Human circulatory systems

The pulmonary circulatory system consists of blood vessels that transport deoxygenated blood from the heart to the lungs and return oxygenated blood from the lungs to the heart. In the systemic circulatory system, blood vessels transport oxygenated blood from the heart to various organs in the body and return deoxygenated blood to the heart.

Pulmonary circulation system

In the pulmonary circulation system, deoxygenated blood leaves the heart through the right ventricle and is transported to the lungs via the pulmonary artery. The pulmonary artery is the only artery that carries deoxygenated blood. It carries blood to the capillaries where carbon dioxide diffuses out of the blood into the alveoli (lung cells) and then into the lungs, where it is exhaled. At the same time, oxygen diffuses into the alveoli, and then enters the blood and is returned to the left atrium of the heart via the pulmonary vein.

The Pulmonary circulation system. Oxygen rich blood is shown in red; oxygen-depleted blood is shown in blue.

Systemic circulation Systemic circulation refers to the part of the circulation system that leaves the heart, carrying oxygenated blood to the body’s cells, and returning deoxygenated blood to the heart. Blood leaves through the left ventricle into the aorta, the body’s largest artery. The aorta leads to smaller arteries that supply all organs of the body. These arteries finally branch into capillaries. In the capillaries, oxygen diffuses from the blood into the cells, and waste and carbon dioxide diffuse out of cells and into blood. Deoxygenated blood in capillaries then moves into venules that merge into veins, and the blood is transported back to the heart. These veins merge into two major veins, namely the superior vena cava and the inferior vena cava. The movement of blood is indicated by arrows on the diagram. The deoxygenated blood enters the right atrium via the the superior vena cava. Major arteries supply blood to the brain, small intestine, liver and kidneys. However, systemic circulation also reaches the other organs, including the muscles and skin.

THE HEART AND ASSOCIATED BLOOD VESSELS

External structure of the heart

The heart is a large muscle, about the size of your clenched fist, that pumps blood through repeated rhythmic contractions. The heart is situated in your thorax, just behind your breastbone, in a space called the pericardial cavity. The heart is enclosed by a double protective membrane, called the pericardium. The region between the two pericardium layers is filled with pericardial fluid which protects the heart from shock and enables the heart to contract without friction.

The heart is a muscle (myocardium) and consists of four chambers. The upper two chambers of the heart are called atria (singular= atrium). The two atria are separated by the inter-atrial septum. The lower two chambers of the heart are known as ventricles and are separated from each other by the interventricular septum. The ventricles have more muscular walls than the atria, and the walls of the right ventricle, which supplies blood to the lungs is less muscular than the walls of the left ventricle, which must pump blood to the whole body.

In addition, there are a number of large blood vessels that carry blood towards and away from the heart. The terms artery′ and vein’ are not determined by what the vessel transports (oxygenated blood or deoxygenated) but by whether the vessel flows to or from the heart. Arteries take blood away from the heart and generally carry oxygenated blood, with the exception of the pulmonary artery. Veins transport blood towards the heart and generally carry deoxygenated blood, except the pulmonary vein. On the right side of the heart, the superior vena cava transports deoxygenated blood from the head and arms and the inferior vena cava transports deoxygenated blood from the lower part of the body back to the heart, where it enters the right atrium. The pulmonary artery carries deoxygenated blood away from the right ventricle of the heart towards the lungs to be oxygenated. On the left side of the heart, the pulmonary vein brings oxygenated blood from the lungs towards the left atrium of the heart and the oxygenated blood exits the left ventricle via the aorta and is transported to all parts of the body.

Since the heart is a muscle, and therefore requires oxygen and nutrients itself to keep beating, it receives blood from the coronary arteries, and returns deoxygenated blood via the coronary veins.

The external structure of the heart: the major part of the heart consists of muscles and is known as the myocardium. The region in which the heart is found is known as the pericardial cavity, which is enclosed by the pericardium.

Internal structure of the heart

As previously mentioned, the heart is made up of four chambers. There are two atria at the top of the heart which receive blood and two ventricles at the bottom of the heart which pump blood out of the heart. The septum divides the left and right sides of the heart. In order to make sure that blood flows in only one direction (forward), and to prevent backflow of blood, there are valves between the atria and ventricles (atrioventricular valves). These valves only open in one direction, to let blood into the ventricles, and are flapped shut by the pressure of the blood when the ventricles contract.

The tricuspid valve is situated between the right atrium and the right ventricle while the bicuspid/ mitral valve is found between the left atrium and the left ventricle. Strong tendinous cords (chordae tendineae) attached to valves prevent them from turning inside out when they close. The semi-lunar valves are located at the bottom of the aorta and pulmonary artery, and prevent blood from re-entering the ventricles after it has been pumped out of the heart.

The internal structure of the mammalian heart.

In the previous sections we have discussed pulmonary and systemic circulation, and we have described the four chamber structure of the heart as well as some of the major arteries and veins that transport blood towards and away from the heart. In order to summarise all this information, study the flow diagram below which describes the passage of deoxygenated blood through one full cycle.

The Flow diagram depicting movement of blood from the heart through the circulatory system. The blue boxes represent deoxygenated blood, the purple boxes represent capillary networks where gaseous exchange occurs and the red boxes represent stages at which the blood is oxygenated.

Major organs and systemic circulation

All the organs of the body are supplied with blood. This is necessary so that the cells can obtain oxygen, which is required for cellular respiration, as well as essential nutrients. Each organ has an artery that supplies it with blood from the heart. Metabolic wastes, including carbon dioxide, need to be removed from cells and returned to the heart. These move into the capillaries which enter into veins that eventually enters either the superior or inferior vena cava which then enters the right atrium.

Arteries and veins have been named according to the organ to which they supply blood. The liver receives oxygenated blood from the heart via the hepatic artery. This artery runs alongside the hepatic portal vein. The hepatic portal vein contains nutrients that have been absorbed by the digestive system. This nutrient-rich blood must first pass through the liver, so that the nutrient composition of the blood can be controlled. Blood passes from the liver to the heart through the hepatic vein. Metabolic waste is circulated in the blood, and if allowed to accumulate, would eventually reach toxic levels. The kidneys are supplied with blood (which contain waste) via the renal arteries. The kidneys filter metabolic waste from the blood, passing it to urine to be excreted safely. Blood leaves the kidney via the renal vein.

The brain is supplied with blood via the carotid arteries and the vertebral arteries. The blood from the brain is drained via the jugular veins. The brain is supplied with 15%of the total amount of blood pumped by the heart. The heart is also a muscle (myocardium) that requires blood flow to work. Blood is supplied to the heart via two coronary arteries, and leaves the heart via four cardiac veins.

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