The Cardiovascular System Explained – Part 1

Oct 17, 2014 |

The cardiovascular system is comprised of the heart, blood vessels, and approximately 5 liters of blood that they circulate within the body. Powered by the hard-working organ known as the heart, which is more or less the size of your fist, the circulatory system’s main responsibility is for getting oxygen, nutrients, hormones, and cellular waste throughout the body. The average heart can pumps over 5 liters of blood every minute, just while at rest.

Anatomy of the Cardiovascular System

The Heart
This strong pumping organ is located between the lungs at the midline of the body. The lower portion of the heart, called the “apex”, turns to the left, allowing approximately two-thirds of the heart to be positioned on the left side with the other third on right side. The upper portion of the heart, referred to as the “base”, connects the aorta, pulmonary trunk, vena cava and pulmonary veins.

Pulmonary & Systemic Circulation
The human body has two main circulatory loops, specifically: the systemic circulation loop and the pulmonary circulation loop.

  • Pulmonary circulation is responsible for carrying de-oxygenated blood from the right atrium and ventricle of the heart and transporting it to the lungs, where the blood is oxygenated and returned to the left half of the heart.
  • Systemic circulation transports the oxygenated blood from the left atrium and ventricle to the rest of the entire body, not including the heart and lungs. Systemic circulation clears waste products from the body’s tissues while simultaneously returning de-oxygenated blood back to the right portion of the heart.

Blood Vessels
Blood vessels are the body’s thoroughfares which enable blood to course freely from the heart, throughout the body, and back again. The larger the blood vessel, the greater amount of blood that is able to flow through it. All blood vessels in the body are comprised of a hollow area, the lumen, where the blood flows through. The lumen is surrounded by the wall of the vessel, which is sometimes very thick (such as with arteries), and other times quite thin (as with capillaries).

Each and every blood vessel is lined with a fine layer of simple squamous epithelium called the endothelium that contains blood cells within the blood vessels and also keeps clots from forming. The entire circulatory system is lined with his layer of endothelium, including the interior of the heart, where it is known as the endocardium.

There are five primary categories of blood vessels: arteries, arterioles, capillaries, veins and venules.

  • Arteries: These blood vessels transport highly oxygenated blood away from the heart, having only just left the lungs on its way to the rest of the body. The only exception to this is the pulmonary trunk and arteries of the pulmonary circulation loop, since they transport de-oxygenated blood away from the heart to the lungs for oxygenation. The walls of these arteries are necessarily thicker and more pliable than other vessels due to the large level of pressure amassed as blood is pushed away from the heart under great force. The smooth muscle walls of the smaller arteries are able to expand or contract in order to regulate blood flow through their lumen, enabling the body to control the amount of blood flowing to various parts of the body under differing circumstances. Blood pressure is also affected by the regulation of blood flow since smaller arteries have less area for blood to flow through which increases the pressure of the blood on the walls of the arteries.
  • Arterioles: Arterioles are smaller arteries that transition from the ends of arteries the capillaries. Due to their lower blood volume, distance from the heart and overall larger numbers, they exhibit considerably lower blood pressures than arteries. Because of this, arteriole walls are much finer than arterial walls.  Similar to arteries, they are also able to use smooth muscle to constrict or dilate to regulate blood flow and pressure.
  • Capillaries: The capillaries are not only the smallest blood vessels in the body, but the most numerous as well, being located throughout virtually every tissue in the body. Capillaries act as a go-between the arterial and venus systems, connecting arterioles at one end and venules on the other. By transporting blood in close proximity to tissue cells, they assist with the exchange of gases, nutrients, and waste products. The capillary walls are composed of a fine layer of endothelium, creating as minimal an amount of substance as possible between the blood and the tissues. This endothelial layer functions as a filter to prevent blood cells from escaping the vessels while simultaneously allowing liquids, dissolved gases and other substances to move into or out of the tissues. Pre-capillary sphincters are found at the arteriole ends of capillaries and regulate blood flow into the capillaries. Depending on the energy and oxygen requirements of the particular tissues, these precapillary sphincters control blood flow by reducing it to less active tissues and increasing it into more active tissues.
  • Venules: Like the arterioles, venules are small vessels that connect to capillaries, but stemming into veins rather than arteries. Venules move blood from a large number of capillaries and transfer to larger veins where it is carried back to the heart.
  • Veins: Veins are the counterpart vessels to arteries and function to return blood back to the heart. Veins and venules exhibit very low blood pressures compared to the arteries, since they absorb the majority of force from the heart’s contractions. Due to this reduced pressure, the walls of the veins are much thinner and less elastic compared to the walls of  the arteries. Since veins must employ a combination of gravity, inertia, and  skeletal muscle contractions to help move blood back to the heart, some veins posess one-way valves that keep blood from moving in the opposite direction from the heart. When muscles in the body contract, they exert force against nearby veins and push the blood through valves toward the heart.When the muscles relax, these valves trap blood until another contraction is able to force it in the direction of the heart.

Digestive & Hepatic Portal Circulation
The veins of the  digestive organs perform a special task: rather than transporting blood directly back to the heart, they move blood to the liver via the hepatic portal vein. The blood that leaves the stomach and intestines is rich in nutrients and other substances absorbed from food. The liver functions to remove toxins, store sugars, and process digestive by-products before they reach the rest of the body’s tissues. Blood then enters the inferior vena cava from the liver and returns to the heart.

Coronary Circulation
The heart possesses its own group of blood vessels which provide it with oxygen and nutrients necessary to perform the task of pumping blood throughout the body. Branching off from the aorta are the left and right coronary arteries which supply blood to the left and right sides of the heart. Returning de-oxygenated blood from the heart to the vena cava is the coronary sinus, which is a vein located on the posterior side of the heart.

The typical human body contains around 4-5 liters of blood. Being a liquid connective tissue, blood is comprised of red and white blood cells, platelets, and liquid plasma, and transports numerous substances throughout the body, helping to maintain the homeostasis of nutrients, wastes and gases.

  • Red Blood Cells: Also referred to as erythrocytes, red blood cells are by far the most numerous type of blood cells, comprising about 45% of blood volume. Erythrocytes are created from stem cells inside of red bone marrow, amazingly producing around 2 million cells per second. The bi-concave shape of erythrocytes gives them a high surface area to volume ratio, allowing them to fold and squeeze into the tiny capillaries. The nucleus of the erythrocyte is ejected from the cell when it reaches maturity in order to achieve it’s unique shape and flexibility. Since red blood cells do not contain DNA they are not able to repair themselves when damaged. Erythrocytes also distribute oxygen in the blood through a red pigment called hemoglobin. Hemoglobin is comprised of both proteins and iron which greatly increase their oxygen carrying capability when combined.
  • White Blood Cells: White blood cells, or leukocytes, have crucial roles in the body’s immune system, even though they only encompass a very small portion of the total blood volume. White blood cells can be divided into two major groups: granular leukocytes and agranular leukocytes.
    1. Granular Leukocytes: There are three classifications of granular leukocytes: eosinophils, neutrophils, and basophils. Each of these types can be differentiated by substances in their cytoplasm that define their function. Eosinophils also contain digestive enzymes, specifically for ingesting viruses bound by antibodies located in the bloodstream. Neutrophils possess digestive enzymes that protect the body against invading bacteria. Basophils deliver histamines which heighten allergic reactions and aid in neutralizing the threat of parasites.
    2. Agranular Leukocytes: There are two basic classifications of agranular leukocytes, lymphocytes and monocytes. Lymphocytes are comprised of T cells and NK (natural killer) cells which neutralize viral infections, and B cells that create antibodies to prevent infections by pathogens. Monocytes change into cells called macrophages that surround and consume pathogens and also dead cells resulting from infection or injury.
  • Platelets : Also called thrombocytes, platelets are tiny cell fragments that play a key role in blood clotting and scab formation. Platelets are formed in the red bone marrow from the sizable megakaryocyte cells. These cells occasionally burst, in the process releasing thousands of bits of membrane that form into platelets. Having no nucleus, thrombocytes will survive for up to a week in the body before macrophages engulf and ingest them.
  • Plasma: The majority of blood volume, approximately 55%, is comprised of plasma. Made of around 90% water, plasma also contains proteins and broken-down substances, including glucose, oxygen, carbon dioxide, electrolytes, nutrients and cellular waste products, which are transported throughout the body. The proteins contained within the plasma include albumins and antibodies. Albumins aid in maintaining osmotic balance in the body by supplying an isotonic solution for cells. Antibodies belong to the immune system whose function is to attach to antigens on the outside of pathogens that cause infection.

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