McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

blood then circulates around the alveoli of the lungs, picking up oxygen and getting rid of carbon dioxide; flows through pulmonary capillaries (the tiny blood vessels that connect arteries and veins) into the pulmon­ ary veins; and then returns to the left atrium. Systemic circulation When the pressure of blood volume in the left atrium is greater than the pressure in the large left ventricle, this oxygenated blood flows into the left ventricle. The left atrium contracts and pushes any remaining blood into the left ventricle, which is stimulated to contract and generates tremendous pressure to push the blood out the aorta, carrying it throughout the body. The aorta and other large arteries have thick, muscular walls. The entire arterial system contains muscles in the walls of the vessels all the way to the terminal branches or arterioles, which consist of fragments of muscle and endothelial cells. These muscles offer resistance to the blood that is sent pumping into the arterial system by the left ventri­ cle, generating pressure. The arterial system is referred to as a resistance system . The vessels can either constrict or dilate, increasing or decreasing resistance, respect­ ively, based on the needs of the body. The arterioles are able to completely shut off blood flow to some areas of the body; that is, they can shunt blood to another area where it is needed more. The arterioles, because of their ability to increase or decrease resistance in the system, are one of the main regulators of blood pressure. Blood from the tiny arterioles flows into the capillary system, which connects the arterial and venous systems. These microscopic vessels are composed of loosely con­ nected endothelial cells. Oxygen, fluid and nutrients are able to pass through the arterial end of the capillaries

and enter the interstitial area between tissue cells. Fluid at the venous end of the capillary, which contains carbon dioxide and other waste products, is drawn back into the vessel. This shifting of fluid in the capillaries, called the capillary fluid shift, is carefully regulated by a balance between hydrostatic (fluid pressure) forces on the arterial end of the capillary and oncotic pressure (the pulling pressure of the large, vascular proteins) on the venous end of the capillary. In a normal situation, the higher pressure at the arterial end of a capillary forces fluid out of the vessel and into the tissue, and the now-concentrated proteins (which are too large to leave the capillary) exert a pull on the fluid at the venous end of the capillary to pull it back in. A disruption in the hydrostatic pressure or in the concentration of proteins in the capillary can lead to fluid being left in the tissue, a condition referred to as oedema. The capillaries merge into venules, which merge into veins, the vessels respons­ ible for returning the blood to the heart (Figure 42.7). The veins are thin-walled, very elastic, low-pressure vessels that can hold large quantities of blood if neces­ sary. The venous system is referred to as a capacitance system because the veins have the capacity to hold large quantities of fluid as they distend with fluid volume. These capacitance vessels have a great deal of influence on the venous return to the heart—the amount of blood that is delivered to the right atrium. Coronary circulation The heart muscle requires a constant supply of oxy­ genated blood to keep contracting. The myocardium receives its blood through two main coronary arteries that branch off the base of the aorta from an area called the sinuses of Valsalva. These arteries encircle the heart

Venous end

Arterial end

Capillary

Direction of blood flow

8 mmHg

–7 mmHg

FIGURE 42.7  The net shift of fluid out of and into the capillary is determined by the balance between the hydrostatic pressure (HP) and the oncotic pressure (OP). The HP tends to push fluid out of the capillary and the OP tends to pull it back into the capillary. At the arterial end of the capillary bed, the blood pressure is higher than at the venous end. At the arterial end, HP exceeds OP and fluid filters out. At the venous end, HP has fallen, and HP is less than OP; fluid is pulled back into the capillary from the surrounding tissue. The lymphatic system also returns fluids and substances from the tissues to the circulation.

Hydrostatic pressure (HP) Driving force of heart tends to push fluid out of capillary Oncotic pressure (OP) Pressure exerted by plasma proteins tends to pull fluid into capillary Filtration pressure Net force on fluid, determined by the balance between HP and OP

36 mmHg

21 mmHg

28 mmHg

28 mmHg

8 mmHg

–7 mmHg

HP > OP Fluid leaves capillary

HP < OP Fluid enters capillary