The Digestive System Part 2

Bile functions

1. required for digestion and absorption of lipids(hydrophobic) bile is emulsifier for lipids-breaks up into bubbles, allows lipids to go into suspension, increases surface area so enzymes can reach it 2. processes toxins that leave liver via bile and excretes as waste

1. the large intestine receives material from the small intestine that was not digested or absorbed, and is a passageway for feces to exit the body

2. the large intestine is active in absorbing water and electrolytes which is critical for maintaining fluid, electrolyte, and acid-base homeostasis

The Small Intestine

6 meter long small bowel is the longest portion of the alimentary canal. 4 main processes that occur in the small intestine 1. secretion 2. digestion 3. absorption 4. propulsion Cells of the small intestine known as enterocytes produce multiple digestive enzymes, hormones, and mucus

2. the mucosa folds into projections called villi in which each villus consists of a layer of enterocytes surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal.

Between villi the mucosa indents to form intestinal crypts which house glands with both enteroendocrine cells and mucus secreting goblet cells

b. Hepatic arterioles and portal venues both drain into large, leaky capillaries called hepatic sinusoids that pass between rows of hepatocytes

Blood flows slowly through sinusoids as materials are exchanged between blood and hepatocytes, eventually draining into central vein central veins merge and drain into hepatic veins which in turn feed into inferior vena cava

1. The largest type of folds, visible with the naked eye as ridges in the wall, are called circular folds or plicae circulars

Circular folds involve both the mucosa and submucosa of the small intestine, which increase surface area. these folds also slow down the transit of chyme through the small intestine, which gives the nutrients more time to be digested and the small intestine cells called enterocytes more time to absorb nutrients

3. the smallest folds, the microvilli, are found in the plasma membrane of the enterocytes.

Each enterocyte has as many as 3000 microvilli which gives the cell the appearance of a bristle brush, or brush border. digestive enzymes produced and secreted by enterocytes catalyze reactions that break down peptides

2. The small intestine undergoes 2 types of movement during eating: peristalsis and segmentation

The vagus nerve appears to regulate both peristalsis an segmentation

The Pancreas, Liver, and Gallbladder

accessory organs an exocrine glands that secrete a product through a duct to the outside of the body secrete their products into the alimentary canal, which appears to be within the body, but the alimentary canal is open to the external environment on each end

the common bile duct joins the main pancreatic duct near the duodenum to form the hepatopancreatic ampulla.

ampulla surrounded by a ring of smooth muscle called the hepatopancreatic sphincter, which controls the emptying of bile and pancreatic fluids into the duodenum. the contents of the hepatopancreatic ampulla empty into the duodenum at the major duodenal papilla.

proximal large intestine consists of

ascending and transverse colon site of water and electrolyte absorption and bacterial activity exhibits segmentation or churning and mass movement of or mass peristalsis

Bacteria in the large intestine

bacteria in the large intestine hosts a staggering number of bacteria; bacteria make up as much as 60% of the dry mass of feces

2. Middle jejunum

begins at the duodenojejunal flexure and sits within the peritoneal cavity measures 7.5 feet long and is most active site for chemical digestion and absorption

1. During fasting, the small intestine exhibits slow, rhythmic contractions along its length in a pattern called the migrating motor complex what is the function of this complex?

clear any remaining food, remove secretions, duodenum to ileocecal valve takes 2 hours

pancreatic juice

collective secretions of the pancreatic acinar and duct cells water, digestive enzymes, proteins duct cells secrete bicarbonate ions a base which make pancreatic juice alkaline alkaline juice helps neutralize the acidic chyme that enters the duodenum from the stomach and protects the duodenum from damage by acid

Liver anatomy

covered by thin connective tissue capsule and visceral peritoneum, is composed of 4 lobes: right,left,caudate, and quadrate

distal large intestine consists of

descending and sigmoid colon, rectum, and anal canal absorption of water stores fecal material until ready to be expelled during defacation when mass movements force fecal matter into the normally empty rectum, it initiates the parasympathetic-mediated defacation reflex

Divisions of the small intestine

duodenum, jejunum, ileum which are distinguishable by histological differences

a. portal triad

found at each of 6 corners of lobule composed of hepatic arteriole,portal venule, and a small bile duct that carries bile.

pancreas

found in left upper quadrant composed of 3 regions main pancreatic duct travel down the middle of the pancreas where it receives secretions from acinar cells

Gallbladder vs liver

gallbladder sits on posterior liver and receives most of the bile from the common hepatic duct releases bile into cystic duct which joins the common hepatic duct to form the common bile duct

Pancreas

gland with endocrine and exocrine functions

1. Duodenum

initial segment of small intestine that begins at pylorus and is the shortest of the 3 divisions measuring 10 inches long houses the major duodenal papilla which is where secretions from the gallbladder and pancreas enter the small intestine duodenal submucosa contains specialized glands called duodenal (Brunner's) glans, which produce alkaline mucus to protect the duodenum from the acidic chyme

The Large Intestine

large bowel, runs along the border of the abdominal cavity, surrounding the small intestine and other abdominal organs like a frame

Functions of the liver

most diverse set of functions releases endocrine and exocrine secretions and converts harmful chemicals into nontoxic substances that the body can eliminate main function is to produce bile which contains water, electrolytes,and organic compounds

liver

right upper quadrant inferior to diaphragm

Motility of the small intestine

small intestinal motility differs during fasting and eating in the following way

3. ileum

small intestine's final segment intraperitoneal measures 3.6 meters long terminates at the portion of the large intestine called the secum the ileocecal valve is a sphincter that controls the movement of materials from the small intestine into the large intestine and prevents backflow of bacteria into small intestine

gallbladder

small sac found on posterior liver

Gross anatomy of the large intestine

the different regions of the large intestine are cecum with vermiform appendix, colon ascending, transverse, descending and sigmoid, rectum, and anal sphincter

Structure and function of the small intestine

the internal surface of the small intestine contains 3 progressively smaller types of folds most of the absorption that occurs within the alimentary canal takes place in this region, and these folds increase the surface area available for absorption about 400 to 600 times

Motility of the large intestine and defacation

the large intestine has 2 functional segments: proximal and distal

Liver histology

the liver lobule, the basic unit of liver, lobules are separated from one another by septa that branch in from the connective tissue capsule of the liver lobules composed of flattened plates of cells, called hepatocytes, arranged in shape of a hexagon and stacked on top of each other with a small central vein

Histology of the large intestine

the mucosa of the large intestine lacks villa and its cells lack microvilli. the muscularis externa of the large intestine is unique in that its longitudinal layer is not continuous throughout most of its length this layer is gathered into 3 bands or ribbons of muscle called taeniae coli Their constant tension bunches the colon into pockets referred to as haustrum. the serosa,or visceral peritoneum, contains fat filled pouches known as epiploic appendages