Human Physiology
The human body is the result of a large number of anatomical systems that have successfully evolved in response to survival needs over a very, very long period of time. Some aspects of our physiology pre-date our Homo Sapien existence, inherited from our mammalian ancestry and even earlier. As such, the Darwinian school of anatomy never starts from a clean sheet, but prefers to adapt from some earlier successful life form.
Physical Anatomy
At a very basic level, our physical anatomy is a transportation system for the brain, which is being carried around perched on the top of our bodies in the skull. This transportation system has some auxiliary features that allow us to breathe air, gather food and convert the food into the necessary nutrients to sustain life. We also have a built-in waste disposal system; part of which is inter-linked to a reproductive system, which some have suggested implies a sense of humour somewhere in the design process. Of course, this is a miserable and woefully inadequate description of our physical anatomy. It completely overlooks the complexity of our hand movements, facial expressions and the myriad of other facets that give us the sense of life and personal identity.
Figure : Human Anatomy
Skeletal System
The average human is born with over 300 bones, but as we grow to maturity some of these bones begin to fuse together. As a result, a normal adult is left with 206 bones joined by ligaments and tendons to form a protective and supportive framework for the attached muscles and the soft tissues. The skeleton also produces red blood cells from the marrow of certain bones and white cells from the marrow of other bones to destroy harmful bacteria as part of the body's wider immune system.
Muscular System
The human body has more than 600 muscles connected via nerves and the spinal cord to our brain. By control of our muscles we are capable of an incredible number of complex actions from delicate touch to brute-force survival.
Digestive System
When eating, our body digests food so that our body cells can use it to make energy. Acids and enzymes in stomach and intestine break down the food into molecules tiny enough for the body cells to use. These molecules are then taken into the blood and distributed around the body.
Circulatory System
Our arteries are vessels that carry blood away from the heart. The heart is a pump that pushes the blood around the body. Red blood cells carry oxygen from the lungs to all the cells of the body. Our veins are vessels that carry blood back to the heart. Blood circulates around the body in about one or two minutes and also carries white blood cells that protect the body via a complex immune system.
Respiratory System
The primary function of the respiratory system is to supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body. When we breathe through our lungs, we inhale oxygen and exhale carbon dioxide. This exchange of gases is the way the respiratory system gets oxygen into the circulatory system
Excretory System
The volume of our blood, approximately 8 pints, passes through the kidneys 300 times a day. Nephrons clean the blood in ~45 minutes and send about six cups of urine to the bladder every day.
Summary
The human anatomy has evolved in respect to the specific challenges of a physical world that it has previously survived, as such, it reflects our evolutionary path from the past up until now. It is now recognised that some basic aspects of human physiology are a result of evolutionary adaptations laid down hundreds of millions of years ago and shared by almost every life form, especially mammals. However, within this ever changing process, humans have evolved some physical characteristics that separate us from our nearest DNA cousins, the apes. The most important possibly being an opposable thumb, in conjunction with an enlarged brain, which gives humans an extended ability to manipulate tools in a far more effective way than any other species of mammal.