Organisation: The Cardiovascular & Respiratory System

The heart and blood vessels

The heart is an organ that pumps blood around the body in a double circulatory system. One circuit carries blood to the lungs to pick up oxygen; the other carries oxygenated blood to the body tissues. The advantage of two circuits is that blood can be pumped to the body at high pressure, giving a faster flow rate.

Blood enters the atria, which contract to push it into the ventricles. The ventricles contract to push blood out of the heart. Valves prevent backflow. Follow the route carefully:

body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body

The left ventricle has a thicker muscular wall than the right, because it pumps blood all the way round the body rather than just to the lungs. The heart's own muscle is supplied by the coronary arteries. Resting heart rate is controlled by a group of cells in the right atrium that act as a natural pacemaker; an artificial pacemaker can be fitted to correct an irregular heartbeat.

Three vessel types, each adapted to its job:

• Arteries — carry blood away from the heart. Thick, muscular, elastic walls withstand high pressure.
• Veins — carry blood towards the heart. Thinner walls, wider lumen, and valves to prevent backflow at low pressure.
• Capillaries — walls one cell thick and permeable, so substances diffuse into and out of cells over a very short distance.

Blood

Blood is a tissue consisting of a fluid called plasma in which three things are suspended:

• Red blood cells — contain haemoglobin, which binds oxygen. They have no nucleus, leaving more room for haemoglobin, and a biconcave shape giving a large surface area.
• White blood cells — part of the immune response; some engulf pathogens, others produce antibodies.
• Platelets — small fragments of cells involved in clotting.

Plasma transports dissolved substances including carbon dioxide, glucose, urea and hormones.

Coronary heart disease

In coronary heart disease, layers of fatty material build up inside the coronary arteries. This narrows them, reducing blood flow, so the heart muscle receives less oxygen.

• Stents are mesh tubes inserted to keep the arteries open. They work for a long time and recovery is quick, but there is a risk of infection from surgery and of a clot forming near the stent.
• Statins are drugs that reduce blood cholesterol, slowing the rate of fatty deposit formation. They must be taken long term and can have side effects.

Faulty heart valves can become stiff or leaky and may be replaced with biological or mechanical valves. In heart failure, a donor heart may be transplanted, or an artificial heart used to keep a patient alive while waiting or to allow the heart to rest and recover.

The lungs and gas exchange

Air travels down the trachea, which divides into two bronchi, then into bronchioles, and finally into millions of alveoli where gas exchange happens. Oxygen diffuses into the blood and carbon dioxide diffuses out.

The alveoli are effective because they provide a very large surface area, have walls only one cell thick giving a short diffusion path, are surrounded by a dense capillary network maintaining the concentration gradient, and are moist so gases dissolve.