biology heart

• Multicellular organisms require transport systems to supply their cells and remove waste products.
• Diffuses from the cells of the liver to the tissue fluid, and then across the capillary walls into the blood plasma.
• In humans, one of the functions of the circulatory system is to transport substances.
• The circulatory system in humans includes the heart, blood vessels, and blood.
• The heart is a pump that circulates blood through the blood vessels.
• Blood is transported in arteries, close arteries, blood vessels that carry blood away from the heart.
• Blood vessels that carry blood away from the heart are called veins.
• A blood vessel with valves that transports blood to the heart is called a venule.
• Capillaries are tiny blood vessels with walls one-cell thick where exchange of materials occurs.
• Blood is pumped from the heart in the arteries.
• Blood is returned to the heart in the veins.
• The capillaries connect the two types of blood vessel and molecules are exchanged between the blood and the cells across their walls.
• Arteries carry blood away from the heart.
• Veins always carry blood back to the heart.
• Arteries carry oxygenated blood, except for the pulmonary artery.
• Veins always carry deoxygenated blood, except for the pulmonary vein.
• Arteries carry blood under high pressure.
• Veins carry blood under low or negative pressure.
• Arteries have thick muscular and elastic walls to pump and accommodate blood.
• Veins have thin walls and have less muscular tissue than arteries.
• A type of supporting tissue called connective tissue provides strength to arteries.
• Veins have less connective tissue than arteries.
• The channel in the blood vessel that carries blood, the lumen, is narrow in arteries.
• Veins have a wide lumen.
• Capillaries connect the smallest branches of arteries and veins.
• The walls of capillaries are just one cell thick, allowing the exchange of molecules between the blood and the body's cells.
• Oxygen diffuses through the capillary wall, into the tissue fluid.
• Carbon dioxide diffuses from the cells into the tissue fluid, then across the capillary walls into the blood plasma.
• Glucose diffuses from the blood plasma, across the capillary walls to the tissue fluid, and then to the cells.
• The waste product urea diffuses from the blood plasma, across the capillary walls to the tissue fluid, and then to the cells.