|
An overview of the anatomy and physiology of the heart.
Introduction
The heart is about the size of a clenched fist and pumps the blood around the body. It essentially consists of 4 chambers connected by one way valves. It pumps at about 60-80 beats a minute at rest and increases to about 160 with exercise.
The human heart is shaped like an upside-down pear and is located slightly to the left of centre inside the chest cavity. The heart is made primarily of muscle tissue that contracts rhythmically to propel blood to all parts of the body.
The heart, blood, and blood vessels make up the circulatory system, which is responsible for distributing oxygen and nutrients to the body and carrying away carbon dioxide and other waste products.
The following diagram identifies the major blood vessels leaving and entering the heart. The left side of the heart pumps the oxygenated blood and the right side pumps the deoxygenated blood (blue).
Inside the Heart Diagram
Arteries always carry blood away from the heart and veins deliver blood to the heart. Normally arteries carry oxygenated blood but note that this is not the case with the pulmonary arteries and veins.
 The blood flow
All the chambers are connected by valves, which allow blood to move in one direction from one chamber to the next. In a normal cardiac cycle the following occurs:
- The right atrium receives blood from all the organs except the lungs through two large veins called the superior vena cava and the inferior vena cava.
- It then passes this oxygen-poor blood to the right ventricle, which then pumps the blood through the pulmonary artery.
- The pulmonary artery brings the blood to the lungs where it picks up oxygen from the air you breathe in and also gets rid of excess carbon dioxide.
- This oxygen rich blood is returned to the left atrium of the heart by way of the pulmonary veins.
- This blood passes from the left atrium into the left ventricle to be pumped out to the rest of the body through the aorta.
The Heart is capable of pumping 4-5 litres blood per minute at rest and 20 litres with exercise.
In normal conditions the liver receives 28%, kidneys 24%, skeletal muscles 15%, brain 14%, heart 5% of the cardiac output.
Layers of the heart
The heart is made up of three layers:
- the endocardium - the inside smooth layer
- the myocardium - the thicker muscle layer
- the pericardium - the outer membranous sac. The strong outer portion of the sac, or fibrous pericardium, is firmly attached to the diaphragm below, the mediastinal pleura on the side, and the sternum in front, and gradually blends with the coverings of the superior vena cava and the pulmonary (lung) arteries and veins leading to and from the heart.
The two layers of serous membrane are normally separated only by 10 to 15 millilitres (0.6 to 0.9 cubic inch) of pericardial fluid, which is secreted by the serous membranes. The slight space created by the separation is called the pericardial cavity. The pericardial fluid lubricates the two membranes with every beat of the heart as their surfaces glide over each other.
The heart muscle is supplied by blood by the coronary arteries. A partial obstruction of these causes the heart muscle to become partially starved of oxygen, this results in a chest pain called Angina.
During panic or fright the adrenal glands release adrenaline into the blood stream which stimulates heart to beat faster, dilating blood vessels and increases the breathing rate. This is the classical fight or flight mechanism.
Patent Foramen Ovale (PFO)
The foramen ovale is a small passageway in the heart that all people have at birth. The foramen ovale permits oxygen-rich blood to pass from the placenta in the mother's womb to the left side of the baby's heart. From there it can travel to the vital organs in the baby's body. Once the baby is born, the lungs expand with air, and the pressures inside the heart change. This forces a flap to close over the PFO passageway. After a few months, this flap seals over with scar tissue and does not permit any blood to mix between the left and right sides of the heart. However, in 10% of people, the foramen ovale passageway does not seal over and it remains patent (which means open). In the vast majority of people, this does not cause any problem.
|
|
Normal Circulation |
With PFO |
Statistics show unequivocally that divers with a PFO have a higher risk of DCS. The mechanism is that bubbles of nitrogen in the blood, which form in the veins of every diver during a dive, can pass from the veins through the PFO into the arteries, instead of going to the lungs where they would be filtered out. Bubbles in the arteries can go anywhere and cause gas embolism (blockage of blood flow) particularly in the brain.
In addition, a Valsalva manoeuvre, used by most divers to equalize their ears during descents and ascents, can increase venous atrial pressure to the point that it forces blood containing bubbles across the PFO into the arterial circulation with the same outcome.
|
