Final answer:
Fats increase blood viscosity by forming plaques in the arteries which narrows them and makes it difficult for the heart to pump blood, leading to cardiovascular diseases.
Step-by-step explanation:
The question is asking which substance, when present in high quantities, increases the viscosity of blood, thus raising the resistance to blood flow and making it harder for the heart to pump. Among the options provided -- sugars, salts, fats, and red blood cells -- the correct answer is C) Fats.
Fats, especially when they are in the form of 'bad cholesterol' or low-density lipoproteins (LDLs), can lead to the buildup of plaques in the arteries. This condition, known as atherosclerosis, narrows the arteries and increases blood viscosity. Because fats do not mix well with the water-based blood due to their lipid nature, they cannot move easily in the bloodstream. This leads to increased resistance in blood flow, and the heart must work harder to pump blood, putting a strain on the cardiovascular system and increasing the risk for heart attack or stroke.
Trans fatty acids, in particular, are known to be harmful to the heart and blood vessels as they further contribute to high cholesterol levels and can lead to cardiovascular diseases.