Describe the structure and function of capillaries and how red blood cells are adapted for efficient oxygen transport.

You're on a roll — here are the answers to questions 4 to 7: Question 4: a) The blood vessels that link arterioles with venules are capillaries. b) Four ways in which capillaries are suited to carrying out their functions: • They have walls that are only one cell thick, which provides a short diffusion distance for efficient exchange of substances between blood and tissues. • They have a very narrow lumen, forcing red blood cells to pass through in single file, which increases their surface area to volume ratio for oxygen exchange. • They form an extensive, highly branched network throughout tissues, providing a large total surface area for exchange. • Their walls have pores (fenestrations) that allow plasma and small molecules to leak out, forming tissue fluid for exchange with cells. c) Two ways in which the composition of blood in the pulmonary arteries differs from that in the pulmonary veins: • Pulmonary arteries carry blood with a lower oxygen concentration and a higher carbon dioxide concentration. • Pulmonary veins carry blood with a higher oxygen concentration and a lower carbon dioxide concentration. Question 5: Red blood cells of mammals are adapted for efficient transport of oxygen in the following ways: • They have a biconcave disc shape, which increases their surface area to volume ratio, facilitating faster diffusion of oxygen. This shape also allows them to be flexible and squeeze through narrow capillaries. • They contain hemoglobin, a protein that reversibly binds with oxygen to form oxyhemoglobin, enabling efficient transport of a large amount of oxygen. • They lack a nucleus and most other organelles when mature, maximizing the internal space available for hemoglobin, thereby increasing their oxygen-carrying capacity. Question 6: a) The blood cells represented by X are platelets (or thrombocytes). b) The metal ion represented by Y is calcium ions (Ca^2+). c) The end product of the mechanism represented Z is fibrin. d) Excess bleeding can result in death due to a significant loss of blood volume, which leads to a severe drop in blood pressure. This reduces the supply of oxygen and nutrients to vital organs like the brain and heart, causing organ failure and ultimately death. e) The process by which the human body naturally stops bleeding is called hemostasis. Question 7: a) Low blood volume can be brought back to normal by: • Drinking fluids (e.g., water, electrolyte solutions). • Intravenous fluid administration (e.g., saline solution, blood transfusion in severe cases). b) One defect of the circulatory system in humans is hypertension (high blood pressure). c) Three functions of blood other than transport are: • Regulation of body temperature. • Defense against pathogens (immunity). • Blood clotting (hemostasis). d) Blood in veins is prevented from flowing backwards by the presence of valves. e) Two ways in which red blood cells are adapted to their function are: • Their biconcave disc shape provides a large surface area for efficient oxygen diffusion. • They contain hemoglobin, which allows them to bind and transport oxygen effectively. What's next?