Digestive System

Digestive System

The gastrointestinal tract (GI tract), also called the digestive tract, alimentary canal, or gut, is the system of organs that takes in food, digests it to extract energy

and nutrients, and expels the remaining waste. The major functions of the GI tract are

digestion and excretion.In a normal human adult male,

the GI tract is approximately 6.5 meters long (20 feet) and

consists of the upper and lower GI tracts. The tract may also be

divided into foregut, midgut, and hindgut, reflecting the

embryological origin of each segment of the tract.

The GI tract can be divided into 4 concentric layers:mucosa the innermost layer of the GI tract, surrounding the lumen, or space within the tube. This layer comes in direct contact with the food (or bolus), and is responsible for absorption and secretion, important processes in digestion.

submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics and nerves branching into the mucosa and muscularis

muscularis externa consists of a circular inner muscular layer and a longitudinal

outer muscular layer. The circular muscle layer prevents the food from going

backwards and the longitudinal layer shortens the tract.

The coordinated contractions of these

layers is called peristalsis and

propels the bolus, or balled-up food,

through the GI tract.

adventitia (or serosa) consists of several layers of epithelia.

Upper gastrointestinal tractThe upper GI tract consists of the mouth, pharynx, esophagus,

and stomach. The upper GI tract roughly corresponds to the derivatives of the foregut, with the exception of the first part of

the duodenum

The primary function of teeth is to

tear, scrape, and chew

food (=mechanical digestion).

The mouth contains the

buccal mucosa, which contains the openings of

the salivary glands; the

tongue; and the teeth.

Summary of salivary gland features

Parotid gland (#1) – secretes a watery fluid rich in amylase.

Sublingual gland (#3) – secretions are thick and stringy which binds food particles and lubricates during swallowing.

Submandibular gland (#2) – secretes watery fluid rich in amylase but more viscous.

Salivary Glands – moistens food particles, helps bind them together and begins the chemical digestion of

food. Saliva contains enzymes that initiate the breakdown of carbohydrates.

Behind the mouth lies the pharynx (refer to our discussion of the respiratory system), which leads to a hollow muscular tube, the esophagus. The esophagus extends through the chest and pierces the diaphragm

to reach the stomach.

Food is passed through the esophagus by using the process of peristalsis.

Specifically, it connects the pharynx with the stomach, where the second stage of

digestion is initiated (the first stage of digestion is in the

mouth, with teeth and tongue masticating food and mixing it

with saliva).

The esophagus is lined with a

mucous membrane, and is more deeply

lined with muscle that

acts with peristaltic

action to move swallowed

food down to the stomach.

The stomach is a bean-shaped hollow muscular organ involved in the second phase of digestion, following

mastication.The stomach is usually a highly acidic environment

due to gastric acid production which produces a pH range usually between 2 and 4 depending on food intake and other factors (a pH lower than 2 generally

indicates a health problem). Such an environment is able

to break down large molecules (such as from food) to smaller ones so

that they can eventually be absorbed from the small

intestine. The stomach can produce and secrete about 2 to 3 liters of gastric acid

per day.

Two smooth muscle bands, or sphincters, keep the contents of the stomach contained. They are the

Cardiac or esophageal sphincter dividing the tract above, and the Pyloric sphincter dividing the stomach

from the small intestine.

In humans, the stomach has a volume of about 50 mL when empty. After a meal, it generally expands to hold

about 1 litre of food, but it can actually expand to hold as much as 4 litres. The human stomach has more nerve

endings than the human brain.

Chyme is the liquid substance found in the stomach before passing through the

pyloric valve and entering the duodenum.

The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous

system and by various digestive system hormones:

Pepsin is a necessity in protein digestion. Other functions include absorbing water, some ions, and some lipid soluble compounds such as alcohol, aspirin, and caffeine. Gastrin

causes an increase in

the secretion of HCl and

pepsinogen.

The stomach is divided into four sections, each of which has different cells and functions. The

sections are:

Cardiawhere the contents of the esophagus empty into the stomach.

Fundus formed by the upper curvature of the organ.

Body or corpus

the main, central region.

Pylorus the lower section of the organ that facilitates emptying the contents into the small intestine.

Most cases of stomach ulcers, Most cases of stomach ulcers, gastritis, and stomach cancer are gastritis, and stomach cancer are

caused by caused by Helicobacter pyloriHelicobacter pylori infection. infection.

Lower gastrointestinal tractThe lower GI tract comprises the intestines and anus.1. Bowel or intestine:

small intestine, which has three parts:

A. duodenum

B. jejunum C. ileum

large intestine, which has three parts:

A.cecum (the vermiform appendix is attached to the cecum).

B.colon (ascending colon, transverse colon, descending colon and sigmoid flexure)

C. rectum 2. anus

The intestinal tract can be broadly divided into two different parts, the small and large intestine. Grayish-purple in color, the small intestine is the first and longest, measuring

twenty feet on average in an adult man.

The lumen is the cavity where digested material passes through and from

where nutrients are absorbed. Along the whole

length of the small intestine are goblet cells. These secrete mucus which

lubricates the passage of food along and protects it from digestive enzymes. Villi are extensions of the mucosa and increase the overall surface area of the

intestine. Micro villi are present on the epithelium of a villus and further increase the surface area over which absorption can take place.

The small intestine is divided into three structural parts:A. duodenum 25 cm (9.84 in) in length B. jejunum 2.5 m (8.2 ft) C. ileum 3.5 m (11.5 ft)

Although the small intestine is much longer than the large intestine (typically 4-5 times longer), it is referred to as such due to its comparatively smaller

diameter. On average, the diameter of the small intestine of an adult human measures

approximately 2.5-3cm, and the large intestine measures about 7.6 cm in diameter.

Absorption of the majority of nutrients takes place in the

jejunum, with the following notable exceptions:

Iron is absorbed in the duodenum Vitamin B12 is absorbed in the

terminal ileum. Water and lipids are absorbed by

passive diffusion throughout

The digestion of proteins into peptides and amino

acids principally occurs in the stomach but some also

occurs in the small intestine. The small

intestine is where the most chemical digestion

takes place:

lipids (fats) are degraded into fatty acids and glycerol. Lipid

digestion is the sole responsibility of the small intestine. Pancreatic lipase

breaks down triglycerides into free fatty acids and monoglycerides.

carbohydrates are degraded into simple sugars (e.g., glucose). In

the small intestine pancreatic amylase breaks down

carbohydrates into oligosaccharides.

Chemical digestion in the duodenum

The duodenum is largely responsible for the breakdown of food in the small intestine.

The pH in the jejunum is usually between 7 and 8 (neutral or slightly alkaline).

The jejunum and the ileum are suspended by mesentery which gives the bowel great mobility within

the abdomen. It also contains muscles to help move the food along.

The function of the ileum is to absorb the products

of digestion. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi, on its surface. In turn, the

epithelial cells which line these villi possess even

larger numbers of microvilli. Therefore the ileum has an extremely

large surface area both for the absorption

(attachment) of enzyme molecules and for the

absorption of products of digestion.

Large Intestine: Parts and locationParts of the large intestine are:Cecum - the first part of the large intestine Vermiform appendix - small appendage on the cecum containing lymphatic tissue

Locations along the colon are:The ascending colon The transverse colon The descending colon The sigmoid colon - the s shaped region of the large intestine

Rectum Anal canal - last segment of the large intestine Internal anal sphincter - involuntary, smooth muscle External anal sphincter - voluntary, skeletal muscle Anus

The large intestine, or the organ which is now more

commonly referred to as the colon, is the last part of the digestive system: the final

stage of the alimentary canal. Its function is to absorb the

remaining water from indigestible food matter, store

these unusable food matter (wastes) and then eliminates

the wastes from the body.

The large intestine hosts several kinds of bacteria (i.e. ecoli) that deal with

molecules the human body is not able to

breakdown itself. This is an example of symbiosis.

These bacteria also account for the

production of gases inside our intestine (this

gas is released as flatulence when removed through the anus). The large intestine houses

over 700 species of bacteria that perform a

variety of functions including synthesis of

vitamin B complex vitamins and vitamin K.

The large intestine takes 12 to 24 hours to finish up the remaining processes of the digestive system. Food is not

broken down any further in this stage of digestion. The large intestine absorbs vitamins that are created by the bacteria.

The most obvious difference between the small intestine and the large intestine is that the large intestine is wider and

lacks villi.

Medical literature shows that the appendix is rich

in infection-fighting lymphoid cells, and therefore, has an important role in

immunity. Appendicitis is the result of a blockage

that traps infectious material in the lumen. The appendix can be

removed with no damage or consequence to the

patient

The cecum is a pouch

connected to the ascending colon of the

large intestine and the ileum.

It is considered to

be the beginning of

the large intestine.

Ascending colonThe ascending colon, on the right side of the abdomen, is about 12.5 cm long.

Transverse colonThe transverse colon hangs off the stomach, attached to it by a wide band of tissue called the greater omentum. More cancers form as the large intestine goes along and the contents become more solid (water is removed) in order to form feces.

Descending colonThe descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon.

Sigmoid colonThe sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped. The walls of the sigmoid colon are muscular, and contract to increase the pressure inside the colon, causing the stool to move into the rectum.Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon.

The rectum is the final straight portion of the large intestine, terminating in the anus. The human rectum is about 12 cm

long. The rectum intestinum acts as a

temporary storage facility for feces. As the rectal walls expand due to the

materials filling it from within, stretch receptors from the nervous system

located in the rectal walls stimulate the desire to defecate. If the urge is not

acted upon, the material in the rectum is often returned to the colon where more

water is absorbed. If defecation is delayed for a prolonged period,

constipation and hardened feces results.The rectum shortens as material is

forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external

sphincter allow the feces to be passed by muscles pulling the anus up over the

exiting feces.

Anus- terminal end of the digestive system

Accessory Organs of the Digestive Accessory Organs of the Digestive systemsystem

The liver is an organ that plays a major role in metabolism and has

a number of functions in the body, including glycogen

storage, protein synthesis, and drug detoxification. This organ also is the largest gland in the

human body. It produces bile, an alkaline compound which aids in

digestion.

The liver is the second largest organ (the largest organ being the skin). The liver lies on the right of the stomach and makes a kind of bed for the gallbladder (which stores bile).

The bile produced in the liver is collected in bile

canaliculi, which merge to form bile ducts.

The gallbladder stores about 50 ml of bile (or "gall") until the

body needs it for digestion. The bile, produced in the liver,

emulsifies fats and neutralizes acids in partly digested food.

After being stored in the gallbladder, the bile becomes

more concentrated than when it left the liver, increasing its potency and intensifying its

effect on fats.

The pancreas is an organ in the digestive and endocrine system. It is both exocrine (secreting pancreatic juice

containing digestive enzymes) and endocrine (producing several important hormones, including insulin and

glucagon).

Excretory System and Excretory System and organsorgans

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In anatomy, the kidneys are bean-shaped excretory organs. Part of the urinary system, the kidneys filter

wastes (such as urea) from the blood and excrete them, along with water, as urine. The medical field that studies

the kidneys and diseases of the kidney is called nephrology.The basic functional unit of the

kidney is the nephron, of which there are more than a million within the cortex and medulla of each normal adult

human kidney. Nephrons regulate water and soluble

matter (especially electrolytes) in the body by

first filtering the blood under pressure, and then

reabsorbing some necessary fluid and molecules back into

the blood while secreting other, unneeded molecules.

Nephron – Nephron – Glomerulus/Bowman’s Glomerulus/Bowman’s

CapsuleCapsuleA A glomerulusglomerulus is a capillary is a capillary

tuft surrounded by tuft surrounded by Bowman's capsuleBowman's capsule in in

nephrons of the kidney. The nephrons of the kidney. The resistance of the arterioles resistance of the arterioles results in high pressure in results in high pressure in the glomerulus aiding the the glomerulus aiding the process of process of ultrafiltrationultrafiltration where fluids and soluble where fluids and soluble materials in the blood are materials in the blood are

forced out of the capillaries forced out of the capillaries and into Bowman's capsule.and into Bowman's capsule.

A glomerulus and its A glomerulus and its surrounding Bowman's surrounding Bowman's

capsule constitute a capsule constitute a renal renal corpusclecorpuscle, the basic filtration , the basic filtration

unit of the kidney. unit of the kidney.

The kidney is one of the major organs involved in whole-body

homeostasis. Among its homeostatic functions are acid-

base balance, regulation of electrolyte concentrations,

control of blood volume, and regulation of blood pressure. The kidneys accomplish these

homeostatic functions independently and through

coordination with other organs, particularly those of the

endocrine system. The kidney communicates with these organs through hormones

secreted into the bloodstream.

Congenital absence of one or both kidneys, known as unilateral or bilateral renal agenesis can occur.

The ureters are the ducts that carry urine from the

kidneys to the urinary bladder. The ureters are muscular tubes that can propel urine along by the motions of peristalsis. In the adult, the ureters are

usually 25-30cm long.The urinary bladder is a hollow, muscular, and

distensible (or elastic) organ that sits on the pelvic floor. It is

the organ that collects urine excreted by the kidneys prior to disposal by urination. Urine

enters the bladder via the ureters.The urethra is a tube which connects the urinary bladder to

the outside of the body. The urethra has an excretory function in both genders to pass urine to the outside, and also a

reproductive function in the male, as a passage for sperm. The external urethral sphincter is a smooth muscle that allows

voluntary control over urination.