Axial Skeleton

The axial skeleton consists of the skull, vertebral column (made up of vertebrae), ribs and sternum. The human skull looks like a three-dimensional puzzle and is made up of flat bones fused together to form immovable joints. These bones are known as the cranial and facial bones. The lower jaw bone or mandible is movable and allows the mouth to open and close while talking and eating. The eyeballs are located in depressions in the facial bones known as eye sockets.

The vertebral column (backbone or spine) consists of 33 small vertebral bones or vertebrae which are attached together by joints to form a slightly curved, strong and flexible column. The vertebral column supports the head and body and provides a canal for the nerve cord and spaces for the nerves that branch out if the spinal cord. The joints allow limited movement, giving the vertebral column a degree of flexibility. The cartilage disc sandwiched between each pair of vertebrae acts to absorb shock when we move. The vertebral column is divided into five parts based on the location and structure of the vertebrae.
 

There are variations in the size and shape of the vertebrae but a typical vertebra has a solid body or centrum, a neural canal and several bone processes. There are five types of vertebrae based on the five distinct regions where they are found. The cervical vertebrae found in the neck, the thoracic vertebrae found in the thorax or chest region, the lumbar vertebrae found in the abdominal region, the sacrum found in the lower back and the coccyx found right in the end of the vertebral column.
 

Human have seven cervical vertebrae. The cervical vertebrae are easily recognised because they possess a pair of vertebrarterial canals. The function of these canals is to allow the vertebral artery to pass through to the brain. The first cervical vertebra is the atlas while the second cervical vertebra is the axis.
There are 12 thoracic vertebrae. They possess long, backwardly pointing spinous processes and short transverse processes. All the thoracic vertebrae articulate with the ribs except the 11th and 12th thoracic vertebrae. These vertebrae serve to support the ribs.
Scoliosis is a condition caused by the abnormal sideway caused by the abnormal sideway curvature of the spine at the thoracic region, for some unknown reason, it is quite common during childhood, particularly in girls. Scoliosis is treated with body braces or surgery before growth ends. If left untreated, it causes permanent deformity and breathing difficulties.
 

There are five lumbar vertebrae. The lumbar vertebrae are the biggest vertebrae in the vertebral column. They have a short but big centrum because they are subject to the greatest stress. The sacrum consists of five vertebrae fused together to form a broad triangular structure. The coccyx consists of four vertebral bones which are fused together to form a sharp triangular structure. The coccyx is fused to the sacrum and in humans, it is not visible externally. It has no special function.
 

The ribs are flattened, curved bones. There are 12 pairs of ribs and they articulate with the sternum or chest bone ventrally and the thoracic vertebrae dorsally to form a cage known as the rib cage which protects the heart and lungs. Not all ribs articulate with the sternum. The sternum is a flattened, kite-shaped bone.
 

Are bones living or none living? Although the phrase ‘dry as a bone’ is often used, bones are actually made of both living and non-living materials. The living materials are bone cells, blood cells and nerve cells while the non-living materials are calcium and phosphorous. These minerals make the bone hard. In the sixteenth century, Andreas Vesalius of Belgium wrote a book that was the first guide ti the musculoskeletal system. His book was used by Leonardo da Vinci, an Italian artist, inventor and scientist, as a guide to help him make accurate sketches of the human body. 

Support and Locomotion

Necessity for Support and Locomotion in Humans
Have you ever wondered about what supports a building or house? Is it just bricks and cement? You may be surprised to know that beneath the cement and mortar are thousands of steel pieces which are assembled to form the frame of a building. Without this rigid frame, the building would not be able to withstand the forces of nature such as strong winds and rain and would collapse in no time.

Like a building, human also have inner framework which is made of bones. The bones are joined together at the joints to form a framework or skeleton. The skeleton has five major functions. It provides shape and support, enables you to move, protects internal organs, produce blood cells and store certain materials such as calcium and phosphate.

Structure of the Human Skeleton
The parts of the skeleton that form an imaginary line down the back of the body are known as the axial skeleton. The limbs and the bones that connect them to the axial skeleton make up the appendicular skeleton. A newborn baby has about 275 bones whereas an adult has 206 bones. This is because as a person grows, some of the bones are fuse together.

Types of Immunity

Did you have measles or chicken pox when you were young? If you did, most likely you will never get them again as you have acquired immunity against these diseases. Do you know what immunity means?
Immunity refers to the ability of an organism to defend itself against infection by pathogens. Immunity refers to the ability of an organism to defend itself against the infection by pathogens. Immunity depends on the presence of lymphocytes and the production of antibodies which give a specific immune response. The various types of immunity are active immunity (natural and artificial) and passive (natural and artificial).
 

Inherited natural immunity is the immunity which is inherited by an individual through the placenta or mother’s milk. Active natural acquired immunity is acquired after a person recovers from an infection, for example, measles or chickenpox. The body then has the ability to produce more antibodies rapidly against further attack by the same type of invading antigen. Antigens are foreign proteins or polysaccharides usually found in the surface of cells, for example, bacteria and viruses. When they enter the body, they stimulate the lymphocytes to produce antibodies. Antibodies destroy the antigens or neutralise the toxins produced by pathogens.
 

A vaccine contains killed or weakened antigens. When the vaccine is injected into the bloodstream, the lymphocytes in the body produce antibodies against that particular antigen. Vaccination produces active artificial acquired immunity: active because the antibodies are produced by the body itself, artificial because it is obtained through vaccination. This process is known as immunisation. When antibodies are transported from the mother across the placenta to the foetus or through the mother’s milk to the young infant, this gives passive natural immunity for a few months.
 

Passive artificial immunity is obtained by injecting serum containing specific antibodies prepared from the blood of humans or other animals. This is normally used to treat patients who are already seriously ill, for example from rabies, botulism, tetanus or snake bites. The antibodies obtained from other individuals give quick temporary immunity. This type of immunity cannot last for several weeks or months because the foreign antibodies break down in the body and are not replaced.

Body Defence Mechanism

The Three Lines of the Body’s Defence Mechanisms
There are thousands of microbial spores and parasites in the environment. Some organisms are pathogenic and can cause diseases when they enter the human body. These pathogens can be transmitted by air, contaminated food or drinking water, as well as by animal vectors such as mosquitoes and houseflies. Some skin diseases can even be transmitted by contact. The human body have three lines of defense to protect against these pathogens.

First Line of Defence
The skin and mucous membranes act as the first line of defence. The skin acts as a physical barrier. It is made up of a dead keratinised layer which is difficult to penetrate. If there is a cut, the blood clots quickly to seal the wound. Tears secreted by tear glands and acidic sebum secreted by sebaceous glands contains lysozymes which destroy bacteria. Mucus secreted by mucous membranes in the nasal cavity and trachea trap dust particles and bacterial spores. The cilia in the respiratory tract sweep the trapped particles to the pharynx. When microorganisms enter the stomach, they are killed by the hydrochloric acid in the gastric juices.

Second Line of Defence
If pathogens get through the first line of defence, they will meet the second line of defence. Some white blood cells, such as neutrophils act as phagocytes. They are attracted by chemicals produced at the sites of infection. The phagocytes move towards the pathogens, for example, bacteria, and engulf them by phagocytosis. Digestive enzymes are secreted into the phagocytic vacuoles to destroy and digest the bacteria. Useful soluble products are absorbed and assimilated by the phagocytes. Sometimes the phagocytes are destroyed by toxins produced by the pathogens. When there is an infection, the number of white blood cells increases in the body to try to destroy the pathogens.

Third Line of Defence
The third line of defence in the body is the lymphocytes. Lymphocytes are white blood cells found in the lymph nodes and in the blood circulatory system. There are two main types of lymphocytes. The T-lymphocytes attack cells infected by pathogens or produce certain chemicals to coordinate immune response. The B-lymphocytes produce antibodies. An antibody is a protein produced by lymphocytes in response to the presence of an antigen. An antigen is a foreign substance which stimulates the body to produce an immune response. It takes some days to produce an immune response. It takes some days to produce the antibodies. The antibodies are specific in action and promote the destruction of antigens in different ways. After an infection, some lymphocytes remain in the body as memory cells which may last for several months or years. The memory lymphocytes help to defend the body against further infection by the same antigen. The body is then said to be immune against the particular diseases.

The Lymphatic System

Formation of the Interstitial Fluid
When blood flows from arteries into capillaries, there is higher hydrostatic pressure at the arterial end of the capillaries. This high pressure forces some fluid out through the capillary walls into the intercellular spaces between the cells. Once the fluid leaves the capillary walls, it is called interstitial or tissue fluid. Interstitial fluid is similar in composition to blood plasma but it has no erythrocytes, platelets or large protein molecules as these are too large to pass through the capillary walls.

Importance of the Interstitial Fluid
Interstitial fluid is important because it forms the internal environment of the body. It bathes the cells and supplies them with their requirements. Oxygen and nutrients diffuse from the blood through the interstitial fluid and into the cells. Excretory waste products, such as carbon dioxide and urea, diffuse out of the cells into the interstitial fluid. The internal environment of the body is kept within a normal range by homeostatic processes.

Fate of the Interstitial Fluid
Approximately 90% of the interstitial fluid flows back into the venous end of the capillary system where the hydrostatic pressure is low. The remaining 10% of the interstitial fluid enters the lymphatic capillaries and is called lymph. If excess interstitial fluid is unable to return to the blood circulatory system, it will accumulate and cause tissue swelling. This condition is called oedema.

Structure of the Lymphatic System
Lymph is the colourless fluid found in the lymphatic vessels. Lymph is similar in composition to blood plasma but has no erythrocytes, platelets or large protein molecules. Lymph contains a higher number of lymphocytes than blood. Lymph travels through the lymphatic vessels by the contraction of the surrounding skeletal muscles. The lymph flows in one direction. One end of the vessels is closed and back flow is prevented by valves present in the larger vessels. The smaller lymphatic vessels join form larger vessels. 

The vessels from the left side of the body, the alimentary canal and the right side of the lower part of the body flow into the thoracic duct. The thoracic duct is the largest lymphatic vessel in the body. It carries lymph to the left subclavian vein and back into the bloodstream. The right lymphatic duct transports lymph from the right side of the head and chest into the right subclavian vein. Lymph nodes are mainly found in the neck, armpits and the groin. The lymphatic system consists of a network of lymphatic capillaries, lymphatic vessels, lymph nodes and certain organs such as the thymus gland, spleen and tonsils.

Role of the Lymphatic System in Transport
The lymphatic system collects the interstitial fluid and returns it to the circulatory system. Lacteals are lymphatic capillaries in the villi of the ileum. They absorb fats and fat-soluble vitamins and transport them to the blood circulatory system. The lymph nodes filter out bacteria and other foreign particles. Phagocytes present in the nodes engulf and destroy these foreign particles. Lymphocytes in the lymphatic tissues produce antibodies which aid in the destruction of pathogens and the neutralisation of toxins.

Consequences of an Impaired Blood Clotting Mechanism

Some people lack the gene for the production of certain clotting factors, for example, factor VIII. They suffer from a disease called haemophilia. This is an impaired clotting mechanism which causes serious bleeding particularly in the joints. In severe cases haemophiliacs may die of internal or external bleeding. Certain clotting factors such as factor VIII can now be produced by genetic engineering and are used in the treatment of haemophilia.
 

Sometimes a local blood clot (thrombus) is formed on the damaged rough inner wall of the artery. This may cause blockage of the artery, a condition known as thrombosis. When the thrombus dislodges and is carried away by blood circulation, it is known as an embolus. The embolus may be trapped in the small artery where it blocks the blood flow. This condition is called embolism.
 

If the coronary artery is partially blocked, it can cause chest pains called angina. A total blockage, which cuts off the supply of oxygen and nutrients to the heart muscles, causes heart attack (myocardial infarction). The affected heart muscles are damaged. If only a small area of the muscle cells died, the victim can recover. Extensive heart muscle damage can cause death. A stroke occurs if there is a blockage of blood to the brain cells. Strokes are commonly caused by an embolus blocking the flow of blood to a portion of the brain.

Clotting of Blood

Necessity of Blood Clotting
Blood clotting is necessary to prevent blood loss from the body when there is damage to the blood vessels. It prevents blood pressure from falling to a low level as pressure is needed to maintain proper blood circulation. A blood clot seals a wound and prevents the entry of microorganisms and foreign particles into the body through the wound.

Blood Clotting Mechanism
What happens when you accidentally cut your fingers and bleeding occurs? In your blood there are cells called the platelets which help in blood clotting. When you get a cut, the blood vessels around the wound immediately constrict to reduce blood loss. The platelets in the blood become sticky and clump together to plug the wound.

Clotting factors are released by the platelets and damaged tissues which set off a chain of reactions. Thrombokinase, in the presence of factor VIII, converts prothrombin into thrombin. The formation of prothrombin in the liver requires vitamin K. thrombin converts a soluble plasma protein, fibrinogen, into insoluble fibrin fibres which form a meshwork of threads over the wound. As the blood flows out, erythrocytes and platelets are trapped in the fibrin fibres and blood clot forms. It dries to form a scab which covers the wound. When the wound heals, new skin is formed and the scab peels or falls off.

Prevention of Cardiovascular Diseases

Cardiovascular diseases are disorders of the heart and the blood circulatory system. Some common disorders are hypertension, atherosclerosis, thrombosis, embolism, angina and heart attack (myocardial infarction). Major risk factors for cardiovascular diseases are high levels of blood cholesterol, family history, age, cigarette smoking obesity, diabetes mellitus and a sedentary lifestyle.

Some cardiovascular diseases begin early in life, practising a healthy lifestyle is an important preventive measure. A good balanced diet low in saturated fats and cholesterol and higher in unsaturated oils should be practised. The diet should have sufficient green leafy vegetables and fruit. Excessive intake of salt and smoking should be avoided. Regular exercise such as brisk walking, moderate jogging, cycling or swimming, together with maintaining an optimal body weight and a stress-free lifestyle, will help to reduce the risk of cardiovascular diseases.

Regulatory Mechanism of Blood Pressure

Blood pressure is the force of the blood exerted on the walls of the arterial blood vessels. Arterial blood pressure is highest during ventricular systole, and lowest during diastole. Normal blood pressure is 120 (systolic)/80 (diastole) mm Hg. Stretch-sensitive receptors known as baroreceptors are located in the walls of the aorta and carotid arteries branch out form the aorta. They monitor the pressure of blood flowing to the body and to the brain. An increase in blood pressure stretches the baroreceptors. Impulses are sent to the cardiovascular control centre in the medulla oblongata of the brain. From there, impulses are then sent via the parasympathetic nerve to the heart. This slows down the heartbeat, resulting in a decrease in blood pressure.

A decrease in blood pressure increases stimulation of the sino-atrial node by the sympathetic nerve. This increases the contraction of the cardiac muscles of the heart and the smooth muscles of the arteries. The blood pressure increases and returns to its normal level. A person’s blood pressure can be measured by using an instrument called a sphygmomanometer.

The Muscular Heart

Blood can carry out its transportation function only if it can circulate in the human body. The organ responsible for generating the pressure to pump the blood through the vessels is the muscular heart. The heart is a dark red cone-shaped muscular organ found in the thoracic cavity. It is the size of a clenched fist and weighs from 350 to 450 grams in an average adult. The heart is located between the lungs with its apex slightly oriented to the left. The heart has four chambers: two upper thin-walled atria and two lower thick-walled ventricles. The septum separates the right chambers from the left chambers.

The valves in the heart ensure that blood flows in one direction. Heart disease is the number one killer in many countries in the world. An important preventive measure is to educate people about heart fitness and a healthy lifestyle. The valve between the left atrium and left ventricle is the bicuspid valve. The valve between the right atrium and right ventricle is the tricuspid valve. The valves at the base of the aorta and pulmonary artery are the semilunar valves. The heart mainly made up of myogenic cardiac muscles. The heart muscles contract and relax automatically throughout life and are not controlled by the nervous system.The heart functions like two pumps with different pressure system, the right pump forces deoxygenated blood to the lungs. The left pump forces oxygenated blood to other parts of the body.

The sino-atrial node (SAN) is a group of specialised cells located in the right atrial wall, near the entrance of the superior vena cava. It acts like a pacemaker which initiates the heartbeat. The SA node generates a wave of excitatory impulses which spread to the two artria, causing them to contract simultaneously. Blood is then forced from the atria into the ventricles.The second node, the atro-ventricular node (AVN), lying at the base of the right atrium, is then stimulated. Impulses from the AV node are conducted by specialised muscle fibres called bundle of His and Purkinje fibres to the ventricular walls. This causes the contraction of both ventricles to pump the blood out of the heart.

 The right ventricle pumps the blood into the pulmonary artery, which forces the blood to the lungs. The left ventricle, which is thicker and more muscular than the right ventricle, generates greater pressure to pump blood through the aorta to the other arteries in the body.
  
The cardiac cycle is the series of events that occur during one complete heartbeat. It includes the contraction (systole) and relaxation (diastole) of both the atria and the ventricles. The average heart rate is about 72 heartbeats per minute.
  
The pumping of the heart generates sufficient force to move the blood through the arteries, arterioles and capillaries. However, when the blood reaches the veins, the pressure produced by the heart is insufficient to force it back into the heart. The blood on the veins also has to flow against gravitational pull. How does the blood in the veins flow back to the heart? When the body moves, the skeletal muscles around the veins contract and press on the veins. The blood pressure increases, forces open the valves and pushes the blood towards the heart. The valves in the veins prevent the blood from flowing backwards.
  
The sino-atrial node can initiate the heartbeat on its own. However, the heart rate may be modified by certain external factors. The sympathetic nerve carrying impulses to the heart can increase the heart rate and the parasympathetic nerve can slow it down. When a person is excited, an increase in the secretion of the hormone, adrenaline, causes the heart to beat faster. An increase in the partial pressure of carbon dioxide in the blood also increases the heart rate. Heart rate also increases when body temperature is elevated. The human heart is a very hard-working organ. It works non-stop day and night. The average heart beats about 72 times per minute. In an average lifetime, the human heart beats more than two and a half billion times. In total it pumps 34 million litres of blood throughout the circulatory system in the body! (1 billion = 1 000 000 000).

Structure of Human Blood Vessels

Blood vessels are tubes that transport blood from one part of the human body to another. Arteries are blood vessels that carry blood away from the heart. Arteries branch out into small vessels called arterioles. The arterioles branch to form tiny vessels with thin walls called capillaries. Capillaries are the sites for the exchange of respiratory gases, nutrients and wastes. Capillaries join with another to form venules. Venules join together to form veins. Veins transport blood back to the heart. It estimated that if all the blood vessels in a human body were to be arranged end to end, the vessels would measure about 100 000 km in length.


  Table 1.Comparison between arteries, capillaries and veins.

Function of Blood Transport

Blood plays an important role in the transport of oxygen from the lungs to other parts of the body. It also transports absorbed food materials from the digestive tract to body tissues. Beside these, it transports waste products, such as carbon dioxide from body tissues to the lungs and urea to the kidneys for excretion. It also transports heat, hormones and water.

Transport of Oxygen

Oxygen diffuses from the lungs, where the partial pressure of oxygen is higher, into the surrounding capillaries. Oxygen combines with haemoglobin in the erythrocytes to form oxyhaemoglobin. The erythrocytes are carried by circulating blood to other parts of the body where the partial pressure of oxygen is lower. Oxyhaemoglobin dissociates into haemoglobin and oxygen, and oxygen is thereby supplied for cellular respiration. Carbon monoxide binds more rapidly with haemoglobin to form carboxyhaemoglobin which does not dissociate readily. As a result there is less haemoglobin to bind with oxygen. People can suffer from carbon monoxide poisoning if they inhale gas fumes or smoke from the exhaust of a vehicle.

Transport of Carbon dioxide

Cellular respiration releases carbon dioxide. The carbon dioxide diffuses into the surrounding blood capillaries. It then combines with water to form carbonic acid. This reaction is catalysed by an enzyme in the erythrocytes. The carbonic acid then ionises to form hydrogen ions and hydrogen carbonate ions. Hydrogen carbonate ions then leave the erythrocytes and remain in the plasma.

CO2 (carbon dioxide) + H20 (water)  H2CO3 (carbonic acid)  H+ (hydrogen ion) + HCO3-(hydrogen carbonate ion)

About 70% of carbon dioxide is transported in the form of hydrogen carbonate ions. Another 23% combines with the amino group in haemoglobin and is transported as carbaminohaemoglobin. A small amount of 7% dissolves directly in the blood plasma.

When the blood reaches the lungs, the carbon dioxide is released and diffuses out of the blood into the alveoli. The carbon dioxide is then breathed out. When the blood reaches the lungs, the carbon dioxide is released and diffuses out of the blood into the alveoli. The carbon dioxide is then breathed out.

Transport of absorbed food material
Soluble digested food material such as simple sugars and amino acids, water soluble vitamin B and C, and mineral salts are absorbed into the capillaries of the villi in the small intestine. They are transported by the hepatic portal vein to the liver and then to the heart for general blood circulation. The fatty acids, glycerol and vitamin A, D, E and K are transported by the lymph into the blood circulatory system via the left subclavian vein.

Transport of excretory waste products
Deamination of excess amino acids occurs in the liver. The amino group is removed from the amino acid and is converted to urea. Urea is transported by blood to the kidneys to be excreted.

Transport of heat
Blood helps to regulate body temperature by distributing heat from heat-producing sites such as the skeletal muscles to areas of heat loss such as the skin.

Transport of hormones
Blood transport hormones such as insulin and glucagon produced by the endocrine glands to the target organs where they exert their effects. For example, insulin and glucagon produced by the pancreas are carried by blood to the liver.

Transport of water to tissues
Water is an important component of protoplasm and is transported by blood to provide a medium for biochemical reactions.

Composition of Human Blood

An average adult human has about five to six litres of blood. Blood is a connective tissue containing 45% cellular components and 55% of a fluid called plasma.

Human blood:
- Cellular components: erythrocytes, leucocytes, platelets
- Plasma: water(90-92%), soluble solutes(dissolved substance)

Erythrocytes (Red Blood Cell)
There are about five million erythrocytes in every cubic millimetre (microlitre) of blood. Erythrocytes are small biconcave disc with a diameter of 8 µm and a thickness of 2 µm. This shape serves to increase the surface area for gaseous exchange through the thin cell membrane.

In human, erythrocytes have no nucleus. As a result, there is space for great quantities of haemoglobin. Haemoglobin is a protein which contains iron. The main function of haemoglobin is to transport oxygen. When haemoglobin combines with oxygen, the cells become bright red. Without oxygen, they are dark red.
When the partial pressure of oxygen is high in the lungs, haemoglobin will combine with oxygen to form oxyhaemoglobin. When the partial pressure of oxygen is low as in respiring tissues, the oxyhaemoglobin dissociates and oxygen is released. A small amount of carbon dioxide can also bind with haemoglobin to form carbaminohaemoglobin to be carried to the lungs.
Erythrocytes are produced in the bone marrow at the rate of about two million cells per second. They circulate in the body for 120 days. After that they are destroyed by the phagocytes in the liver and spleen.

Leucocytes (White Blood Cells)
Leucocytes are responsible for the defence of organisms against diseases. They are different from erythrocytes in several ways. In the blood, they are much less numerous than erythrocytes. There are about 6000 to 10 000 leucocytes in every microlitre of blood. Leucocytes have nuclei but do not have haemoglobin. Leucocytes are larger than erythrocytes and do not have fixed shapes. Phagocytic leucocytes can move by changing body shape.
Leucocytes are manufactured in the bone marrow but may migrate to the thymus gland or lymph nodes for their growth and development stages.
There are two basic types of leucocyte, namely granulocytes and agranulocytes. Granulocytes have granular cytoplasm and lobed nuclei. There are three types of granulocyte: neutrophils, eosiniphils and basophils. The neutrophils are phagocytes. They engulf foreign materials, for example, bacteria by phagocytosis, and destroy them. Eosinophils help tto control allergic responses. Basophils secrete heparin to prevent blood from clotting.
Agranulocytes have relatively clear cytoplasm and their nuclei are not lobed. There are two types of agranulocyte: monocytes and lymphocytes. Monocytes are the largest of the leucocytes. They spend only a few days in the blood and then move to the body tissues to become phagocytic macrophages. They engulf dead cells and bacteria that enters the body. Lymphocytes are the smallest leucocytes. Some lymphocytes produce antibodies to aid in the destruction of pathogens or to neutralise toxins whereas others attack and destroy infected cells.

Platelets

Platelets are small irregularly shaped fragments of large cells in the bone marrow. They are important in the process of blood clotting. Clots form to reduce blood loss and prevent the entry of pathogens through wounds. Each microlitre of blood contains about 250 000 platelets.

Plasma
Plasma is the pale yellow liquid part of the blood. It is made up of 90% water and 10% dissolved solutes. The dissolved solutes consist of digested nutrients, dissolved gases, minerals, hormones, plasma proteins and excretory wastes. Blood serum is the same as plasma except that clotting factors such as fibrin have been removed.

Transport System in Human Body

The River of Life

Why blood is called the river of life? Blood carries oxygen and essential nutrients to body tissues and transports waste products away to be removed from the body. Living organism require substances such as oxygen and nutrients from the external environment for cellular activities. These activities produce toxic waste products that must also be removed. William Harvey (1578-1657) discovered that blood flows in a closed circulatory circuit in the body. His work laid the foundation for modern understanding of the cardiovascular system.

Circulatory System in Human

The circulatory system in a large organism such as humans involves flow of fluid through the tissues and organs, allowing transport and exchange of substances such as nutrients, oxygen and waste products. In humans, the circulatory system includes medium, vessels and pump. The medium is the fluid that flows in the circulatory system. It transports materials around the body. The blood in the body is carried by a system of large and small vessels. These are arteries, veins and capillaries. The pump in the body is the muscular heart. It creates the pressure that force s the blood through the blood vessels throughout the whole body. Nutrients and oxygen are carried by the capillaries to cells where the exchange of material occurs and waste products are carried away from the cells.