Introduction:

In unicellular organisms like amoeba, nutrients and respiratory gases enter cell and waste materials leave cell by diffusion and other cellular processes. Therefore, the organisms are able to exchange matter directly with outside environment. In multicellular organisms, procedure of matter exchange is more complex. Trillions of specialized cells in multicellular organism are organized in functional, structural units, like tissues and organs. Individual cells which make up the structural units need nutrients and oxygen, and they should rid themselves of wastes, simply as unicellular organisms do.

Major Functions of Circulatory System:

Circulatory system has following three main functions:

• It transports gases (from respiratory system), nutrient molecules, and waste materials (from digestive system).
• It regulates internal temperature and transports chemical substances which are essential to health from one part of body to other.
• It protects against blood loss from injury and against disease causing microbes or toxic substances introduced in body.

Main Components of the Circulatory System:

Circulatory system has 3 main components: heart, blood vessels, and blood. Heart is a muscular organ which continuously pumps blood through body and generates blood flow. Blood vessels are a system of hollow tubes by which the blood moves. Together, heart and blood vessels include cardiovascular system (cardio comes from a Greek word meaning "heart," and vascular comes from a Latin word meaning "vessel"). Blood is fluid which transports oxygen, nutrients, carbon dioxide, and several other materials throughout body

Transport System in Lower Animals:

Among unicellular protists, oxygen and nutrients are attained directly from aqueous external environment through simple diffusion. Body wall is only 2 cell layers thick in cnidarians, like Hydra, and flatworms, like Planaria. Every cell layer is in direct contact with either external environment or gastrovascular cavity. Gastrovascular cavity of Hydra expands from body cavity in tentacles, and that of Planaria branches widely to supply every cell with oxygen and nutrients.

Open and Closed Circulatory Systems:

Larger animals, though, have tissues which are several cell layers thick, so that several cells are also far away from body surface or digestive cavity to exchange materials directly with environment. In its place, oxygen and nutrients are transported from environment and digestive cavity to body cells by the internal fluid within the circulatory system.

Open circulatory system:

Several invertebrates have the open circulatory system. It is general in molluscs and arthropods. System is known as open as blood flows freely in body cavity and builds direct contact with organs and tissues. Or we can say, there is no difference between blood and interstitial fluid. Open circulatory systems that evolved in insects, crustaceans, molluscs and other invertebrates, pump blood into hemocoel (blood-filled, open body cavity) with blood diffusing back in circulatory system between cells. Blood is pumped by heart in body cavities, where tissues are enclosed by blood. In invertebrates, like insects and molluscs, mixture of blood and fluids which surrounds cells is known as hemolymph. In insects like grasshopper, hemolymph is pumped through single vessel which runs from head to abdomen. In abdomen, vessel separates in chambers which function as insect's heart. Small holes in heart wall, called as ostia, permit hemolymph to enter heart chambers from body cavity. Hemolymph is pushed from one chamber to next by muscle contractions. Nutrients and wastes are exchanged between hemolymph and cells in heart chambers before hemolymph passes back in transporting vessel to be eradicated from insect's body.

In the closed circulatory system, circulating fluid, or blood, is always surrounded in blood vessels which transport blood away from and back to pump, heart. Annelids and all vertebrates contain closed circulatory system. In annelids like the earthworm, dorsal vessel contracts rhythmically to act as the pump. Blood is pumped by 5 small connecting arteries that also function as pumps, to ventral vessel, that transports blood posteriorly until it finally reenters dorsal vessel.

Composition of Blood:

Vertebrate blood is specialized connective tissue including fluid matrix, plasma and formed elements. Plasma is straw-colored liquid part of blood. In mammals, plasma is approx 90% of water and gives solvent for dissolving and transporting nutrients. The group of proteins (albumins, fibrinogen, and globulins) includes another 7% of plasma. It is concentration of the plasma proteins which influences distribution of water between blood and extracellular fluid. As albumin represents approx 60% of total plasma proteins, it plays significant roles with respect to water movement. Fibrinogen is essential for blood coagulation (clotting), and globulins transport lipids and fat-soluble vitamins. Serum is plasma from which proteins involved in blood clotting have been eliminated. Gamma globulin portion functions in immune response as it comprises mostly of antibodies. Remaining 3% of plasma is made of amino acids, electrolytes, glucose and other nutrients, different metabolic wastes, hormones, enzymes, and traces of several inorganic and organic molecules.

Formed elements fraction is cellular component of vertebrate blood. It comprises of leukocytes (White blood cells); WBCs), platelets (thrombocytes) and erythrocytes (red blood cells RBCs). White blood cells are less in number than red blood cells, usually being 1-2% of blood volume. White blood cells are separated in agranulocytes (with no granules in cytoplasm) and granulocytes. There are 2 kinds of agranulocytes: lymphocytes and monocytes. Granulocytes contain granules in cytoplasm. There are 3 kinds of granulocytes; basophils, neutrophils and eosinophils.

Red Blood Cells (RBCs):

Red blood cells, also known as erythrocytes, build about 44% of total volume of blood. Red blood cells are specialized for oxygen transport. Oxygen having capacity of blood is dependent on number of erythrocytes which are present and amount of hemoglobin which every red blood cell has. The mature mammalian erythrocyte is disk-shaped cell with no nucleus. Every cell is packed with approx 280 million iron-containing molecules of respiratory protein hemoglobin. Big quantities of oxygen transported in blood as hemoglobin has special properties which permit it to pick up, or chemically bind with oxygen. Hemoglobin releases oxygen in presence of cells which require it. Hemoglobin transports some carbon dioxide waste from cells. After carbon dioxide diffuses in blood, it enters red blood cells, where little amount binds to hemoglobin.

White Blood Cells (WBCs):

These acts as scavengers which destroy microorganisms at infection sites eliminate foreign chemicals and remove debris which is outcome of dead or injured cells. Every WBCs are derived from immature cells (known as stem cells) in bone marrow by the process known as hematopoiesis. Granulocytes are composed of 3 types:

• Eosinophils are phagocytic; they consume foreign proteins and immune complexes rather than bacteria. In mammals, eosinophils also discharge chemical which neutralize effects of inflammatory chemicals released in allergic reactions.
• Basophils are the least numerous white blood cells. When they react with the foreign substance, granules discharge histamine and heparin. Histamine causes blood vessels to dilate and leak fluid at the site of inflammation, and heparin stops blood clotting.
• Neutrophils are the most numerous of the white blood cells. They are chemically involved to sites of inflammation and are active phagocytes. Agranulocytes are of 2 kinds; monocytes and lymphocytes. Lymphocytes exist in 2 forms B cells and T cells. B cells begin in bone marrow and colonize lymphoid tissue, where they mature.

T cells are related with and influenced by thymus gland before they colonize lymphoid tissue and play their role in immune response.

Platelets (thrombocytes):

Platelets are disc-like shaped cell fragments which function to start blood clotting. When the blood vessel is injured, platelets instantaneously move to site and start to clump together, attaching themselves to damaged area, and start process of blood coagulation.

The Functions of Blood:

Respiration:

Transportation of carbon dioxide and oxygen is primary function of blood. Transport of oxygen from lungs to different tissues, and transport of carbon dioxide from tissues to lungs are largely performed by blood.

Transport of food materials:

Blood is only medium through which absorbed food materials are transported to different parts of body

Excretion:

Metabolic wastes like water, urea, creatine, carbon dioxide, uric acid, etc., are transported by blood, to skin, lungs, kidney and intestine for removal.

Regulation of blood temperature:

Blood has significant role in regulation of body temperature by giving out heat throughout body. This heat is made in muscles by oxidation of carbohydrates and fats.

Maintenance of acid-base balance:

Blood contain buffering capacity and preserves normal acid-base balance in body.

Regulation of water balance:

Blood serves to keep water balance in body by exchanging water between blood and tissue fluid.

Defense:

Blood gives protection to body against infections and antibodies.

Transport of hormones:

 Blood is only medium that serves to allocate hormones to various parts of body.

Clotting:

Loss of blood from body by injury is prevented by action of thrombocytes of blood.

Transport of metabolites:

Blood is liable for supply of chemicals and essential metabolites.

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