Discovery Science: Earth Biology – Humans – Heart and Circulation

Earth Science: Biology – Humans – Heart and Circulation

The heart is the organ that distributes blood around the body through vessels such as arteries, veins, and capillaries.

The human body’s blood is circulated by one central organ: the heart. A healthy heart sends blood, oxygen, and various nutrients contained in the blood to all the organs and tissues.

Made up almost entirely of muscle, an adult heart only weighs about nine ounces (300 g). The human heart consists of two halves that are divided by a wall. Each half is divided into sub-chambers: the upper atria and the lower ventricles.

Sinus nodes, which act as the heart’s own nervous system, cause the heart’s chambers to contract and relax in a steady rhythm. Contractions pump the blood through the blood vessels that carry it to every part of the body.

Blood circulation

Because all the blood in the human body is contained within vessels, the human circulatory system is called a “closed” system. The heart’s left side pumps oxygen-rich blood throughout the body in arteries. As the oxygenated red blood cells circulate through the body’s tissues, they release oxygen and absorb carbon dioxide.

When the blood returns through the veins to the heart, the heart’s right side pumps it to the lungs to be reoxygenated, after which the blood returns to the left side of the heart, continuing the cycle. Although blood vessels are not the cause of circulation, the layers of muscle that comprise them regulate the quantity of blood flow.


Heart attacks, strokes, and other diseases of the cardiovascular system are common causes of death in industrialized parts of the world. Both heart attacks and strokes are caused by narrowed or blocked blood vessels clotted by blood cells or protein fibers.

The heart can sustain irreversible damage if it is deprived of blood for more than 20 minutes—as can happen during a heart attack—much as nervous tissue may die when blood vessels in the brain are obstructed.


In 1967, Christiaan Barnard and his team performed the first successful heart transplant in South Africa. During the five-hour operation, the patient received a donor heart.

In order to prevent the donor heart from being rejected as a foreign organ, his immune system was sup- pressed. The patient succumbed to pneumonia 18 days fat lowing the surgery  as a consequence.


Cardiac arrhythmia, a condition that causes the heart to beat erratically, sometimes requires more than just medication. A pacemaker, a small, battery-operated device designed to stabilize the heart’s beat, can be implanted in the patient’s chest and an electrode carrying an electric pulse from the pacemaker’s battery can then be pushed through a vein into the heart.

Today, thousands of people living in industrialized nations rely on pace- makers to regulate their heartbeats.