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By Aditya Dharma, Jacob Boos, and Koustuv Datta
  1. Types of Circulation
  2. Circulation Overview
  3. The Heart
  4. Heart Functions
  5. Heart Form
  6. Blood
  7. Blood Pressure
  8. Blood Vessels
  9. Cardiovascular Diseases

I. Types of Circulation

A. Open Circulatory System
Arthropods and most mollusks have an open circulatory system. In open circulatory systems, the circulatory fluid bathes the organs directly it is in contact in. In this system, the the circulatory fluid, hemolymph, is also the interstitial fluid. The fluid is pumped through circulatory vessels by the contraction of one or more hearts. The fluid goes into interconnected sinuses, which are the spaces surrounding the organs. In the sinuses, chemical exchange occurs between the hemolymph and body cells. Hemolymph is drawn back in through the pores by the relaxation of the heart. Body movement also helps circulate the hemolymph by periodically squeezing the sinuses. In the open circulatory systems of larger crustaceans, i.e. lobsters and crabs, a more extensive system of vessels as well as an accessory pump is included.

B. Closed Circulatory System
This system occurs in annelids, cephalopods, and all verterbrates. In this system, blood is confined into ONE vessel and is distinct from the interstitial fluid. One or more hearts pump blood into a large vessel and branches off into smaller ones that are in organs. materials are exchanged between the smallest vessels and the interstitial fluid bathing the cells. The benefits of closed circulatory systems is that they include relatively high blood pressures, enabling Oxygen to be effeciently delivered and nutrients to the cels of larger and more active animals. They are also well suited to regulate the distribution of blood to different organs.

C. Cardiovascular System
In humans and other verterbrates, the closed circulatory systems are called the cardiovascular system. This system is a network of a heart, capillaries, arteries, and veins working together to transport blood. Arteries are tubular structures that carry Oxygen rich blood away from the heart to organs throughout the body. Arteries branch into arterioles, small vessels conveying blood to the capillaries. Capillaries are microscopic vessels with very thin walls. Capillary beds are the network of the vessels. These vessels infiltrate each tissue, passing witghin a few cell diameters of every cell in the body. Diffusion of Oxygen and chemicals occurs across the thin walls of capillaries. At the end of the capillaries are venules which then convert into veins. Veins transport Oxygen deprived blood to the heart. Arteries and veins are distinguished by the direction and flow in which they carry blood. Natural selection has modified the cardiovascular system and made it more complex.

D. Single Circulation
Single circulation occurs in bony fishes, rays, and sharks. This type of circulation gets its name because blood passes through the heart once in a complete circuit. Blood enters the heart through the atrium when the heart relaxes. As the heart contracts, it releases blood through the ventricle and goes into the arteries of the fish. Blood is pumped to the gills where Oxygen is diffused into the blood and CO2 out. Blood leaves the gills through the veins and returns to the heart. In this system, blood passes through two capillary beds before returning back to the heart. When flowing through a capillary bed, blood pressure drops substanially, limiting the rate of blood flow in the rest of the animal's body except for the gills. When the animal begins to swim, the contraction and relaxation of its muscles help accelerate the pace of circulation.

F. Double Circulation
Amphibians, mammals, and reptiles use double circulation in their circulatory system. Double circulation is the arrangement of two distinct circuits through different tissues before blood returns back to the heart. The coordination of the pumping cycles is simplified because two pumps in a single heart take off some of the work that only one pump is doing in a single circulation system. The pump on the right side of the heart delivers oxygen-poor blood to capillary beds of the gas exchange tissues, where there is a net movement of O2 into the blood and CO2 out of the blood. This part of the circulation is called a pulmonary circuit if the capillary beds are involved all in the lungs, such as reptiles and mammals. Pulmocutaneous circuit is if this exchange includes capillaries in both lungs and skin, like amphibians.
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II. Circulation Overview

All circulatory systems have three basic components: a circulatory fluid, a set of interconnecting tubes, and a muscular component i.e. the heart. The circulatory fluid consists of red blood cells, plasma, and platelets. These are all components that make up blood, which travel through the sets of interconnecting tubes. These tubes are called arteries, veins, capillaries, arterioles, and venules. Arteries carry the Oxygen enriched blood throughout the system where they branch off into arterioles, which branch off into capillaries. Capillaries branch off into venules, which then turn into veins. Veins transport the Oxygen deprived blood back to the heart where the process starts all over again. The heart is the muscular component of the circulatory system. The heart is like the control center of this system. It receives and gives blood to the body. Without this, there would be no such thing as complex organisms, such as mammals and animals.

III. The Heart

The heart is the main organ of the Circulatory system. The heart is about the size of a clenched fist and serves as a pump to pump blood around the body to either provide energy and oxygen to muscles or to remove carbon dioxide and other waste produced by the muscles. The heart is in the chest cavity, behind the ribcage and in between the lungs in order to receive oxygen rich blood to pump throughout the body and to provide the lungs with oxygen poor blood to get rid of carbon dioxide and other wastes. The heart is a muscle that works nonstop to pump blood through out the body. In organisms that have hearts, the heart pumps blood either through blood vessels, in a closed circulatory system, or the heart pumps blood into body cavities, in an open circulatory system.

Location of Heart

IV. Heart form

The heart is made up of cardiac muscles, which are strong muscles that work nonstop and involuntarily. This means that the heart is able to work nonstop without yourself having to constantly telling your heart to beat. The heart is roughly the size of a clenched fist and is divided into two halves. The right half of the heart receives oxygen poor blood to be sent to the lungs in order to turn it into oxygen rich blood. The left side of the heart receives oxygen rich blood to be then sent to the rest of the body. The left and right side of the heart is then divided into two more sections, called the atriums and the ventricles. The left and right side of the heart is separated by the walls of the heart, while the atrium and ventricles are separated by muscles that serve as valves. Theses valves contract and relax to increase and release pressure in the heart in order to aid the flow of blood through the heart and body. The valves also form a seal between the atriums and ventricles to prevent back-flow.

The heart of most animals differ. For example humans, mammals, and reptiles have four chambered hearts. This evolved from the three chambered of amphibians, which evolved from the two chambered hearts of fish. The four chambered hearts of reptiles and mammals is most efficient because it keeps oxygen rich blood and oxygen poor blood separate while still maintaining enough pressure to move blood throughout the body efficiently.The four chambered heart is found in mammals and is most efficient in keep oxygenate and deoxygenated blood separate from each other.

Diagram of a Two Chambered Heart
Diagram of a Two Chambered Heart

V. Heart Functions

The heart pumps in a constant rhythm to keep blood flow constant and too avoid the accumulation of too much pressure in any one area of the circulatory system. A complete sequence of pumping and filling of the heart is a cardiac cycle and takes about 0.8 seconds (average). The amount of blood pumped per minute by each ventricle is referred to as cardiac output. The average cardiac output is about 5 L/min. Since the average amount of blood in the body is roughly 5L it takes about a minute for a blood molecule to exit the heart and reenter it.

Blood flows through the heart by first entering the right atrium of the heart. Deoxygenated blood enters the right atrium through both the Inferior and Superior vena cava, the largest veins in the body. The muscles surrounding the right atrium then contracts which increases the pressure in the right atrium. This increase in pressure causes the tricuspid valve (valve that separates the right atrium and right ventricle) to open and lets blood enter the right ventricle. When the muscles of the right ventricle contracts it causes the tricuspid valves to shut and seal off the space between the atrium and ventricle. It also shuts to prevent backflow from the ventricle. The muscles surrounding the right ventricle then contract to increase the pressure within the right ventricle. The increase in pressure then causes the pulmonary valve to open releasing blood into the pulmonary arteries. The pulmonary valve then shuts to prevent backflow into the ventricle. From the pulmonary veins the deoxygenated blood enters the lungs to be rejuvenated with oxygen to rid itself of carbon dioxide and other wastes. After passing through the lungs the blood enters the left atrium. The muscle of the left atrium then contract to open the mitral valve and send blood into the left ventricle. The contraction of the left ventricle then shuts the mitral valve in order to prevent backflow. At the same time the contraction of the left ventricle increases pressure within the left ventricle. This increase in pressure causes the aortic valve to open. Blood then enters the aorta, which is the largest artery in the human body. The muscles of the left ventricle are the most powerful. This is because the left ventricle has to provide enough pressure for the blood to be able to move throughout the rest of the body. From the aorta, the blood moves into smaller arteries that bring blood to muscles. The blood then passes through capillaries where the oxygen is filtered out to provide energy to the muscle and wastes like carbon dioxide is put into the blood. The blood then travels back to the heart by veins where it will reenter the vena cava and restart the entire process over again.

In the three chambered heart of an amphibian, oxygen rich blood enters from the pulmonary veins into the left atrium. Oxygen poor blood enters from the vena cava into the right atrium of the heart. The blood is then pumped into the ventricle of the heart, where the oxygen rich blood and oxygen poor blood mix together. Some of the mixture of blood is pumped into the aorta and the rest is pumped into the pulmonary veins which lead into the lungs. From the lungs, the blood is completely rid of carbon dioxide and other wastes. From the aorta, the blood flows into arteries then to capillaries where the oxygen is filtered out and carbon dioxide is carried out. The blood then enters veins where it will flow back to the heart. The single ventricle of the heart causes the blood in the body to never be completely saturated with oxygen. It also causes the blood to always have some carbon dioxide and other wastes in it before even entering the arteries.

In a two chambered heart system only deoxygenated blood flows through the heart. Blood enters the atrium of the heart and gets pumped into the ventricle and later gets passed into the gills to be rejuvenated with oxygen. This blood is later then moves along blood vessels where it passes systemic capillaries. In these systemic capillaries, oxygen is filtered out of the blood and the blood carries waste products made by muscles. After the deoxygenated blood reenters the heart through the veins where the cycle restarts.

VI. Blood

Here is a diagram about everything in blood:

Blood is probably the most important thing in the body. Blood is the red liquid that circulates in the arteries and veins of humans and other vertebrate animals, carrying oxygen to and carbon dioxide from the tissues of the body. Inside blood there is this plasma that together with the blood cells. They transport, regulate osmosis, and defend the cells. 45% of blood is actually blood cells and the other 55% is plasma.

So what is plasma? Plasma consists of many inorganic salts in the form of dissolved ions but it contains 90% water. Some of these ions help control the pH of the blood. The salts are very important in maintaining osmotic balance in the blood as well. However this is not the only function of proteins in the plasma. Some plasma proteins even combat against viruses; these are the antibodies that protect from virus invasions. Other plasma proteins escort lipids because lipids can only travel in water if they are attached to proteins since lipids are insoluble in water. A third group of plasma proteins help “plug” blood vessel leaks when they are injured. Plasma also transports many things from one part of the body to the other to the other such as nutrients, metabolic wastes, respiratory gases, and hormones.

Next are the contents of the other 45% of the blood. First are the Erythrocytes. This is just a fancy name for the word red blood cells. These are the most numerous red blood cells. Their main function is to transport oxygen and their structure is built to carry out that function. Their shape increases the surface area of the blood cell therefore enhancing the rate of diffusion of oxygen across the cell membrane. An unusual characteristic of these cells though is that when these cells are fully mature they lose their nuclei. This leaves more space for the hemoglobin. Hemoglobin is a protein that contains iron and transports oxygen.

The Leukocytes are essentially the white blood cells of the body. They fight infection. Some fight the infection by eating away the body’s own dead cells. Leukocytes are also found outside the circulatory system as well.

Platelets are cytoplasmic fragments that have specialized bone marrow cells. They serve to clot blood.

VII. Blood Pressure

See there are two portions of the heartbeat you need to know about. One of those portions is called “systole,” and the other is called “diastole.” Remember your heartbeat – the ventricles contracted, and then they relaxed, and then they contracted, and then they relaxed. And when they contract, when the ventricles pump, you are in systole, and the blood is squirting through your arteries; and when you’re in diastole, they’re relaxed. That does not mean your blood has stopped, but the point is that the pressure is not as high.

Regulating blood pressure is another thing See, blood pressure fluctuates over two time scales. The first is the oscillation in arterial blood pressure during each cardiac cycle. Blood pressure also fluctuates on a longer time scale in response to signals that change ethe state of the smooth muscles in the walls. For example, physical/emotional stress can trigger nervous/hormonal responses that cause smooth muscles in arteriole walls to contract. This is called vasoconstriction. When this happens, the arterioles narrow therefore increase the blood flow upstream. Vasodilation is the process that increases the diameter of the arteriole.
This is a diagram on how to take the blood pressure of a person

VIII. Blood Vessels

,The three major types of blood vessels are veins, arteries, and capillaries.
Capillaries, veins, and arteries – they all have at their basis an inner lining called endothelium. Endothelium is a kind of epithelial cell; epithelial cells in general are “covering cells,” and this is an “endo-thelium.” So all three have an endothelium. So there’s something that they all have in common, this layer of flat cells that are tightly packed and tightly joined together – endothelium. On the other hand, that’s it for capillaries. Capillaries – they do have a membrane called a basement membrane, which holds them together. That’s common to all tissues. But, generally speaking, capillaries are one cell layer thick, and they’re made out of endothelium. On the other hand, veins and arteries both have smooth muscle and connective tissue. So what’s the difference? Well, now it comes down to function; and one of these, as you know, is going to have a different function than the other. Arteries carry blood away from the heart; veins carry blood to the heart. By the time the blood gets back to the veins – and it goes like this, it goes capillaries are going to connect arteries and veins. So it’s going to go to the arteries, and the arteries are going to go to small arterials – it’s just a nice vocabulary, it simply means “little arteries” – and eventually go to capillaries; the blood is eventually going to – and remember they’re just branches upon branches upon branches. So we have arteries, arterials, capillaries – and then the capillaries are going to come together, and form venules –small veins – which eventually are going to be veins. And if you take a look at – there’s not going to be that much difference between arteries and veins except as their structure is going to be determined by their function. So, therefore, we’re probably going to differ in these two layers – the muscle layer and the connective tissue, and particularly the muscle layer; we’ll see why. What do arteries do? Well, arteries have strong muscles inside of them that they can even pump along with the heart. So they pump in synchrony with the heart. So as the heart pumps, so also do your arteries to keep the blood flowing along; you can even feel an artery literally pumping by just putting your fingers in your wrist, or right here, and feeling a pulse, as your arteries continue the pumping of blood throughout the body.

IX. Cardiovascular Diseases

Cardiovascular diseases, disorders of the heart and blood vessels, cause more than half of human deaths in the United States. Cardiovascular diseases can range from being minor disturbances of vein and heart function to major life-threatening disruption of blood flow to the heart or brain. Atherosclerosis, the hardening of the arteries by accumulation of fatty deposits, is one reason why cardiovascular diseases cause so many deaths. It causes these deaths because they often aren’t detected until they disrupt critical blood flow. The fatty deposits can cause blockage in the arteries and can lead to heart attack or stoke. These are caused by unhealthy arteries. Healthy arteries, on the other hand, have a smooth inner lining and reduce resistance to blood flow. Damage or infection can roughen the lining and lead to inflammation. Leukocytes attach to the damaged lining and begin to take up lipids, including cholesterol. In addition, a fatty acid, plaque, steadily grows in the damaged lining and adding blockage in the arteries. The walls of the arteries become stiff as the plaque grow, creating an obstruction.
Healthy vs Unhealthy Arteries
Healthy vs Unhealthy Arteries

Cardiovascular diseases, disorders of the heart and blood vessels, cause more than half of human deaths in the United States. Cardiovascular diseases can range from being minor disturbances of vein and heart function to major life-threatening disruption of blood flow to the heart or brain. Atherosclerosis, the hardening of the arteries by accumulation of fatty deposits, is one reason why cardiovascular diseases cause so many deaths. It causes these deaths because they often aren’t detected until they disrupt critical blood flow. The fatty deposits can cause blockage in the arteries and can lead to heart attack or stoke. These are caused by unhealthy arteries. Healthy arteries, on the other hand, have a smooth inner lining and reduce resistance to blood flow. Damage or infection can roughen the lining and lead to inflammation. Leukocytes attach to the damaged lining and begin to take up lipids, including cholesterol. In addition, a fatty acid, plaque, steadily grows in the damaged lining and adding blockage in the arteries. The walls of the arteries become stiff as the plaque grow, creating an obstruction.

If atherosclerosis is unrecognized or untreated, the most common results are heart attack or stroke. The fancy, scientific term for heart attack is myocardial infarction. Myocardial infarction is the damage or death of cardiac muscle tissue resulting from blockage of one or more coronary arteries. Because they are small in diameter, they are vulnerable to blockage. This obstruction of blood flow can cause damage because it deprives the heart of Oxygen, making it unable to survive. Once the heart stops beating, there are ways for a victim to survive a heart attack. The victim can survive through CPR, cardiopulmonary resuscitation or some other emergency procedure within a few minutes of the attack.

In addition to blood flowing to the heart, there is also the blockage of blood flow to the brain. This obstruction is called a stroke. In a stroke, nervous tissue in the brain is damaged due to the lack of Oxygen carried from blood flow. Strikes are usually a result from rupture or blockage of arteries in the head. The effects of a stroke differ on the individual it attacks and the location of the damaged brain tissue. Four signs that you can recognize a stroke are:
1. Your face is feeling droopy or has an odd sensation or feeling.
2. Your arm(s) and/or other limbs or parts or your body begins to feel numb or tingly.
3. Your speech is becoming slurred while talking
What Happens in a Stroke
What Happens in a Stroke

Cholesterol is a major contributor to atherosclerosis. Cholesterol travels in the blood plasma and forms many particles. There are two main types of cholesterol in the blood plasma. They are low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDLs are usually referred to as “bad cholesterol” because it is associated with the deposition of cholesterol in arterial plaques. On the other hand, HDLs are sometimes called “good cholesterol” because they appear to reduce the deposition of cholesterol. A way to reduce your LDL/HDL ratio is to exercise. Trans fats and smoking can increase the ratio, having an opposite effect. People who are at risk with cardiovascular diseases are treated with medicine that reduces the LDL levels. Therefore, the frequency of heart attacks is reduced.
In addition to cholesterol and blockage in the arteries, hypertension is another contributor to heart attack and stoke as well as other health problems. Hypertension is high blood pressure. High blood pressure occurs when people are obese, have high LDL cholesterol levels, and can also be hereditary.