The cardiovascular (or circulatory) system is like a superhighway, it reaches all parts of the body, bringing nutrients and oxygen to cells, then hauling away wastes and carbon dioxide. It is also the avenue through which hormones are distributed throughout the body.
The circulatory system has three primary components: the heart, blood, and blood vessels.
The heart, also known as the cardiac muscle, is a biological miracle of sheer power and sturdiness. The heart of an average person beats over 100,000 times a day; that’s almost 38 million times a year and around three and a half billion beats in a lifetime. Every day the heart pumps around two thousand gallons of blood.
The heart is located between the lungs, behind the sternum. It is enclosed in a double-layered membrane called the pericardium. The outer layer of the pericardium is attached by ligaments to the spinal column and diaphragm. The inner layer of the pericardium is attached to the heart muscle. The two layers are separated by pericardium fluid that enables the heart to move as it beats while still being held in a secure area.
The heart has four chambers: the upper chambers are called the left and right atria and the bottom chambers are called the left and right ventricles. A sheet of muscle called the septum separates the left and right halves of the heart.
The left ventricle is the largest and strongest chamber, because it is the chamber that pushes blood through the aortic valve and into the body.
The Heart Valves
There are four heart valves, each with a specific function, that regulate blood flow through the heart.
One complete heartbeat, called a cardiac cycle, has two phases, systole and diastole.
In the systole phase the ventricles contract, pushing blood into circulation. To prevent blood from flowing backwards into the atria during systole, the atrioventricular valves, the mitral, and tricuspid close. That closure is what creates the first audible sound, S1, of a heartbeat.
When the ventricles finish contracting, the aortic and pulmonary valves, also called the semilunar valves, close to prevent blood from flowing back into the ventricles. This closure is what creates the second heartbeat sound, S2 . The ventricles then relax and fill with blood from the atria and the process starts all over again.
Powering the Heart
The rhythmic beating of the heart is controlled by an electrical impulse. The sinoatrial, or SA node, is a small area of tissue in the wall of the right atrium. The SA node, often referred to as the pacemaker, sends out an electrical signal that first causes the atria to contract; then the impulses travel down to the atrioventricular, or AV node, which acts as a relay. The electrical signal passes through the AV node and travels through the right and left ventricles, causing them to contract and force blood out into the major arteries.
Systemic circulation is the system of blood vessels and associated tissues that supplies oxygen through the blood to all parts of the body, and brings CO2 back to the heart for oxygenation in the lungs. Generally speaking, blood moves out of the left ventricle through the aorta, is dispersed to other arteries which branch into arterioles then capillaries before being collected by veins and returned to the right atrium through the superior and inferior vena cava. Superior and inferior refer to their location above and below the heart.
The capillaries act as a kind of biochemical trading post. While in the capillaries, the blood delivers oxygen and nutrients to the body’s cells and then takes away waste materials. Blood goes back through the capillaries into venules, meaning little veins, and then to larger veins until it reaches the vena cavae. Blood from the head and arms returns to the heart through the superior vena cava, while blood from the torso and lower body returns through the inferior vena cava.
From the right atrium, blood moves to the right ventricle, where it is pumped into the pulmonary circulation for more oxygen.
The blood flowing into the right atrium is low in oxygen and high in CO2. To remove the carbon dioxide and replenish the oxygen, the blood passes into the right ventricle, which pumps it into the double-branched pulmonary artery. The right branch goes to the right lung, the left branch into the left lung. These arteries eventually branch into capillaries throughout the lungs, which are filled with millions of tiny air sacs called alveoli. The blood in the capillaries get an oxygen refill at the same time the CO2 is removed in a process called oxygenation. When we exhale, we are releasing CO2 into the environment.
On a cellular level, oxygenation occurs when oxygen locks onto a protein molecule called hemoglobin, which is what gives red blood cells their color. The newly oxygenated blood exits the lungs through the pulmonary veins and goes back to the heart. It enters the heart in the left atrium and then flows into the left ventricle, which pumps it out into systemic circulation.
The human body contains approximately 10 pints of blood. Whole blood is a mixture of blood cells and plasma.
There are three types of blood cells, each of which is produced in bone marrow:
In addition, there are three types of white blood cells (leukocytes):
In babies and young children, most bones in the body produce blood cells. But as we get older, blood cells are predominantly made by the marrow of the vertebrae, ribs, pelvis, skull, and sternum plus specific parts of the humerus and femur.
Plasma is the yellowish liquid part of the blood that carries the blood cells and other components throughout the body. Approximately half (55%) of the blood in a body is composed of plasma, which is produced in the liver and is 90 percent water. Plasma also contains nutrients, proteins, hormones, and waste products.
There are around 60,000 miles of blood vessels in the human body if you were to string them all together in a single line.
There are three main types of blood vessels:
The walls of arteries and veins have three layers, although they are thinner and more rigid in veins:
As mentioned earlier, the inferior vena cava and the superior vena cava are the largest veins in the body. The superior vena cava returns blood from the upper part of the body back to the heart. The inferior vena cava returns blood from the lower part of the body back to the heart.
The largest artery in the body is the aorta. It is attached directly to the left ventricle of the heart. The left ventricle pumps blood out of the heart through the aortic valve into the aorta. All other major arteries branch out from the aorta delivering blood to the rest of the body.
In addition to circulating blood, blood vessels provide two important ways of measuring vital health statistics, pulse and blood pressure.
Pulse is the rhythmic contraction of the artery that can be felt through the skin. This rhythmic contraction keeps pace with the beating heart. Because arteries are near the surface of the skin we can easily touch the artery and get an accurate measure of the heart’s pulse.
Blood pressure is the force of blood pushing against the artery walls. This force is strongest when the heart contracts, and weakest when the heart relaxes. In addition, this force is stronger in arteries and weaker in veins.
As a result, there are two numbers when reporting blood pressure: the higher number (which is called systolic pressure occurs when the heart contracts) goes over a lower number (which is called diastolic pressure and occurs when the heart relaxes). Normal blood pressure for an adult is “120 over 80” or 120/80.