Human Body Systems: Digestion, Excretion, Respiration

The Digestive System

The digestive system is a set of organs and associated glands. It is responsible for converting food into simple, readily usable substances by the body. From the mouth to the anus, the digestive tract is about eleven meters in length.

Digestion Process

Digestion begins in the mouth. The teeth grind the food, and secretions from the salivary glands initiate and wet the chemical decomposition. Then, the bolus crosses the pharynx, goes into the esophagus, and then into the stomach. The stomach is a muscular bag, with a capacity of about two pints under normal conditions, which secretes potent gastric juice. In the stomach, food is agitated until it becomes chyme.

Small Intestine

At the outlet of the stomach, the digestive tract extends into the small intestine, about six meters long but very folded in on itself. In the first portion, or duodenum, it receives secretions from the intestinal glands, bile, and pancreatic juices. All these secretions contain a large number of enzymes that break down food and turn them into simple, soluble substances.

Large Intestine

The gastrointestinal tract continues through the large intestine, just over five feet in length. Its final piece is the rectum, which ends at the anus, through which the indigestible remains of food are disposed of.

The Excretory System

The excretory system is a set of organs responsible for the elimination of nitrogenous waste products of metabolism, known in medicine as urine, which comprise urea and creatinine. Its architecture consists of structures that filter body fluids (liquid celomatic, hemolymph, blood). In invertebrates, the basic unit of filtration is the nephridium, while in vertebrates, it is the nephron.

Human Urinary System

The human urinary system consists basically of two parts:

• The secretory organs: kidneys that produce urine and carry out other functions.
• Excretory, which collects urine and expels it outside.

It consists of a set of tubes that are:

• The ureters carry urine from the kidneys to the bladder.
• The urinary bladder, a receptacle where urine collects.
• The urethra, the tube that carries urine to the outside, being short in length in women and longest in men, called the penile urethra.

The Respiratory System

The respiratory system is responsible for taking up oxygen (O₂) and removing carbon dioxide (CO₂) from cellular metabolism.

Mechanism

The respiratory system generally includes tubes, such as the bronchi, used to carry air into the lungs, where gas exchange occurs. The diaphragm, like any muscle, can contract and relax. On inhalation, the diaphragm contracts and flattens, and the chest cavity enlarges. This contraction creates a vacuum that sucks air into the lungs. On exhalation, the diaphragm relaxes and returns to its dome shape, and the air is expelled from the lungs.

Components

In humans and other mammals, the respiratory system consists of the airways, lungs, and respiratory muscles that mediate the movement of air both inside and outside the body.

Gas Exchange

Gas exchange is the exchange of oxygen and carbon dioxide between the animal and its environment. Within the alveolar system of the lungs, molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous environment and the blood. Thus, the respiratory system facilitates oxygenation with the concomitant removal of carbon dioxide and other gases that are wastes of metabolism and circulation.

The system also helps maintain the acid-base balance in the body through the efficient removal of carbon dioxide from the blood.