Humans have an endoskeleton that consists of bones, which protect internal organs, provide a framework in which muscles can pull, and give shape and structure to the body. The place where to bones meet is a joint. There are 3 types of major joints known as fixed joints, semi movable joints, and movable joints. Fixed joints restrict movement, semi movable joints allow limited movement, and movable joints allow a wide range of movements. Ligaments are bands of connective tissue that hold bones and joints together. Tendons are connective tissues that connect bone to muscle.

​

The cardiovascular system and lymphatic system make up the circulatory system. It transports nutrients, hormones, and gases; it gets rid of wastes and helps the body maintain a normal body temperature. The 4-chambered heart is divided by the septum; it separates the right and left side of the heart. The right side pumps blood to the lungs and the left side pumps blood around the body. Each side has an upper (atrium) and lower (ventricle) chamber. Valves allow blood in and then to a ventricle where blood goes through a large vessel to the body. Deoxygenated blood enters the right atrium, which sends blood into the right ventricle. This blood goes into the pulmonary arteries and into the lungs where it is oxygenated and sent back into the left atrium, then the left ventricle and around the body through the aorta. Arteries are large muscular vessels that take blood to the parts of the body. Capillaries are little vessels that make up a network formed by the arterioles. A vein is a large vessel that carries blood to the heart and is made up of many venules.  Blood transports nutrients and oxygen to cells and also take carbon dioxide and other wastes way from them.

​

The human respiratory system exchanges cells with the cardiovascular system. It involves external respiration, exchanging gases with the outside atmosphere, and internal respiration, exchanging gases with the cells inside the body. The trachea is also known as the windpipe and it is a cartilaginous tube that allows air in. The lungs are where the gases exchange with the atmosphere and blood. The alveoli are tiny air sacs that exchange the gases in the lungs.

​

The digestive system breaks down food into molecules it can use. It occurs in the digestive tract. The mouth starts breaking up food using 2 sets of teeth, the incisors and molars. Salivary glands produce saliva that is made of water, mucus, and salivary amylase. Salvia breaks up some starch into the disaccharide maltose. The esophagus is a muscular tube that connects the pharynx to the stomach. The stomach is involved in mechanical and chemical digestion and continues to break down food. The small intestine is a 7m long tube that absorbs the nutrients from the broken down food. The large intestine is where the wastes from the food are collected and processed into feces. The liver stores glucose as glycogen, breaks down toxic substances, and makes proteins.

​

The nervous system controls mental and physical activities and maintains homeostasis. The brain and spinal cord make up the central nervous system. The peripheral nervous system includes neurons that have cell bodies that are not included in the brain and spinal cord. A neuron has a cell body with a nucleus. The axon transmits information from the cell body in the form of electrical impulses called action potentials. A lipid layer known as myelin sheath covers the axons. Neurons communicate with other neurons at synapses. The human body has many sense organs, one of them being the eye. The eye focuses light into our brain so we can see images. Light first passes through the cornea, then the pupil where the iris controls how much light enters. The light then goes through the lens where it focuses light into the retina, where the photoreceptor cells, rods and cones, send the image into the optic nerve to the occipital lobe in the brain.