Arteries
On a hot summer day, you go jogging outside and notice that your cheeks are gradually turning red. What causes them to become red? It is the arteries in your cheeks dilating in an attempt to release the heat in your body. Not only do arteries help regulate body temperature, but they also play a crucial role in delivering blood to organs, keeping you alive. Damage to arteries can have life-threatening consequences. For instance, cerebral arteries that are ruptured or obstructed in a stroke can block blood supply to the brain, resulting in death if immediate action to restore blood supply is not taken.
To recap, arteries transport blood (rich in oxygen and nutrients) from the heart to bodily organs. As arteries carry blood away from the heart, they branch into smaller vessels. Arteries are classified into three types based on their size and proximity to the heart: elastic, muscular, and arterioles.
Elastic Arteries
Elastic arteries are closest to the heart and have the largest diameter, usually more than 10 mm. Arteries leaving the heart, such as the aorta and its branches, are classified as elastic arteries. They must bear the high pressure by which blood is pumped by the heart. They contain a protein known as elastin, enabling the artery to expand and recoil as blood flows with high speed. Elastic arteries contain large lumens with low resistance, allowing them to conduct blood that flows with high pressure to muscular arteries which do not have the ability to withstand very high pressure. For this reason, elastic arteries are also known as conducting arteries; they ease the pressure by which blood flows into muscular arteries. The thickest layer of elastic arteries is the tunica media which is composed of mainly elastic fibers.
Muscular Arteries
Elastic arteries branch into muscular arteries, which have a diameter ranging from 0.3 mm to 10 mm. By the time blood reaches muscular arteries, blood pressure has significantly slowed down. Unlike elastic arteries, they contain little elastin. However, they still contain elastic membranes surrounding the tunica media. Muscular arteries are known for the high amount of smooth muscle in their tunica media. This is why they are mainly involved in vasodilation and vasoconstriction, which are mechanisms that regulate blood pressure by adjusting the diameter of arteries. Muscular arteries are also known as distributing arteries because they supply blood to certain regions of the body. This is why most named arteries (such as the external carotid artery, splenic artery, femoral artery, etc.) are classified as muscular arteries.
Arterioles
Muscular arteries branch into arterioles, which have a diameter of approximately 0.01 mm to 3 mm. The smallest type of arteries, arterioles contain a thin layer of each tunic, which gradually disappears as arterioles diverge. The tunica media is at most two muscle cell layers thick, and in the tiniest arterioles, one cell layer thick. Arterioles branch into capillary beds, where gas and nutrient exchange occurs between blood and cells immersed in the interstitial fluid. Arterioles control both the rate at which blood flows into capillaries and whether the cells receive the oxygen and nutrients from the blood. If they are constricted, blood does not enter capillaries; if dilated, blood enters capillary beds, enabling gases, nutrients, and wastes to diffuse between the blood and interstitial fluid. In the same way elastic arteries act as resistance vessels to ease the pressure by which blood flows into muscular arteries, arterioles reduce blood pressure before blood enters the fragile, microscopic capillary beds.
Arterial Blood Pressure*
As the ventricles (pumping chambers of the heart) contract, the rate at which blood is pumped rises to its peak, which brings about a rise in blood pressure. This period is known as ventricular systole. Blood pressure is extremely high when blood is pumped by the left ventricle into elastic arteries, causing their walls to expand. In fact, if a hole was made in the aorta's* wall during this period, blood would jump upward to a height of six feet! The pressure that blood exerts on the walls of arteries during ventricular systole is called systolic pressure. Healthy adults have a systolic pressure of about 120 mm Hg. It is important that the systolic pressure does not exceed this value. A systolic pressure above 130 mm Hg is classified as hypertension and signifies that the ventricles are working too hard to pump blood into the aorta. A blockage in several arteries is one risk factor and forces the ventricles to pump contract harder in order to push blood past the blockage. The energy with which the heart pumps blood during systole is restored during diastole.
Figure 5: Systole and Diastole
The ventricles must have enough blood in their chambers to pump blood out of the heart. The period during which blood pours from the atria into the ventricles is known as ventricular diastole. At this point, the ventricles are relaxed; their chambers are simply filling with blood. Blood pressure significantly drops during this period because the ventricles are not contracting, and the walls of arteries recoil. The pressure that blood exerts on the walls of arteries during ventricular diastole is known as diastolic pressure. In healthy adults, diastolic pressure is approximately 80 mm Hg. Similar to how systolic pressure should not exceed 120 mm Hg, diastolic pressure must remain at or below 80 mm Hg. A diastolic pressure above 80 mm Hg is classified as hypertension, and signifies that the arteries have lost their ability to stretch and recoil as blood flows, making their walls vulnerable to rupture. During diastole, the aortic valve closes to prevent newly pumped blood in the arteries from leaking back into the heart. Because it precedes ventricular systole, ventricular diastole can be thought of as the pressure of blood between heart beats.
Pulse pressure is the difference between systolic and diastolic pressure. For instance, if an individual had a systolic pressure of 115 mm Hg and a diastolic pressure of 70 mm Hg, the pulse pressure would be 45 mm Hg. In healthy individuals, pulse pressure is usually between 40 mm Hg and 60 mm Hg. A pulse pressure above 60 mm Hg is a sign of heart disease (especially buildup of plaque in arteries) whereas a pulse pressure below 40 mm Hg could indicate heart failure.
Figure 6: Relationship between time and systolic pressure, diastolic pressure, mean arterial pressure, and pulse pressure
The pressure by which blood travels in arteries to reach cells and tissues is the mean arterial pressure (MAP). Because diastole lasts longer than systole, the MAP cannot be the average of the systolic and diastolic pressures. It can be calculated by adding the diastolic pressure to one-third of the pulse pressure. If an individual has a diastolic pressure of 80 mm Hg and a systolic pressure of 116 mm Hg, the MAP would be 92 mm Hg. The MAP is extremely high when blood is pumped out of the heart, but as blood moves away from the heart into smaller arteries, MAP decreases, protecting the delicate capillaries from rupture.
Despite the fact that blood pressure slows down as arteries diverge, blood is still able to reach the distal arterioles and capillary beds because it flows with high velocity out of the heart. Otherwise, if blood pressure were not high as blood exits the heart, it would have a hard time reaching the thousands of tiny arterioles and capillary beds.
*Read my first blog about the journey of blood through the heart to get a better understanding of systole and diastole
*The aorta is the biggest artery in the body. Freshly pumped blood leaves the heart through the aorta.
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