The blood leaving the tissues becomes richer in (a) carbon dioxide.
Explanation: As cells in the tissues use oxygen for respiration, they produce carbon dioxide as a waste product. This carbon dioxide then diffuses into the bloodstream, making the blood leaving the tissues richer in carbon dioxide.
Why other options are incorrect:
(b) water:
While water is a component of blood, its concentration doesn’t significantly change as blood passes through tissues. Water is primarily involved in transporting nutrients and other substances, not as a waste product of cellular respiration.
(c) hemoglobin:
Hemoglobin is a protein in red blood cells that carries oxygen. It remains relatively constant in concentration, as its primary function is to transport oxygen to tissues, not carbon dioxide.
(d) oxygen:
The blood leaving the tissues is depleted of oxygen because cells have used it for respiration. Therefore, the blood becomes poorer in oxygen, not richer.