VMBO Year 2 · HAVO/VWO Years 2–3 · Circulation · Section 3.2
The circulatory system
Blood vessels
The circulatory system consists of the heart and blood vessels. Three main vessel types are arteries, capillaries and veins.
The heart pumps blood into arteries, which branch into smaller vessels and eventually capillaries. Capillary walls are only one cell layer thick. Blood then passes into small veins and larger veins returning to the heart.
The blood vessel system
Oxygenated blood is brighter red; deoxygenated blood is darker red. Both are red, even when diagrams use blue to represent deoxygenated blood.
Many vessels are named after the organs or regions they supply or drain, such as arteries in the legs or veins in the arms. Other names include the aorta, superior and inferior vena cava, and hepatic portal vein.
The hepatic portal vein carries relatively oxygen-poor but nutrient-rich blood from the digestive organs to the liver, especially after a meal.
Double circulation
Humans have double circulation: blood passes through the heart twice during a complete circuit.
- Pulmonary circulation: Heart → lungs → heart. Blood gains oxygen and loses carbon dioxide.
- Systemic circulation: Heart → body tissues → heart. Blood supplies oxygen and nutrients and takes up waste products.
Birds and other mammals also have this fully separated double circulation.
Arteries
Arteries carry blood away from the heart. Their pressure is generally higher than in veins. Thick, muscular, elastic walls withstand pressure changes and help maintain flow.
The pressure wave from each heartbeat can be felt as a pulse in suitable arteries. Many major arteries lie deep in the body, helping protect them from damage.
Capillaries
Capillaries are very narrow vessels. Blood pressure falls as blood passes through the small vessels and capillary networks.
Their thin walls allow substances such as oxygen and nutrients to pass towards tissues, while carbon dioxide and other wastes enter the blood. White blood cells can also leave the circulation through small vessel walls when needed.
Veins
Veins return blood towards the heart at relatively low pressure. Their walls are generally thinner and less muscular than artery walls, and no strong arterial pulse is normally felt.
Some veins lie near the skin and may look blue because of how light passes through tissues, not because their blood is blue.
Many veins, particularly in the limbs, contain valves preventing backflow. Together with muscle contractions, these help blood return against gravity.
Glossary
- Veins
- Blood vessels carrying blood towards the heart, usually at relatively low pressure.
- Circulation
- The movement of blood through the body to transport useful substances and wastes.
- Circulatory system
- The heart and blood vessels together.
- Double circulation
- A complete route in which blood passes through the heart once for the lung circuit and once for the body circuit.
- Systemic circulation
- The route from heart to body tissues and back.
- Capillaries
- Tiny blood vessels with very thin walls allowing exchange between blood and tissues.
- Pulmonary circulation
- The route from heart to lungs and back for gas exchange.
- Valves
- Structures preventing blood from flowing backwards, including those in many veins.
- Hepatic portal vein
- A vein transporting blood rich in absorbed nutrients from the digestive organs to the liver.
- Arteries
- Vessels carrying blood away from the heart, generally with thick, elastic walls and higher pressure.