Final answer:
Parathyroid hormone increases blood calcium levels by targeting the skeleton, the intestines through vitamin D activation, and the kidneys. The heart is not a target tissue for PTH in this context.
Step-by-step explanation:
You asked which target tissue parathyroid hormone (PTH) does not increase blood calcium levels in. PTH primarily targets the skeleton, the kidneys, and the intestines to increase blood calcium levels. It stimulates osteoclasts in the bones to release calcium into the bloodstream, enhances dietary calcium absorption in the intestines indirectly via vitamin D activation, and increases calcium reabsorption in the kidneys.
Among the options given:
A. The heart is not a target tissue for PTH to increase blood calcium levels.
B. The intestines are affected indirectly by PTH.
C. The bones are a direct target of PTH.
D. The kidneys are a target for increased calcium reabsorption due to PTH.
Therefore, the correct answer is A. The heart.