Here's what I know, I looked at Costanzo and than Guyton both are great. I would prefer Costanzo for medical school the reason is it is easy to read and the material is concise. There is only a limited amount of time in medical school to learn but, remember Medicine is a dedication to a lifetime of learning. The book that I really prefer is Vander's Physiology but, they all have strengths and weakness. Vander also has a small book on Renal Physiology that in my opinion is the best introductory I've seen and heard of on renal physiology. There is also a book my ex-wife who is a physician, (although she's my ex-wife I love her and value her opinion greatly) used during medical school. In addition to the school recommended Textbooks in Anatomy and Physiology she used a excellent textbook for conceptual understanding in Anatomy and physiology which is Tortora and Grabowski, "Principles of Anatomy and Physiology." Also, remember get the books with the questions like Costanzo's Board Review Series (BRS), if you don't do the problems your not going to test your knowledge.
Here's my list for medical school physiology textbooks in order of preference.
1. Costanzo's book and her BRS review book.
2. Tided with Guyton and Hall and Vander's (my Preference is Vander's) also, if you get Guyton and Halls book in addition get the other book from them called Guyton and Halls Physiology Review.
3. Vander's Renal Physiology and West's Respiratory Physiology book.
4. Walter Boron & Emile Boulpaep. Medicine is a dedication to a lifetime of learning. I haven't read this book yet but, every person I've read recommends that this book is a great detailed book on Physiology but, states it's to difficult to read during medical school. I personally, am looking forward to the challenge in the future.
Good luck and never give up,
Tom TL