Substances Transported by the Cardiovascular System Include Nutrients, Waste Products, Hormones, Electrolytes, and Water

The blood transports the metabolic substrates needed by every cell of the body, including oxygen, glucose, amino acids, fatty acids, and various lipids. The blood also carries away from each cell in the body various metabolic waste products, including carbon dioxide, lactic acid, the nitrogenous wastes of protein metabolism, and heat.

Blood also transports vital chemical messengers: the hor­mones. Hormones are synthesized and released by cells in one organ and are carried by the bloodstream to cells in other organs, where they alter organ function. For example, insulin, which is produced by cells of the pancreas, is carried by the blood to cells throughout the body, where it promotes the cellular uptake of glucose. Inadequate insulin production (as in type 1 diabetes) results in inadequate entry of glucose into cells, whereas glucose concentrations in the blood rise to very high levels. The low intracellular glucose concentration is par­ticularly disruptive to neural function, and the consequences can be serious (diabetic coma) or lethal. Another hormone, adrenaline (a mixture of epinephrine and norepinephrine), is released into the bloodstream by cells in the adrenal medulla during periods of stress. The epinephrine and norepinephrine circulate to various body organs, where they have effects that prepare a threatened animal Ibr⁴Tight or flight.” These effects include an increase in heart rate and cardiac contractility, dilation of skeletal muscle blood vessels, an increase in blood pressure, increased glycogenolysis, dilation of the pupils and airways, and piloerection (hair standing on end).

Finally, the blood transports water and essential electro­lytes, including Na*, CΓ, K⁺, Ca²*, H∖ and HCO₃'. The kidneys are the organs primarily responsible for maintaining normal water and electrolyte composition in the body. The kidneys accomplish this by altering the electrolyte concentra­tions in blood as it flows through the kidneys. The altered blood then circulates to all other organs in the body, where it normalizes (he water and electrolyte content in the extra­cellular fluids of each tissue.