The Cardiovascular System - ONLINE FIRST AID

Lesson no. 6

The Cardiovascular System

The cardiac system is divided into three parts: pump (heart), pipes (blood vessels) and fluid (blood).

The Functions of the Cardiovascular System

Transportation of gases (oxygen, carbon dioxide).
Transportation of hormones like adrenaline and insulin from the organ that produces them to the organ that needs them.
Body heat regulation – when it’s hot outside, the blood vessels dilate (expand) and emit heat, but when it’s cold they contract to keep the heat trapped inside.
Disposal of waste materials.
Immune function – to be discussed later in the lesson.

Blood – the Fluid

The blood volume in an average adult is about 6 liters. It consists of 55% plasma – the fluid itself, and 45% percent hematocrit.
The plasma is made up of 90% water, proteins, glucose, lipids, amino acids, hormones, coagulation factors, and more.
The hematocrit is a measure of the volume of red blood cells relative to the total blood sample volume and is expressed as a percentage. It consists of red blood cells, white blood cells, and platelets (though the amount of platelets is negligible).
Red blood cells (erythrocytes) are cells whose function is to carry oxygen from the alveoli to the body’s cells and to carry back the carbon dioxide from the body’s cells to the alveoli. Oxygen transport is done through the hemoglobin protein.

White blood cells (leukocytes) are cells whose role is to take part in the body’s immune response and protect us from foreign invaders, bacteria, contaminants, and more. These cells can exit the blood vessels, move against the flow’s direction, and change shape.
Platelets (thrombocytes) are cell fragments originating from a giant cell. When a blood vessel is injured/ruptured, the platelets attach to each other and to the blood vessel, sealing the site by activating the system that causes coagulation (clotting). That is, they activate the coagulation process until the blood clot is formed.

The Blood Vessels – The Pipes

These consist of three types of blood vessels:

Artery – a blood vessel that exits from the heart.

These are usually blood vessels that contain oxygen-saturated blood (except for pulmonary arteries which pump carbon dioxide-saturated blood). It is the most massive blood vessel of them all because it is required to carry the greatest pressure.

It consists of three layers:

Tunica Intima – the innermost layer. It’s very smooth and prevents the blood’s contents from “getting stuck” to the walls of the blood vessels and damaging them.
Tunica Media – the middle layer. A muscular layer that is able to expand and contract, and affects the blood pressure in the body.
Tunica Adventitia – the outermost layer.

Important Arteries:

The first and largest artery – the aorta. It exits directly from the heart and splits into the other arteries.
Carotid arteries – two arteries (right and left) that exit directly from the aortic arch and carry blood to the brain.
Radial artery – the artery that runs along the thumb line. It is used to check the pulse.
Femoral artery – the thigh’s artery. Like the organ it’s found in, it is very large.

Coronary arteries: the arteries that nourish the heart muscle itself.

The arteries come from the very beginning of the aorta and engulf the entire heart.

It is important to remember that, unlike the entire circulatory (cardiovascular) system, the heart is nourished by the coronary arteries during diastole as opposed to during systole!

Vein – a blood vessel that enters the heart.

These are usually blood vessels that contain carbon dioxide-saturated blood (except the pulmonary veins that carry oxygen-saturated blood).

Capillaries – the smallest blood vessel. Consists of a single-celled layer. This feature allows it to exchange oxygen and carbon dioxide with the body’s cells. It transports gases, water, and essential substances directly into and out of cells.

Blood Pressure – the pressure exerted by circulating blood against the walls of the blood vessels.

Blood pressure is affected by many factors such as the amount of blood, the heart’s force of contraction, the number of beats per minute, the diameter of blood vessels (which can vary), etc. Blood pressure is divided into systolic pressure (when the rooms of the heart contract) and diastolic pressure (when the rooms of the heart relax).

In a normal adult, systolic blood pressure ranges from 90-140mg and normal diastolic blood pressure ranges from 60-90mg. The number is presented as follows: 120/70.

Proper blood pressure in children and infants (systolic) is calculated as 80 + (age of child multiplied by 2).

Pulse Pressure is defined as the difference between systolic and diastolic blood pressure, where the normal range ranges from 30-60mg.

The Heart – The Pump

The heart is a muscular organ used to pump blood. Throughout our lives, the heart never stops beating, not even for a moment. This makes the heart muscle very unique. It is roughly the size of a person’s fist, and is located in the center of the thorax, behind the sternum, with a slight tilt to the left.

We’ll look at the heart like a box divided into four chambers:

Divided horizontally – a left side and a right side, with the left side rich in oxygen and the right-side oxygen-depleted.

Divided vertically – atrium and ventricles. At the top of the box are two atria, and at the bottom of the box are two ventricles.

The heart’s muscle (myocardium) is responsible for circulating blood throughout the body and uses a mechanism that utilizes contraction and relaxation.

When the heart contracts, blood flows from the heart to the blood vessels, while during relaxation the heart fills with blood. Between the atrium and ventricles and between the ventricles and large arteries (the aorta and pulmonary artery) are valves that allow blood to flow in only one direction, which keeps the blood volume in the chambers that later circulate to the blood vessels.

The heart consists of three layers:

Endocardium – a smooth inner layer
Myocardium – the thick middle layer. An extremely strong muscle layer
Epicardium – the outer layer

Around it is the pericardium (or: the heart’s sac) and between them is the pericardial fluid that acts as a lubricant.

How does the system work?

First, the atria contract with the ventricles soon following. When the atria are contracted, the ventricles are relaxed and the valves between the atria and ventricles are open and allow blood flow from the atria to the ventricles (diastole). After the blood passes from the atria to the ventricles, the rooms will contract to pump the blood into the large blood vessels, which will circulate blood into the body and lungs. When the ventricles are contracted, the valves between the atria and ventricles are closed, preventing blood flow back to the atrium (systole), as the atria are filled with blood.

The normal adult heart rate is 60-100 beats per minute.

The normal heart rate for a child is 80-120 beats per minute.

The normal heart rate for babies is 100-160 beats per minute.

A heart rate above 100 beats per minute (in an adult) is called tachycardia, while a heart rate below 60 beats per minute (in an adult) is called bradycardia.

We will learn about the cardiovascular structure by following the journey of blood circulation.

Systemic Circulation: Oxygen-saturated blood found in the left atrium passes through the mitral valve to the left ventricle. From the left ventricle, the blood exits through the aortic valve to the aorta. From the aorta, blood flows to the entire body. Blood flowing through the artery is saturated with oxygen and supplies oxygen to the body’s organs through capillaries. In the capillaries, gases are exchanged with the cells. They pass along oxygen to the cells and receive carbon dioxide back. Carbon dioxide flows from the capillary system and continues to the veins until it reaches the two venae cavae: the superior vena cava which returns blood from the upper part of the body, and the inferior vena cava which flows blood from the lower part of the body back to the heart. The venae cavae enter the right atrium. This blood is oxygen-depleted.

Pulmonary Circulation: Blood saturated in carbon dioxide (CO₂) and oxygen-depleted, which is found in the right atrium, flows to the right ventricle through the tricuspid valve. From the right ventricle, the blood flows through the pulmonary valve to the pulmonary artery. The pulmonary artery splits into two and enters the lungs. The lungs then carry out an exchange of gases, where carbon dioxide is exchanged for oxygen. From the lungs, the blood drains into the four pulmonary veins and from there to the left atrium. This blood is oxygen-rich.

Electrical Conduction System: The heart is unable to perform mechanical contraction without electrical stimulation and because of its importance, the heart has its own conduction system.

The electrical current is created in the heart’s main pacemaker (sino-atrial node) which is located in the right atrium. The electric current allows for atrial contraction. After pausing for about a sixth of a second, the current continues to the secondary pacemaker (atrioventricular node), which is positioned at the junction between the atria and the ventricles. The secondary pacemaker transfers the current to the ventricles and causes them to contract. When the heart’s system is functioning properly, the atria will contract before the ventricles, so that blood can fill the ventricles and be ejected into the rest of the circulatory system.

did you know?

The Cardiovascular System

In an average adult, the heart beats a hundred thousand times and circulates about 7,570 liters of blood a day. The total length of blood vessels, if connected, is approximately 100,000 kilometers.
During sleep, heart rate drops significantly due to changes in metabolism, and chemical reactions in the body. During rest and sleep, the parasympathetic nervous system is active. This system is responsible for the body’s activity during rest and directs energy where it’s required. For example, during a meal, you will channel most of your energy and blood to the stomach. During sleep, it causes the heart to slow down and the body to relax.
New research shows that sitting in a chair during the day is linked to many health problems including heart issues.

Heart Disease

Studies indicate that people who fall asleep late are at higher risk of developing heart disease, even if they get eight hours of sleep.
Negative emotions and depression increase the chance of having a heart attack or a stroke.
The main factors in increasing the risk of heart disease in descending order: age, gender, family history, smoking, obesity, lack of physical activity.

Circulatory System

Microcirculation is a term that describes circulation in the smallest of blood vessels. Before we begin with its description, we will describe circulation from the heart on. The blood flows out of the heart through the arteries. These arteries gradually get smaller until the blood reaches the arterioles (blood vessels from about ten to one hundred micrometers in diameter). The blood flows from the arteriole to a branched system of capillaries (about five to ten micrometers in diameter). From this capillary system the blood goes out to the venules (about ten to two hundred micrometers in thickness) and gradually expands to the veins and back to the heart. Most of these blood vessels have smooth tissue walls and are surrounded by contracting cells called pericytes. The tissue provides smooth passage for the blood and also secretes molecules that prevent the blood from clotting, unless there is a leak. The contracting cells are used to contract and dilate the blood vessels, thus controlling the blood’s velocity and blood pressure.
When the blood reaches the arteriole it slows down, with the help of said cells. From there it moves to the capillary system where metabolism between the tissues and blood takes place. Because the thickness of the capillaries is about one cell in diameter, the metabolic process is carried out very efficiently. From there, the blood goes out to the wider venules which increase the flow’s velocity once again.

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