Oxygenated blood from the heart is circulated around the whole body via a network of arteries. The heart must pump the blood out of the heart at high pressure to push it through the system of arteries which supply the entire body. The arterial walls reflect this by being strong and muscular to resist this surge of high blood pressure.
Deoxygenated blood is returned to the heart via a network of veins. By the time the blood gets into the vascular system the blood pressure has greatly reduced. The venous walls reflect this by being thin and flaccid as they do not experience the same surges of high blood pressure.
The walls of the arteries have a slightly different structure to those of the veins, however they both consist of the same three layers (tunica).
Arteries carry blood away from the heart. They begin as large conducting vessels such as the aorta, common carotid and common iliac arteries. Both the aorta and pulmonary trunk are conducting vessels which arise directly from the left and right ventricles of the heart respectively. The conducting arteries have the thickest muscular walls (tunica media) and expand when ventricles are squeezing blood into them, recoiling when the ventricles relax.
The conducting arteries divide and give off smaller 'named' distributing arteries which travel to specific parts of the body. The distributing arteries divide into smaller unnamed branches until eventually dividing into tiny arterioles which further divide into minute capillaries. As the arteries get smaller the relative thickness of the tunica media also decreases.
The capillaries join together to form a capillary network; it is in this network where the nutrients from the blood are exchanged into the surrounding tissues.
Veins carry blood towards the heart. Venules, the minute beginnings of the veins emerge from the capillary network. Like the capillaries the venules form a network; it is in this network that waste products from the surrounding tissues are absorbed into the blood. The venules join together to form small unnamed veins which join together to form the larger 'named' veins.
A number of these named veins possess semilunar valves to prevent the blood from flowing backwards in the wrong direction. These larger veins eventually converge to form the large superior and inferior vena cavae which empty into the right atrium of the heart.
The circulation of blood can be described in three parts;
The pulmonary circulation functions to get rid of the waste product carbon dioxide from the blood and to saturate the blood with oxygen. The cycle begins when deoxygenated blood is pushed from the right ventricle into the pulmonary trunk. The pulmonary trunk soon divides into two large pulmonary arteries, left and right. These arteries are the only arteries in the body that carry deoxygenated blood. The right pulmonary artery enters the hilum of the right lung and divides into 3 lobular arteries to the 3 lobes of the right lung. The left pulmonary artery enters the hilum of the left lung and divides into 2 lobular arteries to the 2 lobes of the left lung. The lobular arteries continue to divide until they become a fine network of capillaries surrounding the tiny alveoli. It is here where gas exchange takes place, and CO2 is released from the blood and O2 is taken into the blood. From here, the now oxygenated blood is carried back to the heart by venules which continually unite to eventually exit each lung as two pulmonary veins; superior and inferior. The 4 pulmonary veins drain into the left atrium which squeezes the oxygenated blood into the left ventricle where it is pushed into the systemic circulation. These veins are the only veins in the body that carry oxygenated blood.
The portal circulation or 'hepatic portal system' functions to filter the deoxygenated but nutrient rich blood received from the digestive system of toxins and bacteria before it is distributed to the rest of the body. The liver receives venous blood from the oesophagus, stomach, small and large intestines, gallbladder, pancreas and spleen via the large portal vein. On reaching the liver the portal vein divides into two large branches (left and right), which enter the liver and continually branch to form hepatic sinusoids within all of the lobes of the liver. Superiorly the hepatic sinusoids unite to form the hepatic veins which exit the liver from its posterior side and enter the inferior vena cava. From the inferior vena cava the blood is transported to the right atrium where it enters into the pulmonary circulation.
The systemic circulation functions to deliver oxygenated blood and remove the waste products from all of the tissues in the body. The cycle begins when oxygenated blood is pushed from the left ventricle into the ascending aorta. From here the aorta arches over the heart and descends into the thorax and abdomen giving rise to all of the systemic arteries. These arteries continually divide until they form a network of capillaries that surround all of the tissues of the body. It is here where gas exchange takes place; O2 is released from the blood and CO2 is taken into the blood. The now deoxygenated blood begins its journey back to the heart in a network of tiny venules. The venules continually unite to form small veins and then large veins. These large veins eventually empty into the superior and inferior vena cava which delivers the deoxygenated blood to the right atrium where it is then circulated around the pulmonary system.
The aorta is the largest artery in the body and is the source of all of the systemic arteries. It arises from the left ventricle of the heart, arches backwards over the heart and descends through the thorax and abdomen where it eventually divides into the common iliac arteries.
Because of its size the aorta is described in 4 parts;
The right and left common carotid arteries supply a large proportion of the head and neck with blood. The left common carotid artery emerges from the arch of the aorta and the right from the right subclavian artery. They both ascend at the side of the neck and divide to form the internal and external carotids. At this division is an important swelling, the carotid sinus, which is supplied by sensory fibres of the glossopharyngeal (IX cranial) nerve and functions to control the pressure of the blood travelling into the brain; keeping it constant.
The vertebral arteries are important as they supply the cervical vertebrae, the cerebellum and the spinal cord with blood. They arise from the subclavian arteries and travel to the base of the brain via the holes (transverse foramen) in the transverse processes of the cervical vertebrae and enter the skull via the foramen magnum.
The Circle of Willis is formed at the base of the brain by the cerebellar branches of the basilar artery, the internal carotid arteries and the cerebral arteries of the internal carotid artery. The vessels join together via communicating branches to form a circle of anastomosing vessels around the pituitary gland and optic chiasma. This arrangement is very important if one of the vessels becomes occluded or damaged, as it provides an alternative continuous blood supply to the brain.
The blood supply to the upper limbs is derived from branches of the subclavian arteries. The left subclavian artery arises directly from the arch of the aorta, and the right subclavian artery arises from the brachiocephalic trunk. Both subclavian arteries travel laterally towards the shoulder and pass under the clavicles. Once they pass over the lateral border of the first rib the arteries are renamed the axillary arteries. The axillary arteries pass through the axilla (armpit) giving off branches to the shoulder joint. They are renamed the brachial arteries as they pass under teres minor and descend into the arm. The brachial arteries descend along the medial side of the arm supplying the flexors of the arm. It divides in the front of the elbow to become the radial and ulnar arteries. The ulnar artery passes along the ulnar (medial) side of the arm to the wrist where it forms the superficial palmar arch. The radial artery passes along the radial (lateral) side of the arm to the wrist where it forms the deep palmar arch. The superficial and deep palmar arches anastomose, and give rise to the digital arteries which supply the thumb and fingers.
The blood supply to the lower limbs is derived from the common iliac arteries, which are a direct continuation of the descending abdominal aorta. The common iliac arteries are short and soon divide into internal and external iliac arteries. The internal iliac arteries divide into anterior and posterior trunks which supply the gluteal region (buttocks) the pelvic muscles and the external genitalia. The external iliac arteries and its branches supply the entire lower limb. It passes along the medial edge of psoas major to pass underneath the inguinal ligament where it is renamed the femoral artery. The femoral artery travels through the femoral triangle on the front of the thigh, leaving it via its apex to reach the adductor canal where it is surrounded by the adductor muscles. The femoral artery leaves the adductor canal via the adductor hiatus in adductor magnus to enter the back of the knee (popliteal fossa) as the popliteal artery. The popliteal artery passes through the popliteal fossa and divides into the anterior and posterior tibial arteries. The anterior tibial artery descends through the leg on the front of the interosseous membrane. On reaching the ankle joint it becomes the dorsalis pedis artery which supplies the dorsum (top) of the foot. The posterior tibial artery descends along the back of the leg on top of the tibialis posterior muscle. On reaching the back of the ankle it splits into medial and lateral plantar arteries which travel into the sole (plantar aspect) of the foot which they supply.
It is this stretching and recoiling of the arteries as the heart beats that can be felt as a pulse, in the large arteries located close to the skin. The pulse can be used to monitor the rate at which the heart is beating; usually at rest an adult heart beats 60-70 times a minute.
It reaches its highest when the left ventricle is pushing blood into the aorta which is known as systolic pressure, and reaches its lowest when the ventricles are relaxed, known as diastolic pressure. When blood pressure is measured it is recorded using two numbers to represent the systolic and diastolic pressures, which in a healthy person should be 120/80mg Hg (120 over 80)
It is normal for your blood pressure to fluctuate and can increase when active or excited. Some people suffer with continuous high blood pressure (hypertension), the cause unknown, which can go undetected for years as it generally does not show any symptoms. Other people suffer with high blood pressure as part of another condition they have such as diabetes or kidney disease. High blood pressure means that the heart has to work a lot harder and can lead to a large number of adverse affects including stroke, heart disease and kidney disease. Unusually low blood pressure (hypotension) can also cause problems such as fainting and dizziness and should be investigated in case there is an underlying cause.
Veins return the deoxygenated blood from the body back to the heart. Their route is more variable than that of the arteries and they tend to anastomose (join with other vessels) to create complex networks. Veins can be described as being deep or superficial. The superficial veins form a variable network in the the subcutaneous fat just below the skin. The deep veins lie deeper and usually follow the arteries of the same name.
Blood from the brain, scalp and face all drain into the internal jugular veins which can be found either side of the neck under sternocleidomastoid. The internal jugular vein unites with the subclavian vein to form the brachiocephalic vein which drains into the superior vena cava. Blood from the parotid gland, base of the skull, maxilla and neck all drain into the external jugular veins which can be found either side of the neck on top of the sternocleidomastoid muscle. The external jugular vein drains into the subclavian vein. The superior vena cava receives blood from the head, neck, upper limb and breast. It is formed by the union of the two brachiocephalic veins and drains blood into the right atrium.
Blood from the upper limb is drained by a network of deep and superficial veins. Blood from the deep parts of the hand and forearm is drained into the ulnar and radial veins. At the elbow these veins unite to form the brachial vein which at the shoulder becomes the axillary vein. As the axillary vein passes over the lateral border of the 1st rib it is renamed the subclavian vein which drains into the brachiocephalic vein.
Blood is drained from the trunk into the inferior vena cava.
Blood from the lower limb is drained by a network of deep and superficial veins. Blood from the deep parts of the sole of the foot is drained by the medial and lateral plantar veins which unite at the back of the ankle to form the posterior tibial vein. Blood from the deep parts of the dorsum of the foot is drained by the dorsalis pedis vein which becomes the anterior tibial vein as it passes into the ankle. The anterior and posterior tibial veins ascend in the calf to the back of the knee where they unite to form the popliteal vein. This travels through the popliteal fossa until it passes through the adductor hiatus into the anterior thigh where it is renamed the femoral vein. The femoral vein ascends to the groin and passes under the inguinal ligament to become the external iliac vein.