The Circulatory System
The circulatory system is made up of the heart and the blood vessels, which are the arteries, veins and capillaries, and the fluid or the blood itself. Failure of any of these components can result in a catastrophic failure of the entire system. The heart provides the driving force behind the circulatory system as it acts as the hydraulic pump for the body constantly moving the blood in a continuous flow though the various vessels. The action of the heart is dynamic and measurable: for example, blood pressure can be measured as the result of the heart contracting and relaxing. The blood vessels act as the carrying agent. Capillaries (the smallest of the blood vessels) provide the mechanism for oxygen to transport via diffusion through the vessel walls to all of the cells in the body. Blood is a combination of red blood cells, white blood cells, platelets, and blood plasma. It is the red blood cells that facilitate the transport of oxygen from the lungs into the circulatory system; this is accomplished by the iron-based protein hemoglobin which allows for the transport and binding of oxygen as hemoglobin enhances oxygen solubility.
Heart
The heart represents the basic organ that is understood at least in part by, most individuals. Young children understand at least basic functions of the heart. They understand it beats rhythmically over and over again. They draw pictures of a heart (granted, not anatomically correct), most understand that blood flows because of the heart, and most seem to understand that without the heart functioning you die. These are easy concepts to understand, unfortunately, we usually do not go much beyond that in the middle school classroom. The following represents a bit more information, which should be provided in order to have students understand concepts at their age level.
The heart is a hydraulic pump in an organic system that works without rest through the course of a lifetime. During a lifetime the heart will beat approximately 3 billion times, pushing 5 liters of blood around the body every minute. This represents a tremendous amount of work; there are few manmade devices being used in the world today that could stand up to these same rigors for 80 years.
The structure of the heart is related to its function. The heart is asymmetrical in appearance with the right side being slightly smaller in muscle size than the left. Upon dissection, the heart has four chambers, each isolated by a set of four valves. In the upper hemisphere of the heart are the atria, and the lower hemisphere are the ventricles (figure 1).
figure 1 (drawn by Stephen Griffith)
Oxygen-poor and carbon dioxide-rich blood flows into the right atrium from the body and is contained there momentarily until the opening of the tricuspid valve, which allows it to travel into the right ventricle (this process happens during diastole, one part of a complete cardiac cycle). During systole, the second part of the cardiac cycle, the heart muscles contract. The atrium contracts first, producing an additional blood flow from the right atrium to the right ventricle. Once the right ventricle fills completely the ventricle contracts allowing for a rush of blood up through the pulmonary valve and into the pulmonary artery where the blood will be transported to the lungs in order to allow the carbon dioxide to exit the blood and allow for an intake of oxygen. The structure of the heart with its valves allows for the heart to work while preventing backflow into the chambers due to low pressure drops when certain chambers are empty or filled.
This process is repeated as oxygen rich blood returns to the left atrium from the lungs via the pulmonary veins. When the mitral valve opens, blood flows into the left ventricle. During the same beat which previously allowed the right ventricle to contract (systole) the left ventricle contracts pushing the blood through the aortic valve and into the aorta. The aorta carries oxygen rich blood to the systemic circulation, which provides the entire body oxygen.
The heart also has its own circulatory system allowing for transport of oxygen to the muscle tissue of the heart. The heart is made from cardiac muscle, which is unlike the skeletal muscle that allows the body to accomplish voluntary motion. Cardiac muscle contractions are involuntary: the heart will continue beating whether we are thinking about it or not. This occurs due to the electrical impulses that originate in the sinoatrial node located near the top of the right atrium. When humans exercise the increase in the heart rate is a direct result of the cells needing oxygen at a faster rate and therefore the blood must deliver it at a higher flow rate through the body.
Blood Pressure
Blood pressure is so much more then the test that we take when we go to the doctor. It is important to get these tests as they do help to indicate stresses on the body and possible cardiac problems. A routine blood pressure measurement is concerned with arterial blood pressure, that is, the pressure as blood is leaving the heart, where the pressure is highest. But blood pressure varies throughout the body.
Blood pressure measured when you go to the doctor is recorded in millimeters (mm) of mercury (Hg). Average air pressure is 760 mm of Hg. Average blood pressure is 120 mmHg over 80 mmHg, with the first number representing systole, and the second measuring diastole. The numbers represent the relative pressure in the blood, which is above atmospheric pressure. This means that the actual pressure in the body is 760mm of Hg plus the systole or the diastole number.
Blood pressure is generated by the rhythmic pumping of the heart. Here, I focus on the action of the left side of the heart. Figure 2 shows the cardiac cycle. The length of time for the cardiac cycle is .8s for a person with an average heartbeat of 72-74 beats per minutes. As blood flows into the heart from the lungs it enters the left atrium and is prevented from entering the left ventricle by the closed mitral valve. The mitral valve opens as the upper hemisphere of the heart contracts forcing the blood into the ventricle. Contraction of the muscle in the ventricle forces blood up through the aortic valve and into the aorta at high pressure. The pressure produces blood flow through the body. As the ventricle relaxes, the aortic valve closes, preventing backflow into the ventricle. 1
In figure two, the top picture follows the cardiac cycle starting in late diastole. The graph below the diagram emphasizes changes in pressure in different regions of the heart. The aortic pressure goes through a periodic increase and decrease in pressure, between diastole and systole, with its lowest (diastole) being about 40 mm of Hg less than the greatest (systole). Blood pressure measured by your doctor reflects the pressure changes in the aorta. 2
figure 2 (drawn by Stephen Griffith)
Blood Vessels
The blood vessels make up the vascular system. The three main parts are: arteries, which consist of vessels taking blood away from the heart; veins, which are vessels that take blood to the heart; and capillaries, which are the vessels that connect the arteries and veins together to form the complete circuit.
The vascular tree allows the vascular system to create multiple pathways for the blood to flow, so that it can ultimately reach all areas of the body and deliver oxygen to the cells. These pathways appear as branches from aorta. At each branch or bifurcation, the original is split into two new pathways. At each bifurcation, the diameter of the vessel also gets smaller.
Arteries
Arteries are the vessels that carry the flow of blood from the heart. The arteries can be broken down into smaller constituent components beginning with the aorta, followed by the large arteries, which branch into progressively smaller arteries, and finally the arterioles.
All arteries have similar layered structures, with the primary difference being their diminishing size. The layers are: the innermost, or tunica intima, the center section, or tunica media, and the outermost, or tunica adventitia. These vessels are made up primarily of elastin (which acts as a connective tissue that resumes its primary shape after stretching), collagen (the connective tissue between the layers), and smooth muscle (which provides active constriction and relaxation of the vessels). 3 There is also a network of very fine blood vessels traveling through the arterial walls allowing for the diffusion of oxygen to take place for the cells and tissue that make up this region.
A cross section of an artery reveals that the innermost section consists of a layer of endothelial cells, connective tissue, and basement membrane. The inner section of the blood vessels (tunica intima) has a thin layer of the endothelial cells in order to prevent blood clotting and lower turbulence created by the flow of blood. The middle layer of the arteries (tunica media) are made up primarily of a "prominent layer of elastic tissue" which helps to provide the flexibility needed for vessels to constrict and relax in order to help regulate blood flow in the system. The outermost layer (tunica adventitia) is made up "mostly of stiff collagenous fibers". 4
The elastic tissue in the arteries are concentrically distributed and attached by smooth muscle cells and connective tissue. As bifurcations occur, from the aorta down through large and small arteries, the number of elastic laminae decreases with distance from the aorta, but the amount of smooth muscles increases as well as the relative wall thickness of the vessels. This is a needed byproduct of the function of the arteries: their ability to constrict and relax helps to control the flow of blood and thus blood pressure. 5
Coronary Arteries
The coronary arteries are a network of small arteries found on the surface of the heart. These vessels are extremely important to the health of an individual, as they provide the pathways for blood flow, and therefore oxygen delivery, to the tissue of the heart. Due to the small diameter of these vessels, they are susceptible to blockage.
Arterioles
The arterioles are small diameter blood vessels at the end of the arterial system. This is the area where the greatest amount of pressure drop takes place in the circulatory system, as this is where regional variations in blood flow are regulated. Although similar in structure to the rest of the artery system, there are some key differences that should be noted. Similar to the other parts of the arterial system these areas have the ability to contract restricting blood flow, and also relax allowing for greater flow.
Capillaries
Capillaries are largely ignored in middle school textbooks, simply being referred to as the region where the arteries and the veins are connected. There is little discussion about why they are so small or what there purpose is. The reality is that the capillaries are an integral part of the circulatory system for without them there would not be any transport of oxygen to the cells of the body and those cells would perish.
Capillary structure is simpler than arterial structure. The capillaries are made of endothelial cells in a single layer joined together with molecular cement. The diameter of the capillaries varies throughout the circulation, but is often the width of a red cell, meaning the cells need to line up in order to pass through these vessels. This confluent monolayer of endothelial cells does not allow blood cells to seep through the capillary wall, but it does allow for a leakage of oxygen, glucose, carbon dioxide, and even some proteins. This leakage, which is often driven by molecular diffusion, gives the capillary bed an essential role in the circulatory system. 6
Veins
The veins carry the flow of blood back to the heart. In the systemic circulation, venous blood is low in oxygen content but high in carbon dioxide. In the pulmonary veins, which are bringing blood back to the heart from the lungs, the veins contain oxygenated blood. The smallest veins, called venules, are directly connected to the capillaries. Smaller veins join together to form larger veins, with the largest vessels returning blood to the heart. The blood flowing through the veins is passive in nature as the veins do not have the smooth muscle that the arteries have and therefore do not contract and relax in order to promote higher or lower resistance to flow. Flow through veins, which occurs at low pressure, is unidirectional by the presence of valves within many veins, allowing the blood to only flow towards the heart. The walls of veins are collagenous, similar to the arterial wall, and veins have the same trilayer structure as arteries, but the layers are less distinct. There are smooth muscles associated with the tunica media of some veins, but they are not as organized or abundant as in arteries. Veins, like all vessels in the circulatory system, are covered with endothelial cells on their lumenal surface (the side containing the blood). 7
Vascular Tree
The blood vessels that lead away from the heart go through a series of bifurcations. These bifurcations are associated with a diminishing size of the blood vessels. Although bifurcation is the most common form of branching in the system there is also trifurcation and multiple (more than three) branching throughout the circulatory system in order to meet the demands of supplying oxygen to all cells and tissue of the body. This bifurcation continues to occur at regular intervals as the vessels get further from the heart and nearer regions that the oxygen is needed. In order for oxygen diffusion to occur properly there must be both a sufficient drop in size of the blood vessel and a sufficient number of vessels to reach cells within all regions of the body.
As in any system that is produced by sequential bifurcations, the overall surface area of the blood vessels increases with each generation of bifurcation. By analogy, imagine the bifurcating branches of an oak tree: the majority of the trees total surface area is contained on the smallest branches, which outnumber the trunk and large branches. Because the total cross-sectional area of all smallest branches also increases, the velocity of blood is slowest in the capillaries. The total flow rate from the heart is 5 l/min and remains constant throughout the cycle; the flow rate at any point in the bifurcating network (that is, at any distance from the heart) is equal to the velocity of flow times the total cross-sectional area at that point.
The variation of velocity throughout the circulatory system has important consequences. Because the same volume of blood must flow through each segment of the circulation each minute, the velocity of blood flow is inversely proportional to vascular cross-sectional area. The cross sectional area for an adult human aorta is approximately 2.5cm 2 compared to the cross sectional area of the capillary bed which is approximately 2500 cm 2. But the overall flow rate of blood is 5 l/min in both of these sections. Due to this difference in area though the velocity of the blood at the aorta is ~2,000 cm/min (5,000 cm 3/min divided by 2.5 cm 2) whereas the velocity in the capillaries is ~2 cm/min (or 0.3 cm/sec). This tremendous slowing of the blood velocity in the capillary section provides the time needed for oxygen (and other molecules) to diffuse across the thin vascular walls of the capillaries. 8
To understand this in more general terms, consider a garden hose that is split into two hoses at a Y junction, and then each of these two hoses is further split by Y junctions thus creating a tree-like structure, in which one tube flows into four. Also consider a reduction in overall diameter of the hose at each Y junction from a 1" hose, down to a ¾" hose continuing down to hose with a diameter of ¼". The flowrate of water going through each segment of the hose must be identical, replicating total flow through the cardiovascular system. Although the flowrate is identical, the velocity of through each section is different, due to the different cross sectional area in the different regions that are produced by bifurcation.
Figure 3 shows the blood pressure variations throughout the systemic circulation. As the blood leaves the heart, the blood pressure fluctuates between the systolic and diastolic pressure. As you progress to the right on the diagram, multiple branchings have taken place and then you see the biggest drop in pressure at the arteriole region. Finally there is a drop at the capillary region down to almost zero (note that zero on the graph is not zero pressure, it is equal to one atmospheric pressure of 760mm of Hg). Pressure differences are the driving force for flow through the circulatory system: the higher pressure in the aorta drives flow into the lower pressure regions: to arteriole and then to capillary. 9
As blood flows through the capillary bed, and diffusion is taking place, oxygen is lost from the blood and carbon dioxide is collected. This oxygen-poor, carbon dioxide-rich blood flows into the veins. Instead of a decrease in size associated with each branching from the aorta to the capillaries, the numerous capillaries join together to create larger venules, and the venules join together to create larger veins. There is an increase in overall diameter of the veins until the branching ceases at the vena cava and heart. The veins are a passive conduit for carrying the blood (controlled by the initial compression and contraction of the heart, arteries and arterioles) emphasizing the need for unidirectional valves inside of the veins in order to prevent backflow of blood.
The right side of the heart pumps blood to the lung region, which is geographically centered around the cardiac region. Figure 3 shows that the pressure in the pulmonary circulation is much lower than the systemic pressure. As the blood leaves the heart through the pulmonary artery it must again go through a vast array of branching to allow for a significant amount of capillaries in the lung region. The reverse is then happening once again as it ravels back to the left atrium of the heart. All of this is possible due to the complexity of the vascular branching, which results in the vascular tree. 1 0
figure 3 (drawn by Stephen Griffith)
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