If it were hypertonic, water would flow into the parasite’s cell, causing it to burst (lysis). If it were hypotonic, water would flow out of the parasite’s cell, causing it to shrivel. Both of these situations would cause the parasite to die. Only if it were isotonic (same concentration of solutes in the fluid) as the blood would there be an osmotic equilibrium, thus it would not lose or gain too much water, allowing it to survive.
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b: isotonic
If it were hypertonic, water would flow into the parasite’s cell, causing it to burst (lysis). If it were hypotonic, water would flow out of the parasite’s cell, causing it to shrivel. Both of these situations would cause the parasite to die. Only if it were isotonic (same concentration of solutes in the fluid) as the blood would there be an osmotic equilibrium, thus it would not lose or gain too much water, allowing it to survive.