Digestive System

6.1 Digestive Functions

The food we eat is a combination of a huge assortment of molecules

Our system Breaks these molecules down via physical and chemical digestive processes to obtain forms that can be absorbed and used in our body

The parts that can not be used are released

The digestive functions are:

Ingestion

Physical and chemical digestion

Absorption

Defecation

6.2 Digestive Structures

Imagine the digestive system as a long tube (around 7 m long) contained within the body

Food enters one end, the mouth, is processed throughout its length; nutrients absorbed, and wastes are released from the other end, the anus

Much of the treatment that food is exposed to comes from the specializations of this tube, the alimentary canal (gastrointestinal tract)

Its muscular walls control the movement of materials through it; glands along its walls release water, enzymes, and some hormones; specialized structures absorb nutrients

The pancreas and the liver are attached to the alimentary canal and produce secretions that are used in the digestive process

Both of these secretions enter at the duodenum

Pancreas

The pancreas is located behind the stomach

It is specialized both as an endocrine and exocrine gland

It is an endocrine gland because it produces and secretes the hormones insulin and glucagon from cells called islets of Langerhans into the blood stream

Insulin lowers blood sugar levels by increasing cell’s ability to use it and promoting its storage in the form of glycogen by the liver

Glucagon allows the liver to release glucose from glycogen when blood sugar levels drop

Together, these hormones help regulate the blood’s level of glucose

The pancreas is an exocrine gland because it produces the digestive juice called pancreatic juice, which it releases to the duodenum through the pancreatic duct

Pancreatic juice is a combination of water, sodium bicarbonate, lipase, trypsinogen (which becomes trypsin), nucleases, and pancreatic amylase

Liver

The liver is the bodies largest internal organ

It is located behind the diaphragm

The liver is richly supplied with blood, and has the following functions:

1) Regulate the glucose level in blood

Glucose should comprise about 0.1% of blood plasma

If the level is too high, glucose will be converted to and stored as glycogen by the liver

If it is too low, glycogen breaks down into glucose

2) Produce urea

Body cells produce ammonia, a toxic by-product from the metabolism of amino acids and nucleotides

It is transported though blood to the liver where it is converted into, the less toxic, urea

Urea is removed from the blood by the kidneys during urine formation

3) Detoxify blood

The liver destroys other toxins in the blood (like alcohol)

The liver metabolizes alcohol into fatty acids, which increases the fatty nature of liver tissue

Removes and stores fat-soluble vitamins, which have toxic effects on cells when in high concentrations

4) Produce bile

Bile is stored in the gall bladder until a hormone signals its release

Bile emulsifies fat clusters into smaller pieces

This increases their surface area, increasing the efficiency of the lipase enzyme, which chemically breaks them down into fatty acids and glycerol

5) Produce blood proteins (globulins)

Albumin (helps maintain osmotic pressure)

Fibrinogen

Prothrombin (needed for blood clotting)

6) Destroy old red blood cells

RBC have an average life span of 4 months (millions die every day)

The liver disassembles the dead RBC and recycles the useful components

For example, iron from hemoglobin is returned to bone marrow for the manufacture of new RBC; the rest of the hemoglobin is worn out and added to bile to be excreted through the digestive system

Review

The function of the gall bladder is to

a) Store bile

b) Synthesize lipids

c) Produce enzymes

d) Stimulate the release of pancreatic juice

Review

The function of the gall bladder is to

a) Store bile

b) Synthesize lipids

c) Produce enzymes

d) Stimulate the release of pancreatic juice

Glucose is stored in the liver as

a) Fat

b) Starch

c) Protein

d) Glycogen

Glucose is stored in the liver as

a) Fat

b) Starch

c) Protein

d) Glycogen

The release of insulin is triggered by

a) Low levels of blood sugar

b) High levels of blood sugar

c) Low amounts of sugar in one’s diet

d) High amounts of sugar in one’s diet

The release of insulin is triggered by

a) Low levels of blood sugar

b) High levels of blood sugar

c) Low amounts of sugar in one’s diet

d) High amounts of sugar in one’s diet

One function of the liver is the production of

a) Urea

b) Lipase

c) Mucus

d) Hydrochloric acid

One function of the liver is the production of

a) Urea

b) Lipase

c) Mucus

d) Hydrochloric acid

Which of the following pancreatic secretions doesn’t enter the duodenum?

a) Insulin

b) Lipase

c) Trypsin

d) Bicarbonate ions

Which of the following pancreatic secretions doesn’t enter the duodenum?

a) Insulin

b) Lipase

c) Trypsin

d) Bicarbonate ions

MOUTHSTOMACH

SMALL INTESTINESCOLON (LARGE INTESTINES)

Processes of Digestion

6.3 Mouth

First, food is chewed up (physical digestion), increasing the surface area so that chemical digestion can take place faster.

Salivary glands release saliva (water and salivary amylase, also known as ptyalin) through ducts into the mouth

Water is used as a lubricant to aid in swallowing food and as a reactant in hydrolytic reactions that occur during digestion

Salivary amylase (or ptyalin) breaks the bonds between the sugar molecules of starch to begin its chemical conversion to maltose

The tongue is a muscle that rolls chewed food into lumps (bolus) and pushes them to the back of the mouth for swallowing

Swallowing is a reflexive action resulting from contractions of the two layers of smooth muscles that line the pharynx and esophagus

The pharynx is a common passageway for both food and air

In order to prevent food materials from going down the trachea, the epiglottis covers it

The muscle contractions called peristalsis move each bolus down the esophagus

There is a constriction called the cardiac sphincter at the base of the esophagus that must relax and open before the food materials can enter the stomach

6.4 Stomach

The stomach is a large J-shaped organ with walls that have three layers of muscle that churn food material , causing the release of gastrin (a hormone that travels from the cells of the stomach walls into the blood stream

As gastrin circulates the body, it affects the stomach and causes the release of gastric juices

Gastric juice contains water as well as HCl and pepsinogen

At this point the bolus becomes known as acid chyme (runny)

The HCl creates an environment with a low pH to kill any bacterial growth that may be in the food and reacts with pepsinogen, converting it to pepsin (which converts proteins into polypeptides)

Pepsin is secreted as pepsinogen because a cell would not survive the production of it in its active form

The inner walls of the stomach also produce mucus to protect themselves from the acid chyme

Bacterial infection of the mucosal lining by Helicobacter pylori can lead to an ulcer

Review

Which of the following digestive enzymes is NOT correctly matched with its substrate?

a) Lipase – fat

b) Pepsin – protein

c) Trypsin – nucleic acid

d) Salivary amylase - starch

Review

Which of the following digestive enzymes is NOT correctly matched with its substrate?

a) Lipase – fat

b) Pepsin – protein

c) Trypsin – nucleic acid

d) Salivary amylase - starch

The part of the digestive tract where starch first undergoes chemical digestion is the

a) Mouth

b) Stomach

c) Small intestine

d) Large intestine

The part of the digestive tract where starch first undergoes chemical digestion is the

a) Mouth

b) Stomach

c) Small intestine

d) Large intestine

When salivary amylase enters the stomach, it becomes

a) Basic

b) Acidic

c) Buffered

d) denatured

When salivary amylase enters the stomach, it becomes

a) Basic

b) Acidic

c) Buffered

d) denatured

Which region has openings for both the trachea and esophagus?

a) Throat

b) Pharynx

c) Stomach

d) epiglottis

Which region has openings for both the trachea and esophagus?

a) Throat

b) Pharynx

c) Stomach

d) epiglottis

Which of the following correctly matches a digestive enzyme with its source?

a) Pepsin – pancreas

b) Bile – gall bladder

c) Trypsin – stomach

d) Amylase - pancreas

Which of the following correctly matches a digestive enzyme with its source?

a) Pepsin – pancreas

b) Bile – gall bladder

c) Trypsin – stomach

d) Amylase - pancreas

6.5 Small Intestines

The pyloric sphincter is similar to the cardiac sphincter, except it is located at the posterior of the stomach where it controls the passage of the liquid acid chyme into the duodenum (the first part of the small intestines)

The duodenum is specialized by the presence of chemoreceptors (chemical sensitive nerve endings that are able to detect the different biochemicals in the food material), that regulate which secretions are released

When signaled to, the gall bladder releases the fat emulsifier bile and the pancreas releases pancreatic juice

The biocarbonate ions (HCO3-) “over-

neutralize” the acid chyme and buffer it at a pH of about 8.3

The enzymatic components of pancreatic juice are active at this alkaline pH

Lipase converts emulsified lipids into fatty acids and glycerol

Trypsin breaks many peptide bonds to convert various lengths of polypeptides into shorter peptide units

Pancreatic amylase breaks remaining starch into maltose

Nuclease converts some of the nucleic acids into nucleotides and nucleotidases (which break nucleotides into their component molecules)

All of these enzymes are active in the duodenum

The duodenum also produce and secrete its own enzymes

Disaccharidases (maltase and lactase)Peptidase (to break remaining polypeptides

into amino acids)nuclease

The longest part of the small intestines is called the ileum (absorbs the products of digestion)

It has a large surface area and is lined with specialized structures for absorption by active trasnport

These structures are called villi

The epithelial cells lining the villi are equipped with mitochondria to produce the required ATP

Internally, each villus has a capillary bed and a lacteal (an absorptive end of the lymphatic system)

The products of fat digestion are reconstructed into neutral fats by the epithelial cells before they enter the lacteals, the rest of the products of digestion enter the blood stream

6.6 Colon

The undigested food move from the ileum through the ileo-caecal valve (another sphincter) to the large intestine (colon)

The first segment of the colon is the caecum, then the appendix (a vestigial structure)

The major roles of the colon are to absorb a great amount of the water and store E. coli

E. coli is a bacterium that metabolizes some of what our body cannot (waste material)

Their metabolism releases minerals and manufactures some vitamins and amino acids

These nutrients (from E. coli) get absorbed along with water into the circulatory system

The bacteria begin the decomposition of the waste materials and convert them to feces with a changed color, smell, and texture

The last part of the colon is the rectum that stores feces until defecation

It ends with the anal sphincter, which controls defecation

Review

The bacterium E. coli is normally abundant in

a) Food

b) Feces

c) Bile salts

d) Stomach juices

Review

The bacterium E. coli is normally abundant in

a) Food

b) Feces

c) Bile salts

d) Stomach juices

Which of the following is absorbed into lacteals?

a) Glucose

b) Fatty acids

c) Nucleotides

d) Amino acids

Which of the following is absorbed into lacteals?

a) Glucose

b) Fatty acids

c) Nucleotides

d) Amino acids

Which digestive organ is the appendix attached to?

a) The stomach

b) The gall bladder

c) The large intestine

d) The small intestine

Which digestive organ is the appendix attached to?

a) The stomach

b) The gall bladder

c) The large intestine

d) The small intestine

The removal of a large section of the colon is likely to affect the

a) Release of bile

b) Digestion of fats

c) Absorption of glucose

d) Water balance in the body

The removal of a large section of the colon is likely to affect the

a) Release of bile

b) Digestion of fats

c) Absorption of glucose

d) Water balance in the body

Which of the following structures does NOT produce digestive enzymes?

a) Stomach

b) Pancreas

c) Small intestine

d) Large intestine

Which of the following structures does NOT produce digestive enzymes?

a) Stomach

b) Pancreas

c) Small intestine

d) Large intestine