The 19th century German philosopher, Ludwig Feuerbach, said, "A man is what he eats." Food fuels the body's cells and is used as building material for repair and in some cases, is stored for future use. The purpose of digestion is to break food down to molecules that can be used by cells. Digestion involves three principle processes: mechanical digestion, chemical digestion and absorption.

The process begins in the mouth. Chewing initiates the mechanical breakdown of food and is followed by secretion of saliva, which moistens and lubricates food for swallowing. Saliva also contains amylases (enzymes), which start the chemical breakdown of carbohydrates.
 
The swallowing reflex begins in the pharynx and initiates rhythmic waves of smooth muscle contractions called peristalsis. Peristaltic contractions transport food to the stomach and allow a person to swallow even if he/she are upside down.

The stomach contains an extra layer of muscle that aids in mechanically mixing and churning food into a semi liquid form called "chyme." Chemical digestion begins with proteins through the action of hydrochloric acid and the enzyme, pepsin. Only water and a few substances, such as aspirin and alcohol, are absorbed by the lining of the stomach.

As food enters the small intestine (so named because of diameter, not the length), secretions from the liver, gall bladder and pancreas are added. The small intestine completes digestion of food materials and the nutrients are absorbed into the blood. Fingerlike projections called villi (covered with microvilli) line the interior of the small intestine. Villi increase the surface area for absorption and secretion.

The large intestine is much shorter than the small intestine but is greater in diameter. The large intestine does not contain villi, which greatly reduces its available surface area for absorption. It has no role in digestion. Only water and vitamin K, synthesized by bacteria living in the the colon are absorbed into the bloodstream from the large intestine. Undigested or unabsorbed food is eliminated.
 
A nutrient is a substance the body needs for growth, repair and maintenance.  Nutrients include carbohydrates, proteins, lipids, vitamins, minerals and water.  Carbohydrates must be broken down into monosaccharides and are the body's main source of energy. Proteins are broken down to amino acids and supply the raw materials for growth and repair. The body requires 20 amino acids, 10 of which it cannot make and must obtain from outside sources. Lipids are reduced to fatty acids and glycerin. They are used to make steroid hormones, cell membranes and also store energy. Vitamins are organic molecules that aid in the regulation of body processes. Finally, water is required for metabolism and chemical reactions within the body, for transport of substances around the body, and for regulation of body temperature. Approximately two-thirds of the body weight is water.

The digestive system includes the gastrointestinal tract (mouth, pharynx, esophagus, stomach, small and large intestines) along with accessory structures (teeth, tongue, salivary glands, liver, gall bladder and pancreas). Food materials are broken down to usable nutrients and absorbed into the bloodstream. They are used by the body for metabolism, building and repair. Some nutrients are stored within the body.
 
The lining of the small intestine consists of tiny folds, called villi, which, in turn, are covered by microvilli. Microvilli have brush border enzymes to hydrolyze lactose and sucrose. The villi contain capillaries and lymphatic vessels for the absorption of nutrients.

Normal metabolic activity generates waste products that must be eliminated to sustain homeostasis. Ammonia (a product of protein metabolism) is converted to urea for elimination. Carbon dioxide, generated from cellular respiration, is removed from the lungs. Excess water and salts must be eliminated in order to maintain osmotic and pH balance.

The skin removes water, salts and nitrogen wastes in the form of sweat.
 
The lungs eliminate carbon dioxide, water vapor and heat in exhaled air.

The kidneys excrete the majority of metabolic waste products from the body. Each kidney contains about one million functional filtering units called nephrons.

The urinary system, consisting of the kidneys, ureters, urinary bladder and urethra, is responsible for eliminating the majority of metabolic wastes from the body.
 
The functional unit of the kidney is the nephron. Each nephron is made of a cup-shaped portion called Bowman's capsule, tubules and a network of capillaries.  Blood pressure within a knot of capillaries (called the glomerulus) increases, causing most of the fluid of the blood to enter Bowman's capsule. This fluid is called filtrate. As the filtrate passes through the tubule portion of the nephron, materials needed by the body are reabsorbed and the remainder of the filtrate becomes urine. Proper functioning of the kidney is essential to maintaining homeostatsis in the body.

The reproductive system is responsible for producing, storing and releasing specialized cells called gametes and then transporting them to a place where fertilization can occur. As the fertilized egg (called a zygote) begins to divide, it becomes an embryo and must be maintained within the body during a critical period of development.

The testes produce the male gamete (called sperm) in the seminiferous tubules.  From there, sperm are moved through the epididymus and the vas deferens, past the seminal vesicle (where seminal fluid is produced), through the prostate gland, past the bulbourethral gland (Cowper's gland) and out of the body through the urethra. Testosterone, the primary male androgen, is produced by the testes and is regulated through a negative biofeedback system.

Eggs are produced in the ovary of the female before birth. Following the onset of puberty, they are matured and released in a process called ovulation. After ovulation, the egg is swept into the fallopian tube where fertilization can take place.  If the egg is fertilized, it passes to the uterus, where implantation occurs in the blood rich inner layer of the uterus (the endometrium). The outer end of the uterus is called the cervix. Beyond that is a canal called the vagina. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland along with estrogen and progesterone from the ovary, are the primary hormones controlling the monthly preparation of the endometrium and maturation of the egg (called the menstrual cycle).

The testes are located outside of the body cavity in the scrotum. Sperm are produced in the seminiferous tubules within the testes (Production is controlled by two hormones from the pituitary gland). Sperm are transported into the epididymus which stores and improves the fertility of sperm. From the epididymus, sperm are moved by cilia through the vas deferens and out through the urethra. Seminal fluid with nutrients for sperm is added by the seminal vesicle and prostate gland, producing semen. The bulblurethral gland (Cowper's gland) secretes an alkaline fluid to neutralize acidic traces of urine in the urethra. Semen is ejaculated through the penis into the vagina of the female during sexual intercourse.

At birth, the ovaries contain all the immature eggs a female ever will produce.  After puberty, eggs are released monthly from the ovaries. The fingerlike projections of the fallopian tube are lined by cilia that draw the eggs into the fallopian tube, where fertilization can take place. If the egg is fertilized, it implants in the endometrium of the uterus. The vagina, a canal leading from the uterus, is where semen is deposited by the male during sexual intercourse.

Glands produce and release secretions. Endocrine glands (ductless) produce chemical substances called hormones and release them into the bloodstream. These glands include the hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, ovary and testes. Exocrine glands deliver secretions through a duct or tube to specific locations (salivary glands, sweat glands, mucous glands).

Hormones are chemical messengers that bind to target cells and affect cell activities. In general, the response of the body to hormones is slower and more long lasting than the response to nerve impulses. The release of hormones is controlled through a negative feedback system. Negative feedback systems are control mechanisms that operate by inhibiting a process or activity that originally led to starting the process or activity. For example, as glucose levels in the body increase, sensors in the pancreas signal the release of the hormone insulin.  Insulin promotes the entry of glucose into cells, lowering glucose levels. As the glucose levels in the blood decrease, the pancreas stops secreting insulin.

Steroid hormones are produced from the lipid, cholesterol and are able to pass through cell membranes and bind to receptor proteins. Nonsteroid hormones are made from amino acids and generally cannot pass through the cell membrane. These hormones bind to a receptor site on the outside of a cell in order to initiate chemical reactions inside the cell.

• Hypothalamus - coordinates activities of the nervous and endocrine systems and produces hormones to regulate the pituitary gland
• Pituitary - produces hormones that direct the activities of other endocrine glands
• Pineal - releases melatonin which is involved in rhythmic or cyclic activities
• Thyroid - produces hormones that regulate metabolism and development
• Parathyroid - helps maintain appropriate calcium levels
• Thymus - involved with immune development during childhood
• Adrenal - regulates the body's stress response (epinephrine, norepinephrine, cortisol, aldosterone)
• Pancreas - controls glucose levels in the blood (insulin, glucagon)
• Ovary - secretes estrogen and progesterone to regulate reproduction
• Testes - produces testosterone to control formation of sperm and sexual behavior

The nervous system coordinates the body's response to changes in the internal and external environments. It is responsible for gathering, integrating, interpreting and responding to information about conditions throughout the body.
 
Neurons are specialized cells that transmit electrical signals, called impulses, throughout the body. Motor neurons relay impulses from the brain and spinal cord to other parts of the body. Sensory neurons transmit impulses toward the brain and spinal cord. Interneurons relay impulses between sensory and motor neurons. A basic neuron is made of a cell body (which contains the nucleus), dendrites and a long axon.

The central nervous system (CNS) consists of the brain and spinal cord and is responsible for relaying, processing and analyzing information. The brain is the primary processing center for the body. The spinal cord links the brain with the peripheral nervous system.

The peripheral nervous system (PNS) consists of 12 pairs of cranial nerves, 31 pairs of spinal nerves and all the nerves that extend beyond them into the body. Sensory nerves bring information into the CNS. Motor neurons carry impulses to muscles and glands. The motor division of the PNS is further divided into the somatic nervous system (regulates activities under voluntary control) and the autonomic nervous system (regulates involuntary activities). The portion of the autonomic nervous system that is most active during normal activities is called the parasympathetic system. The portion that dominates during physical or emotional stress is the sympathetic system.