Structural Biochemistry/Cell Signaling Pathways/Circulatory Systems

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The circulatory system is made up of the vessels and the muscles which help and control the flow of the blood around the body. The main components of the circulatory system are the heart, the blood and the blood vessles. It also includes the pulmonary circulation, a loop through the lungs for the blood to be oxygenated and the systemic circulation.

It serves to move blood to a site or sites where it can be oxygenated, and where wastes can be disposed. Circulation then serves to bring newly oxygenated blood to the tissues of the body. The waste produces diffuse into the blood cells to be carried away, when oxygen and other chemicals spread out of the blood cells and into the fluid surrounding the cells of the body's tissues. Blood circulates through organs where wastes are removed, and back to the lungs for a fresh dose of oxygen, and then the process repeats itself.

Open Circulatory Systems In higher animals, there are two primary types of circulatory systems -- open and closed. In an open circulatory system, blood leaves the blood and flows freely within the tissues. There is neither a true heart nor capillaries as are found in humans. Instead, there are blood vessels that act as pumps to force the blood along. Instead of capillaries, blood vessels join directly with open sinuses. A fluid called hemolymph, is forced from the blood vessels into large sinuses, where it actually baths the internal organs. Other vessels receive blood forced from these sinuses and conduct it back to the pumping vessels. This is a very inefficient system. Insects can get by with this type system because they have numerous openings in their bodies that allow the "blood" to come into contact with air. Closed Circulatory Systems The closed circulatory system separates the tissue fluid from blood. The blood is pumped through a closed system of arteries, veins, and capillaries. Capillaries surround the organs, making sure that all cells have an equal opportunity for nourishment and removal of their waste products.

The simplest type of closed circulatory systems is an earthworm. Earthworms have two main blood vessels, a dorsal and a ventral vessel, which carry blood towards the head or the tail. Blood is moved along the dorsal vessel by waves of contraction in the wall of the vessel. In the anterior region of the worm, there are five pairs of vessels, that connect the dorsal and the ventral vessels. These connecting vessels function as basic hearts and force the blood into the ventral vessel. There are also special organs in the earthworm for the removal of nitrogenous wastes. Still, blood can flow backward and the system is only slightly more efficient than the open system of insects.

The closed circulatory system has more advantages over the open circulatory system.

1. The blood transfers faster in the closed system, thus oxygen, nutritients, and wastes transport fast also.

2. Specialized cells help carry nutrients.

3. The blood and the tissue fluid are distinguished easily.