The Nervous System
The nervous system is a very complex system in the body. It has many, many parts. The nervous system is divided into two main systems, the central nervous system (CNS) and the peripheral nervous system. The spinal cord and the brain make up the CNS. Its main job is to get the information from the body and send out instructions. The peripheral nervous system is made up of all of the nerves and the wiring. This system sends the messages from the brain to the rest of the body.
Central Nervous System
Peripheral Nervous System
Endocrine System
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digestive system

Human Digestive System
The human digestive system is a complex series of organs and glands that processes food. In order to use the food we eat, our body has to break the food down into smaller molecules that it can process; it also has to excrete waste.
Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food as it makes its way through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals.
The Digestive Process:The start of the process - the mouth: The digestive process begins in the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smaller molecules).
On the way to the stomach: the esophagus - After being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to the stomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach. This muscle movement gives us the ability to eat or drink even when we're upside-down.
In the stomach - The stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested and mixed with stomach acids is called chyme.
In the small intestine - After being in the stomach, food enters the duodenum, the first part of the small intestine. It then enters the jejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder), pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food.
In the large intestine - After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food. Many microbes (bacteria like Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process. The first part of the large intestine is called the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon.
The end of the process - Solid waste is then stored in the rectum until it is excreted via the anus.

blood circulatory system

There are many systems in the human body: Circulatory System (heart, blood, vessels) Respiratory System (nose, trachea, lungs) Immune System ,nervous system,digestive system ,excretory system,and muscular system.
circulatory system

Blood flows through 96,500 km/60,000 mi of arteries and veins, supplying oxygen and nutrients to organs and limbs. Oxygen-poor blood (blue) circulates from the heart to the lungs where oxygen is absorbed. Oxygen-rich blood (red) flows back to the heart and is then pumped round the body through the aorta, the largest artery, to smaller arteries and capillaries. Here oxygen and nutrients are exchanged with carbon dioxide and waste products and the blood returns to the heart via the veins. Waste products are filtered by the liver, spleen, and kidneys, and nutrients are absorbed from the stomach and small intestine.

Cross sections of an artery and a vein. Arteries have thicker walls than veins as they have to withstand a higher blood pressure than do veins. Veins have valves to prevent blood from flowing backwards.

The direction of blood flow in the human body, through the major blood vessels to the principal organs.
System of vessels in an animal's body that transports essential substances (blood or other circulatory fluid) to and from the different parts of the body. It was first discovered and described by English physician William Harvey. All animals except for the most simple – such as sponges, jellyfish, sea anemones, and corals – have some type of circulatory system. Some invertebrates (animals without a backbone), such as insects, spiders, and most shellfish, have an ‘open’ circulatory system which consists of a simple network of tubes and hollow spaces. Other invertebrates have pump-like structures that send blood through a system of blood vessels. All vertebrates (animals with a backbone), including humans, have a ‘closed’ circulatory system which principally consists of a pumping organ – the heart – and a network of blood vessels.
Fish have a single circulatory system in which blood passes once around the body before returning to a two-chambered heart. In birds and mammals, there is a double circulatory system – the lung or pulmonary circuit and the body or systemic circuit. Blood is first pumped from the heart to the lungs and back to the heart, before being pumped to the remainder of the body and back. The heart is therefore a double pump and is divided into two halves. In all vertebrates, blood flows in one direction. Valves in the heart, large arteries, and veins prevent backflow, and the muscular walls of the arteries assist in pushing the blood around the body. A network of tiny capillaries carries the blood from arteries to veins. It is through the walls of capillaries that materials are transported to and from the blood.
Although most animals have a heart or hearts to pump the blood, in some small invertebrates normal body movements circulate the fluid. In the open system, found in snails and other molluscs, the blood (more correctly called haemolymph) passes from the arteries into a body cavity (haemocoel), and from here is gradually returned by other blood vessels to the heart, via the gills. Insects and other arthropods have an open system with a heart. In the closed system of earthworms, blood flows directly from the main artery to the main vein, via smaller lateral vessels in each body segment.
The human circulatory system performs a number of functions: it supplies the cells of the body with the food and oxygen they need to survive (see nutrition); it carries carbon dioxide and other waste products away from the cells; it helps to regulate the temperature of the body; and protects the body from disease. In addition, the system transports hormones, which help to regulate the activities of various parts of the body.m,and muscular system.