BBC  Science & Nature - Human Body and Mind - Organ Layer
Physical description: Grapefruit-sized and cone-shaped
Function: To pump oxygen-rich blood throughout your body and oxygen-poor blood to your lungs
Cardiac muscle
Your heart is an incredibly powerful organ. It works constantly without ever pausing to rest. It is made of cardiac muscle, which only exists in the heart. Unlike other types of muscle, cardiac muscle never gets tired.
Four chambers
Your heart is divided into four hollow chambers. The upper two chambers are called atria. They are joined to two lower chambers called ventricles. These are the pumps of your heart.
One-way valves between the chambers keep blood flowing through your heart in the right direction. As blood flows through a valve from one chamber into another the valve closes, preventing blood flowing backwards. As the valves snap shut, they make a thumping, 'heart beat' noise.
Double pump
Blood carries oxygen and many other substances around your body. Oxygen from your blood reacts with sugar in your cells to make energy. The waste product of this process, carbon dioxide, is carried away from your cells in your blood.
Your heart is a single organ, but it acts as a double pump. The first pump carries oxygen-poor blood to your lungs, where it unloads carbon dioxide and picks up oxygen. It then delivers oxygen-rich blood back to your heart. The second pump delivers oxygen-rich blood to every part of your body. Blood needing more oxygen is sent back to the heart to begin the cycle again. In one day your heart transports all your blood around your body about 1000 times.
Your right ventricle pumps blood to your lungs and your left ventricle pumps blood all around your body. The muscular walls of the left ventricle are thicker than those of the right ventricle, making it a much more powerful pump. For this reason, it is easiest to feel your heart beating on the left side of your chest.
Pacemaker
Unlike skeletal muscle cells that need to be stimulated by nerve impulses to contract, cardiac muscle cells can contract all by themselves. However, if left to their own devices, cardiac muscle cells in different areas of your heart would beat at different rates. Muscle cells in your ventricles would beat more slowly than those in your atria. Without some kind of unifying function, your heart would be an inefficient, uncoordinated pump. So, your heart has a tiny group of cells known as the sinoatrial node that is responsible for coordinating heart beat rate across your heart. It starts each heartbeat and sets the heartbeat pace for the whole heart.
Damage to the sinoatrial node can result in a slower heart rate. When this is a problem, an operation is often performed to install an artificial pacemaker, which takes over the role of the sinoatrial node.
Heart rate
Without nervous system control, your heart would beat around 100 times per minute. However, when you are relaxed, your parasympathetic nervous system sets a resting heart beat rate of about 70 beats per minute, (resting heart rate is usually between 72-80 beats per minute in women and 64-72 beats per minute in men).
When you exercise or feel anxious your heart beats more quickly, increasing the flow of oxygenated blood to your muscles. This is triggered by your sympathetic nervous system. Your heart rate also increases in response to hormones like adrenalin.
On average, your maximum heart rate is 220 beats per minute minus your age. So a 40 year old would have a maximum heart rate of 180 beats per minute.
Oxygen supply to your heart
Although your heart is continually filled with blood, this blood doesn't provide your heart with oxygen. The blood supply that provides oxygen and nutrients to your heart is provided by blood vessels that wrap around the outside of your heart.