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an ha 6. Describe the factors that affect mean arterial pressure. Include rect mean arterial pressure. Include in your description the terms cardiac output, peripheral resistance blood volume, and distribution of blood between arteries and veins. For each the effect (does it increase or decrease MAP?) and mechanism (how does it increase or distribution of blood between arteries and veins. For each factor, make sure you discuss Fase MAP?) and mechanism (how does it increase or decrease MAP?).

Answer 1

Blood pressure increases with increased cardiac output, peripheral vascular resistance, volume of blood, viscosity of blood and rigidity of vessel walls.

Blood pressure decreases with decreased cardiac output, peripheral vascular resistance, volume of blood, viscosity of blood and elasticity of vessel walls.

Five factors influence blood pressure:

1.CARDIAC OUTPUT-

Cardiac output is the volume of blood flow from the heart through the ventricles, and is usually measured in litres per minute (L/min). Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow. These factors include sympathetic stimulation, the catecholamines epinephrine and norepinephrine, thyroid hormones, and increased calcium ion levels. Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume or both, will decrease arterial pressure and blood flow. These factors include parasympathetic stimulation, elevated or decreased potassium ion levels, decreased calcium levels, anoxia, and acidosis.

When MAP is elevated, increasing baroreceptor stimulation, the nucleus tractus solitarius decreases sympathetic output and increases parasympathetic output. The increase in parasympathetic tone will decrease myocardial chronotropy and dromotropy, with less pronounced effects on inotropy and lusitropy, via the effect of acetylcholine on M2 muscarinic receptors in the myocardium. The result is a decrease in cardiac output and a subsequent decrease in MAP.

2.PERIPHERAL VASCULAR RESISTANCE-

Peripheral vascular resistance refers to compliance, which is the ability of any compartment to expand to accommodate increased content. Veins are more compliant than arteries and can expand to hold more blood. When vascular disease causes stiffening of arteries, compliance is reduced and resistance to blood flow is increased. The result is more turbulence, higher pressure within the vessel, and reduced blood flow. This increases the work of the heart.

Mean arterial blood pressure (MAP) and systemic peripheral resistance are increased (up to 35% and 160%, respectively) at operative levels of pneumoperitoneum (12 to 15 mm Hg), presumably as a result of sympathetic vasoconstriction from hypercarbia. Cardiac index may increase 20%. As intra-abdominal pressure increases more than 20 mm Hg, cardiac output falls and abdominal venous compliance decreases.

3.VOLUME OF CIRCULATING BLOOD-

Volume of circulating blood is the amount of blood moving through the body. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. Baroreceptors are pressure-sensing receptors. As the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac centre responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase HR. The opposite is also true.

4.VISCOSITY OF BLOOD-

Viscosity of blood is a measure of the blood’s thickness and is influenced by the presence of plasma proteins and formed elements in the blood. Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water. The viscosity of blood has a dramatic effect on blood pressure and flow.

Increasing the blood volume increases the preload, increasing the stroke volume, and therefore increasing cardiac output. Afterload also affects the stroke volume in that an increase in afterload will decrease stroke volume. Heart rate is affected by the chronotropy, dromotropy, and lusitropy of the myocardium.

5.ELASTICITY OF VESSEL WALLS-

Elasticity of vessel walls refers to the capacity to resume its normal shape after stretching and compressing. Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibres allow them to expand as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump and maintain adequate pressure and flow. Artery walls would have to become even thicker in response to this increased pressure.

Systemic vascular resistance is determined primarily by the radius of the blood vessels. Decreasing the radius of the vessels increases vascular resistance. Increasing the radius of the vessels would have the opposite effect. Blood viscosity can also affect systemic vascular resistance. An increase in hematocrit will increase blood viscosity and increase systemic vascular resistance.