Human Body
...dermis is the outside layer of the skin, and the dermis is the inside layer of skin. Hair is made in hair follicles which are small folds of epidermis that extend into the dermis. The follicle is nourished at the base by small blood vessels. The shaft is the part of hair that you see which is made of dead epidermis. It is found all over the body except on the bottoms of the feet, the palms of the hands, and the lips. Nails are dead cells made of keratin that protect the tips of fingers and toes. The fourth major system of the human body is the digestive system. The digestive system is composed of the mouth, the pharynx, the esophagus, the liver, the stomach, the pancreas, the small intestine, the large intestine, the rectum, and the anus. The major function of the digestive system is to digest food. The tube like passageway that runs from the mouth to the anus is called the alimentary canal. The alimentary canal contains all the organs used in digestion. The mouth starts digestion by grinding the food into smaller pieces. It is then mixed with the saliva, which lubricates the food and starts decomposing the starches. You then swallow the food and it goes through the pharynx to the esophagus. Food is then pushed through the esophagus by a motion called peristalsis. This motion pushes food into the stomach where it is digested into smaller parts. There are three muscles in the stomach that help to digest food mechanically and many types of acid in the stomach that help break down food chemically. The food leaves the stomach in a semi-liquid form called chyme. It then enters the small intestine where the majority of chemical digestion and absorption happens. The small intestine is pumped with chemicals that aid in digestion from the liver and pancreas. The food that was not absorbed by the small intestine then travels into the large intestine where absorption is completed. The leftover substance is called feces. It is stored in the rectum until released through the anus. The fifth major system of the human body is the circulatory system. The circulatory system transports waste, nutrients, gases, and chemicals throughout the body. It consists of the heart, the major arteries, veins, blood vessels, and capillaries. The heart is a muscular organ that pumps blood throughout a person’s body. The heart is about the size of the person’s fist of whom the heart belongs. The right and left sides of the heart are two separate pumps. Both sides have an upper side and a lower side. A one-way valve separates the upper side (the atrium) and the lower side (the ventricle). A vein brings blood to the heart. The superior vena cava brings blood from the upper part of the body. The inferior vena cava brings blood from the lower part of the body to the heart. An artery carries blood away from the heart. On the right side, the blood travels from the heart through the pulmonary artery to the lungs where it exchanges carbon dioxide for oxygen. On the left side, blood is pumped from the left ventricle to the aorta then through the rest of the body. The circulatory system also contains a system called the lymphatic system. It absorbs excess proteins and liquids and returns them to the bloodstream. The sixth major system of the human body is the respiratory system. The respiratory system consists of the lungs, the pleura, the diaphragm, vocal cords, trachea, nostrils, and the nasal cavities. The major function of the Respiratory system is respiration. Respiration is the process by which the body takes in oxygen, uses it to produce energy, and then gets rid of some waste products of cellular respiration. Air travels through the nostrils when a person inhales. The air then travels through the pharynx to the trachea through the vocal cords to the bronchi, which are inside the lungs. Breathing begins when the diaphragm drops causing the air pressure out side the body to be greater than that inside so air flows in. Every person has two lungs. A lung is a cone shaped, sac-like organ. Inside the lung oxygen diffuses into the blood through the capillaries and in turn carbon dioxide is differs into the air in the lungs. The seventh major body system is the excretory system. The major organs of the excretory system are the kidneys. “The main functions of kidneys are to remove urea and other wastes, regulate the amount of water in the blood, and adjust the concentrations of various substances.”(Biology Today) The body rids itself of metabolic wastes and nitrogen compounds through the process of excretion. A person’s liver combines carbon dioxide and ammonia forming a less toxic substance called urea. The urea is filtered out in a process called glomerular filtration. This process takes place in the kidneys. It also removes glucose, water, and other life sustaining substances. The nutrients are reabsorbed in a process called reabsorption. This takes place in a part of the kidney called the renal tube. The waste and water remaining in the kidneys after reabsorption are mixed together to form urine. Urine comes together from many different tubes and is deposited into the renal pelvis and then into the urinary system. The first part of the urinary system is the ureter which is a long tube coming from each kidney and connecting to the urinary bladder. When the bladder fills with urine the brain receives a response from the bladder wall to relax the sphincter muscles. Then the urine is forced out of the body through the urethra. The eighth major system of the human body is the nervous system. There are two main parts to the nervous system which are the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord. The central nervous system receives stimuli which tells the body how it should react. The brain controls the whole body. It is divided into three different sections: the cerebrum, the cerebellum, and the brain stem. The cerebrum is the largest part of the brain. The cerebrum is split into two halves called cerebral hemispheres, which are connected by 200 million nerve cells. These nerve cells are called the corpus callosum. The gray area of the cerebrum is called the cerebral cortex. The main objective of the cerebral cortex is to receive impulses and coordinate motor responses to them. Also in the cerebrum is the thalamus, a relay center for impulses and the hypothalamus, which controls the body’s temperature. The next part of the brain is the cerebellum, which coordinates the voluntary muscle movement. The final part of the brain, the brain stem, controls breathing, swallowing, digestive processes and action of the heart and blood vessels. The spinal cord is like a highway transporting orders to and from the brain. The peripheral nervous system provides the path that the impulses sent from the central nervous system take. Neurons are the nerve cell that does most of the work in the nervous system. There are three types of neurons. They are sensory neurons, which receive stimuli and send them to the central nervous system. There are motor neurons, which take impulses to muscles or glands from the central nervous system. Finally there are interneurons which link sensory and motor cells together. The ninth system of the human body is the endocrine system. The endocrine system controls hormones which are very powerful chemicals that control the body’s growth and maturation. Hormones are produced in many glands which are referred to as endocrine glands. The th...