As main agents important in the circulatory system are:

As the surface area per volume ratio of anorganism increases, so would it’s need for oxygen and other components, and itwould be harder to effectively distribute these components. Which is why multi-cellularorganisms have to adapt and develop a specialized exchange surface, one ofthese being a good blood supply which is part of the circulatory system I willbe talking about below.The circulatory system is a closed systemwhich passes through the heart twice during a complete circuit.

It has 2circuits which are: Pulmonary (pumps blood to lungs for oxygenation) andSystemic (pumps oxygenated blood to head and body.  Three main agents important in thecirculatory system are: the arteries( they have thick, muscular and elasticwalls to whist and the pressure and to exert force which is what gives you yourpulse, and they serve to take blood away from the heart at high pressure); thecapillaries( which allow exchange of materials between the tissues and theblood with they permeable walls) and veins (they have a large diameter to offerleast flow resistance and valves to prevent back flow, they  returns low pressure oxygenated blood toheart). The main component of the circulatory system is “blood”, itis a connective tissue which plays three main important functions which are:Transportation, regulation and protection.

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Transportation: The human bodyrequires: oxygen and carbon dioxide to be exchanged for respiration, nutrientsfor energy, waste to be removed from the body so that we do not becomeintoxicated, and an even transport of hormones.For the transport of gases, wehave red blood cells which are specialized cells (formed from stem cells in thebone marrow) that circulate around the body delivering oxygen to other cellswhile also removing carbon dioxide from the latter. It does not have a nucleus,mitochondria or ribosomes which means there is a larger surface area for therapid diffusion of oxygen and carbo dioxide, it also has hemoglobin which is amolecule made of iron that gives red blood cells its red color.    Waste and nutrients are alsoeffectively transported by the blood. As it flows through the capillaries andinto the lungs, the hemoglobin contained in the red blood cell releases carbondioxide which is in turn removed from the lungs when we breathe out. Waste isalso removed from our body tissues and organs and removed from the blood in afilter like system as it goes through the kidneys. The waste material will thenexit the body in the form of urine.

Blood also flows through the intestine’scapillaries and picks up nutrients (glucose, vitamins and minerals) which arethen delivered to the rest of the body and cells that need it by blood flow.Hormones are chemicalmessengers produced by different glands which affect distant organs, such anexample is the hormone insulin which is produced in the pancreas and affectsthe sugar levels in the blood. Our blood also transports hormones made byendocrine systems to organs and tissues. The circulatory system also delivershormones such as peptides hormones which are secreted by the heart atrial cellsto help regulate blood volumes and pressures. Estrogen is also transported hereto promote vascular health in women and men. Regulation:Heat is absorbed and exchanged throughoutthe body by the blood which moves farther and closer from the skin in everyareas of the body.

This heat is produced by the tissues when they break downnutrients for energy, releasing waste and repairing themselves. As the bodytemperature rises, the blood vessels closer to the skin will increase in size,so that heat can be loss through the skin. If body temperature falls, the bloodvessels closer to the skin will become smaller so that they can retain heat.

Italso distributes heat generated by muscular contractions. Th circulatory systemregulates blood volume and pressure by regulating fluid loss in the form ofurine, and transports gonadotropic hormones which regulates reproductivefunctions. Protection:The circulatory system is very importantwhen it comes to fighting diseases, it makes up most of the immune system andcontains cells known as ” white blood cells”, which patrol the bodyin search of foreign bodies. As soon as a foreign body is detected, chemicalsignals are sent through the bloodstream, this then alerts the white bloodcells of the situation which then are transported to the site of infectionwhere they will engulf (phagocytizing) or release chemicals at the bacteria orbody responsible for the infection.

Also, during tissue damage or injury,messenger chemicals are released which starts the activation of platelets orattract platelets to the site. The platelets in the blood will then sticktogether to form a plug which will then cover the wound and coagulate to stopfurther blood loss and bacteria to enter the bloodstream through the wound. The circulatory system transports and makestissue fluid not been able to be reabsorbed by the vascular capillaries back tothe lymphatic system for processing. Tissue fluid is formed due to the pressure from the pumping of the heart.It is composed of water molecules, fatty molecules and carbon dioxide.

They arefound in capillaries; specially adapted one-cell thick tubes which occur afterthe arteries.The pressurecauses, small molecules to be pushed out of the blood plasma through thecapillaries and to form interstitial fluid (tissue fluid). Plasma proteins fromthe blood remain in the blood plasma since these are too large to pass throughcapillaries. Water is reabsorbed into the blood plasma by osmosis to maintainblood pressure.Diffusion is the spreading of the particlesof a gas, or of any substances in a solution, resulting in a net movement froma region where they are of a higher concentration to a region with a lowerconcentration. The greater the difference in concentration, the faster the rateof diffusion. As organisms get bigger, the surface per volume ratio changes, itgets smaller and as a result diffusion is no longer enough. There is not a bigenough surface area for diffusion to take place.

This is particular concerningin humans as they are active, this means food and oxygen can’t reach the cellsfast enough to supply cellular respiration. The waste products can’t be removedfast enough to prevent them damaging the cells. A transport system is needed!