Final answer:
The first sign that a newly transplanted kidney is working is the production of urine, as this shows the kidney is actively filtering blood and maintaining homeostasis.
Step-by-step explanation:
The first sign that a newly transplanted kidney is working is D. the kidney produces urine. When a kidney transplant is successfully completed, the newly transplanted kidney will begin filtering blood, a process that involves removing waste and excess substances from the blood. This filtering results in the production of urine, which is the first observable evidence that the kidney has begun to function properly in the recipient's body. This process is crucial as it maintains the necessary balance of water, salts, and minerals in the bloodstream.
There is a three-step process by which kidneys filter blood, including glomerular filtration, tubular reabsorption, and tubular secretion. Together, these processes ensure that essential substances are retained while waste products and excess water are excreted as urine. Blood flow and filtration rate are also carefully regulated by the kidneys to optimize filtering processes and maintain homeostasis.