3.2

• All living organisms need to exchange substances with their surrounding environment
• Organisms need to take in oxygen and nutrients and release waste products
• Exchange sites within an organism are where substances enter or leave
• Small organisms like Chlamydomonas can exchange substances directly with the environment due to their large surface area: volume ratio
• Larger organisms require specialised mass transport systems due to increasing transport distances, surface area: volume ratio, and levels of activity
• Surface area and volume are important factors in the exchange of materials in organisms
• As organisms increase in size, their surface area: volume ratio decreases
• Single-celled organisms have a high surface area: volume ratio allowing for efficient substance exchange via diffusion
• Larger organisms have lower surface area: volume ratios, resulting in less efficient substance exchange
• Increasing levels of activity in larger organisms lead to a higher metabolic rate and the need for efficient transport mechanisms
• Larger organisms have evolved mass flow transport systems for efficient movement of nutrients and waste
• Mass flow systems involve bulk movement of materials with some diffusion at specific exchange sites
• Circulatory systems help bring substances quickly from one exchange site to another and maintain diffusion gradients
• The circulatory system in mammals is a well-studied example of a mass transport system
• There are two types of circulatory systems: single and double circulatory systems
• Single circulatory systems involve blood passing through the heart once during one circuit of the body
• Double circulatory systems involve blood passing through the heart twice during one circuit of the body
• Fish have a single circulatory system while mammals have a double circulatory system
• In mammals, the left side of the heart contains oxygenated blood and the right side contains deoxygenated blood
• Double circulatory systems maintain higher blood pressure and speed of flow compared to single circulatory systems
• Closed circulatory systems contain blood within blood vessels, while open circulatory systems pump blood directly into body cavities
• Humans have a closed double circulatory system with blood passing through the heart twice in one circuit
• Insects have one main blood vessel called the dorsal vessel
• The tubular heart in the abdomen pumps haemolymph (blood in insects) into the dorsal vessel
• The dorsal vessel delivers the haemolymph into the haemocoel (body cavity)
• Haemolymph surrounds the organs and eventually reenters the heart via one-way valves called ostia
• Oxygen is delivered directly to insect tissues via tracheae (a system of tubes) that connect directly to the outside
• Arteries transport blood away from the heart to tissues at high pressure
• Arterioles branch into narrower blood vessels called arterioles which transport blood into capillaries
• Veins transport blood to the heart at low pressure
• Venules transport blood from the capillaries to the veins
• Arteries have a narrow lumen to maintain high blood pressure
• Veins have a larger lumen to ensure blood returns to the heart at an adequate speed
• Veins contain valves to prevent the backflow of blood
• Arteries have three layers in their walls: tunica adventitia/externa, tunica media, and tunica intima
• The tunica intima is made up of an endothelial layer, a layer of connective tissue, and a layer of elastic fibres
• The tunica media is made up of smooth muscle cells and a thick layer of elastic tissue
• The tunica adventitia covers the exterior of the artery and is mostly made up of collagen
• Arteries have a pulse and a narrow lumen to maintain high blood pressure
• Arterioles have a muscular layer that allows them to contract and partially cut off blood flow to specific organs