(blood pressure) is 120/80 mmHg. The higher the resistance, the higher the
blood pressure.
The total volume of blood expelled by the heart in a minute is referred to as
cardiac output. The average cardiac output is 4 to 8 L/min. The amount of blood
ejected from the left ventricle during each heartbeat is called the stroke volume.
The average stroke volume is 70 mL/beat.
As blood flows into the ventricles the ventricles fill and stretch. The force
in which blood flows into the ventricle is called preload. The force used to con-
tract the ventricle is called the heart’s contractility. The resistance to blood
ejected by the ventricle is called the afterload. Afterload is the opposing pressure
in the aorta and systemic circulation to the contraction of the ventricle.
Drugs can be used to ease the workload of the heart by increasing or decreas-
ing the preload and afterload resulting in adjusting the stroke volume and car-
diac output. Vasodilators decrease the preload and afterload decreasing arterial
pressure and cardiac output. Vasopressors increase the preload and afterload
increasing the arterial pressure and cardiac output.
CIRCULATION
There are two types of circulation.
1.Pulmonary circulation. The pulmonary circulation is when the heart pumps
deoxygenated blood that contains CO 2 from the right ventricle through the
pulmonary artery to the lungs. Oxygenated blood returns to the left atrium
by the pulmonary vein.
2.Systemic (peripheral) circulation. In systemic circulation, the heart pumps
blood from the left ventricle to the aorta into the general circulation.
Blood is carried by the arteries to the arterioles down to the capillary beds
where nutrients in the blood are exchanged with waste products at the
cellular level. Blood then returns through the venules to the veins back to
the heart.BLOOD
Blood is composed of plasma, red blood cells (erythrocytes), white blood cells,
(leukocytes) and platelets. Plasma is the fluid component of blood that is
comprised of 90% water and 10% solutes, and constitutes 55% of the total
blood volume.
CHAPTER 19 Cardiac Circulatory Medications^349