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TRANSPORT IN ANIMALS

The blood

Blood is one of the most important components of life. Almost any animal that possesses a circulatory system has blood. From an evolutionary perspective, blood was speculated to have risen from a type of cell that was responsible for phagocytosis and nutrition. Billions of years later, blood and the circulatory system have drastically helped the evolution of more complex lifeforms.

Blood is a fluid connective tissue that consists of plasma, blood cells and platelets. It circulates throughout our body delivering oxygen and nutrients to various cells and tissues. It makes up 8% of our body weight. An average adult possesses around 5-6 litres of blood.

THE BLOOD CELLS

Functions of Blood

Blood is responsible for the following body functions:

1. Fluid Connective Tissue

Blood is a fluid connective tissue composed of 55% plasma and 45% formed elements including WBCs, RBCs, and platelets. Since these living cells are suspended in plasma, blood is known as a fluid connective tissue and not just fluid.

2. Provides oxygen to the cells

Blood absorbs oxygen from the lungs and transports it to different cells of the body. The waste carbon dioxide moves from the blood to the lungs and exhaled.

3. Transports Hormone and Nutrients

The digested nutrients such as glucose, vitamins, minerals, and proteins are absorbed into the blood through the capillaries in the villi lining the small intestine.

The hormones secreted by the endocrine glands are also transported by the blood to different organs and tissues.

4. Homeostasis

Blood helps to maintain the internal body temperature by absorbing or releasing heat.

5. Blood Clotting at Site of Injury

The platelets help in the clotting of blood at the site of injury. Platelets along with the fibrin form clot at the wound site

6. Transport of waste to the Kidney and Liver

 Blood enters the kidney where it is filtered to remove nitrogenous waste out of the blood plasma.

The toxins from the blood are also removed by the liver.

7. Protection of body against pathogens

The White Blood Cells fight against infections. They multiply rapidly during the infections.

The Components of the Blood 

Because blood has many functions you might be able to conclude that the blood is composed of many different parts.  The liquid part of the blood is called plasma.  Plasma takes up about 55% of the total volume of the blood.  The remaining 45% of the blood is made up of red blood cells, white blood cells, and platelets.  An adult human has between four and six liters of blood in the body.

There are many cellular structures in the composition of blood.  When a sample of blood is spun in a centrifuge machine, they separate into the following constituents: Plasma, buffy coat and erythrocytes.

Plasma – Plasma is the clear liquid portion of the blood.  90% of plasma is water.  The other 10% contains many types of molecules, including nutrients, glucose, vitamins, cellular wastes, salts, and proteins.  There are three major types  of proteins which exist in plasma.  These are albumin, fibrinogen, and globulins.  Each proteins has a specific function to perform.  The albumin keeps water from leaving the blood and entering the surrounding cells by osmosis.  It does this by helping to keep the concentration of the water within the blood the same as the concentration in the body tissues.  The fibrinogen aids in the clotting of the blood.  Some globulins transport proteins and other substances from one part of the body to the next.  Other globulins are known as antibodies, which help to fight of infection.  Antibodies are proteins that attach to and help destroy foreign substances in the body.

 Red Blood Cells – These cells are red and carry oxygen and carbon dioxide.  They are present in huge numbers within the blood.  The human body contains 30 trillion red blood cells, or approximately 5 million cells per cubic millimeter of blood.

RBCs are biconcave cells and without nucleus in humans; also known as erythrocytes. RBCs contain the iron-rich protein called haemoglobin;  give blood its red colour.  The red blood cells transport oxygen from the lungs to the tissues in the body.  They also carry carbon dioxide from the body tissues to the lungs.  In humans, the matured red blood cells do not contain a nuclei.

Their cytoplasm is filled with an iron-containing protein called hemoglobin.  Hemoglobin is the substance that gives the blood its red colour.

 Human red blood cells are constructed by bone marrow and have and a average life span of up to 120 days. New cells are produced at the same rate red blood cells are destroyed .  This occurs at pace of about 2 million per second. The old red cells are removed from the body by the spleen and liver and are then broken down. The iron from the hemoglobin is then collected and reused.

When a person has an insufficient amount of hemoglobin or too few red blood cells, this is referred to as anemia.  Both of these conditions lowers the amount of oxygen that can be carried throughout the blood. Anemia causes the cells not to recieve the proper amount of oxygen.  This is a heriditary disorder, and is caused by an abnormal form of hemoglobin.

White blood cells ( Leucocytes)

Leucocytes are colourless blood cells.. WBCs mainly contribute to immunity and defence mechanism. They protect the body from bacteria and viruses, which are disease-causing organisms.  Unlike red blood cells, the white blood cells contain a nucleus and are larger than the red blood cells. There are fewer white blood cells than white, but there are still about 60 billion in an adult human body.  The bone marrow and lymphatic tissue produce approximately 1 million white blood cells every second.  The white bloods cells are distribute themselves throughout the body by moving through the circulatory system.  When there is an infection within the body, the white blood cells collect in the infected area and attack the foreign organisms.

There are five different types of white blood cells.  The majority of them function to protect the body in some form.  A portion of the white blood cells are what are called phagocytic(monocytes & neutrophils).  They protect the body by fighting the bacterial invaders, and anything which does not belong in the body.  The lymphocytes take care of the production of antibodies and the cells that destroy certain substances and uncommon cells. Usually, there are 7000 to 10 000 white blood cells present per cubic millimeter of blood. When an infection of the blood occurs, the numbers of white blood cells may increase to 30,000 or more per cubic millimeter.  The phagocytic white blood cells eat the bacteria which they encounter.  After these phagocytic cells eat the bacteria some of them die.  This is what pus is when it forms around an infected area.

 Platelets – The part of the blood which is involved in the clotting of blood.  Platelets are formed when bits of cytoplasm are pinched.  Even though these bits of cytoplasm contain no nuclei, they surrounded by a membrane.  There about a total of 1.5 trillion platelets in the blood of an adult human.  There are about 300,000 platelets existing in a cubic millimeter of blood. Their life lasts for about seven days and are produced at about 200 billion per day.

 Blood Clotting – Unless blood is a free-flowing liquid it will not be able to circulate easily throughout the body’s blood vessels.  However, in being a liquid it could cause a large variety of problems.  If there was an injury that broke a large blood vessel it could lead to a large loss of blood. This problem is resolved by the complex mechanism of clotting.  The clots form a temporary barrier to prevent blood loss until the vessel walls have healed.  

The Clotting Process  – When a blood vessel is injured, platelets begin to collect near the injury, which forms a barrier known as the platelet plug.  When the platelets come in contact with an injured area, they swell up, become sticky, and release certain chemicals.

Blood clotting requires many precise reactions to maintain a certain balance between quick and efficient clot formation.  This balance has to be kept exact so that your blood will not clot at the wrong time. Prothrombin and fribrinogen are two proteins that are produced by the liver that are always present in the plasma of the blood.  The injured tissues and platelets release prothrombin activator and calcium ions (Ca²⁺) to change prothrombin into the enzyme thrombin.  Then the thrombin splits two short amino acid chains from each fibrinogen molecule.  The ends of the fibrinogen then join together, forming threads of fibrin.  Thesefibrin surround the platelet plug in the damaged area of the blood vessel and provide the shape for the clot.  Red blood cells are present within the fibrin which makes the clot appear red.  After this the clot stops the bleeding, gets smaller, and hardens.  Over time the injury is repaired by the growth of new cells which will replace the cells lost because of the injury.  When all the healing has finished an enzyme called plasmin in activated and dissolves the fibrin clot.

Some Clotting Problems – There are many conditions which can cause the clotting process to be disrupted.  People who have the hereditary disease haemophilia, lack the essential Factor VIII (Antihaemophilic Globulin, or AHG) of blood clotting.  These people can recieve certain injections which will enable ther blood to clot properly.  If you don’t have enough platelets in the blood or lack vitamin K, this will reduce the ability to clot.

Blood Groups – The ABO grouping and the Rh factor are the most often used to determine blood type.  The physician Karl Lansteiner determined that there were four major blood groups among humans.  He designated them as A, B, AB, and O.  This same system is used today.  It is now a known fact that blood type is based on a type of glycoprotein present in the red blood cells.  Type A blood has A-type glycoproteins, type B blood has B-type glycoproteins, type AB blood has both of these glycoproteins, and type O blood has neither of them.  The A and B glycoproteins function as antigens, and they combine specifically with antibody molecules.  When this kind of reaction occurs, the red blood cells agglutinate (join together). People who have type O blood are called universal donors.  It can be given to anyone without fear of agglutination because it does not contain any antigens that could combine with anti-a or anti-b antibodies  The name universal recipient is given to a person with type AB blood. AB blood does not have anti-a or anti-b antibodies that could combine with any antigens.

The Rh factors are another group of antigens found on the surface of red blood cells.  They are called Rh factors simply because they were first discovered in rhesus monkeys.  About 85% of  humans are Rh⁺, which means they have Rh factors on their red blood cells.  The remaining 15% of humans are Rh^–, which means that they do not contain the Rh factor.  These Rh factors may present a problem when a mother is Rh^– and the baby is Rh⁺.  If their blood is to mix and some of the Rh⁺ blood cells enter the mother’s circulatory system, her immune system will from anti-Rh antibodies.  In future pregnancies, these anti-Rh antibodies could enter the babies blood stream.  If this were to happen and the baby was Rh^–,  the antibodies would destroy the babies red blood cells.  This problem can be eliminated if the mother is given an injection of anti-Rh antibodies to destroy the baby’s Rh⁺ cells shortly after the birth of each Rh⁺ child.  This will prevent the mother’s immune system from forming it’s own antibodies