Annelids
Annelids are coelomate animals. The presense of coelom separaes the body wall from the internal organs and gives the advantage of independence of movement of internal structures such as the gut. However, this means there is a need for some form of transport systems in the body. A body system has evolved which connects gut and body wall. The earthworm, for example, has a well developed blood system in which blood circulates around the body through a system of blood vessels.

Two main blood vessels run the length of the body, one dorsal and one ventral. They are connected by the blood vessels in each segment. Near the front of the animal, five pairs of these connecting vessels are contractile and act as pumps. The main blood vessels can also pump blood. The blood contains haemoglobin dissolved in the plasma rather than being carried in red blood cells. Haemoglobin transports oxygen around the body.

Anthropods
Anthropods have an open blood system. The coelom is drastically reduced and its place taken by the haemocoel. This is a network of blood filled spaces called sinuses in which the internal organs are suspended. Gaseous exchange in most arthropods is achieved by the tracheal system and the blood vascular system is not used for transporting respiratory gases. Arthropods blood is colorless and contains no haemoglobin.

Vertebrates
The blood system of all vertebrates possesses a muscular heart, lying in a ventral position near the front of the animal. The heart is responsible for pumping blood rapidly to all parts of the body. Arteries carry blood away from the heart and veins carry blood from the body back to the heart. Oxygen is carried by haemoglobin in red blood cells. The lymphatic system will also be examined briefly where its functions overlap with those of the blood system.

The simpler animals such as cnidarians and Platyhelminths lack specialized systems for the transport and distribution of materials. The organisms in these groups possess a large surface area to volume ration and diffusion of gases over the whole body is surface is sufficient for theirs needs. Internally the distance that materials have to travel is again small enough for them to move by diffusion or Cytoplasmic streaming.

An organisms increase in size and complexity so the quantity of materials moving in and out of the body increase. The distance that materials have to travel within the body also increases, so that diffusion becomes inadequate as a means for their distribution. Some other organism to another is therefore necessary. This generally takes the form of a mass flow system. There are two circulatory systems which rely on mass flow in animals, namely the blood vascular system and the lymphatic system. A vascular system is one which contains fluid-filled vessels involved in transport.

General characteristics of a blood vascular system
The purpose of a blood vascular system is to provide mass flow of materials from one part of the body to another over distances where diffusion would be too slow. On reaching their destination the materials must be able to pass through the walls of the circulatory system into the organs or tissues. Likewise, materials produced by these structures must also be able to enter the circulatory system. In other words, exchange systems are linked to mass flow systems. Every blood system possesses three distinct characteristics:

Two distinct types of blood system are found in animals. They are the open and closed blood systems.
The open blood system:
Blood is pumped by the heart into an aorta which branches into a number of arteries. These open into a series of blood spaces collectively called the haemocoel. In other word, blood does not stay in the blood vessels, hence the terms ‘open’. Blood under low pressure moves slowly between the tissues, gradually percolating back into the heart through open ended veins. Distributions of blood to the tissues are poorly controlled.
The closed blood system (echinoderms, most cephalopod mollusks, annelids, vertebrates including humans).

Blood vessels are named according to their structures and function. Vessels conveying blood away from the heart are called arteries. These branch into smaller arteries called arterioles. The arterioles divide many times into microscopic capillaries which are located between the cells of nearly all the body tissues. It is here that exchange of materials between blood and tissues takes place.
Within the organ or tissues the capillaries reunite to form Venules which being the process of returning blood to the heart. The Venules join to form veins. Veins carry blood back to the heart.

When there are problems with regularity of heartbeat and artifical pacemaker can be used to gain control over the electrical activity of the heart. A pacemaker has two basic components, a pluse generator containing a power sources and one or two pacing leads, each with an electrode on its tip.

Pacemaker can be temporary or permanent. When long term control of the heart is required, a permanent pacemaker is implanted under the skin. The two most common modes of pacing are :
Demand: this detects the heart’s own rhythms and simulates depolarization of the heart muscle, and therefore contraction, as necessary.
Fixed rate: this fires at a predetermined rate irrespective of the heart’s own activity.

Heart transplant suragery
In case of heart disease where all other treatments are inadequate or inappropriate, a heart transplant may be advised. The first heart transplant was carried out in 1967 in south Africa. At first survival rates were low but great resulted in the majority of transplant patients surviving more than five years. The operation itself is relatively simple, with life being sustained by a heart-lung machine during the opereation. The difficult task is caring for the patients afterward, and few institutions are equipped to do this. Although a relative common procedure, demand greatly exceeds the supply of donor hearts. Attempts to develop artificial hearts have not been successful. The last proposal to meet the demand is to use heart from pigs which have been genetically engineered to avoid the potential rejection problems. This may not only raises animal right issues but the possibility of transferring disease from one species to another. This is serious concern, especially in the light of the transfer of BSE to humans from beef. Other ethical issues are raised by transplant surgery, particularly heart transplants. Choices have to be made about who to treat when there is a scarcity of organs. Should smokers be as entitled to treatment as non-smokers, for example? The high cost of transplant surgery also raises the question of how limited resources are best allocated within the National Health Service.

A number of factors are known or believed to be involved in development of atherosclerosis, and hence cardiovascular disease. Some are more important than others and usually several act together to bring it about. The three most important are diet, hypertension (high blood pressure) and smoking.

Diet: Atheroma contains fats and cholesterol, and it has been shown clearly that experimental animals fed on a high fat diet develop diseased arteries. In countries such as Greece and Japan where the average diet is relatively low in fat, cardiovascular disease is much less common. The main problem is caused by saturated fats which cause a rise in blood cholesterol levels. In 1992 the UK government recommended that the population from saturated fatty acids should be reduced from 17% (proportion in 1990) to no more than 11% by 2005 (a 35% reduction). A 12% reduction in total fat was also proposed (from about 40% to 35% of food energy). On the other hand, polyunsaturated f atty acids, found in unsaturated fats, are thought to help reduce cholesterol levels in blood and are therefore beneficial to health.

Hypertension (High Blood Pressure): Raised blood pressure can considerably increase the chance of developing cardiovascular disease. It has been shown that men under the age of 50 years with a blood pressure of 170/100 are twice as likely to die of coronary heart disease as men with normal blood pressure of about 120/80. High blood pressure itself is associated with a number of different factors, including stress, obesity, smoking, drinking excessive amounts of alcohol and lack of exercise. There is also a genetic predisposition in some people. Some of these factors can obviously be avoided by changes in lifestyle.

Drug known as β-blockers can be used to reduce hypertension. The hormone adrenaline has an excitatory effects on the body. It binds to specific receptors known as α- and β-receptors in its target organs. Β-blockers block the β-receptors, thus inhibiting the action of adrenaline and reducing heart rate. The same drugs are used to treat angina because they reduce the need for oxygen in the heart muscle.

Smoking: Heavy smokers are more likely to develop cardiovascular diseases. For example, people under the age of 45 years who smoke more than 25 cigarettes a day are 15 times more likely to die of heart disease than non-smokers. Nearly 40% of cardiovascular deaths re due to smoking. Smoking increase atherosclerosis and decreases the ability to remove blood clots that build up at atheromatous plaques.

The effects of smoking re many and complex.

Smoking is the largest single cause of premature death in Britain. More than one third of the extra deaths caused by smoking are due to cardiovascular disease.

Physical exercise: Many studies have shown that the more a person is physically active at work or during their leisure time, the less chance they have of suffering from cardiovascular disease. The effects of suffering from cardiovascular diseases.

Gender: Death rates from cardiovascular disease in women are less than half those for men and women rarely suffer from it before the menopause. There is a protective effect from the female sex hormones and a harmful effect from the male hormone testosterone. After the menopause women show and increase in blood fats and a sharp increase in rates of cardiovascular disease.
Lipids are insoluble in plasma and are therefore carried in a combined form with other molecules. In particular they are combined with protein to form spherical particles called Lipoproteins. The size and density of these particles varies. Two common types are the low density and high density Lipoproteins. The size and density of these particles varies. Two common types are the low density and high density lipoproteins (LDLs and HDLs respectively). HDLs contain 21%cholesterol while LDLs contain 55% cholesterol. Generally speakingm the higher t he amount of HDLs and the lower the amount of LDLs in the blood, the better for health. LDLs tend to stickto artery walls and unload their fats, contributing to atheroma. In males, HDL levels tend to drop at puberty and LDL levels gradually increase with age. This is thought to be due to testosterone. Oestrogen on the other hand increases HDLs and therefore tends to protect women between puberty and menopause. They still get heart disease but on average about 10 years later than men.

Heredity: If one parent suffers from premature coronary heart disease then the risk of a man suffering a myocardial infarction is doubled. If both parents suffered, the risk is increased by a factor of five. This indicates that there could be a genetic predisposition to heart disease. It is anticipated that the genes responsible will be identified fairly soon and tests may become available to identify those at risk.

Stress: There is great difficulty in defining stress and quantifying it. However, there is general agreement that psychological and emotional stress is often an important factor in triggering attacks of angina or even a myocardial infarction.

Age: Arteriosclerosis appears to be an inevitable consequence of ageing and this increase the risk of cardiovascular disease.

In the developed countries infectious diseases are no longer the major causes of death. The average life expectancy is now about 76 years. As people age however they become prone to disease of the heart and blood vessels, and cancers, and these two types of disease account for about two thirds of all deaths in developed countries.

Cardiovascular disease in Britain’s biggest killer, accounting for about 40% of all premature deaths. The two major cardiovascular diseases are coronary heart disease, which accounted for about 25% of all deaths in Britain in 1990 and strokes, responsible for about 10% of all death in Britain in 1990. Overall this is between 300 to 400 people per day and represents about a five fold increase since the Second World War. It is not surprising then than cardiovascular disease has been called the “modern epidemic” as it is on a scale comparable to major infectious diseases of the past. Unfortunately Britain has one of the highest rates of cardiovascular disease in the world and in 1992 the Government set targets aimed at reducing its incidence. The targets acknowledge the fact that to alarge extent the deaths are avoidable and that it is important to understand their causes and to try to develop more effective strategies to reduce the numbers of deaths.

Atherosclerosis
By far the most common cause of cardiovascular disease is atherosclerosis. The process leading to atherosclerosis stats with the deposition of yellow fatty streaks containing a high proportion of cholesterol in the inner coat of arteries. The deposits form beneath the inner are deposited in the cholesterols and these often start to calcify and become hard, a process known as arteriosclerosis. The deposits are referred to as athermanous plaques. As a plaque increases in size it protrudes into the occurs in the aorta and coronary arteries which supply the muscles of the heart. If the plaque breaks through the smooth endothelium, its rough surface commonly cause a blood clot to develop. This is called a thrombus which may build up until it is large enough to block the artery. If clot breaks away, it may block an artery at another location. A clot that breaks away like this is called and embolus.
The artery wall is made weaker by athermanous plaques and may stretch as a result. Local stretching is called aneurysm. It may rupture, a process known as hemorrhage. This is more likely if arteriosclerosis have occurred. Once an artery is blocked the tissue it supplies will suffer oxygen starvation and will be severely damaged or die. If thrombosis occurs in a coronary artery the heart is damaged and a heart attack any occurs. This medical term for heart attack is myocardial infarction. (Myocardial refers to heart muscles; infraction means suffocation due to lack of oxygen.) If thrombosis occurs in the brain (cerebral thrombosis) a stroke may occurs. Strokes are sometimes referred to as cerebra vascular accidents. They are also caused by cerebral hemorrhage. They usually result in permanent damage to the cerebral hemispheres due to oxygen starvation. The cerebral hemispheres are the conscious part of the brain and control may functions such as speech and motor coordination. Both heart attacks and strokes may result in death.

A muscle that is excercised without an adequate blood supply will give rise to pain as a result of cramp. When the heart is involved, such as pain is called angina. An angina attack may be brought on even by gentle execise such as climbing stairs. The pain may spread out from the center of the chest to the neck, jaws, arms and back.

Thus coronary heart disease has two main forms, angina and mycocardial infraction (heart attack). A heart attack may be caused by a coronary thrombosis or simply by narrowing of the artery by atherosclerosis until the blood supply by narrowing of the artery by atherosclerosis until the blood supply is sufficiently restricted. About half a million people a year in Britain have heart attacks and about one third die as a result. Half of these die within one hour. There are now great efforts taken to try to avoid these deaths by carrying special equipment in ambulances and by suitable treatment in hospital casualty units. Drugs can be used to restore normal heart rhythms and a heart which has stopped beating can sometimes be restarted by administration of an electrical shock across the chest wall.

Malaria has been one of the world’s worst killer diseases throughout recorded human history. Despite attempts to eradicate it, it remains one of the worst diseases in terms of deaths annually, and has actually increased in incidence since the 1970s. About 200 to300 million new case occurs worldwide each year and about 1.5 million deaths over two third of which occurs in Africa. It is particularly common in Africa south of the Sahara and is widespread through Asia and Latin America. It is used to be common in Europe and North America (Oliver Cromwell died of malaria and Sir Walter Raleigh suffered from it.). Malaria provides a good example of how social, economic and biological factors are all important in controlling disease.

Transmission, signs and symptoms
The causative agents of malaria are the protozoan parasite Plasmodium. Four species cause malaria, but most cases are caused by one of two species. Plasmodium vivex is found in the subtropics; Plasmodium falciparum is more common and more lethal, and is found in both the tropics and subtropics. It is the largest single cause of death in Africa. Malaria is transmitted by female mosquitoes of the genus Anopheles. This mosquito is itself a parasite, the females visiting humans for occasional meals of blood. During feeding, infected mosquitoes pass on the malaria parasite from their salivary glands. The mosquito is described as a vector.

An immature form of Plasmodium is injected into the blood of humans by the mosquito. This form disappears from the bloodstream as it enters various cells of the body, particularly the liver. Here it multiples to produce large numbers of a form (the merozoite) which can infect other liver cells. Finally it leaves the liver and enters red blood cells. Each parasite cell in a red blood cell undergoes further division. The red cell bursts and the released parasite cells can enter other red blood cells. As a result of this extensive division, millions of these parasites can be present in the blood. Some of the parasites transform in to male and female forms of the parasite (gametocytes).

If infected red blood cells are now sucked up by a mosquito the parasites survive digestion in the stomach of the mosquito and transform into male and female gametes. Fertilization occurs and the zygote penetrates the wall of the stomach where it grows to produce a swelling containing immature parasites.

People who have been exposed to infection since birth and who have survived attacks of malaria develop a certain amount of tolerance to the disease. People with no history of previous infection will develop serious disease very rapidly. Ten days after infection a few fever develops and the body temperature increases rapidly to 40.6-41.7ºC. The fever may last as long as 12 hours accompanied by headache, generalized aches and nausea. After the fever sweating starts and then the temperature falls. The area of the abdomen over the spleens is tender.

The symptoms and fever coincide with the multiplication of the parasite when the red blood cells burst open and the parasites are released. The fevers occurs every third day in P.vivex and P.falciparum malarias. The attacks can be complicated, however, as a result of successive infections by mosquitoes. The infections by P.falciparum cause malignant malaria in which the fever is accompanied by other complications. The parasite tends to accumulate in blood vessels in the brain, causing convulsions or coma. Other common complications include kidney failure and pneumonia. Malaria caused by P.falciparum can be fatal within two or three days.

Prevention and treatment
Prophylaxis that is the use of medicines (drug) to prevent disease can be used by people entering areas endemic for malaria. The usually accepted prevention is chloroquine or mefloquine which is taken weekly before and during a visit to endemic areas. The dose must also be taken for six weeks after leaving the area. Other synthetic drugs are proguanil hydrochloride and pyrimethamine. The effectiveness of these drugs has declined as the parasite developed resistance. P.falciparum in now resistant in America, East Africa and S.E. Asia. New drugs are constantly being sought, but pharmaceuticals companies are tending to scale down work on ant malarial drugs because there is little profit to be made from developing countries.

A drug called Fansidar and another called Lariam (mefloquine) are effective against chloroquine-resistant Plasmodium. Mefloquine can cause unpleasant side effects such as nausea, dizziness, diarrhea, vomiting, headaches, abdominal pain and occasionally psychiatric disturbance. It was introduced in 1985 but already half the malaria cases in Thailand are resistant to it. A traditional drug, quinine has come back into favors, but resistance is also growing to this.
Together with the use of drugs, other measures such as clothing (long sleeves trousers) and mosquito nets at night should be used in mosquito area. These are now often impregnated with new insecticides. It may be necessary to administer drugs by injection. This then produces a high concentration of the chosen drug to kill the malaria organisms in the blood.

Eradication of malaria
In 1955 the World Health Organization launched a programs designed to eradicate malaria from the world. Large sums of money were devoted to this project and the methods employed were as follows:
Drainage of stagnant water
Destruction of the breeding stages of the mosquito
Destruction of the adult mosquitoes.

Another common condition caused by the undernutrition is marasmus. Originally this was thought to be duet to a lack of energy in the diet, but it may not be as clear cut as this, as discussed below. The appearance of a child suffering from some symptoms of marasmus is shown in the fig. and can be compared with the child suffering from kwashiorkor. Followings are the signs and symptoms of protein disease marasmus:

As we have learned more about malnutrition caused by undereating , the distinctions between the causes of marasmus and kwashiorkor have become less and less clear. Different children in the same family have developed t he two different conditions while feeding on the dame diet, and sometimes a child develops marasmus after kwashiorkor. The child in the fig shows the thin limbs associated with the kwashiorkor. There is a tendency to refer to both conditions now simply as malnutrition or protein energy malnutrition (PEM). This always involves stunted growth and reduced resistance to infectious diseases.