心脏 (xīnzàng) refers to the physical heart as an organ. Unlike 心 (xīn), which can mean "heart" or "mind" in an emotional sense, 心脏 specifically refers to the anatomical organ that pumps blood.
医生说他的心脏非常健康。
Yīshēng shuō tā de xīnzàng fēicháng jiànkāng.
The doctor said his heart is very healthy.
剧烈运动后,她感觉心脏跳得特别快。
Jùliè yùndòng hòu, tā gǎnjué xīnzàng tiào de tèbié kuài.
After intense exercise, she felt her heart beating especially fast.
心脏是人体最重要的器官之一。
Xīnzàng shì réntǐ zuì zhòngyào de qìguān zhī yī.
The heart is one of the most important organs in the human body.