THE DIGESTIVE SYSTEM

The digestive system is a group of organs working together to convert food into energy and basic nutrients.

Food passes through a long tube inside the body known as the alimentary canal or the gastrointestinal tract. The alimentary canal is made up of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

Accessory organs of the digestive system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

The mouth is the first part of the alimentary canal. It includes salivary glands, teeth, and the tongue.

The pharynx (throat) is a part both of the digestive system and the respiratory system.

The esophagus (oesophagus) is the organ which consists of a muscular tube through which food passes from the pharynx to the stomach.

The stomach is a muscular sac located on the left side of the abdominal cavity under the diaphragm. In an average person, the stomach is about the size of their two fists, so, it has a capacity of from 2.14 to 4.28 l (liters). The stomach contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth.

The small intestine is a long, thin tube about 6.5 meters long. The large intestine is a long, thick tube about 1.5 meters long.

The liver is the largest gland in the human body. It is located in the right upper part of the abdominal cavity under the diaphragm. The weight of the liver is 1,500 g. The liver produces bile which aids digestion.

The gall bladder is a hollow sac lying on the lower surface of the liver.

The pancreas is a major organ functioning as an accessory digestive gland in the digestive system. It is a long thin gland lying behind the stomach.

The digestive system is constructed to perform its specialized function of turning food into the energy we need to survive.

The functions of the digestive system are:

· Ingestion – eating food

· Digestion – breakdown of the food

· Absorption – extraction of nutrient from the food

· Defecation – removal of waste products

During digestion two main processes occur at the same time:

- mechanical digestion which refers to the physical breakdown of large pieces of food into smaller pieces.

- chemical digestion when enzymes break down food into the small molecules which the body can use.

Gastroenterology is the branch of medicine focused on studying and treating digestive system disorders.

ENDOCRINE GLANDS

Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into the blood to help your body function properly.

The endocrine system works to regulate certain internal processes. Note: endocrine shouldn’t be confused with exocrine. Exocrine glands, such as sweat and salivary glands, secrete externally and internally via ducts. Endocrine glands secrete hormones internally, using the bloodstream.

The endocrine system helps control the following processes and systems:

- growth and development

- homeostasis (the internal balance of body systems)

- metabolism (body energy levels)

- reproduction

- response to stimuli (stress and/or injury)

The endocrine system completes these tasks through its network of glands, which are small but highly important organs that produce, store, and secrete hormones.

The glands of the endocrine system are hypothalamus, pineal gland, pituitary gland (hypophysis), thyroid, parathyroid, thymus, adrenals, pancreas, ovaries, testes .

These glands produce different types of hormones. Hormones are molecules released by a group of cells in the body that influence the behavior of another group of cells. Hormones are the chemicals of the endocrine system. In response to a signal from the brain, hormones are secreted directly into the blood by the endocrine glands.

Like the nervous system, the endocrine system is one of your body’s main communicators. But instead of using nerves to transmit information, the endocrine system uses blood vessels to deliver hormones to cells.

The pituitary gland is often referred to as the "master gland”. It greatly influences other organs in the body, and its function is vital to the overall well-being of a person.

THE NERVOUS SYSTEM

The nervous system consists of the brain, spinal cord, sensory organs, and all of the nerves that connect these organs to the rest of the body. Together, these organs are responsible for the control of the body and communication among its parts.

The brain and the spinal cord form the control center known as the central nervous system (CNS), where information is evaluated and decisions made.

The sensory nerves and sense organs of the peripheral nervous system (PNS) monitor conditions inside and outside of the body and send this information to the CNS.

The brain, a soft, wrinkled organ, is located inside the cranial cavity. The brain is one of the largest and most complex organs in the human body. Its weight is about 1.5 kg.

The brain's nerve cells are known as neurons, which make up the organ's so-called "gray matter". The neurons transmit and gather electrochemical signals that are communicated via a network of millions of nerves. These are the brain's "white matter."

The cerebrum is the largest part of the brain, accounting for 85 percent of the organ's weight. The distinctive, deeply wrinkled outer surface is the cerebral cortex, which consists of gray matter. Beneath this the white matter lies. The cerebrum has two halves, or hemispheres. Experimental studies have determined that the motor cortex controlling many body movements becomes tired rapidly.

The approximately 100 billion neurons of the brain form the main control center of the body. Scientists have determined that each cell is connected to the other by nerve fibers. Numerous investigations have shown that nervous cells are the most delicate cells of the human body.

The spinal cord is a long, thin mass of neurons that carries information through the vertebral cavity of the spine. The spinal cord contains the white and grey matter.

Nerves act as information highways to carry signals between the brain and spinal cord and the rest of the body.

The nervous system has 3 main functions: sensory, integration, and motorю

REFLEXES

A reflex is an automatic response to a stimulus.

All reflexes are divided into unconditioned and conditioned ones.

An unconditioned reflex is a normal, instinctive, unlearned reaction to a stimulus that occurs naturally and is not dependent on previous experience. Unconditioned reflexes are also called inborn reflexes.

Unconditioned reflexes serve as the foundation for the rest of the nervous activity of the organism.

There are a lot of unconditioned (unlearned) reflexes. The simplest ones include swallowing, salivation, blinking, scratching, sucking (in infants), sneezing, yawning, and others.

Probably the best-known reflex is the pupillary reflex. If light is flashed near one eye, the pupils of both eyes contract. Greater intensity light causes the pupil to become smaller, whereas lower intensity light causes the pupil to become larger.

Another reflex involving the eye is known as the lacrimal reflex. When something irritates the eye, the lacrimal reflex causes nerve impulses to stimulate the lacrimal glands.

A conditioned reflex is some action or feeling that you learn to do in response to a specific situation or stimulus. It is an automatic reaction learned through training to a stimulus that does not normally elicit such response. In Pavlov’s classic experiment, dogs learned to associate the sound of a ringing bell with feeding time so that they salivated at the sound of the bell, regardless of whether or not food was given to them. Conditioned reflexes are also called acquired reflexes, behavior reflexes, or trained reflexes.

Unlike conditioned reflexes, the unconditioned reflexes are mostly stable. The conditioned reflexes are not only unstable but can be modified and extinguished.

A leading role in the performance of unconditioned reflexes is played by the lower divisions of the higher nervous system, the subcortical nuclei, brain stem, and spinal cord. Conditioned reflexes, in contrast, are a function of the cerebral cortex.

The investigation of conditioned reflexes became the basis for the theory of how organisms learn. Learning theory grew into the foundation of Behaviourism, a school of physiology that had great societal influence in the mid-20^th century.