The blood flow in cardiovascular system - Biofluid Mechanics

The human body is made up of billions of cells, which are the elementary units like the bricks of a building, endowed with their own life but united in many aggregations (the tissues), which are gradually more complex to form the whole of the human organism.

To live, and the cells continually need oxygen (O2) and nutrients. At the same time, to maintain their vitality and carry out their activities, they need to be freed from the waste they produce and, in particular, from carbon dioxide (CO2). This is done by the blood that flows through the body's circulatory system due to the pumping action of the heart.

The cardiovascular system is a complex system consisting of the heart and a series of conduits, the blood vessels. The heart, rhythmically contracting, pumps the blood into the blood vessels which, spreading throughout the body, distribute it to all tissues and organs. In this way, a continuous round-trip of the blood is carried out, which brings the substances for the nourishment (in particular Oxygen = O2) of the cells and removes the waste products (in particular Carbon Dioxide = CO2). 

The blood releases CO2 and is enriched with O2 when passing through the lungs. The lungs replenish themselves with O2 from the air and eliminate CO2 in the air. The easiest way to imagine the cardiovascular system is to compare it to a machine consisting of a muscle pump (the heart) and a system of connected tubes (blood vessels) that start from the heart (the aorta and the other large arteries). They progressively divide into smaller and smaller ducts (medium-sized arteries and arterioles) up to a very thin-walled tube (the capillaries through which the gases and nutrients pass through the walls).

The Heart

It is a fundamental organ of the human body. It constitutes the most important part of the cardiovascular system and acts as a pump. Through the arteries, a very branched network of blood vessels that branches out throughout the body from the heart, it supplies all organs, tissues, and cells with the energy necessary for life in the form of blood rich in oxygen and nutrients.

The heart is a muscle pump that contracts constantly and rhythmically to send blood to the lungs and throughout the body. It works tirelessly as a suction pump that collects non-oxygenated blood from the whole body and as a pressing pump that pushes the blood into the lungs to be oxygenated and then into the arteries to be distributed to the whole organism.

This powerful muscle contracts in response to electrical signals produced by its electrical system. It pumps 5 to 6 liters of blood in one minute at rest, but more than 20 liters per minute during exercise.

In daily life, the heart automatically adapts to different needs. When the body requires more nourishment and energy (for example, when carrying out a physical activity such as climbing stairs or running), the heart responds accordingly. It beats more quickly and energetically, to circulate a greater quantity of blood, bringing more oxygen and nutrition to the various muscles and organs.

In order to contract, the heart muscle needs a constant supply of oxygenated blood that is supplied by the coronary arteries.

It is located in the thorax, behind the lower part of the sternum, slightly shifted to the left between the two lungs and the diaphragm. It has a vaguely conical shape, with the base at the top and to the right, slightly inclined backward. The volume of an adult heart is similar to that of a clenched hand. In an average individual, it measures approximately 13 centimeters long by 8 centimeters wide and weighs less than 500 grams.

The heart is wrapped in a protective bag called the pericardium. The most superficial part of the heart is called the epicardium and is a thin membrane that covers its surface. Under the epicardium, there is a thick muscle layer, the myocardium, which contracts rhythmically by acting as a pump. The inside of the heart is covered with a glossy membrane called the endocardium, which covers the internal walls and heart valves and continues with the internal lining of the arteries and veins (endothelium).

The heart is a hollow muscle that contracts and relaxes rhythmically as a pump and pushes the blood from the center to the periphery of the organism.

It is divided into four cavities (chambers)

- two upper atria (left and right)

- And two lower ventricles (left and right).

Each atrium is in communication with its own ventricle, but the two atria and the two ventricles are separated from each other.

- The atrium and ventricle on the same side communicate with each other through a valve (tricuspid on the right and bicuspid or mitral on the left).

- In turn, the ventricles are provided with another orifice, which allows the blood to be pushed into the pulmonary circulation on the right, and the general circulation on the left. They are also provided with a valve apparatus to avoid regurgitation; through the pulmonary valve, the blood passes from the right ventricle into the pulmonary artery and through the aortic valve, the blood passes from the left ventricle to the aorta.

The right section with the respective atrium and ventricle is separated from the left section with the respective atrium and ventricle through a muscular wall, which is called respectively the atrial septum, the upper one, and the inter-ventricular septum, the lower one. The septum as a whole is called the atrioventricular septum.

- Venous blood loaded with CO2 and low in O2 flows in the right part of the heart, as it is the one that returns from the periphery of the body and will be pushed into the lungs.

- In the left part of the oxygenated heart, blood arrives from the lungs, which are pushed with great power into the general circulation towards all the tissues of the organism.

- The valves act so that, by closing, the flow of blood from the atria cannot go back during the contraction of the ventricles; therefore they direct the blood in a direction towards the front (in fact they are defined as one-way valves)

The weight of the heart is approximately 300 grams and has an overall capacity of approximately 560 cubic centimeters

During an average lifetime, the heart contracts more than 2.5 billion times.

At rest, the heart contracts 60-80 times per minute and pumps approximately 95 ml of blood for each beat (6 liters per minute). During a very intense physical effort, the frequency can reach 200 contractions per minute by pumping almost 200 ml per beat (50 liters per minute).

Blood reaches the heart through channels or vessels called veins and is expelled from the heart through other vessels called arteries. Therefore, regardless of the quality of the blood contained in them, all the vessels that carry blood to the heart and arteries are all the vessels that carry blood from the heart to the periphery.

Two large veins collect all the blood that comes from the periphery and convey it to the right atrium. They are the upper vena cava, which collects the blood coming from the upper part of the body and the lower vena cava, which collects the blood that comes from the lower body.

Author: Vicki Lezama