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Chemical digestion

Although the glands of the large intestine secrete mucus, they do not secrete digestive enzymes. Therefore, chemical digestion in the large intestine occurs exclusively because of bacteria in the lumen of the colon. Through the process of saccharolytic fermentation    , bacteria break down some of the remaining carbohydrates. This results in the discharge of hydrogen, carbon dioxide, and methane gases that create flatus    (gas) in the colon; flatulence is excessive flatus. Each day, up to 1500 mL of flatus is produced in the colon. More is produced when you eat foods such as beans, which are rich in otherwise indigestible sugars and complex carbohydrates like soluble dietary fiber.

Absorption, feces formation, and defecation

The small intestine absorbs about 90 percent of the water you ingest (either as liquid or within solid food). The large intestine absorbs most of the remaining water, a process that converts the liquid chyme residue into semisolid feces    (“stool”). Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body. Of every 500 mL (17 ounces) of food residue that enters the cecum each day, about 150 mL (5 ounces) become feces.

Feces are eliminated through contractions of the rectal muscles. You help this process by a voluntary procedure called Valsalva’s maneuver    , in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis.

The process of defecation begins when mass movements force feces from the colon into the rectum, stretching the rectal wall and provoking the defecation reflex, which eliminates feces from the rectum. This parasympathetic reflex is mediated by the spinal cord. It contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and initially contracts the external anal sphincter. The presence of feces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed. If you decide to delay defecation, it takes a few seconds for the reflex contractions to stop and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate.

If defecation is delayed for an extended time, additional water is absorbed, making the feces firmer and potentially leading to constipation. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and diarrhea can result. This can be caused by the ingestion of foodborne pathogens. In general, diet, health, and stress determine the frequency of bowel movements. The number of bowel movements varies greatly between individuals, ranging from two or three per day to three or four per week.

By watching this animation you will see that for the various food groups—proteins, fats, and carbohydrates—digestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine?

Chapter review

The three main regions of the small intestine are the duodenum, the jejunum, and the ileum. The small intestine is where digestion is completed and virtually all absorption occurs. These two activities are facilitated by structural adaptations that increase the mucosal surface area by 600-fold, including circular folds, villi, and microvilli. There are around 200 million microvilli per square millimeter of small intestine, which contain brush border enzymes that complete the digestion of carbohydrates and proteins. Combined with pancreatic juice, intestinal juice provides the liquid medium needed to further digest and absorb substances from chyme. The small intestine is also the site of unique mechanical digestive movements. Segmentation moves the chyme back and forth, increasing mixing and opportunities for absorption. Migrating motility complexes propel the residual chyme toward the large intestine.

The main regions of the large intestine are the cecum, the colon, and the rectum. The large intestine absorbs water and forms feces, and is responsible for defecation. Bacterial flora break down additional carbohydrate residue, and synthesize certain vitamins. The mucosa of the large intestinal wall is generously endowed with goblet cells, which secrete mucus that eases the passage of feces. The entry of feces into the rectum activates the defecation reflex.

Watch this animation that depicts the structure of the small intestine, and, in particular, the villi. Epithelial cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems. In the small intestine, the products of food digestion are absorbed by different structures in the villi. Which structure absorbs and transports fats?

Answers may vary.

Got questions? Get instant answers now!

By watching this animation , you will see that for the various food groups—proteins, fats, and carbohydrates—digestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine?

Answers may vary.

Got questions? Get instant answers now!

References

American Cancer Society (US). Cancer facts and figures: colorectal cancer: 2011–2013 [Internet]. c2013 [cited 2013 Apr 3]. Available from: (External Link) .

The Nutrition Source. Fiber and colon cancer: following the scientific trail [Internet]. Boston (MA): Harvard School of Public Health; c2012 [cited 2013 Apr 3]. Available from: (External Link) .

Centers for Disease Control and Prevention (US). Morbidity and mortality weekly report: notifiable diseases and mortality tables [Internet]. Atlanta (GA); [cited 2013 Apr 3]. Available from: (External Link) .

Questions & Answers

what is the consequence of performing CPR on a functioning heart.
Pyefa Reply
The central part of the body
Alex Reply
i thought a cell is the fuctional unit of an orgarnism.
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what is the name for inflammation of middle ear?
Madu
otitis media
Chidiebere
Explain the major features and properties of the cell membrane
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it is made up of protein,fat and a small portion of carbohydrates. it is semipermeable but impermeable to uncharged water molecules.
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Describe an experiment to verify the law of constant composition
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thx
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unimarwat
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Noah
What are blood vessels of will's circle
Madu Reply
vertebral arteries and cerebral arteries
Chidiebere
What are sutures of cranial cavity ?
Madu Reply
there are four. the coronal, sagittal, squamous and lambhoidal sutured
Chidiebere
structure of a cell
Robina
pelvic cavity contents?
unimarwat Reply
ilium ,ischium ,pubis
Madu
but that is the three parts of the hip bone
Chidiebere
Describe the muscular skeletal system in terms of definition.. Skeleton Apendicular skeleton Axial skeleton Joints
Sherrine Reply
the basic framework of body made and cartilage is called skeleton skeleton which form limbs is called appendicular skeleton skeleton which form main axis of body is called axial skeleton the points at two or more bones meets is called joints
unimarwat
write short notes on ligaments,curves and moverment of vertebral column.
mutesi Reply
cranial nerves notes
unimarwat
what is the Analysis
ROHIT Reply
what is Anatomy
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it's a organs and bouns reading
AJITH
Cutting Up
Nonie
Cutting Up
Nonie
to dissect
Nonie
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too much salt in the diet
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older age
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yes
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too much salt in the diet
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stress is the leading factor
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smoking and too much alcohol consumption
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obesity can also cause hypertension
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high temperature of the body high salt of the body
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sex
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Male
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guys, read the question, involuntary pumping of heart causes the blood pressure in the arteries, he has not asked about high or low BP.
Jess
explain the cellular mechanism that produces tetanus and summation
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epithelial cells polerity
jitendra Reply
tissues
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demonstrate the fluid replacement in the body
John Reply
the red blood cells is in the long bones or flat bones?
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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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