The Digestive System


The Digestive System is composed of several organs, glands, secretions, etc., that work together to break down food and drink consumed by individuals. Digestion is successful due in part to not only mechanical processes, but chemical processes as well. There are many components that contribute to the overall function of the digestive system. All major organs that take part in the digestion of food can be separated into two groupings. The organs that make up the alimentary canal include those through which food being digested passes through directly. Accessory organs also play a part in digestion. Food does not necessarily pass through these organs, however, these organs are capable of secreting chemicals compounds and enzyme-rich fluids that allow further breakdown of ingested food. The overall breakdown of substances in the human body can be even further divided in observing the differences between the anatomical and physiological aspects of digestion.


Anatomical Digestion

The anatomical digestion of substances in the body is observed by the path that all substances travel. Although this does not include elements of chemical digestion that occur within these parts of the body, anatomical digestion provides insight as to where the processes of digestion occur.

The Alimentary Canal

Another synonymous term used to refer to the alimentary canal is the gastrointestinal (GI) tract. Both terms are used to describe a continuous, muscular tube that extends through the body, remaining open at both ends by the oral and anal cavities. The organs that make up the alimentary canal include: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

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The Mouth

The mouth is the primary site of digestion, where food enters the digestive tract. Another term used to describe the mouth is the oral cavity. The lips protect its anterior opening while the cheeks form its lateral walls. It is here in the mouth where a process called mastication occurs. Mastication is the process of chewing one's food. The teeth work to break the food down into tiny pieces which make it easier for degluttion, or swallowing, to occur.


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The Pharynx

The pharynx acts as a passageway between the mouth and the esophagus. It is made up of three parts: the nasopharynx, the oropharynx, and the laryngopharynx. The interior walls or the pharynx are made up of two layers. The inner layer is made up of cells that span longitudinally while the outer layer has cells that run around the opening in a circular fashion. Due to alternating contractions of these two layers, peristalisis, a propelling mechanism in which food is propelled through the alimentary canal, occurs, pushing the food (now referred to as a bola) into the esophagus.

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The Esophagus

The esophagus is a long, flexible passageway that extends from the pharynx, all the way through the diaphragm, to the stomach. Much like the pharynx, it uses peristalsis to mechanically push the bola downwards and continue through the process of digestion.

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The Stomach

From the stomach, the bola passes through the cardioesophageal sphincter. From here, the bola is dumped into the stomach where segmentation can occur. Segmentation is the contraction and relaxation of muscles that occur in segments of different digestion organs. Opposite to peristalsis, segmentation occurs in more of a horizontal manner while peristalsis is vertical. Throughout this churning and other elements of chemical digestion, the bola becomes a thick fluid known as chyme.
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The Small Intestine

The chyme empties from the stomach, through the pyloric sphincter, into the small intestine. The first part of the small intestine is the duodenum. The small intestine also utilizes the process of segmentation. Chyme passes through the duodenum, into the jejunum, and into the ileum. From the ileum, chyme passes into the first section of the large intestine through the ileocecal valve.
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The Large Intestine

From the ileocecal valve, chyme passes through the first part of the large intestine, the cecum. Hanging from the cecum is appendix. From the cecum, chyme travels through various portions of the colon: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. Following this, it passes through the rectum and into the anal canal by means of peristalsis, finally being ejected from the body in stools.

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Chemical Digestion

Chemical digestion works hand in hand with anatomical digestion to further break down food into molecules of nutrients that are necessary to sustain life. The importance of chemical digestion can be observed through the release of enzymes and other body fluids and substances that are vital to the catabolizing of the food.

Carbohydrate Digestion

The digestion of carbohydrates is essential to the body because it produces glucose which is used as a major source of energy for cells. It is also used to produce energy known as ATP. Starch digestion begins in mouth when salivary amylase is secreted from salivary glands. There are three pairs of salivary glands: the parotid glands, the submandibular glands, and the sublingual glands. The enzymes secreted from these glands begins to catabolize the carbohydrates by breaking them down into simpler sugars. However, the work completed by these glands and enzymes is not the sole source of carbohydrate digestion. Starch digestion continues when the food enters the small intestine. Here, pancreatic enzymes are released through pancreatic ducts and work to finish breaking down the carbohydrates and transforming them into useful means of energy.

Protein Digestion

The stomach is the initial site of protein digestion. It contains gastric juices that help in the chemical breakdown of proteins. Chief cells are responsible for the production of protein-digesting enzymes, mainly pepsinogens. Parietal cells are responsible for producing hydrochloric acid. HCL creates the acidic environment that causes the enzymes to activate. HCL does so by transforming pepsinogens into pepsin, another enzyme essential in the breakdown of proteins. Similar to carbohydrate digestion, protein digestion ceases in the small intestine following its encounter with brush border enzymes which further break down the proteins. Pancreatic juices also play a role in the digestion of proteins with assistance from trypsin, chymotrypsin, carboxypeptidase, etc.

Fat Digestion

The small intestine is the sole site of fat digestion. Here, the prescence of fats trigger the release of pancreatic lipase that helps to break down fat. The fats are also met by bile salts released by the liver through the bile duct. Both work hand in hand to break the fats down into simpler structures. These structures include fatty acids and glycerol. It is then that the fats are either stored as fats or immediately utilized as a form of energy.

The Endocrine System

The endocrine system works together with the nervous system as the body's main mean of controlling functions of the body. While the two serve similar purposes, they are actually very different in their own ways. The nervous system uses nerve impulses to trigger muscles and glands to react in order to produce immediate results and respond accordingly. However, the endocrine system is known for its use of chemical messengers called hormones. The endocrine system uses much more lesiurely techniques to produce change in the body. It is beneficial for these processes to be slower because many of the endocrine's major purposes are served over relatively long periods of time. This includes: reproduction, growth and development, mobilizing body defenses, maintining electrolyte, water, and nutrient balance, and regulationg cellular metabolism. Although the nervous and the endocrine systems serve different purposes, they work together to perform necessary functions to sustain human life.

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Hormones

Hormones, as previously stated, are chemical substances that are secreted by cells to uphold various functions. All hormones can be classified as either amino-acid based or steriods. Hormones are capable of travelling through the blood stream to affect nearly all tissue cells and organs of the body. However, each hormone is only able to directly affect specific cells and organs, better known as target cells and target organs. Such an effect is due to specific protein receptors that are located on the plasma membrane, to which the hormone is able to attach. Following this binding of hormones to receptor organs or cells can result in a change in the permeability of the plasma membrane, the synthesis of proteins, the activation and inactivation of enzymes, and even the stimulation of mitosis.

Release of Hormones

The release of hormones throughout the body is typically regulated with the help of the mechanism of negative feedback. Negative feedback is a means of regulating bodily substances. When there is a lack or surplus of a particular ion, fluid, or possibly even another hormone in a particular area, a stimuli triggers the release of a set of hormones that are meant to determine the problem and counteract the levels by either building up or breaking down the buildup or lack of substance.

There are three main types of stimuli that influence the release of hormones into the body: hormonal, humoral, and neural.
Hormonal stimuli occurs when the release of a hormone is triggered by other hormones. Humoral stimuli are present in cases of varying levels of ions, nutrients, and other substances found in the blood. Neural stimuli are observed in isolated cases of which nerve fibers are what stiumulate a response from target cells and organs.

Pituitary Gland

The pituitary gland is known for its small size and snug location in "Turk's Saddle" settled among the spheniod bone. This gland is particularly well known due to the two functional lobes, both the anterior and the posterior.

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Anterior Pituitary Gland

There are several hormones that are secreted by the anterior pituitary gland. Aside from growth hormone and prolactin, all other hormones produced and secreted by the anterior pituitary gland are considered to be tropic horomones. Such hormones trigger their target organs to secrete other hormones. All included hormones that are secreted from the anterior pituitary gland are as follows:
  • Growth Hormone
  • Prolactin
  • Adrenocoticotropic
  • Thyroid-stimulating hormone
  • Gonadotropic horone
    • Follicle Stimulating Hormone
    • Lutenizing hormone
The anterior pituitary gland is responsible for many functions. It is considered to be a "master endocrine gland" because it controls a great deal of the release of hormones from other endocrine glands. However, none of this would be possible without the help of the hypothalamus. The hypothalamus produces releasing and inhibiting hormones that control the release of hormones in the anterior pituitary gland. The hypothalamus is also the site of production of some hormones as well, such as oxytocin and ADH.

Posterior Pituitary Gland

The posterior pituitary gland is not considered to be an endocrine gland. This is because it does not actually make the hormones that it is releasing. Rather, it acts as a storage counsel to help monitor hypothalmic neuros. Hormones secreted from the posterior pituitary gland include:
  • Oxytocin
  • Antidiuretic Hormone (vasopressin)

Thyroid Gland

The thyroid gland is located at the base of the throat. The thyroid gland is responsible for the production of hormones that serve purpose in metabolic processes. These hormones include thyroid hormone and calcitonin. Thyroid hormone has two main classifications. The first being thyroxine, also known as T4, and the second being triodothyronine, or T3. Thyroid hormone controls the breakdown of glucose and the rate at which it is oxidized and converted to energy. Calcitonin is responsible for decreasing levels of calcium in the blood. It is able to do so by storing calcium in bone deposits, working antagonistically to hormones produced by the parathyroid glands.

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Parathyroid Glands

The parathyroid glands are located on the posterior surface of the thyroid gland. As could be expected, the main hormone that is produced by the parathyroid glands is parathyroid hormone, better known as PTH or parathormone. PTH is responsible for regulating calcium levels in the blood. However, contrarily to calcitonin, it is considered to have a hypercalcemic effect because it breaks down calcium in bone deposits and releases it into the blood to increase calcium levels.

Adrenal Glands

The adrenal glands are located just above the kidneys. There are two parts that make up the adrenal glands, both the adrenal cortex and the adrenal medulla. Unlike all other endocrine organs, the adrenal glands are made up of both glandular and neural tissue parts.
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Adrenal Cortex

The adrenal cortex is responsible for the production of steroid hormones. Collectively, these hormones are called coricosteroids. From here, these steroids can be divided into three groups of hormones. The mineralocorticoids are produced by the outer layer of the adrenal cortex. These hormones are responsible for regulating the mineral content of the blood. These hormones target sodium and potassium in particular. A prime example of a mineralocorticoid is aldosterone. Glucocorticoids are produced in the middle layer of the cortex. Glucocorticoids consist of cortisone and cortisol. These hormones work to promote cell metabolism and resist long-term stressors. These hormones are considered to be hyperglycemic because they increase blood glucose levels. Glucocortioids suppress inflammation and are released in response to rising blood levels of ACTH. Male and female sex hormones are produced by the adrenal cortex as well.

Adrenal Medulla

The adrenal medulla is stimulated by sympathetic nervous system neurons. The hormones secreted by the adrenal medulla are collectively called catecholamines. The two hormones that make up these catecholamines are epinephrine and norepinephrine. The release of these hormones is triggered by feelings of distress that bring about a "fight or flight" response to help cope with stressful situations. The adrenal medulla pumps the hormones into the blood as a way to prolong the effects of neurotransmitters of the sympathetic nervous system. These catecholamines result in an increase of blood pressure and glucose levels. Adrenaline is being pumped through the body, preparing it to handle both long-term and short-term stressors.

Pancreatic Islets

Beta cells of the pancreatic islets are responsible for the production of insulin. This insulin is then used to breakdown glucose in the blood. Alpha cells are responsible for producing glucagon which has the opposite effect. Glucagon is considered to be hyperlycemic because it is used to build up levels of glucose in areas that are lacking. Pancreatic islets are considered to be the best-hidden endocrine glands. The millions of islets that can be found on the pancreas are necessary due to the importance of the jobs that these cells have.
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Disesases

Iodine Deficiency: Goiter (enlargement of thyroid gland)
Hyposecretion of Thyroxine: Cretinism, leads to Dwarfism
Hyperthyroidism: Grave's Disease, leads to Exophthalmos
Low blood calcium levels: Tetany
Hyperparathyroidism: Massive bone destruction
Hyposecretion of adrenal cortex hormones: Addison's Disease
Hypersecretion of adrenal cortex hormones: Tumor


The Muscular System


Muscles are a very important and essential building block of the human body. Without muscular tissue, we would be incapable of completing more tasks than we may be aware of. The essential function of muscle is contraction and shortening. In doing this, muscles become responsible for all body movement. There are three types of muscular tissue: skeletal, cardiac, and smooth. These types differ in cell structure, body location, and how they are stimulated to contract.

Skeletal Muscle

Skeletal muscle fibers are packaged into skeletal muscles that attach to the body's skeleton. They help to form smoother contours of the body. Skeletal muscle fibers are very long, cylindrically shaped, multinucleate cells. They are considered the largest of the muscle fiber types. This type of muscle is also considered striated and voluntary, as it is the only type of muscle that is subject to conscious control. But they are also subject to activation through reflexes.

Smooth Muscle

Unlike, skeletal muscle, smooth muscle has no striations and is involuntary. Smooth muscle is typically found in the walls of hollow visceral organs such as the stomach, bladder, respiratory passages, etc.