Sunday, December 28, 2008

Internal Transport in Animals

All animals must maintain a homeostatic balance in their bodies. This requires the circulation of nutrients, metabolic wastes, and respiratory gases through the animal's body. Multicellular animals do not have most of their cells in contact with the external environment and so have developed circulatory systems to transport nutrients, oxygen, carbon dioxide, and metabolic wastes. Components of the circulatory system include:

Blood: a connective tissue of liquid plasma and cells.
Heart: a muscular pump to move the blood.
Blood vessels: arteries, capillaries and veins that deliver blood to all tissues.

There are several types of circulatory systems. Organisms, such as hydra, have a fluid-filled, internal gastrovascular cavity. This cavity supplies nutrients for all body cells lining the cavity, obtains oxygen from the water in the cavity, and releases carbon dioxide and other wastes into it. The gastrovascular cavity of a flatworm, such as the planarian, is more complex than that of the hydra. The open circulatory system is common to molluscs and arthropods. Open circulatory systems (evolved in insects, mollusks, and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. The resulting blood flow is sluggish.

Higher animals such as vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems have the blood enclosed within blood.

Higher animals such as vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems have the blood enclosed within blood vessels of different sizes and wall thicknesses. It is not released in between the cells. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. Blood flow is not sluggish. Hemoglobin causes vertebrate blood to turn red in the presence of oxygen; but more importantly, hemoglobin molecules in blood cells transport oxygen. The closed circulatory system present in humans is called the cardiovascular system. A circulatory or cardiovascular system is a specialized system that moves the fluid medium, hemolymph or blood, in a specific direction determined by the presence of unidirectional blood vessels.

The vertebrate cardiovascular system includes a heart, which is a muscular pump that contracts to propel blood out to the body through arteries, and a series of blood vessels. The upper chamber of the heart, the atrium, is where the blood enters the heart. Passing through a valve, blood enters the lower chamber, the ventricle. Contraction of the ventricle forces blood from the heart through an artery. The heart muscle is composed of cardiac muscle cells. Arteries are blood vessels that carry blood away from heart. Arterial walls are able to expand and contract. The aorta is the main artery leaving the heart. The pulmonary artery carries deoxygenated blood to the lungs. In the lungs, gas exchange occurs-earbon dioxide diffuses out and oxygen diffuses in. Arterioles are small arteries that branch into collections of capillaries known as capillary beds. Capillaries are thin-walled blood vessels in which gas exchange occurs. Capillaries are concentrated into capillary beds. Nutrients, wastes, and hormones are exchanged across the thin walls of capillaries.

The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones and the removal of carbon dioxide, ammonia, and other metabolic wastes. Capillaries are the points of exchange between the blood and surrounding tissues. Materials cross in and out of the capillaries by passing through or between the cells that line the capillary. Blood leaving the capillary beds is collected into a progressively larger series of venules (venules are smaller veins that gather blood from capillary beds into veins), which in turn join to form veins. Veins carry blood from capillaries to the heart. With the exception of the pulmonary veins, blood in veins is oxygen-poor. The pulmonary veins carry oxygenated blood from lungs back to the heart.

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