Lesson 1, Topic 1
In Progress

Physiology of the Blood

April 11, 2024

Learning Objective: Examine the physiology of the blood, including coagulation and the factors that affect the blood pressure.
      The components in the blood play an important role in the process of blood clotting or coagulation. Blood is also important because it carries nutrients and oxygen to the cells. Maintaining an adequate blood pressure helps with the delivery of nutrients and oxygen to the cells. Both the process of coagulation and an adequate blood pressure are needed to sustain life. The following sections examine the process of coagulation and the factors that affect blood pressure.

Learning Objective: Describe the process of coagulation.
      The process required to form a blood clot is complicated, with many specific chemical reactions and clotting factors involved. What follows is a simplified version of this process.
      First, a damaged vessel will constrict to slow the flow of blood through the vessel. In response to the injury, platelets become sticky and clump together (aggregation). Platelets then stick to the area of injury (adhesion). Because of platelet aggregation and adhesion, a platelet plug is formed over the injury, and platelets release clotting factors, which aid in the process of coagulation. The injured blood vessel tissue also activates clotting factors in the blood plasma. The interaction between the clotting factors works to form fibrin, a white, filamentous, tough protein strand that creates a netlike structure. The fibrin net traps red blood cells and more platelets to form a thrombus. This process is called blood clotting or coagulation. When the body stops the flow of blood through coagulation, the process is called hemostasis.

Factors Affecting the Blood Pressure
Learning Objective: Describe the factors that affect the blood pressure.
      About 1 in 3 American adults has high blood pressure. This means that many of the patients seen in an ambulatory care center may be diagnosed with high blood pressure, or hypertension. More information on hypertension will be provided later in the chapter. By contrast, low blood pressure, or hypotension, can be life threatening in some cases.
The pressure of the blood is highest in the arteries and lowest in the veins. Thus, we measure arterial blood pressure. Blood pressure (BP) can be defined as the resulting force of blood against the walls of the arteries. Two measurements are taken during the cardiac cycle (a complete heartbeat):
            • Systole or the contractive phase: Systolic pressure is measured when the heart is contracting and pumping out the blood.
            • Diastole or the relaxation phase: Diastolic pressure is measured when the heart is resting between contractions.
You will learn more about the procedure to take blood pressure in Chapter 20. This section addresses the factors that can influence blood pressure.

Blood Volume
Blood volume, or the amount of circulating blood, has a direct influence on blood pressure. The greater the blood volume, the more force it exerts on the arterial walls. If the blood volume is low, less force or pressure will be put on the arterial walls. Think of a garden hose attached to a faucet. If you turn on the faucet to the maximum level, there will be a lot of water pressure in the hose. If you turn on the faucet to get a trickle, there is very little water pressure in the hose.
      Let us briefly examine the factors that increase and decrease blood volume. The blood volume can be raised by the following actions:
            • Blood, plasma, and fluid (intravenous [IV]) transfusions
            • Increased sodium intake (because water follows sodium, more water will be drawn into the bloodstream due to the elevated sodium levels)
Blood volume can decrease due to hemorrhaging (bleeding), dehydration, and diuretic medications. Diuretics help pull water and sodium from the blood, thus lowering blood volume.

Strength of Ventricular Contractions
The left ventricle pumps blood to the body. The greater the force of the contraction, the more blood is pumped into the arteries. This increases the blood pressure. If the left ventricular contraction is weak, less blood is pumped out of the heart, and thus the blood pressure is lower. Digoxin is a medication that reduces the heart rate and strengthens the contractions of the heart.
      With heart disease, tests are done to check how the left ventricle is functioning. The stroke volume is the amount of blood pushed out of the left ventricle compared to the total volume of blood that filled the ventricle. It is a measure of the ejection fraction of the cardiac output. As heart disease occurs, the stroke volume can decrease.

Resistance to Blood Flow
Any factor that increases the resistance to blood flow through the arteries will increase the blood pressure. Factors that increase resistance include the size of the lumen (inner opening) of the arteries, the elasticity of the arterial walls, and the viscosity of the blood.
      The peripheral resistance of blood vessels refers to the size of the lumen and the amount of blood flowing through it. The smaller the vessel’s lumen, the greater the resistance to blood flow, thus increasing blood pressure. Several dynamics lead to decreased lumen size, including the following:
            • Plaque (waxy substance) builds up in the arteries and hardens over time. This buildup narrows the arteries and causes higher blood pressure.
            • Smoking.
            • Constriction of smooth muscles, causing vasoconstriction (VAE zoe kon strik shahn). Several medications—including benazepril, lisinopril, and losartan—relax smooth muscles, thus reducing blood pressure.
      Another factor that increases blood flow resistance is the loss of vessel elasticity. The inner layer of an artery contains elastic-like fibers that allow the vessel to expand and contract. Arteries dilate as the blood is pumped out of the heart and narrows between heartbeats to help maintain blood pressure. Increasing age and plaque buildup reduce the elasticity of the vessels. To understand the effect of plaque on the vessels, think of dried glue on a balloon. The dried glue prevents that section of the balloon from expanding. The plaque is like the dried glue. It prevents the walls from expanding.
      Lastly, the viscosity of blood influences the resistance to blood flow. As the viscosity of the blood increases, so does the resistance, and the blood pressure rises. The thickness of blood increases when more blood cells are present (e.g., polycythemia and blood transfusion).
      In summary, blood pressure can be influenced by blood volume, the strength of ventricular contractions, and the resistance to blood flow. Resistance can be increased by the following:
            • Narrowed lumen (e.g., due to plaque, smoking, and vasoconstriction)
            • Loss of vessel elasticity due to aging and plaque
            • Increased viscosity of the blood

25.5 Critical Thinking Application
Because many of their patients have high blood pressure, Rebecca wants to make sure Lizzy understands the factors that affect blood pressure. She asks Lizzy to summarize factors that increase blood pressure. How might Lizzy respond?