Through the miracles of medicine and technology, researchers have had some success with growing and transplanting hollow, relatively simple organs such as tracheas and bladders. building solid organs like the kidneys or complex organs like the heart have proved to be much more difficult. considering how the body is organized, give one reason why it is so difficult for scientists to build a heart.

Complex systems are confined in a small space with multi directional functionality which is difficult for humans to achieve and hence it is difficult for scientists to build a heart.

Explanation:

The special complex organ such as heart and kidney have cells that are specialized to perform specific functions.

Creating cells that perform special functions is a difficult task. Along with this, the internal structure of such organs is very very complex and also the entire system is limited to only a small volume or area.

Thus, complexity further rises with space limitation.

Another major issue is that these special organs are connected to multiple systems with in the body in a way that all  systems work in co-ordination.

Establishing such co-ordinations is a next to impossible task.

Hence, it is difficult for scientists to build a heart.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.