cardiovascular system

The blood vessels serve as the irrigation system that connects the heart to all other structures in the body. If all of the vessels in one adult human were laid end to end, they would span more than 100,000 miles; that would be 4x around the equator! The three main types of blood vessels are arteries, veins, and capillaries.

Veins and Arteries

Veins carry blood toward the heart, and arteries carry blood away from the heart. As arteries leave the heart, they split and become smaller; these smaller arteries are called arterioles. These arterioles then connect with even thinner vessels called capillaries. From here, the blood goes to tiny veins called venues, with continuation into bigger veins that bring the blood back to the heart. The acronym VEAL: veins enter (the heart), arteries leave (the heart) can help you remember the blood’s direction through the major vessels.

Layers of Arteries and Veins

The arteries and veins are comprised of three layers. From superficial to deep, these are the tunica externa, media, and intima. The tunica externa is primarily protective collagen. The tunica media contains smooth muscle that can contract and relax. Contraction is called vasoconstriction, and relaxation is called vasodilation. The inner, the tunica intima or endothelium, layer is made of a single layer of epithelial cells. When this inner layer is damaged, the clotting process is initiated.

Arteries carry blood away from the heart and come in three major varieties: elastic arteries, muscular arteries, arterioles. The elastic arteries are also called conducting arteries. The muscular arteries are also called distributing arteries. The arterioles are the tiny end branches of arteries, which connect to capillaries.

Artery structure and types (Betts et al., 2013)

Veins

Veins carry blood toward the heart. They begin as tiny venules branching off the capillaries, then merge together into medium and then large veins that enter the heart. Veins have valves to help prevent backflow. When valves, often in the lower extremity, become dysfunctional varicose veins may emerge.

Vein structure and types (Betts et al., 2013)

Capillaries

The capillaries connect arterioles and venules. The capillaries have only two layers: the tunica media and tunica intima. Their thin layers allow them to perform their primary function of diffusing gasses, nutrients, and wastes through their walls.

Circulatory system overview (Betts et al., 2013)

Now that we have examined the anatomical foundations, we will learn more about how they function together with the heart.

The cardiac cycle is a term used to describe how all of the events, mechanical and electrical, coordinate the blood flow through the heart in a single contraction of the heart (heart beat).

Cardiac cycle (Betts et al., 2013)

Cardiac Output

Cardiac output is one way to assess the overall cardiac cycle. Cardiac output is the amount of blood pumped out of the heart per minute. It is often symbolized as either Q or CO. In an average adult human, this is 4900 mL per minute. This output is dependent on the heart rate, or number of beats per minute, and the stroke volume. Stroke volume is the amount of blood ejected from the left ventricle with each beat. Stroke volume is symbolized as SV. The equation: Q = HR x SV represents how cardiac output is a product of the heart rate and stroke volume. If heart rate and/or stroke volume increase, cardiac output increases. Think of exercise. When you exercise, the heart rate goes up, and the stroke volume may increase as well. This elevates cardiac output. If the HR and/or SV decrease, cardiac output decreases. This is not only dependent on exercise, as body position, stress, pathology, medication, and other factors can also alter heart rate, stroke volume, and cardiac output.

Cardiac output as a product of heart rate and stroke volume

Stroke Volume

Stroke volume is determined by the difference between end diastolic volume (EDV) and end stroke volume (ESV). EDV is the amount of blood left in the heart when it fills with blood. ESV is the amount of blood left in the heart after the ventricles contract. SV is therefore represented by the equation SV = EDV – ESV. This stroke volume process can be visualized as filling a plastic water bottle or balloon, then squeezing out the water:

Process of stroke volume as determined by the EDV and ESV.

1. Diastole: The heart is at rest and it fills with incoming blood from the heart and lungs.

2. EDV: The heart is full of blood. This is called preload.

3. Systole: When you squeeze the water bottle or balloon, water comes out. This is like the ventricles contracting to squeeze out the blood. The fluid must work against the narrow neck of the bottle, balloon, or blood vessel. This narrow neck represents the resistance that the blood must overcome to leave the heart, which is called afterload. The fluid coming out is the stroke volume, or amount of blood leaving the heart (left ventricle) in one contraction.

4. The fluid remaining in the bottle is called the ESV.

Preload versus afterload

Influences on heart rate and stroke volume

Ejection Fraction

Ejection fraction is a common clinical measure to assess heart health. This is a common measure to assess heart failure. Ejection fraction is the amount of blood pumped out of the left ventricle divided by the total amount of blood entering the left ventricle. Normal function is indicated by greater than 55% ejection fraction.

Blood Pressure and Hypertension

Blood pressure helps assess blood vessel health. Blood pressure readings have a top number called the systolic pressure, and a bottom number called the diastolic blood pressure. Since original sphygmomanometers used mercury, the units mm Hg (millimeters of mercury) follow the reading. The systolic pressure is the pressure in the vessels when the ventricles contract. The diastolic pressure is the pressure in the vessels when the ventricles relax. Typically, blood pressure is assessed at the brachial artery in the arm, as blood pressure is not consistent throughout the body. Since blood pressure varies based on activity and body position, it is typically measured after a few minutes of sitting quietly. A normal blood pressure is 120/80 mm Hg or lower. Elevations are staged as pre-hypertension, stage I hypertension, or stage hypertension. A hypertension classification does not require elevation of both the systolic and diastolic, as a person could have isolated systolic or isolated diastolic blood pressure as well.

Blood pressure health classification

Hypotension

Low blood pressure may also occur, and this is termed hypotension. There is some variation in the defining numbers for hypotension. In general, a systolic lower than 90 mm Hg and/or diastolic lower than 60 mm Hg may be considered as hypotension. Hypotension may have medical significance, such as an allergic reaction or shock. However, elite athletes may naturally have blood pressures in this range without medical concerns. If the hypotension occurs with a change in body position such as supine to sitting, or sitting to standing, this is called postural hypotension; this is often of particular consideration when helping older adults or patients on prolonged bed rest get out of bed. If they experience postural hypotension, they may become dizzy and need to sit for a minute before standing to allow time for their blood pressure to self-regulate.