Biology 238

Class Notes

The Circulation |

Types and structures of the vessels:

Elastic arteries - the aorta and its branches to the organs and tissue areas. Virtually all of the arteries listed in your objectives are elastic arteries. They are called conducting arteries because they conduct blood to these major areas. Along with the connective tunica adventitia (externa) and endothelial tunica interna (intima) elastic arteries have thick walls with a tunica media made of smooth muscle and elastic tissue, including an internal and sometimes an external elastic lamina. The elastic tissue enables these arteries to withstand the pulse pressure.

Smaller muscular arteries are the distributing vessels which carry blood into an organ and tissue area. They have little or no elastic tissue, experience much less pulse pressure, but their muscle is important for vasoconstriction in regulating blood pressure.

Capillaries: These are the thinnest vessels, functioning to allow transport through their walls to and from the blood and tissues. They come in several types:

Continuous capillaries are formed of a single layer of simple squamous epithelium, a continuation of the endothelial lining, held together by tight junctions allowing only small molecules to pass. They are found in the brain, muscles, the skin, the lungs and many other organs.

Fenestrated capillaries have pores to increase transport. They are found in the absorptive capillaries in the GI tract, in glands, in the glomerulus of kidneys, etc.

Sinusoids are chamber-like vessels which allow blood from several sources to mix. They are found in the liver, spleen, and lymph nodes.

Arteriovenous anastomosis - a direct connection between arteries and veins which allows blood to bypass the capillary bed. This is important in the skin and GI tracts especially. In the skin blood is shunted away from the skin to conserve heat, and into the skin release heat to the environment. In the GI tract the blood is shunted way during sympathetic stimulation in "Fight or Flight", exercise and the like. The vessel which acts as the shunt is called a metarteriole and capillary sphincters regulate the blood flow.

Venules and veins:

Blood leaving the capillaries returns to the heart through the venous system, beginning with venules and progressing to larger and larger veins which lead to the superior and inferior vena cavae which enter the right atrium. These veins are low resistance conduits back to the heart. They are thin walled, usually flowing partially collapsed, and are larger in internal diameter that arteries and the same level. The pressure in veins is very low and actually dips below zero during right ventricular diastole. At rest about 60% of your total blood volume is in your systemic veins, this blood acting as a reservoir which can be moved into the systemic arterial system to distribute to the muscles or skin during exercise. Exercise stimulates venous return (and lymph return) through the skeletal muscular pump  and the semilunar valves of the large veins and lymph vessels.

As blood flows through the capillaries it experiences a cycle of filtration and osmosis (sometimes referred to as Starling's Law of the Capillaries). At the arterial end capillary blood pressure (hydrostatic pressure) is highest and overcomes osmotic pressure to produce a net outward pressure (NFP, Net Filtration Pressure) of about 10 mmHg. This causes water and dissolved substances to leave the capillaries. At the venous end the hydrostatic pressure is much less and there is a net pressure into the capillary due to osmosis of about 8 mmHg. This brings water back into the blood and helps to maintain the volume and pressure of the blood. Not all the filtered water return, however, and the function of the lymphatic system is to return this excess water to the circulation. Without effective lymphatic action the tissues accumulate fluid and experience edema.


Atherosclerosis - the development of fatty cholesterol-containing plaque along the lining of arteries. The stimulus is thought to be damage to vessel walls, from hypertension for instance, or from certain chemical agents, bacteria, viruses and the like. However very often the stimulus is unknown. Macrophages responding to this damage cause inflammation and begin to accumulate abundant cholesterol and LDL (Low Density Lipoproteins). Smooth muscle cells grow into the lining and form the framework of the plaque incorporating the fat-laden macrophages.

This plaque reduces the blood causing ischemia (reduced oxygenated blood supply). When this happens in the coronary arteries of the heart it leads to impaired myocardial metabolism. Myocardial cells cannot function without oxygen quickly forming lactic acid and becoming fatigued. The lactic acid produces the burning felt as chest pain. In the brain the result is TIAs, Transient Ischemic Attacks with symptoms of dizziness or fainting, visual impairment, slurred speech, etc.

Damage to the endothelium exposes the blood to collagen plus other clot- stimulating chemicals. This encourages the formation of a clot or thrombus which can totally block oxygenated blood flow. This results in myocardial infarction in the coronary arteries, a heart attack. (Infarction means tissue death resulting from inadequate oxygenated blood supply.) In the brain, infarction causes a stroke, now often called a "brain attack".

Varicose veins are hereditary malformations in which the veins and/or their semilunar valves are stretched and ineffective in returning blood to the heart. The condition is exacerbated by standing for long periods, or other activities which allow the blood to pool due to inertia. A sedentary lifestyle also contributes because exercise is an important component in venous return via the muscular pump. (See Figure 20.6) Varicose veins also form the "spider veins" seen under the skin, hemorrhoids when they occur in the large intestine, and esophageal varicies in alcoholics. Varicose veins usually lead to phlebitis and subsequent thrombosis.

Blood Distribution:

Examining Figure 20.12 shows how blood is distributed at rest and compares it to the distribution with exercise. Notice that distribution to the brain does not change, while much more blood goes to the muscles and the skin at the expense of the kidneys and GI tract. A very important component which provides blood for this redistribution is the venous reservoir.

The Vasomotor Center:

The vasomotor center in the medulla of the brain is responsible for the overall control of blood distribution and pressure throughout the body. Impulses from the vasomotor center are mostly in the sympathetic nervous system (exception: those to the genitalia) and mostly cause vasoconstriction (exception: the skeletal muscles and coronary arteries which are vasodilated). Inputs to the vasomotor center are similar to those innervating the cardiac center: baroreceptors located throughout the body and the hypothalamus. The baroreceptors allow maintenance of normal blood pressure. The hypothalamus stimulates responses associated with exercise, emotions, "Fight or Flight", and thermoregulation.


Autoregulation produces local control of blood flow to an organ or tissue area according to tissue needs. Autoregulation is usually short-term and can enhance or override the vasomotor center.

There are two mechanisms of autoregulation:

myogenic - a direct response of smooth muscle to maintain normal blood flow. In response to increasing pressure the small arteries and arterioles vasoconstrict, thus maintaining constant blood flow despite the increase in pressure.

metabolic - a response through a local neural circuit which controls blood supply in response to oxygen, carbon dioxide, nutrients, wastes, metabolites, pH, etc.

Examples of autoregulation:

skeletal muscle: the response to increased carbon dioxide, increased metabolites, and decreased oxygen is vasodilation of the supplying blood vessels. This results in the most blood directed to the most active muscles. This is part of the exercise hyperemia (increased blood supply) which can reach tenfold.

the heart: coronary arteries vasodilate mostly in response to increased carbon dioxide. Circulation to the heart can increase 3 to 4 times with exercise.

the brain: blood supply to the brain is maintained nearly constant under all conditions. The main mechanism responsible for this is myogenic autoregulation of cerebral arteries. In addition, the brain is very sensitive to carbon dioxide and the lowered pH it brings. In response to a moderate increase in CO2 the cerebral arteries vasodilate somewhat to flush more blood through the brain. In response to a significant increase in CO2 however the arteries vasoconstrict. Since CO2 is coming from outside the brain this effectively shuts of the source. The brain will literally go into a coma to protect it from the damaging effects of low pH.

the skin: during low temperature the vasomotor center will reduce blood flow to the skin to conserve heat. In order to protect local areas from damaging ischemia blood will be restored briefly through local auto-regulatory vasodilation.

the lungs: [See Figure 23.19] since the lungs are the source for blood oxygenation the auto-regulatory mechanisms here are opposite in effect to those in systemic organs. The object of autoregulation in the lungs is to route blood to the best ventilated alveoli (air sacks which allow gas transport into/out of the blood). In response to increased oxygen in blood draining a segment of the lungs arteries leading to that segment will vasodilate, thus increasing the blood entering that area. Low oxygen will stimulate vasoconstriction of the vessels, thus routing blood away from those less well ventilated areas.

Selected Circulatory Pathways | See Also: [Upper Vessels] [Lower Vessels]

Coronary circulation:

Coronary arteries leave the aorta behind the semilunar valves. This means that they fill during ventricular diastole. The right coronary artery branches to smaller arteries including the marginal, which leads down the margin or edge of the right ventricle. The main portion of the right coronary artery proceeds to the back of the heart becoming the posterior interventricular. The left coronary artery divides to form the circumflex which curves to the back of the heart, and the anterior inter-ventricular which descends between the two ventricles. The arteries anastomose  to provide collateral circulation to the ventricular myocardium.

Coronary veins drain the myocardium from the anterior interventricular area through the great cardiac (or coronary) vein, from the right atrial area through the small cardiac vein, and from the posterior interventricular area through the middle cardiac vein. All of these come together to form the coronary sinus which drains directly into the right atrium, the only systemic venous drainage not through the vena cavae.

The Circle of Willis - provides collateral blood flow to the brain.

The Circle of Willis is supplied by:

the right vertebral artery, a branch off the right subclavian, and the left vertebral artery, a branch off the left subclavian. The vertebral arteries merge to become the basilar artery before entering the Circle of Willis. The left and right internal carotid arteries also lead to the Circle of Willis. (See Table 20.4). Communicating arteries complete the circle and the anastomosis which then leads to anterior, medial, and posterior cerebral arteries which carry blood into the brain.

The hepatic portal system carries the blood from the GI tract and spleen to the liver before it enters the inferior vena cava and the general circulation. This is needed because this blood has digestive end-products and absorbed toxins from the GI tract and bilirubin from hemoglobin destruction in the spleen. The liver is in charge of processing these substances. Blood in the splenic vein from the spleen receives blood from the inferior mesenteric vein draining the large intestine, and then combines with the superior mesenteric vein coming from the small intestine to form the portal vein (hepatic portal vein) which enters the liver. The liver also receives oxygenated blood through the hepatic artery and blood from these two sources mixes in liver sinusoids which are lined with the hepatocytes (liver cells). Once processed by these hepatocytes the blood returns to the circulation through the hepatic vein.

The Fetal Circulation  differs from that after birth because the lungs are mostly bypassed. This is because the fetus receives oxygen and gets rid of carbon dioxide and wastes through the placenta. Umbilical arteries, branches from the internal iliacs, take blood from the fetal circulation to the placenta. Oxygenated blood returns from the placenta to the fetal circulation through the umbilical veins. These veins travel through the ductus venosus into the inferior vena cava. At this point the blood becomes mixed as it travels back to the right atrium.

From the right atrium there are two alternatives. Blood may travel in the usual route to the right ventricle and then to the pulmonary arteries to the lungs, and about half the blood goes toward this pathway. But most travels through a detour, the foramen ovale (literally oval hole) to the left atrium. What does pass to the right ventricle and then the pulmonary trunk is mostly diverted by a shunt, the ductus arteriosus, into the aorta. Only about 10% of the blood flow enters the pulmonary circuit.

The fetal structures close at or shortly after birth. The ductus venosus becomes part of the falciform ligament which supports the liver. The foramen ovale reflexively closes due to strong heart contractions after birth and normally grows completely closed within a couple of weeks. Failure of the foramen ovale to close produces atrial septal defect. The ductus arteriosus constricts at birth and later becomes the ligamentum arteriosum seen as a remnant attaching the aorta and pulmonary trunk. Failure of the ductus arteriosus to close is called a patent ductus. In both cases surgery is necessary.

Revised: September 17, 2006