Describe the characteristics of circulatory systems, differentiate between open and closed systems, and explain the process of blood clotting.

• A circulatory fluid (like blood or hemolymph) that transports substances.
• A pump (like a heart) to move the fluid.
• A system of vessels (like arteries, veins, or open sinuses) to carry the fluid throughout the body.

Open circulatory systems differ from closed circulatory systems in how the circulatory fluid interacts with body tissues.

• In an open circulatory system, the circulatory fluid (hemolymph) is pumped by a heart into open spaces called sinuses, directly bathing the organs and tissues. There is no distinction between blood and interstitial fluid.
• In a closed circulatory system, blood is confined within a network of blood vessels (arteries, capillaries, veins) and is pumped by a heart, never directly mixing with the interstitial fluid. Substances are exchanged between blood and tissues across capillary walls.

• Bound to Hemoglobin: The vast majority of oxygen (about 98.5%) is transported by binding to hemoglobin, a protein found in red blood cells. Each hemoglobin molecule can bind up to four oxygen molecules, forming oxyhemoglobin. This allows blood to carry a large amount of oxygen efficiently.
• Dissolved in Plasma: A small amount of oxygen (about 1.5%) is transported directly dissolved in the blood plasma. This dissolved oxygen contributes to the partial pressure of oxygen in the blood, which drives the loading and unloading of oxygen from hemoglobin.