Digestive
The digestive system breaks down food for absorption of nutrients into the body. Mechanical breakdown of food begins in the mouth by chewing of food. Chemical digestion also begins in the mouth by the saliva. The saliva begins to breakdown starches. Once the food is ready it travels down the esophagus into the stomach. Once the food is in the stomach mechanical digestion continues by muscle contractions churning the food. Chemical digestion continues in the stomach with the breakdown of proteins. Digestion does not end here. The food then moves into the small intestine. Here the chemical digestion continues and the nutrients leave the small intestine and go into the blood stream. Any food that is not absorbed into the blood stream goes into the large intestine and leaves the body through the anus.
Circulatory
The circulatory system is responsible for bringing materials and removing unwanted material from cells. The heart pumps the blood to different points in the body. Arteries carry blood away from the heart while the veins bring the blood back to the heart for re-circulation. Lets start with the right side of the heart. The heart pumps blood from the right ventricle to the lungs via the pulmonary arteries. In the lungs the blood fills with oxygen. The pulmonary veins return the blood to the left atrium. The left atrium passes blood to the left ventricle. The left ventricle pumps the blood to the body via the aorta (the largest artery in the body). From the aorta the blood passes through the body from arteries to capillaries to veins back to the heart. The oxygen poor blood from the body returns into the heart from the inferior vena cava (lower body) and the superior vena cava (upper body).
Nervous and Endocrine
The nervous and endocrine systems both interact with internal (inside) and external (outside) changes of the body.
There are two main parts of the nervous system the peripheral and central. Sensory neurons send signals to the brain via a chain of inter neurons. The brain sends a signal back causing motor neurons to respond. This all happens at the speed of light. There are two types of responses voluntary and involuntary. For example, you can breath and your heart beats with out you even thinking.
The endocrine system is responsible for regulating reproduction, growth and development. Glands in your body produce hormones. These hormones cause your body to change how it grows and develops. For example, your thyroid gland is responsible for regulating your metabolism (how your body digests food).
Respiratory
The respiratory system is responsible for supplying oxygen to your blood and removing wastes such as carbon dioxide. You inhale though your nose. Small hairs in your nose filter out dirt and unwanted particles. From your nose the air you breath travels down your trachea and into your bronchi. Your bronchi deliver the air to your lungs. Oxygen from your lungs gets absorbed by alveoli which contain capillaries. The capillaries are part of the circulatory system.
Excretory
The excretory system is responsible for getting rid of wastes. Your lungs, circulatory system, kidneys, urinary structures, large intestines and yes your skin all make up parts of your excretory system.
Skeletal and Muscular
The skeletal system work together to provide support and locomotion for the organism.
Reproductive
The reproductive system helps the organism produce new offspring. Failure to reproduce would cause in a decrease for that particular species.
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