6: Module 4- The Cardiovascular System- Blood Vessels and Circulation - Biology

6: Module 4- The Cardiovascular System- Blood Vessels and Circulation - Biology

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6: Module 4- The Cardiovascular System- Blood Vessels and Circulation

Heart & Circulatory System

The heart is a four-chambered muscular pump which pumps blood round the circulatory system.

The right side of the heart pumps de-oxygenated blood to the lungs to pick up oxygen.

The left side of the heart pumps the oxygenated blood from the lungs around the rest of the body.

  • 1. Deoxygenated blood enters through the vena cava into the right atrium
  • 2. It&rsquos then pumped through a valve into the right ventricle chamber
  • 3. And then up through the pulmonary valve into the pulmonary artery towards the lungs
  • 4. Oxygenated blood enters through the pulmonary veins into the left atrium
  • 5. It&rsquos then pumped through a valve into the left ventricle
  • 6. And then through the aortic valve and out of the aorta to the rest of the body

This video explains how the heart works

How the Blood is transported

Arteries (thick walled muscular tubes) carry blood away from the heart at high pressure in thick walled lumen

Capillaries (very narrow tubes) have thin walls to allow glucose and oxygen to diffuse through

Veins (thin walled tubes) carry low pressure blood back to the heart. Veins have thinner walls and valves to prevent backflow of blood

This video overviews the heart and explains how blood is transported around the body

Heart Disease

Blood vessels called the coronary arteries supply blood to the heart muscles. If they become blocked, a heart attack can happen.

A heart attack can happen after a sequence of events,

  1. fatty deposits build up in the coronary arteries
  2. a blood clot can form on a fatty deposit
  3. the blood clot can block a coronary artery
  4. some heart muscle cells do not get the oxygen and nutrients they need
  5. these cells start to die.

In the UK about 300,000 people have a heart attack every year.

Factors for Heart Disease

The risk of developing heart disease is increased by several factors, including:

  • smoking
  • high blood pressure
  • high levels of salt in the diet
  • high levels of saturated fat in the diet.

High levels of salt in the diet can lead to increased blood pressure. High levels of saturated fats in the diet lead to a build of cholesterol in the arteries, causing a plaque and narrowing of the arteries.

Structure of the Heart

The heart muscle is asymmetrical due to the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit, it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in Figure 2. In humans, the heart is about the size of a clenched fist, and it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs.

In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the biscuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, by-passing the semilunar valve (or pulmonic valve) to the lungs for re-oxygenation.

After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.

Art Connection

Figure 2. The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers.

Which of the following statements about the heart is false?

  1. The mitral valve separates the left ventricle from the left atrium.
  2. Blood travels through the bicuspid valve to the left atrium.
  3. Both the aortic and the pulmonary valves are semilunar valves.
  4. The mitral valve is an atrioventricular valve.

The heart is composed of three layers the epicardium, the myocardium, and the endocardium, illustrated in Figure 2. The inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.

Figure 3. Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.

The heart has its own blood vessels that supply the heart muscle with blood (Figure 3). The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as angina, and complete blockage of the arteries will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack.

Worms, Annelida

Circulatory System

Circulatory systems may be complete, consisting of a dorsal, often contractile vessel, in which the blood flows anteriorly, one or two major ventral vessels, and capillaries connecting these vessels to the gut and to the rest of the organs. Some polychaetes lack the capillary beds and thus have an open circulatory system. And small species often lack a circulatory system entirely, presumably relying on diffusion for transportation of oxygen, carbon dioxide, and so forth. Shared absence of a circulatory system is not considered a feature of phylogenetic importance it seems to be linked to a reduced body size.

Interactive resources for schools

Deoxygenated blood

Blood which has given up the oxygen it was carrying to the tissues

Double circulation

A circulation made up of two separate systems, one flowing from the heart to the lungs and back and the other flowing around the rest of the body

Oxygenated blood

Blood which is carrying oxygen in the form of oxyhaemoglobin


A large organ in the upper abdomen which manufactures, stores and breaks down substances as required by the body

A common term for the digestive system.

The circulatory system

The body transport system

A large multicellular organism such as a person needs a transport system to move substances around to the cells. In the human body the transport system consists of:

  • The blood (the liquid which carries substances around your body)
  • The blood vessels (the pipes which carry the blood)
  • The heart (the pump which moves the blood around the body)

Together they make up the circulatory or cardiovascular system.

Functions of the cardiovascular system

The circulatory system has a number of different functions in the body. These include:

  • Carrying food from the gut to the cells
  • Carrying oxygen from the lungs to the cells
  • Carrying carbon dioxide from the cells to the lungs
  • Carrying urea and other toxins from the liver to the kidneys
  • Carrying chemical messages (hormones) around the body from the glands where they are made to their target organs
  • Distributing heat around the body

The double circulation

We are very active animals so our cells need lots of oxygen. We have a very efficient system which makes sure our cells get the oxygen they need.

Your heart is not a single pump - it is in effect two pumps joined and working together. One part of the heart pumps deoxygenated blood to the lungs to get rid of waste carbon dioxide and pick up oxygen. The other part of the heart pumps this oxygenated blood all around the body. This is a double circulation.

A simple model of a double circulation

However your circulatory system is not a simple system of boxes. There are different types of blood vessels (arteries, veins and capillaries) and the blood flows through lots of different organs.

Every time the heart pumps the blood is pushed around the circulatory system. It flows in a constant stream (see the animation below).

Function of the Heart

The heart's only function is to pump blood.

The right side of the heart: Pumps blood to the lungs, where oxygen is added to the blood and carbon dioxide is removed

The left side of the heart: Pumps blood to the rest of the body, where oxygen and nutrients are delivered to tissues and waste products (such as carbon dioxide) are transferred to the blood for removal by other organs (such as the lungs and kidneys)

A Look Into the Heart

This cross-sectional view of the heart shows the direction of normal blood flow.

Blood travels the following circuit: Blood from the body, which is depleted of oxygen and laden with carbon dioxide, flows through the two largest veins—the superior vena cava and the inferior vena cava, known collectively as the venae cavae—into the right atrium. When the right ventricle relaxes, blood in the right atrium pours through the tricuspid valve into the right ventricle. When the right ventricle is nearly full, the right atrium contracts, propelling additional blood into the right ventricle, which then contracts. This contraction closes the tricuspid valve and propels blood through the pulmonary valve into the pulmonary arteries, which supply the lungs. In the lungs, blood flows through the tiny capillaries that surround the air sacs. Here, the blood absorbs oxygen and gives up carbon dioxide, which is then exhaled.

Blood from the lungs, which is now oxygen-rich, flows through the pulmonary veins into the left atrium. When the left ventricle relaxes, the blood in the left atrium pours through the mitral valve into the left ventricle. When the left ventricle is nearly full, the left atrium contracts, propelling additional blood into the left ventricle, which then contracts. (In older people, the left ventricle does not fill as well before the left atrium contracts, making this contraction of the left atrium especially important.) The contraction of the left ventricle closes the mitral valve and propels blood through the aortic valve into the aorta, the largest artery in the body. This blood carries oxygen to all of the body except to the lungs.

The pulmonary circulation is the circuit through the right side of the heart, the lungs, and the left atrium.

The systemic circulation is the circuit through the left side of the heart, most of the body, and the right atrium.

Circulatory System Structure

As a whole, the circulatory system has a general pattern, structure, and flow. Blood starts in the heart, where it is split into two patterns of circulation. The pulmonary circulation goes to the lungs and back to the heart. This circuit is used to oxygenate the lungs. Then, the blood reenters the heart and is pumped through the systemic circulation.

These veins and arteries serve the body and have a standardized setup. First, arteries carry oxygenated blood toward the tissues. As the arteries get closer to their target tissue they get smaller and smaller, eventually leading to capillaries. Capillaries are the smallest of all vessels, and they serve as the site of gas exchange in the tissues. On the other side of capillaries, the veins start. Veins carry deoxygenated blood, along with various waste products, back towards the heart. The waste products will be excreted in the lungs, or they are filtered out by the liver or kidneys.

In other animals, the circulatory system can vary widely. This article describes the closed circulatory system of humans and other mammals. Fish, on the other hand, have only a 2-chambered heart and the entire circulatory system is much simpler. Other organisms, such as insects and other invertebrates, may have an open circulatory system. This form of a circulatory system simply bathes the organs and tissues in a blood-like fluid but does not contain veins or arteries. Still, other animals like the octopus have multiple hearts to accomplish the tasks of the circulatory system.

What is the circulatory system?

The circulatory system, also known as the cardiovascular system, is an extensive network of organs and structures that allow the blood to circulate throughout the body, transporting oxygen, carbon dioxide, nutrients or other blood cells. Inside the circulatory system, we can also find the lymphatic system, which is in charge of transporting a substance called lymph.

While the blood is a fluid that contains plasma, red and white blood cells and platelets the lymph is, primarily, an excess of recycled blood plasma used to remove unwanted material from the body. This recycling occurs after the interstitial fluid has been filtered out and returned to the system. The oxygen enters the bloodstream through small pulmonary membranes that absorb it while it is inhaled.

As our body uses the oxygen and processes the nutrients, the carbon dioxide is created, which is expelled when exhaled. The right functioning of the cardiovascular system is possible due to constant pressure exerted by the heart and valves throughout the body.

This blood pressure ensures that the veins take the blood to the heart and the arteries transport it far away from it. In an average adult human being, about 7.5 liters of blood are transported daily along more than 96 kilometers of blood vessels. According to studies, a person has about 4-5.5 liters of blood in his or her body.

9.2) Heart

The heart pumps blood through the circulatory system to all the major organs of the body.

In general, blood flows into the heart from a vein, goes into an atrium, then a ventricle, and out through an artery.

The heart contains valves to prevent the blood flowing backwards:

  • the right side has a tricuspid valve (a valve with three flaps)
  • the left side has a bicuspid valve (a valve with two flaps)
  • Both sides have semi-lunar valves (at the entrances to the pulmonary artery and aorta).
  • The left ventricle has thicker walls than the right because it needs to pump blood to most of the body while the right ventricle fills only the lungs.
  • The ventricles of the heart have thicker muscular walls than the atria. This is because blood is pumped out of the heart at greater pressure from these chambers compared to the atria.
  • The septum keeps blood from the right (deoxygenated) and left (oxygenated) sides of the heart from mixing. This is important because the blood in the left ventricle is loaded with oxygen for the rest of the body to use.
  • In pumping the blood, the muscle in the walls of the atria and ventricles contracts and relaxes. The atria walls contract first and force blood into the ventricles. Then the ventricles contract and send blood into the arteries.
  • Valves prevent blood flowing backwards during or after heart contractions.

The activity of the heart may be monitored by:

  • ECG (electrocardiogram)
  • Pulse rate
  • Heart sound using a stethoscope, ‘lub-dub’ sound caused by the closure of the valves

The effect of physical activity on the pulse rate:

  • At rest, the heart beats about 70 times a minute, but varies according to age, gender and fitness.
  • An increase in physical activity increases the pulse rate, up to 200 beats per minute.
  • After exercise has stopped, the pulse rate gradually drops to its resting state, the rate depends on the fitness of the person.
  • During exercise, the muscle cells need more energy than usual. They therefore need to respire more and, as a consequence, need more oxygen and glucose, and they produce more waste, carbon dioxide.
  • If the muscle does not get enough oxygen, it will start to respire anaerobically, producing lactic acid, which cause muscle fatigue, leading to cramp.
  • The coronary arteries supply blood to the heart muscle. These may become blocked by a buildup of fatty plaques containing cholesterol, resulting in coronary heart disease.
  • If a coronary artery is blocked, the blood supply to part of the heart muscle is cut off. That part of the heart cannot continue to contract, causing a heart attack.
  • Lack of exercise
  • Diet high in fat and cholesterol
  • Diabetes
  • High Blood Pressure
  • Genetics
  • Stress
  • Smoking
  • Maintaining a healthy, balanced diet will result in less chance of a person becoming obese. Also be a low intake of saturated fats, so the chances of atheroma and thrombus formation are reduced.
  • Exercising increases muscle tone, good heart muscle tone leads to an improved coronary blood flow and the heart requires less effort to keep pumping.
  • Regular dose of aspirin (salicylic acid). Aspirin prevents the formation of blood clots in the arteries, which can lead to a heart attack.
  • Angioplasty and stent. Angioplasty involves the insertion of a long, thin tube called a catheter into the blocked blood vessel. A wire attached to a deflated balloon is then fed through the catheter to the damaged artery. The balloon is then inflated to widen the artery wall, freeing the blockage. Stent can be used. This is a wire-mesh tube that can be expanded and left in place.

By-pass surgery. The surgeon removes a section of blood vessel from a different part of the body, such as the leg. The blood vessel is then attached around the blocked region of artery to by-pass it, allowing blood to pass freely.

Evidence Summary

Exposure to tobacco smoke is associated with accelerated atherosclerosis and an increased risk of acute MI, stroke, PAD, aortic aneurysm, and sudden death. Smoking appears to have both causal relationships and multiplicative interactions with other major risk factors for CHD, including hyperlipidemia, hypertension, and diabetes mellitus.

The cardiovascular risk attributable to cigarette smoking increases sharply at low levels of cigarette consumption and with exposure to secondhand smoke. The risk then tends to plateau at higher levels of smoking. This finding indicates a low threshold for effect and a nonlinear dose-response relationship. Some of the nonlinearity of the relationship between the number of cigarettes smoked per day and CVD risk may be due to impreciseness of this measure of actual exposure to smoke. However, the data on risk associated with exposure to secondhand smoke indicate a true nonlinear relationship between exposure and CVD risk. Cardiovascular risk is not reduced by smoking cigarettes of lower machine-delivered yields of nicotine or tar.

The constituents of tobacco smoke believed to be responsible for cardiovascular disease include oxidizing chemicals, nicotine, CO, and particulate matter. Oxidizing chemicals, including oxides of nitrogen and many free radicals, increase lipid peroxidation and contribute to several potential mechanisms of CVD, including inflammation, endothelial dysfunction, oxidation of LDL, and platelet activation.

Nicotine is a sympathomimetic drug that increases heart rate and cardiac contractility, transiently increasing blood pressure and constricting coronary arteries. Nicotine may also contribute to endothelial dysfunction, insulin resistance, and lipid abnormalities. However, international epidemiologic evidence and data from clinical trials of nicotine patches suggest that chemicals other than nicotine contribute to an elevated risk of death from MI and stroke. CO reduces the delivery of oxygen to the heart and other tissues and can aggravate angina pectoris or PAD and can lower the threshold for arrhythmias in the presence of CHD. Exposure to particulates is associated with oxidant stress and cardiovascular autonomic disturbances that potentially contribute to acute cardiovascular events.

Cigarette smoking causes acute cardiovascular events such as MI and sudden death by adversely affecting the balance of myocardial demand for oxygen and nutrients and coronary blood flow. Smoking results in increased myocardial work, reduced coronary blood flow, and enhanced thrombogenesis. Enhancement of thrombogenesis appears to be particularly important in that smokers with acute MI have less severe underlying coronary artery disease than do nonsmokers with MI, but smokers have a greater burden of thrombus.

Several potential mechanisms appear to contribute to the effects of smoking in accelerating atherosclerosis. These mechanisms include inflammation, endothelial dysfunction, impaired insulin sensitivity, and lipid abnormalities. Cigarette smoking is a risk factor for diabetes and aggravates insulin resistance in persons with diabetes. The mechanism appears to involve both the effects of oxidizing chemicals in the smoke and the sympathomimetic effects of nicotine.