Physiology was one of the most interesting classes I took in Medical School. Derived from the ancient Greek words for "nature" or "origin" and "speech", physiology is the study of how living things function. Although there are many branches of physiology for many different types of life, such as plants and other animals, my experience is with human physiology and will be my focus here.
Physiology is intimately tied to anatomy. Anatomy is simply the description of the structures in a living organism. However, structure and function are closely related. A hand can pick up a pen because of it's structure. A heart pumps blood because of it's structure. A kidney filters blood also because of it's structure. Similarly, the limitations of a part are related to structure as well. A human arm will never allow a person to fly, not matter how hard you may flap your arms about in an open field pretending you are a bird.
The study of physiology is a vital aspect of human knowledge and key to a doctor's training in Medical School. Medical School can be very roughly broken down in to four phases:
1 - The structure of the body (anatomy and histology)
2 - How the body works (physiology)
3 - What can go wrong with the body (pathology)
4 - How to fix the body when something does go wrong (the rest of Medical School)
As you can see, physiology falls nicely in to the second phase. If you don't understand HOW the human body works, it becomes very hard to have a good idea on how to fix it. You could say that anatomy and physiology are the foundation sciences of modern medicine.
Human physiology is studied by organ system. Some of the organ systems include the cardiovascular, neuro, gastrointestinal, endocrine, respiratory, urinary, immune, and reproductive systems. Each system is made of several organs and structures that work together to achieve a particular function. Of course the human body is more than just the sum of it's parts and all of these systems interact with each other quite closely.
These categories can be somewhat arbitrary, as many of the organs in the body function as part of more than one system. For example, the kidneys are involved in the urinary system, but also play an important role in the cardiovascular system by helping to regulate blood pressure. So which system do the kidneys belong to, the urinary or cardiovascular? The answer is simple: both.
When an organ or system is damaged by disease, there is often a loss of function from normal. The study of how normal physiology is effected by damage and disease is known as pathophysiology. Pathophysiology is also an entire course in Medical School and signals the beginning of a students training in how disease effects the body and causes dysfunction.