Blood physiology and cardiovascular lifespan changes
Physiology of the blood
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.
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
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 (resting) phase: Diastolic pressure is measured when the heart is resting between contractions.
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. Stroke volume is the amount of blood pumped out of the left ventricle with each heartbeat. Ejection fraction is stroke volume expressed as a percentage of the total blood that filled the ventricle. As heart disease progresses, 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 (VAY zoh kon STRIK shun). Several medications - including benazepril, lisinopril, and losartan - relax smooth muscles, thus reducing blood pressure. Lisinopril, for example, is an ACE inhibitor: it blocks the conversion of angiotensin I to angiotensin II, which reduces vasoconstriction and lowers 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 narrow 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 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
Life span changes of the cardiovascular system
An unborn child receives oxygen and nutrients from their mother’s blood through the placenta. The ductus venosus, ductus arteriosus, foramen ovale, and umbilical vessels are special structures that support fetal circulation. After birth, these structures are no longer necessary. The heart rate decreases as a child grows and matures. The systemic vascular resistance increases with age. This means that the resistance for blood flow increases, and thus the blood pressure increases with age.
Changes in pregnancy
During pregnancy, the mother’s cardiovascular system changes:
- Cardiac output (the amount of blood pushed out of the heart in 1 minute) increases.
- Extracellular fluid volume increases; thus, the blood volume is greater.
- Total peripheral resistance decreases, thus reducing the blood pressure.
- Blood flow to various organs increases.
- As the pregnancy progresses into the third trimester, the blood pressure increases.
Changes in older adults
As a person ages, the heart and blood vessels change. The following heart changes occur:
- SA node loses some cells; thus, the heart rate can be slower.
- The left ventricle may increase in size, thus reducing the amount of blood it can hold.
- Normal ECG changes can occur with age.
- Valves can become thicker and stiffer, causing a heart murmur.
- Arterial walls become stiffer, thus increasing the blood pressure.
Baroreceptors in the carotid arteries and aorta detect changes in blood pressure and help maintain a fairly constant blood pressure with position changes. With age, the baroreceptors become less sensitive, making older people more at risk for orthostatic hypotension when changing positions.