Blood and Cardio-vascular System

Structure and functions of the circulatory system

Blood is river of the life. In humans and other animals, blood is unique providing important nourishment to all body organs and tissues. The blood flows from the heart to every part of body and returning to heart. Plasma is the liquid part of the blood which makes up about 55% of the blood volume. The plasma volume in the body is about 2.75 litters of total of 5 litters of blood in the body of average adult. Plasma is yellowish solution consisting of about 91% water, and the other 9% is a host of substances indispensable to life.

The plasma is to help transport food and oxygen to the cells of the body and to carry wastes away from the cells. Plasma plays a crucial role in maintaining the body's chemical balance, water content, and temperature at a safe level. The plasma serves the body by helping to maintain homeostasis, or a stable internal environment in the body.

The cellular portion of blood normally makes up about 45% of the blood volume. There are three cellular components. They are White Blood Cells (WBC), also known as leukocytes, platelets and Red Blood Cells (RBC) also known as erythrocytes.

There are nutrients such as glucose, fats, and amino acids. The chemicals such as sodium, potassium and calcium are important to the body. Special proteins, such as fibrinogen, albumin, and various globulins that produce antibodies fight off viruses and other unwelcome intruders in the body. The hormones such as insulin and epinephrine speed up the heart rate when emergency blood flow to the muscles is required. 

The formation of blood cells is called Hematopoiesis, or Hemopoiesis. It is a continuous process. Blood cells are divided into three groups: the red blood cells, the white blood cells, and the blood platelets. The white blood cells are subdivided into three broad groups: granulocytes, lymphocytes, and monocytes.

Red blood cells, white blood cells, and platelets are produced in the bone marrow. There are soft fatty tissues inside the bone cavities. Within bone marrow, all blood cells are originated from a single type of unspecialised cells which is called a stem cell. When a stem cell divides, it first becomes an immature red blood cell, white blood cell, or platelet-producing cell. The immature cell then divides, matures further, and ultimately becomes a mature red blood cell, white blood cell, or platelet.

The life of blood cells last for a limited time. White blood lasts from a few hours to a few days. Platelets last about 10 days and Red Blood cells last about 120 days. All decaying blood cells must be replaced constantly. The bone marrow produces and releases more white blood cells in response to infections. It produces and releases more platelets in response to bleeding.

The RBC, also known as Erythrocytes comprises 99% of all blood cell. It is responsible for carrying oxygen and removing carbon dioxide from about 30 trillion cells in the human body. During formation, the nucleus is lost and organelles degraded, allowing more internal space to be filled with haemoglobin. Without a nucleus RBC can never divide. The reddish colour of RBC is derived from haemoglobin.

Haemoglobin is an iron-protein protein and carries CO2 from the cells, which is returned to the lungs. 95% of CO2 generated its carried by RBC. About 5% of CO2 is dissolved in blood plasma. It is essential that blood pH is maintained, and haemoglobin acts as a powerful buffer in maintaining a pH of about 7.4.

It carries oxygen from the lungs, where blood is oxygenated, to body cells. In addition to oxygen and carbon dioxide, hemoglobin releases a third gas, nitric oxide. Nitric oxide plays an important role in regulating blood pressure by relaxing the blood vessel walls, thus increasing blood flow. Hemoglobin controls the expansion and contraction of blood vessels, and thus blood pressure, by regulating the amount of nitric oxide to which the vessels are exposed.

The White blood Cells or Leucocytes is making up less than 1% of the total blood volume. It likes red blood cells, are mostly formed from stem cells in the bone marrow. They have a defensive role in destroying invading organisms, and also assist the removal of dead or damaged tissue cells. White cells consist of lymphocytes and monocytes, with relatively clear cytoplasm and 3 types of granulocyte-neutrophils, eosinophils and basophils, whose cytoplasm is filled with granules. Unlike RBC's, WBC's contain nuclei, they are also much larger and colourless. They use the blood to reach tissues and cells, where they migrate to perform specific functions.

The WBCs constitute the blood's mobile security system. The WBCs can move like an amoeba, slipping through thin walls of capillaries and wandering among cells and tissues. They converge together in great numbers wherever invading bacteria, viruses, fungi, or parasites gain entry into the body, destroying them by swallowing them or by synthesizing antibodies, which are complex proteins that react with and destroy these foreign substances. Whenever white cells mobilize for action, the body compensates by manufacturing more. Double the usual number may appear in the blood within hours.

Platelets, or thrombocytes are small and irregularly shaped cell fragments, 2–3 µm in diameter. The average lifespan of a platelet is normally just 5 to 9 days. They circulate in the blood and are involved in hemostasis, leading to the formation of blood clots.

If the number of platelets is too low, excessive bleeding can occur. However, if the number of platelets is too high, blood clots can form thrombosis, which may obstruct blood vessels and result in such events as a stroke, myocardinal infarction, pulmonary embolism or the blockage of blood vessels to other parts of the body, such as the extremities of the arms or legs. 

Haemostasis is normal blood clotting process that prevents excessive blood loss in the body following vascular injury. It is essential for survival. There are three primary mechanisms to control bleeding. The mechanisms are vascular spasm, platelet plug, and blood clotting.

Vascular spasm occurs When a blood vessels is punctured. The smooth muscles surrounding the blood vessel hold the blood supply to the area. It can hold for minutes to hours.

Platelet plug formation is a more complex process than vascular spasm. There are three phases in this mechanism: Platelet adhesion, Platelet release reaction and Platelet aggregation. Platelet adhesion is the first phase. When the damage to a blood vessel has been detected by platelets, it begins to adhere to the exposed surfaces. Once platelets stuck to a damaged area, the platelets begin to change, which is called Platelet release reaction phase. Firstly Platelet created extensions in order to contact each other, so that they can release their contents. There are two types of chemical packages known as granules, which are held within the cytoplasm of platelets. Two types of granules are alpha granules and dense granules. Alpha granules contain clotting factors, growth factors, and fibroblasts. Dense granules contain ADP, ATP, Calcium ions, and Serotonin. In the Platelet aggregation phase, the ADP make the nearby platelets sticky and combine together with the other recruited platelets and form a platelet plug helping to stop the loss of blood through holes in small vessels.

Blood Clotting or Coagulation is the most complex haemostatic process. The main purpose of the process is to turn liquid blood into a gel. The gel is called a clot and is composed of protein fibres called fibrin in which the formed elements of blood are trapped. The gel effectively forms a cap over a wound. There are several substances in clotting process, which are called clotting factors. These factors are one to three, using Roman numerals.  These clotting factors are a cascade of interactions and resulting in clot formation. Clotting occurs in three stages. The first stage occurs via two distinct pathways, intrinsic or extrinsic, results in the formation of the enzyme prothrombinase. In the second stage of the process, prothrombin is catalysed by prothrombinase and calcium ions to the enzyme thrombin. In stage three, thrombin converts fibrinogen to loose fibrin threads.  Vitamin K is a key factor required for the formation of several clotting factors.

The clot caps a damaged area to stop the bleeding. In addition, the fibrin within the clot sews the damaged area pulling back together. Once the damage has been repaired, tissue repair take place.

Once the damaged vessels have been repaired, the fibrinolytic system prevents clots from getting too large and helps to dissolves clots. Fibrinolysis is the process of dissolving a clot, by removing the fibrin within it.

Haemostasis process is very sophisticated mechanism to protect from the loss of blood from ruptured blood vessels. However, if there is large ruptures occur on the blood vessels, medical treatment is necessary to prevent the excessive loss of blood.

The human body maintains functionality in a variety of environments. Adaptability is the body’s ability to maintain homeostasis. Homeostasis means "equilibrium". When equilibrium within the body is maintained, homeostasis is occurred. The human body maintains a steady internal environment for the proper functioning of the body. Maintaining a constant internal environment requires the body to make many adjustments. Adjustments within the body are referred to as regulation of homeostasis.

Homeostatic regulation is comprised of three parts: a receptor, a control centre and an effector. The receptor functions by receiving information about any changes that are occurring in the environment while the control centre processes that information and the effector executes the commands of the control centre by making changes in response. 

The constant monitoring and regulation of the internal environment is crucial for survival. Various factors that the body regulates help maintain homeostasis.  Those various factors are: 

1. Temperature: a set temperature of internal environment. Liver and muscle contractions generate heat within the body.
2. Osmoregulation: regulated pressure of bodily fluids.
3. Sugar: Sugar levels within the body are also regulated to maintain homeostasis.
4. Calcium: regulate calcium levels.

5.   Balance of Fluids: maintenance of adequate balance of fluids within the body

A blood type is a classification of blood depending on the presence or absence of inherited antigenic substances on the surface of Red Blood Cells. These antigens may be proteins, carbonhydrates, glycoproteins or glycolipids. Some of these antigens are also present on the surface of other types of cells.

There are 30 substances on the surface of RBCS, which is described as a complete blood type. There are many possible combinations of blood-group antigens in an individual's blood type.

The ABO blood group is the most well-known and medically important blood types. It was discovered in 1900 and 1901 at the University of Vienna by Karl Landsteiner. There are four principal types in all human beings. They are A,B,AB and O. There are two antigens and two antibodies that are mostly responsible for the ABO types.

The ABO system is the most important blood-group system in human-blood transfusion. The associated anti-A and anti-B antibodies are usually immunoglobulin M, abbreviated IgM antibodies. ABO IgM antibodies are produced in the first years of life by sensitization to environmental substances such as food, bacteria and viruses.

The specific combination of these four components determines an individual’s type in most cases. The table shows the possible permutations of antigens and antibodies with the corresponding ABO type.

For example, people with type A blood will have the A antigen on the surface of their red cells (as shown in the table below).  As a result, anti-A antibodies will not be produced by them because they would cause the destruction of their own blood.  However, if B type blood is injected into their systems, anti-B antibodies in their plasma will recognize it as alien and burst or agglutinate the introduced red cells in order to cleanse the blood of alien protein.

ABO Blood Type
		Antigen    A	Antigen  B		Antibody  anti-A	Antibody  Anti-B
A		yes	no		no	yes
B		no	yes		yes	no
O		no	no		yes	yes
AB		yes	yes		no	no

The Rh System has 50 antigens. It is the second most significant blood group system in human blood transfusion. The most significant Rh antigen is the D antigen, because it is the most likely to provoke an immune system response of the five main Rh antigens. It is common for D-negative individuals not to have any anti-D IgG or IgM antibodies, because anti-D antibodies are not usually produced by sensitization against environmental substances.

Red blood cell compatibility

The individual with AB Blood group have both A and B antigens on the surface of their RBCs, and does not contain any antibodies against either A or B antigens. Therefore, an individual with type AB blood can receive blood from any group, but cannot donate blood to either A or B group. They are known as universal recipients.

The individual with A Blood Group has the A antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the B antigen. Therefore, a group A individual can receive blood only from individuals of groups A or and can donate blood to individuals with type A or AB.

The individual with B Blood group has the B antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the A antigen. Therefore, a group B individual can receive blood only from individuals of groups B or O and can donate blood to individuals with type B or AB.

The individual with O Blood group do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM anti-A and anti-B antibodies against the A and B blood group antigens. Therefore, a group O individual can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group (i.e., A, B, O or AB). They are known as universal donors.

Red blood cell compatibility table
Recipient	Donor[1]
	O−	O+	A−	A+	B−	B+	AB−	AB+
O−
O+
A−
A+
B−
B+
AB−
AB+

The heart is a muscular pump that pushes blood around the body. It is located between the breastbone and the ribs. It has four chambers. They are left atrium and left ventricle, right atrium and right ventricle. The atriums are in the upper chambers, which receive blood, and the ventricles are in the lower chamber, which eject blood into the arteries. It has four valves separating the chambers. The Tricuspid separates the right atrium from the right ventricle. ThepPulmonary valve separates the right ventricle from the pulmonary artery. The bicuspid valve separates the left atrium from the left ventricle and the Aortic valve separates the right ventricle from the aorta.

The heart is made up of three layers. They are Epicardium, myocardium and endocardium. Epicardium gives a smooth texture. Myocardium is responsible for the pumping action which is made up of muscle fibres. Endocardium is inner layer that connect to large blood vessels. The left side of the heart receives oxygenated blood and the right receives deoxygenated blood.

The main function of the heart is to pump out the blood throughout the body.  The right-hand side of the heart receives de-oxygenated blood from the body tissues and entered into the right atrium. This de-oxygenated blood passes through the tricuspid valve into the right ventricle. This blood is then pumped under higher pressure from the right ventricle to the lungs via the pulmonary artery.

The left-hand side of the heart receives oxygenated blood from the lungs via the pulmonary veins into the left atrium. This oxygenated blood then passes through the bicuspid valve into the left ventricle. It is then pumped to the aorta under greater pressure. This higher pressure ensures that the oxygenated blood leaving the heart via the aorta is effectively delivered to other parts of the body via the vascular system of blood vessels.

Coronary circulation is the circulation of blood in the blood vessels of the heart muscle. This vessel delivers oxygen-rich blood to the myocardium. It is called coronary arteries. The vessels that remove the deoxygenated blood from the heart muscle is called cardiac veins. These include the great cardiac vein, the middle cardiac vein, the small cardiac vein and the anterior cardiac veins.

The coronary arteries that run on the surface of the heart are called epicardial coronary arteries. These arteries are capable of auto regulation to maintain coronary blood flow at levels appropriate to the needs of the heart muscle. These relatively narrow vessels are commonly affected by atherosclerosis and can become blocked, causing a heart attack.

The coronary arteries are classified as "end circulation". They are only the source of blood supply to the myocardium.

The arteries carry the blood away from the heart. Arteries are thick, tough and elastic tubes. Arteries’ walls are thicker than veins. The largest arteries are as thick as a thumb. The smallest arteries are thinner than hair. These thinner arteries are called arterioles. The aorta is the largest artery in the body. It is on the top of the left ventricle of the heart.

Veins carry the blood to the heart. The smallest veins are called venules. The veins carry dark red blood that doesn’t have much oxygen. Veins have thin walls and don’t need thick wall as the arteries as the blood is returned to the heart.

( copy fromhttps://www.google.co.uk/search?q=cardiovascular+system&hl=en&tbo=u&tbm=isch&source=univ&sa=X&ei=Usv7UNz9J67a0QWIu4HQDg&sqi=2&ved=0CEMQsAQ&biw=1024&bih=471)

 

The heart is a muscular organ which supplies blood to the entire part of the body by blood circulation system. The blood circulation system is made up of the vessels, the arteries and muscles to control the flow of the blood.

The heart is divided into four main sections. There are left and right atrium and the left and right ventricle. The blood enters the heart through the superior and inferior vena cava into the right atrium. Then it passes by the tricuspid valve into the right ventricle.

The blood vessels are a network of interconnecting veins and arteries. Blood leaves the left side of the heart and travels through arteries, which gradually divide into capillaries.

Blood vessels can be divided into five groups, according to the sequence of blood flow from the heart. Arteries carry blood carry blood away from the heart and toward the tissues. The heart’s ventricles pump blood into the arteries. Arterioles are small subdivisions of the arteries. They carry blood into the capillaries. Capillaries are tiny, thin-walled vessels that allow for exchanges between systems. These exchanges occur between the blood and the body cells and between the blood and the air in the lung tissues. The capillaries connect the arterioles and venules. Venules are small vessels that receive blood from the capillaries and begin its transport back toward the heart. Veins are vessels formed by the merger of venules. They continue the transport of blood until it is returned to the heart.

The blood vessels are made up of muscle fibres. It is capable of contraction and expansion of the size of the vessel. The contraction of the muscles reduces the size of the vessel, allowing the blood flow faster. During relaxation the vessel has more room for blood to flow, thus blood flow rate is reduced. During a moment of panic, the body will release chemicals to stretch the blood vessels in order to allow for maximum blood flow so that the muscles will have the oxygen they need. In case of loosing blood through injuries, the vessel sizes are reduced to prevent excessive loss of blood.

 

References and Bibliography

Bevan J. (1978) Anatomy and Physiology MITCHEL BEASLEY PUBLISHERS LTD

Tucker, L. (2000) An Introductory Guide to Anatomy & Physiology, 3 Ness Road, Burwell, Cambridge, CB5 0AA, An Imprint Ruben Publishing Ltd.

Jones, M and Jones G. (2004) Human Biology for AS, The Pitt Building, Trumpington Street, Cambridge, UK

Givens, P and Reiss M.(2002).Human Biology and health studies.2^nd Edition. Delta Place, 27 Bath Road, Cheltenham GL53 7^TH, UK.

Jenkins, M. (1997) Human Biology, Human Physiology and Health. GCSE Study Guide. New Edition. Letts Educational. Aldine House, Aldine Place, London W12 8AW.

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http://anthro.palomar.edu/blood/ABO_system.htm 13th December 2012

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