PMT Scribe Academy

Circulatory System

No organ quite signifies love like the heart. The circulatory system is an all-embracing network responsible for the movement of blood, nutrients, hormones, oxygen, and other gasses. Without it, the body cannot maintain homeostasis, which is the body’s way of managing a constant internal environment when dealing with external changes. For example, sustaining a consistent pH and a steady body temperature during a hot summer day.

Learn about the mechanism behind the “Lub-Dub” — how the heart first develops; how blood flows through the heart; and where the blood goes after it exits the heart.


The human embryo develops in a very particular order. Vasculogenesis, vas- meaning vessel and -genesis meaning development, takes place over several weeks before the heart resembles a four-chambered structure. The heart pumps from about four weeks after conception and never stops until a person dies.

The heart develops in four phases:

  • In the first phase, the heart looks like a tube. Rapid cellular growth then causes the tube to bend backward, beginning the formation of the familiar heart shape.
  • The second phase is the creation of two chambers.
  • In the third phase, the top two chambers of the heart, the atrium, separate and the bottom two chambers, the ventricles, are just beginning to separate.
  • Finally, the masterpiece; the ventricles completely separate. This four-chambered structure distinguishes the human heart from all other creatures. Before reaching this final phase, the embryo’s heart resembles other animals such as a fish, frog, turtle and snake.

The blood flow before birth is more complicated than after the baby is born. The placenta is doing the work that the lungs do after birth. Before birth, the fetus receives oxygen and nutrients through the placenta via the umbilical cord from the mother. The lungs are bypassed via an arterial trunk that originates from the ventricles, called the truncus arteriosus.

Two Divisions of The Circulatory System

The closed circulatory system is an active transport service for two types of fluid: blood and lymph. Collectively, the cardiovascular system and the lymphatic system make up the circulatory system.

  • Cardiovascular system – blood circulates through the heart and consists of red blood cells (RBCs), white blood cells (WBCs), platelets, and plasma.
  • Lymphatic system – lymph, a helper to the immune system, is basically filtered blood plasma from the fluid between the cells, known as the interstitial fluid.

Mechanisms of the Cardiovascular System


The hollow fist-size organ is a muscular organ that contracts to force blood through the body. Even with all the enormous workload of the heart, the adult heart weighs between 8-12 ounces.

It sits behind the breastbone, between the lungs, and in the center of the chest. The fully developed heart rests in a compartment called the pericardial cavity that is protected by the surrounding ribcage and the diaphragm. The pericardium, a membrane-like sac, secretes fluid to lessen friction as the heart beats.

The average heart rate changes through the human lifespan:

  • Infant – 120 beats per minute.
  • Child – 90 beats per minute.
  • Adolescent and Adults – 60–100 beats per minute.

The heart wall is composed of three layers:

  1. Epicardium - outer protective layer of the heart. The epicardium protects the inner heart layers and assists in the production of pericardial fluid.
  2. Myocardium - muscular middle layer wall of the heart, which enables the heart to contract.
  3. Endocardium - inner layer of the heart that lines the chambers and covers the heart valves.

The heart is a part of the autonomic nervous system and cannot generate its own electrical rhythm. Electrical signals instruct the ventricles when to contract by impulses passed down to the Purkinje fibers, modified muscle cells collectively known as the Bundle of His.

Patterns of Circulation

Blood continuously moves through the body. It’s important to remember that the left side pumps oxygenated blood from the lungs to the body, and the right side pumps blood to the lungs from the body. The septum with vertically divides the left and the right sides of the heart—prevents mixing of oxygen-rich blood and deoxygenated blood.

  • Left and right sides of the heart divide into four chambers.

  • The left side has two chambers, and the right side has two chambers; each side is subdivided into an upper and a lower chamber. The upper chamber is referred to as the atrium while the lower chamber is known as the ventricle.

  • Right atrium sits on top of the right ventricle on the right side of the septum.
  • Left atrium sits on top of the left ventricle on the left side of the septum.
  • The right side of the heart, right atrium and right ventricle, pumps deoxygenated blood to the lungs, where the blood picks up oxygen. This oxygen-rich blood returns to the left side of the heart, the left atrium and left ventricle.

Systemic Circulation

  • Carries oxygenated blood throughout the body and returns deoxygenated blood to the heart through the right atrium.

  • A full systemic cycle is when the blood has traveled from the left ventricle, through the body, and then to the right atrium.
  • Left ventricle → right atrium is a full systemic cycle.

Three components of systemic circulation:

Coronary circulation – the supply of blood to the heart muscle through the coronary artery. The heart itself receives its blood supply from the two coronary arteries leading from the aorta.

  • Coronary arteries → heart → capillaries → veins → right atrium

Renal circulation – supply of the blood to the kidneys through the renal artery; nearly 25% of the blood that exits the heart pumps to the kidneys, which are filters for waste.

Hepatic portal circulation – nutrients from capillaries in the small intestines are carried directly to the liver through the hepatic portal vein—this is approximately 70% of the blood supply to the liver. The other 30% is oxygenated blood from the hepatic artery, which branches off the aorta. Blood leaves the liver via the hepatic vein.

Pulmonary Circulation

Another term for pulmonary circulation is bronchial circulation—it supplies blood to the tissue of the larger airways of the lung.

When a person takes a breath air comes in the nose or mouth then travels down the trachea, also known as the windpipe. The air then passes through the bronchial tubes, or airways, into the lungs for gas exchange, or diffusion, to take place—carbon dioxide is expired while oxygen is absorbed.

  • Deoxygenated blood pumps from the right ventricle → lungs via the pulmonary arteries, the only arteries that transport deoxygenated blood.

  • Oxygenated blood then returns to the left atrium through the pulmonary veins. These are the only veins to carry oxygen-rich blood.

Complete blood flow cycle through the heart, body, and lungs:

Left ventricle → body (systemic circulation) → right atrium→ right ventricle → lungs (pulmonary circulation) → left atrium → left ventricle → body

Cardiac Cycle

In its simplest form, the cardiac cycle is one heartbeat. It is the synchronized contraction of the two atria, followed by the synchronized contraction of the two ventricles—”Lub-dub”.

Two phases of the cardiac cycle:

  • Phase 1 – Systole, or contraction, occurs when the ventricles contract.
  • Phase 2 – Diastole, or relaxation, occurs when the ventricles relax.

Blood passes through valves that act as one-way inlets for blood flowing into a ventricle and one-way outlets for the blood leaving a ventricle. The Four valves of the heart open and close at precisely synchronized times during one heartbeat.

The four valves include:

  • Tricuspid valve – between the right atrium and right ventricle.
  • Pulmonic Valve – between the right ventricle and the pulmonary artery.
  • Mitral valve – between the left atrium and left ventricle.
  • Aortic valve – between the left ventricle and the aorta.

The valves open and shut, as the heart muscle contracts and relaxes—this lets the blood flow into the ventricles and atria at alternate times.

The order of circulation through the valves is as follows:

  • Tricuspid valve – Closes off the right atrium, known as the upper right chamber that holds blood arriving from the body. Then it opens to allow blood to flow from the right atrium to the lower right ventricle.
  • Pulmonic Valve – Closes off the lower right ventricle, which is the lower right chamber. Then opens to allow blood to be pump from the heart to the lungs through the pulmonary artery to get oxygenated.
  • Mitral valve – Closes off the left atrium or upper left chamber to collect the oxygenated blood coming in from the lungs. Then it opens to allow blood to flow from the top left atrium to the lower left ventricle.
  • Aortic valve – Closes off the lower left ventricle that holds the oxygenated blood before it is pumped out to the body. The aortic valve opens to allow blood to leave the left ventricle through the aorta and on to the body.

Blood Vessels

Blood stays contained in the closed circulatory system and flows in one direction. The path doesn’t change:

Blood vessels, except capillaries, have walls made of three layers.

  • Inner layer – epithelial tissue.
  • Middle layer – smooth muscle.
  • Outer layer – connective tissue.

Arteries and Arterioles

Arteries transport oxygenated blood from the heart to the capillaries within an organ. The thick walls, elastic fibers, and smooth muscle enable them to sustain the high pressure of blood pumped from the heart. The force exerted on the walls is known as blood pressure.

A secondary circulation occurs to reduce the workload of the heart. Each artery has elastic recoil of its fibers that cause it to spring back after blood passes through.

The aorta has the largest diameter and carries oxygenated blood from the left ventricle to the body, except for the lungs. The smallest are called arterioles. Arterioles dilate and constrict to alter the flow of blood through the organs. Examples include muscles when running (more blood flow) or the guts between meals (less blood flow).


Arterioles branch into networks of capillaries, tiny and narrow vessels. These have thin walls and a large surface area to aid in diffusion, which consists of the exchange of oxygen and nutrients between the tissues and the blood. The narrowing of capillaries slows the movement of the blood to allow more time for diffusion.


After leaving the capillaries, deoxygenated blood feeds into very small veins called venules. The veins, in turn, transport the blood to the heart.

Similar to arteries, the walls of veins contain smooth muscle; however, veins are more flexible because the walls are thinner and less elastic.

Larger veins contain valves to maintain the flow of blood—this is important when blood flows against the force of gravity. Skeletal muscles also help force blood back to the heart by squeezing against the veins. When muscles contract they squeeze against the veins and help to propel the blood back towards the heart—this is also considered a secondary circulation.

Blood flow through the heart and vessels of the cardiovascular system:

Ventricles (heart) → arteries → arterioles → capillaries (organ) → veins → atrium (heart)

Physiology Facts

  • The kidneys help adjust blood pressure by removing water and salt from the body.
  • The carbon dioxide level is monitored by the carotid artery.
  • When blood pressure is too high, a person is at risk for thrombosis, a blood clot.
  • Blood pressure that is too low puts a person at risk for syncope, or fainting.
  • Blood pressure readings are two numbers—the systolic, the higher number and the diastolic, the lower number. The systolic figure is not as concerning as the diastolic.
  • The lymph system does not have a pump and relies on skeletal muscles to contract and move it through the vessels and lymph nodes.
  • Lymph nodes produce one type of WBC called the lymphocyte, which helps fight infection.
  • The tonsils, appendix, spleen, and thymus gland are a part of the lymph system.
  • An adult has 4–6 liters of blood.
  • Other than oxygen, nutrients, and gases, the blood transfers heat to the surface of the body.
  • The platelets in the blood play a significant role in blood clotting.


Fetal Circulation:

Circulatory System Diagram:

Pulmonary Circulation:

Hepatic Circulation:

Blood Pressure:

Systolic and Diastolic Blood Pressure Readings:

American Heart Association:

Heart, How it Works:

Your Living Blood:

Signaling Pathways in the Specification of Arteries and Vein:

Differences in the Development of Coronary Arteries and Veins:

Written by and last updated Oct 17, 2017

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