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There are 6 liters of blood in the human body. Blood is itself composed of many different cells. These cells include:

• Red blood cells – carry oxygen around the body. They are known clinically as erythrocytes. We breathe in oxygen with every passing breath. That oxygen is carried to the lungs. Red blood cells bypass blood vessels that border the lung and pick-up that oxygen, transferring it to other parts of the body, such as muscle. Muscles and organs need oxygen to function. Without it, cells die. At the core of red blood cells is iron; and its iron that traps oxygen within red blood cells.
• White blood cells – are responsible for fighting infection. Clinically, white blood cells are known as leucocytes. There are many different types of white blood cell that contributes to the overall strength of the immune system; that system that protects the body from bacteria, viruses and disease.
• Platelets – if we suffer a cut, blood cells called platelets plug the gap to prevent bleeding. Platelets are known clinically as thrombocytes.

Blood – with all three of these cell families – must be transferred around the body. It does not move itself. Instead, the heart is used to pump blood around the body. On average, the heart pumps 72 times per minute and is composed of 4 chambers:

• 2 upper atria
• 2 lower ventricles

One-half of the heart receives blood and pumps it toward the lungs, where it picks up oxygen. That blood then returns into the opposite half of the heart and gets pumped around the body by the largest artery in the human body – the aorta.

The heart helps transfer red blood cells – with its oxygen – to every cell in the body. As well as this, red blood cells can pick-up waste carbon dioxide and transfer it back to the lung – which we then exhale.

The heart and blood are both part of the circulatory system – the name given to the system that pumps blood around the body. The circulatory system is itself composed of the following blood vessels:

• Arteries – which pump blood away from the heart (A for artery, A for away). Arteries that feed the heart are called coronary arteries. Arteries that feed the kidney are known as renal arteries. Arteries that feed the lungs are known as pulmonary arteries.
• Veins – which pump blood toward the heart.
• Capillaries – the finest branches that permeate blood to the fine corners of the body, such as to the fingers.

The circulatory system is one part of the cardiovascular system.

We are familiar with diseases of the cardiovascular system. They are among the most widely experienced medical problems – high blood pressure and heart attack, for example.

When arteries become blocked, it means that oxygenated blood cannot reach the muscle – and so the muscle cells die. The more cells that die, the greater the pain that’s felt by the affected person. In the case of heart attack then, occluded (blocked) blood vessels mean that oxygen-rich blood is unable to reach cardiac cells (cells of the heart, or myocardium), and so those cells begin to die.

The greater the degree of blockage, the greater degree of risk.

Sometimes, blockage is partial – so patients can be treated quickly and effectively without much damage to heart muscle. In other, more severe cases, blockage is almost complete – which can lead to almost immediate death.

To succeed at the ASVAB science test, you must have a rounded knowledge of the life sciences.

Here, we’ve reviewed the fundamental facts about the cardiovascular system that you need to know. You must be able to talk about types of blood cell, the circulatory system, how the heart works and functions, and the terms associated with the various types of blood vessel; artery, vein and capillary, for example.

With these facts to hand, you can focus on other aspects of the ASVAB science test.

Check back to our ASVAB blog soon for even more great facts on the cardiovascular system and other key components of the ASVAB general science test!