6 Distribution of materials

Blood is an important transporting tissue. The blood of mammals is made up of several different components. Arteries are thick-walled vessels that transport blood away from the heart. Veins are relatively thin-walled vessels that transport blood to the heart. Arteries and veins are connected by networks of arterioles, very thin walled vessels called capillaries and venules.

Nutrients and oxygen diffuse from capillaries into tissue fluid then into surrounding tissue cells. The muscular heart contracts rhythmically and acts as a pump to maintain circulation of the blood. Various kinds of biotechnology are used to assist diseased hearts.

Some animals have a closed circulatory system, and some have an open circulatory system. Excess fluid is returned from the tissues to the bloodstream via lymphatic vessels in the lymphatic system. Lymphatic vessels contain one-way valves. The composition of blood varies as it circulates the body. The liver has a major role in storing and converting organic compounds.

Animals exchange gases with their external environment through special surfaces in their respiratory system. Mammals exchange gases with their external environment via respiratory surfaces in their lungs. The gas exchange surfaces of fish are located in gills. In insects, air-filled tubes called trachea transport oxygen to the respiratory surfaces. Trachea branch into very fine tracheoles that come into direct contact with cells. Transport of gases

Transport of wastes Metabolism of protein results in the production of nitrogenous wastes. Different animals excrete their nitrogenous waste in different ways and through different compounds. In vertebrates, the kidney of the urinary system is the organ responsible for removing nitrogenous waste from the blood. The functional unit of the kidney is the nephron. In insects, Malpighian tubules remove nitrogenous waste from the blood. Different animals excrete different nitrogenous wastes because they have different amounts of water available to them.

Transport in plants Vascular tissue in plants comprises xylem and phloem. Water and mineral ions enter a plant through the roots and are transported throughout a plant in the xylem. Sugars are made by chloroplasts in chlorophyll-containing tissues and are conducted to other parts of the plant through phloem.

Gaseous exchange in plants Plants have a variety of surfaces through which gases are exchanged with the external environment. Photosynthetic plants require carbon dioxide from their external environment. Photosynthetic plants release oxygen into their external environment.

Excretion in plants Plants do not have excretory organs equivalent to those found in animals. Plants are able to store wastes in ways that are not detrimental to the plant.