The Small Intestine

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Most digestion, as well as absorption of digested food, occurs in the small intestine. This narrow, twisting tube, about 2.5 cm (1 in) in diameter, fills most of the lower abdomen, extending about 6 m (20 ft) in length. Over a period of three to six hours, peristalsis moves chyme through the duodenum into the next portion of the small intestine, the jejunum, and finally into the ileum, the last section of the small intestine. During this time, the liver secretes bile into the small intestine through the bile duct. Bile breaks large fat globules into small droplets, which enzymes in the small intestine can act upon. Pancreatic juice, secreted by the pancreas, enters the small intestine through the pancreatic duct. Pancreatic juice contains enzymes that break down sugars and starches into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. Glands in the intestinal walls secrete additional enzymes that break down starches and complex sugars into nutrients that the intestine absorbs. Structures called Brunner’s glands secrete mucus to protect the intestinal walls from the acid effects of digestive juices.

The small intestine’s capacity for absorption is increased by millions of fingerlike projections called villi, which line the inner walls of the small intestine. Each villus is about 0.5 to 1.5 mm (0.02 to 0.06 in) long and covered with a single layer of cells. Even tinier fingerlike projections called microvilli cover the cell surfaces. This combination of villi and microvilli increases the surface area of the small intestine’s lining by about 150 times, multiplying its capacity for absorption. Beneath the villi’s single layer of cells are capillaries (tiny vessels) of the bloodstream and the lymphatic system. These capillaries allow nutrients produced by digestion to travel to the cells of the body. Simple sugars and amino acids pass through the capillaries to enter the bloodstream. Fatty acids and glycerol pass through to the lymphatic system.

The Stomach

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The stomach, located in the upper abdomen just below the diaphragm, is a saclike structure with strong, muscular walls. The stomach can expand significantly to store all the food from a meal for both mechanical and chemical processing. The stomach contracts about three times per minute, churning the food and mixing it with gastric juice. This fluid, secreted by thousands of gastric glands in the lining of the stomach, consists of water, hydrochloric acid, an enzyme called pepsin, and mucin (the main component of mucus). Hydrochloric acid creates the acidic environment that pepsin needs to begin breaking down proteins. It also kills microorganisms that may have been ingested in the food. Mucin coats the stomach, protecting it from the effects of the acid and pepsin. About four hours or less after a meal, food processed by the stomach, called chyme, begins passing a little at a time through the pyloric sphincter into the duodenum, the first portion of the small intestine.

The Esophagus

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The presence of food in the pharynx stimulates swallowing, which squeezes the food into the esophagus. The esophagus, a muscular tube about 25 cm (10 in) long, passes behind the trachea and heart and penetrates the diaphragm (muscular wall between the chest and abdomen) before reaching the stomach. Food advances through the alimentary canal by means of rhythmic muscle contractions (tightenings) known as peristalsis. The process begins when circular muscles in the esophagus wall contract and relax (widen) one after the other, squeezing food downward toward the stomach. Food travels the length of the esophagus in two to three seconds.

A circular muscle called the esophageal sphincter separates the esophagus and the stomach. As food is swallowed, this muscle relaxes, forming an opening through which the food can pass into the stomach. Then the muscle contracts, closing the opening to prevent food from moving back into the esophagus. The esophageal sphincter is the first of several such muscles along the alimentary canal. These muscles act as valves to regulate the passage of food and keep it from moving backward.

The Human Digestive System

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If a human adult’s digestive tract were stretched out, it would be 6 to 9 m (20 to 30 ft) long. In humans, digestion begins in the mouth, where both mechanical and chemical digestion occur. The mouth quickly converts food into a soft, moist mass. The muscular tongue pushes the food against the teeth, which cut, chop, and grind the food. Glands in the cheek linings secrete mucus, which lubricates the food, making it easier to chew and swallow. Three pairs of glands empty saliva into the mouth through ducts to moisten the food. Saliva contains the enzyme ptyalin, which begins to hydrolyze (break down) starch—a carbohydrate manufactured by green plants.

Once food has been reduced to a soft mass, it is ready to be swallowed. The tongue pushes this mass—called a bolus—to the back of the mouth and into the pharynx. This cavity between the mouth and windpipe serves as a passageway both for food on its way down the alimentary canal and for air passing into the windpipe. The epiglottis, a flap of cartilage, covers the trachea (windpipe) when a person swallows. This action of the epiglottis prevents choking by directing food from the windpipe and toward the stomach.

Sub-topics:
• The Esophagus
• The Stomach
• The Small Intestine
• The Large Intestine

Blood Pressure

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The pressure generated by the pumping action of the heart propels the blood to the arteries. In order to maintain an adequate flow of blood to all parts of the body, a certain level of blood pressure is needed. Blood pressure, for instance, enables a person to rise quickly from a horizontal position without blood pooling in the legs, which would cause fainting from deprivation of blood to the brain. Normal blood pressure is regulated by a number of factors, such as the contraction of the heart, the elasticity of arterial walls, blood volume, and resistance of blood vessels to the passage of blood.

Blood pressure is measured using an inflatable device with a gauge called a sphygmomanometer that is wrapped around the upper arm. Blood pressure is measured during systole, the active pumping phase of the heart, and diastole, the resting phase between heartbeats. Systolic and diastolic pressures are measured in units of millimeters of mercury (abbreviated mm Hg) and displayed as a ratio. Blood pressure varies between individuals and even during the normal course of a day in response to emotion, exertion, sleep, and other physical and mental changes. Normal blood pressure is less than 120/80 mm Hg, in which 120 describes systolic pressure and 80 describes diastolic pressure. Higher blood pressures that are sustained over a long period of time may indicate hypertension, a damaging circulatory condition. Lower blood pressures could signal shock from heart failure, dehydration, internal bleeding, or blood loss.

Circulatory System: Additional Functions

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In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body. These nutrients enter the bloodstream by passing through the walls of the intestine. The nutrients are absorbed through a network of capillaries and veins that drain the intestines, called the hepatic portal circulation. The hepatic portal circulation carries the nutrients to the liver for further metabolic processing. The liver stores a variety of substances, such as sugars, fats, and vitamins, and releases these to the blood as needed. The liver also cleans the blood by removing waste products and toxins. After hepatic portal blood has crossed the liver cells, veins converge to form the large hepatic vein that joins the vena cava near the right atrium.

The circulatory system plays an important role in regulating body temperature. During exercise, working muscles generate heat. The blood supplying the muscles with oxygen and nutrients absorbs much of this heat and carries it away to other parts of the body. If the body gets too warm, blood vessels near the skin enlarge to disperse excess heat outward through the skin. In cold environments, these blood vessels constrict to retain heat.

The circulatory system works in tandem with the endocrine system, a collection of hormone-producing glands. These glands release chemical messengers, called hormones, directly into the bloodstream to be transported to specific organs and tissues. Once they reach their target destination, hormones regulate the body’s rate of metabolism, growth, sexual development, and other functions.

The circulatory system also works with the immune system and the coagulation system. The immune system is a complex system of many types of cells that work together to combat diseases and infections. Disease-fighting white blood cells and antibodies circulate in the blood and are transported to sites of infection by the circulatory system. The coagulation system is composed of special blood cells, called platelets, and special proteins, called clotting factors, that circulate in the blood. Whenever blood vessels are cut or torn, the coagulation system works rapidly to stop the bleeding by forming clots.

Other organs support the circulatory system. The brain and other parts of the nervous system constantly monitor blood circulation, sending signals to the heart or blood vessels to maintain constant blood pressure. New blood cells are manufactured in the bone marrow. Old blood cells are broken down in the spleen, where valuable constituents, such as iron, are recycled. Metabolic waste products are removed from the blood by the kidneys, which also screen the blood for excess salt and maintain blood pressure and the body’s balance of minerals and fluids.

Pulmonary Circulation

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A single blood cell takes roughly 30 seconds to complete a full circuit through both the pulmonary and systemic circulation.

In pulmonary circulation, deoxygenated blood returning from the organs and tissues of the body travels from the right atrium of the heart to the right ventricle. From there it is pushed through the pulmonary artery to the lung. In the lung, the pulmonary artery divides, forming the pulmonary capillary region of the lung. At this site, microscopic vessels pass adjacent to the alveoli, or air sacs of the lung, and gases are exchanged across a thin membrane: oxygen crosses the membrane into the blood while carbon dioxide leaves the blood through this same membrane. Newly oxygenated blood then flows into the pulmonary veins, where it is collected by the left atrium of the heart, a chamber that serves as collecting pool for the left ventricle. The contraction of the left ventricle sends blood into the aorta, completing the circulatory loop. On average, a single blood cell takes roughly 30 seconds to complete a full circuit through both the pulmonary and systemic circulation.