Blood vessels
Pulmonary circulation
Systemic circulation
Blood pressure
Nutrient and waste material exchange
Lymphatic circulation
The blood constantly circulating in blood vessels interacts with the interstitial fluid surrounding the cells. The interstitial fluid is the immediate environment for the cells. In order for cells to function the interstitial fluid needs to have a more or less constant composition, temperature and volume. The blood transfers nutrients and oxygen to the interstitial fluid and carries away the by-products of cell metabolism, such as carbon dioxide.
Humans have a closed circulatory system, meaning that blood circulates in a continuous blood vessel network. The vascular system is made up from the arteries that convey blood from the heart, the veins that carry blood towards the heart and the capillaries that connect the arteries and the veins. The circulatory system has two components. The pulmonary circulation carries blood between the heart and the lungs, while the systemic circulation does so between the heart and other tissues of the body.
Arteries are blood vessels with muscular, elastic walls that carry blood from the heart towards the capillaries. They carry oxygen-rich blood in the systemic circulation and blood rich in carbon dioxide in the pulmonary circulation. The pulmonary artery originates from the heart’s right ventricle, and the aorta originates from the left ventricle. Both branch into smaller and smaller arteries and eventually the smallest arteries become capillaries. Thanks to their elastic walls the arteries are able to maintain the blood pressure despite the pulsating nature of the heart contractions.
Veins carry blood from the capillaries towards the heart with smaller veins collecting into larger and larger ones. They carry blood rich in oxygen in the pulmonary circulation and blood rich in carbon dioxide in the systemic circulation. There is a thinner layer of smooth muscles and elastic fibers in the walls of veins than in the walls of arteries. As a result, they can expand and can carry a larger amount of blood. The valves found in veins prevent blood from flowing backwards.
Capillaries connect small arteries (arterioles) and small veins (venules). The capillaries of the pulmonary circulation form a dense network on the outer surface of the alveoli of the lungs, while the capillaries of the systemic circulation form dense networks in the tissues of the body. The single layer endothel that makes up the capillary wall is easily crossed over by small particles such as oxygen or carbon dioxide or water. Nutrient and waste exchange between the blood and the tissues take place in the capillaries. Through the wall of the capillaries of the pulmonary circulation oxygen transfers into the blood stream and carbon dioxide passes through into the alveoli. Oxygen and nutrients pass through the capillary walls of the systemic circulation into the interstitial fluid, while the blood takes up carbon dioxide and various waste products.
The vascular network of the pulmonary circulation connects the heart and the lungs. The pulmonary artery that originates from the right ventricle of the heart carries blood rich in carbon dioxide towards the lungs. The gas exchange takes place on the surface of the alveoli and the oxygen-rich blood flows towards the heart in larger and larger veins. The pulmonary vein enters the heart via the left atrium.
In resting state approximately 5 dm3 of blood passes through in the pulmonary circulation every minute.
The blood vessel network of the systemic circulation connects the heart with the tissues of the body. The aorta - the main artery that originates from the left ventricle - carries oxygen-rich blood towards the capillaries that enmesh the tissues. The gas exchange takes place through the walls of capillaries, and the blood rich in carbon dioxide flows towards the heart in larger and larger veins. The superior vena cava ends in the right atrium. In resting state approximately 5 dm3 of blood flows through the systemic circulation every minute. Roughly 80% of all the blood in the body can be found in the blood vessel network of the systemic circulation.
Blood pressure is the blood’s hydrostatic pressure exerted on the vessel wall. In the blood vessel network blood flows from higher pressure areas towards lower pressure areas. Differences in pressure between arteries stemming from the heart and veins ending in the heart are a result of the contractions of the heart.
There is a significant exchange of nutrients and waste material between the interstitial fluid and the blood in the capillaries connecting the small arteries and small veins of the systemic circulation. Nutrients and oxygen necessary for cellular respiration exit the capillaries, while carbon dioxide and waste products of cellular metabolism enter the capillaries from the interstitial fluid.
Blood arriving from a small artery exerts a relatively large pressure on the wall of the initial section of capillaries. Through the endothel of the capillary protein free plasma, the so called lymph transfers into the interstitial fluid. Approximately 90% of this lymph gets back into the capillaries, while the remaining 10% is transported back into the blood stream via lymphatic capillaries.
One function of lymphatic circulation is to return excess tissue fluid into the blood stream. Lymphatic circulation starts with the lymphatic capillaries found in the tissues. Lymphatic capillaries collect into larger and larger lymphatic vessels. The largest lymphatic vessel, the lymphatic duct directs the lymph into a systemic vein near the heart. There are lymph nodes along the lymphatic vessels that filter the fluid that flows through them and also play a role in producing lymph cells and in defending the body from pathogens. Apart from lymph nodes there are other, larger lymphatic organs in the body (tonsils, appendix, spleen).
