantly--sequential--contraction. Finally, the bundle of His run through the ventricles to ensure the impulse to contract takes an orderly path.
  • VALVES: There are two pairs of heart valves; the atrioventricular (AV) valves and the semilunar valves. Both pairs exist to prevent blood flowing backward when the heart contracts and refills, respectively.
  • SEPTA (singular, septum): some of the simplest structures in the heart, the Inter-arterial, and Inter-ventricular septa separate the right/left atria and right/left ventricles respectively. The Atrioventricular septa separate the atria from the ventricles.



Arterial Side of the Circulatory System

The arterial side of the circulatory system delivers oxygenated blood from the lungs to body tissue and organs. The arterial system includes the aorta which bifurcates (branches off) into smaller arteries, which also bifurcate and become the even smaller arterioles.

  • The AORTA is the main artery which connects the left ventricle to the rest of the body.
  • The PULMONARY ARTERY connects the right ventricle to the lungs for blood re-oxygenation and is the only artery which transports de-oxygenated blood.
  • The carotid arteries travel up each side of the neck and are the main arteries that supply blood to the brain.
  • The femoral artery can be palpated on the inner thigh, near the groin, and supplies blood to the lower abdomen, legs, and genitalia.
  • The radial artery is the major artery in the forearm and can be palpated on the side of the wrist, next to the thumb.


Venous Side of the Circulatory System

The venous system carries de-oxygenated blood back to the heart and lungs. The venous system begins at the tissue and organ level, the vessels there called VENULES, formed when several capillaries come together. These small vessels then flow into VEINS proper, which feed into the VENA CAVA that flows blood to the right atrium of the heart.

The vena cava is divided into the superior and inferior vena cava: the superior vena cava collects returning blood from the head and upper arms; and the inferior vena cava collects returning blood from the rest of the body below the heart.

The pulmonary veins are the four veins that receive oxygenated blood from the lungs to be sent to the left atrium of the heart. This makes them the only veins that carry oxygenated blood! (Except for the Umbilical Vein in the fetus, but that is another story.) However, they still follow the generalization that veins flow TO the heart and arteries AWAY from the heart.



Blood is primarily composed of red blood cells, white blood cells, and plasma. 

  • RED BLOOD CELLS (RBCs): The red blood cells contain a protein known as hemoglobin that allows them to carry large amounts of oxygen from the lungs to the tissues of the body. These cells are also vital to the process of carrying waste carbon dioxide from the cells of the body to the lungs for removal. 
  • WHITE BLOOD CELLS (WBCs): The white blood cells (leukocytes) are immunological cells that take part in the body's defense against foreign organisms.   
  • PLASMA is the liquid component of the blood that contains a large variety of vital proteins interspersed within. Some of the essential proteins are
    • albumin, which acts to maintain water balance between the tissue and blood;
    • clotting proteins, which ensure that excessive blood is not lost when vessels are damaged; and
    • immunoglobulins, which protect the body from infection.



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