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1. Blood flows through a network of specialized vessels that can be broadly categorized as arteries, capillaries, and veins.

Histological Organization

1. The walls of arteries and veins contain three layers, the tunica externa, tunica media, and tunica interna.

2. The tunica externa forms a connective tissue sheath that stabilizes the position of the vessel.The tunica media contains smooth muscle and connective tissue fibers, often elastin. The tunica interna includes the endothelium and its underlying elastic membranes.


1. The arterial system includes the elastic arteries, muscular arteries, and arterioles. As you proceed toward the capillaries the number of vessels increases but the diameter of the individual vessels decreases and the walls become relatively thin.


1. Capillaries may be continuousor fenestrated. Sinusoids are irregular, flattened capillary channels found in specialized tissues.

2. Diffusion across capillary walls depends on the organization of the endothelium, the size of the diffusing molecule, and its lipid solubility. Suggestion: Emphasize that exchange with the interstitial fluids only occurs across capillary walls.

3. Individual capillaries are usually part of an organized capillary plexus, or capillarybed. Precapillary sphincters determine the relative volume of flow through each of the capillaries.

4. Blood flow through a capillary plexus changes asvasomotion occurs. The entire network may be bypassed by blood flow through arteriovenous anastomoses or via preferred channels within the capillary plexus.


1. Venules are small veins that collect the blood leaving a capillary network. They merge into medium-sized veins ; these convey the blood to large veins including the superior and inferior venae cavae.

2. The arterial system contains blood under relatively high pressures. Venous blood is under relatively little pressure, and special valves are necessary to prevent the backflow of blood.

3. Pressure changes in the pleural cavity as a result of respiratory movements assists in moving blood towards the heart.

The Distribution of Blood:

1. Peripheral venoconstriction may represent an important means of increasing the blood volume following hemorrhaging.The venous reserve accounts for about 20 percent of the normal blood volume.


The Pulmonary Circulation

1. The pulmonary circuit includes the pulmonary trunk, the right and left pulmonary arteries, pulmonary capillaries, and four pulmonary veins that empty into the left atrium.

The Systemic Circulation

The Arterial System:

1.The ascending aorta gives rise to the coronary circulation. The aortic archcommunicates with the descending aorta.

2.The aortic arch gives rise to the brachiocephalic, left common carotid, and left subclavian arteries.The right common carotid and right subclavian arteries arise from the brachiocephalic.

3. The carotid divides into an external carotid servicing the superficial structures of the head and an internal carotid that enters the skull at the carotid foramen.

4. The vertebral arteriesarise from the subclavians, ascend within the transverse foramina of the cervical vertebrae, and enter the skull through the foramen magnum.

5. The carotids supply the rostral portions of the cerebrum, and the vertebrals supply the rest of the brain. Anastomoses between these vessels at the circle of Willisensure a reliable supply of blood.

6. The capillaries of the brain are relatively impermeable to materials other than dissolved gases; this represents the blood- brain barrier.

7. The descending aorta travels through the thoracic cavity within the mediastinum. The diaphragm marks the boundary between the thoracic and abdominal divisions of the descending aorta.

2. The thoracic aorta provides blood to the intercostal and superior phrenic arteries. The abdominal aorta services the inferior phrenics as well as the celiac, superior and inferior mesenterics, the gonadal, thesuprarenal, the renal, the lumbar, and the common iliac arteries.

The Venous System:

1. Peripheral structures often receive two sets of veins, superficial and deep. Shunting blood from one to another can play an important role in thermoregulationby encouraging or preventing heat loss to the external environment.

2. The small veins of the brain drain intovenous sinusesof the dura, such as the superior sagittal sinusof the falx cerebri. These sinuses drain into the internal jugular vein. The internal jugular vein descends parallel to the course of the superficial external jugular vein draining superficial structures.

3. The venous distribution generally resembles that of the arteries. Thesubclavian veins are joined by the external and internal jugulars to form the brachiocephalics (innominates). These unite to form the superior vena cava , which also receives blood from the body wall via the azygos complex of veins.

4. Blood returning fom the pelvis and legs enters the common iliac veins. These unite to form the inferior vena cava. The IVC also receives blood from the lumbar, gonadal, renal, suprarenal, phrenic, and hepatic veins en route to the heart.

5. The digestive viscera are drained by tributaries of the hepatic portal vein. This vessel breaks down into sinusoidal networks within the liver, where it mixes with arterial blood delivered by the hepatic artery, a branch of the celiac trunk.

Development of the Circulatory System

1. During fetal development the umbilical arteriescarry blood to the placenta. It returns via the umbilical vein and enters a network of vascular sinuses within the liver. The ductus venosus collects this blood and returns it to the inferior vena cava.

2. At this time the interatrial septa are incomplete, and the foramen ovale allows the passage of blood from the right atrium to the left atrium. The ductus arteriosus also permits the flow of blood between the pulmonary trunk and the aortic arch.These connections are closed shortly after birth.


1. Cardiovascular regulation maintains adequate blood flow to vital tissues and organs.

2. Flow rate is directly proportional to the pressure applied. Blood flows from regions of higher pressure to areas of relatively lower pressure.

3. Friction against the walls of the vessels and interactions between fluid molecules create a resistance that opposes fluid movement. The flow rate is inversely proportional to the resistance.

Blood Pressure

1. Blood circulates because the heart establishes a pressure gradient, with the pressure highest at the aorta and lowest at the entrance to the right atrium.

2. The difference in pressure from one end of the systemic circuit to the other represents the circulatory pressure.

Arterial Pressures:

1. Arterial pressures are highest during ventricular systole and lowest in ventricular diastole. The difference between these pressure readings constitutes the pulse pressure , and the mean arterial pressure lies partway between the diastolic and systolic pressures.

2. Blood pressure can be measured by determining the force exerted against the walls of muscular arteries; values are reported as systolic/diastolic pressures.

3. The blood pressure and pulse pressures fall as you proceed along the arterial network toward the peripheral capillaries due to elastic components in the vascular walls. Arteriolar pressures are relatively steady at about 30 mm Hg.

4. Capillary pressures decline from 30 mm Hg near the arterioles to 18 mmHg or less near the venules.

Venous Pressures:

1. Arterial pressures determine the rate of peripheral blood flow; venous pressures affect the venous return, and so influence cardiac output.

2. Venous pressures are low, and several mechanisms assist in propellng blood to the heart: valves, muscular pumps, and the thoracoabdominal pump are important factors that assist the venous return.

Peripheral Resistance

1. Most of the total peripheral resistance is provided by the peripheral resistance of the arterioles.

2. Vasodilation reduces peripheral resistance; vasoconstriction increases it. The status of peripheral arterioles is controlled by the vasomotor center in the medulla, which determines the vasomotor tone.

Circulatory Dynamics

1. The relationships between flow, pressure, and resistance can be summarized as: F (flow rate) = P (pressure)/R (resistance). For the arterial system, this relationship can be restated as:

CO = AP (arterial pressure) X PR (peripheral resistance)

(cardiac output is flow)

Variations in Cardiac Output:

1. Cardiac output may vary as the result of local factors, autonomic controls, or the presence of hormones.

2. Local factors include intrinsic regulation (Starling's law), changes in ionic composition, and physical damage to the myocardium.

3. Autonomic commands are issued by the cardiac centers of the medulla.

4. Hormonal control involves the adrenal hormones epinephrine and norepinephrine.

Variations in Peripheral Resistance:

1. Alterations in peripheral resistance may occur under local control, in response to commands issued by autonomic centers, or in response to circulating hormones.

2. Local adjustments occur as smooth muscles of the precapillary sphincters contract or relax in response to alterations in the characteristics of the surrounding interstitial fluid.

3. Autonomic commands are issued by the vasomotor center of the medulla.

4. Most of the hormones that affect peripheral resistance cause vasoconstriction; examples include ADH, angiotensin II, epinephrine and norepinephrine. Atrial natriuretic factor causes vasodilation.

Alterations in Blood Pressure:

1. Cardiac output and peripheral resistance are controlled directly, by local, neural, or hormonal mechanisms. Blood pressure is controlled indirectly, primarily via alterations in cardiac output, peripheral resistance, and blood volume.


1. Vasomotion in peripheral tissues does not normally affect blood pressure. But if a general vasodilation occurs, blood pressure and peripheral blood flow declines.

2. Central control mechanisms respond to changes in blood pressure or alterations in the concentration of carbon dioxide or oxygen in the blood or cerebrospinal fluid.

3. Short-term responses adjust cardiac output and peripheral resistance to stabilize blood pressure and maintain tissue blood flow.

4. Long-term adjustments involve alterations in blood volume that affect cardiac output and the transport of oxygen to peripheral tissues.

5. The nervous and endocrine systems direct short-term responses; hormones and local factors control long-term adjustments. Neural Mechanisms and the Short-Term Regulation of Blood Pressure

The Baroreceptor Reflexes:

1. A rise in blood pressure inhibits the cardioacceleratory center, stimulates the cardioinhibitory center, and inhibits the vasomotor center. Cardiac output and peripheral resistance decline, and so blood pressure falls as a result.

2. A decline in blood pressure stimulates the cardioacceleratory center, inhibits the cardioinhibitory center, and stimulates the vasomotor center. The increased cardiac output and peripheral resistance elevates blood pressure and improves peripheral blood flow.

3. Examples of baroreceptor reflexes include the aortic, carotid sinus, and atrial reflexes.

The Chemoreceptor Reflexes:

1. The chemoreceptor reflexes respond to alterations in dissolved oxygen and/or carbon dioxide concentrations at the carotid bodies, the aortic bodies, or the cerebrospinal fluid.

2. Decreased oxygen or increased carbon dioxide concentrations leads to elevations in peripheral resistance, cardiac output, and blood pressure, increasing peripheral blood flow.

Autonomic Activation and Higher Centers:

1. The cardiac and vasomotor centers may also be influenced by activities in other areas of the brain. Sympathetic activation leads to stimulation of the cardioacceleratory and vasomotor centers; parasympathetic activation stimulates the cardioinhibitory center.

Hormones and Cardiovascular Regulation

1. Hormones involved with blood pressure regulation over the short-term include epinephrine and norepinephrine. These hormones promote peripheral vasoconstriction and venoconstriction, and increase the cardiac output.

2. Antidiuretic hormone and angiotensin II promote peripheral vasoconstriction in addition to their other functions.

3. ADH and aldosterone promote the retention of water and electrolytes and stimulate thirst.

4. Erythropoietin stimulates red blood cell production, adding to the volume of whole blood and assisting in the delivery of oxygen to peripheral tissues.

5. Atrial natriuretic factor encourages fluid loss, reduces blood volume, and inhibits thirst.

Local Factors Affecting Blood Volume and Blood Pressure

1. Local controls exist because changes in capillary hydrostatic and osmotic pressures alter the rates of filtration and resorption across capillary walls.

2. Under normal conditions a capillary loses slightly more fluid than it gains along its length. The excess interstitial fluid gets removed by the lymphatic system.

3.Shifting the dynamic center can result in the recall of fluidsor the production of tissue edema.


Exercise and the Cardiovascular System

1. During exercise blood flow to the skeletal muscles increases, at the expense of circulation to non-essential organs.Cardiac output can rise 6 times, as the result of increased heart rate and a reduced end systolic volume.

2. Cardiovascular performance improves with training. Athletes have a larger stroke volume, slower resting heart rates, and increased cardiac reserves.

The Cardiovascular Responses to Hemorrhaging

The Elevation of Blood Pressure:

1. Hemorrhaging provokes an increase in cardiac output, a mobilization of venous reserves, peripheral vasoconstriction, and the liberation of hormones that promote the retention of fluids and the manufacture of erythrocytes.

2. A fall in blood pressure at the carotid sinus to below 50 mm Hg will produce the central ischemic response. If tissue controls override this mechanism, a circulatory collapse occurs and death results.

The Restoration of Blood Volume:

1. After a severe hemorrhage, several days pass before blood volume returns to normal. Adjustments over this period are primarily directed by ADH, angiotensin II, aldosterone, and erythropoietin.