Part 3

Blood Pressure

Although this force is present throughout the vascular system, the term blood pressure most commonly refers to pressure to the arteries supplied by branches of the aorta (systemic arteries). This is known as clinical blood pressure and is what is measured for medical applications.

Arteries

Are strong, elastic vessels adapted for transporting blood away from the heart under relatively high pressure.

Arterioles

Arteries subdivided into progressively thinner tubes and eventually give rise to these which are finer and branched.

Blood Pressure

BP is determined by cardiac output (CO) and peripheral resistance (PR) according to BP= CO X PR. Maintenance of normal arterial pressure therefore requires regulation of those two factors.

Baroreceptors

Cardiac output and peripheral resistance are controlled in part by baroreceptor reflexes. Are sensory receptors in the aortic arch and carotid arteries that sense change in the blood pressure. If arterial pressure increases, impulses travel from the baroreceptors to the cardiac center of the medulla oblongata. This center relays parasympathetic impulses to the SA node in the heart, and the heart rate decreases in response. As a result of this CARDIOINHIBITOR REFLEX, cardiac output fails, and blood pressure decreases toward the normal level. Conversely decreasing arterial blood pressure initiates the CARDIOACCELERATOR REFLEX, which sends sympathetic impulses to the SA node. AS a result, the heart beats faster, increasing cardiac output and arterial pressure.

Viscosity

Difficulty with which the molecules of the fluid flow past one another. The greater the viscosity, they greater the resistance flow. Blood cells and plasma increase viscosity. The greater the blood's resistance to flowing, the greater the force needed to move it through the vascular system. Thus, it is not surprising that blood pressure rises as blood viscosity increases and drops as viscosity decreases.

Veins

Formed by venules that continue from the capillaries and merge. Transport blood back to the atrial, follow pathways that roughly parallel those of the arteries. The walls of veins are similar to those of arteries in that they are composed of three distinct layers. However, the middle later of the venous wall is much thinner than that of the arterial wall. Consequently, veins have thinner than that of the arterial walls that have less smooth muscles and less elastic connectives tissue than those of comparable arteries. The lumens of veins have greater diameter. Many veins, particularly those in the upper and lower limbs, are buried in skeletal muscles and have valves, which project inward from their linings. Most valves are composed of two leaflets that close to prevent backflow of blood in a vein. The contraction of the muscles squeezes the vessels, pushing blood toward the heart. The valves also aid in returning blood to the heart because they open as long as the blood flow is toward the heart, but prevent flow in the opposite direction.

Atherosclerosis

In the arterial disease, deposits of fatty materials, particularly cholesterol, form within and on the inner lining of the arterial walls. Such deposits, called plaque, protrude int o the lumens of vessels and interfere with blood flow. Furthermore, plaque often forms a surface texture that can initiate formation of a blood clot, increasing the risk of developing thrombin or emboli that result in inadequate blood flow (ischemia) downstream from the obstruction causing tissue death (necrosis). Risk factors or developing this include a fatty diet, elevated blood pressure, tobacco smoking, obesity, and lack of physical exercise. Genetic factors may also increase the risk.

Hypertension (high blood pressure)

Is persistently elevated systemic arterial pressure. It is one of the more common diseases of the cardiovascular system.

Systolic Pressure

Maximum pressure during the ventricular contraction.

Capillaries

Microscopic, thin-walled vessels connect the smallest arterioles and the smallest venules. Capillaries are extensions of the inners linings of arterioles in that their walls are endothelium. The thin wall of capillaries form the semipermeable layer through which substances in the blood are exchanged for substances in the tissue fluid surrounds body cells. Exchange of oxygen, carbon dioxide, and nutrients between blood and tissue fluid.

Tunica Media

Middle layer makes up the bulk of the arterial wall. It includes smooth muscle cells, which encircle the tube, and a thick layer of elastic connective tissue.

Vasomotor Center

Of the medulla oblongata continually send sympathetic impulses to smooth muscle in the arteriole walls, keeping them in a state of tonic contraction. This action helps maintain the peripheral resistance associated with normal blood pressure. Because the vasomotor center responds to changes in blood pressure, it can increase peripheral resistance by increasing its outflow of sympathetic impulses, or it can decrease such resistance by decreasing its sympathetic outflow. In the latter case, the vessels vasodilate as sympathetic stimulation decreases.

Tunica EXterna

Relatively thin and chiefly consists of connective tissue with irregular elastic and collagen fibers. This layer attaches the artery to the surrounding tissues.

Arterial Blood Pressure

Rises and falls in a pattern corresponding to the phases of the cardiac cycle. That is, contraction of the ventricles (ventricular systole) squeezes blood out and into the pulmonary trunk and aorta, which sharply increases the pressure of the arteries. Depends on a variety of factors. These include cardiac output, blood volume, peripheral resistance, and blood viscosity.

Colloid Osmotic Pressure

Th pressure of an impermeant solute on one side of a cell membrane creates an osmotic pressure. Plasma proteins trapped in the capillaries creat an osmotic pressure that draws water into the capillaries. He term colloid osmotic pressure describes this osmotic affect due solely to the plasma proteins. The effect of capillary blood pressure, which favors filtration, opposes the plasma colloid osmotic pressure, which favors reabsorption. At the arteriolar end of the capillaries, the blood pressure is higher than the colloid osmotic pressure, so filtration predominates here. At the venular end, the colloid osmotic pressure is essentially unchanged, but the blood pressure has decreased due to peripheral resistance through the capillary, so reabsorption predominates.

Venules

The microscopic vessels that continue from the capillaries and merge to form veins.

Pulse

The surge of blood entering the arterial system during a ventricular contraction distends the elastic arterial walls, but the pressure begins to drop almost immediately as the contraction ends, and the arterial walls recoil. This alternative expanding and recoiling of the arterial wall can be felt as a pulse in an artery that runs close to the body surface. The radial artery is commonly used to take a person's pulse. Other sites where an arterial pulse is easily detected includes the carotid, brachial, femoral, and dorsalis pedis arteries.

Vasomotor Fibers

The sympathetic branches of the autonomies nervous system innervate smooth muscle in the artery and arteriole walls. Stimulates the smooth muscle to contract, reducing the diameter of the vessel, lessening the blood flow.

Cardiac Output

The volume discharged from the ventricle per minute. It is calculated by multiplying the stroke volume by the heart rate in bets per minute. For example, If the stroke volume is 70 milliliters per beat and the heart rate is 72 beats per minute, the cardiac outside is 5,040 milliliters per minute. Blood pressure varies with cardiac output. If either the stroke volume or the heart relate increases, so does the cardiac output, and blood pressure increases. Conversely, if the stroke volume or the heart rate decreases, the cardiac output decreases, and blood pressure decreases.

Tunica INterna.

The wall of an artery consists of three distinct layers. Inner most layer, composed of squamous epithelium called ENDOTHELIUM. Rests on a connective tissue membrane that is rich in elastic collagen fibers. Helps prevent blood clotting by providing a smooth surface that allows blood cells and platelets to flow through the vessel without being damaged and by secreting biochemicals that inhibit platelet aggregation. Endothelium also may help regulates local blood flow by secreting substances that dilate or constrict blood vessels. For example, endothelium releases the gas nitric oxide, which relaxes the smooth muscle of the vessel.

Vasoconstriction

Vasomotor fibers stimulate the smooth muscle to contract, reducing the diameter of the vessel, lessening blood flow.

Diastolic Pressure

When the ventricles relax (ventricular diastole), the arterial pressure drops, and the lowest pressure that remains in the arteries before the next ventricular contraction

Blood volume

equals the sum of the formed elements and plasma volumes in the vascular system. Although the blood volume varies somewhat with age, body size, and sex, it is usually about 5 liters for adults, or 8% of body weight in kilograms. Normally blood pressure is proportional to blood volume in the cardiovascular system. Thus, any change in blood volume can initially alter the blood pressure. For example, if a hemorrhage reduces blood volume, blood pressure initially drops.

Peripheral Resistance

friction between blood and walls of blood vessels, which hinders blood flow. Factors that alter the peripheral resistance change blood pressure. For example, contracting these vessels. Blood backs up into the arteries supplying the arterioles, and the arterial pressure rises. Dilation of arterioles has the opposite effect- peripheral resistance decreases, and arterial blood pressure drops in response.

Vasolidation

if vasomotor impulses are inhibited, the smooth muscle cells relax, the diameter of the vessels increases, allowing greater blood flow.

Stroke Volume

the volume of blood discharged from the ventricle with each contraction and equals about 70 milliliters in an average-weight male at rest.