When you breath in oxygen it is then drawn in through the mouth and down into the lungs. The oxygen then passes across the thin lining of the capillaries and into the blood. The oxygen molecules are carried to the body cells by the blood. Carbon dioxide from the body cells is carried by the blood to the lungs where it is released into the air. The process of getting oxygen into the body and releasing carbon dioxide is called respiration.

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Your body sometimes needs to remove the waste that build up from cell activity and from digestion. These wastes include carbon dioxide, urea, and certain plant materials. If these wastes are not removed, your cells can stop working and you can get very sick. The excretory system can also help to release wastes from the body. Excretion is the process of removing wastes from the body.


Digestion is the process of breaking down food into nutrients. There are two types of digestion, mechanical and chemical. In mechanical digestion, large chunks of food are broken down into small pieces. This is a physical process. In chemical digestion, large food molecules are broken down into small nutrient molecules. This is a

chemical process.


Systemic circulation is the part of the cardiovascular system that carries oxygen-rich blood away from the heart, to the body, and returns oxygen poor blood back to the heart. Pulmonary circulation is the part of the cardiovascular system that carries oxygen poor blood away from the heart to the lungs, and returns oxygen-rich blood back to the heart.


Fertilization occurs when an egg and sperm come together to form a zygote. A zygote develops into an embryo and then a fetus. This happens when cells divide, different types of cells develop, and organs form. A person grows quickly and develops new abilities during infancy and childhood. A child becomes mature and changes in many other ways during adolescence.