SECTION 3, CHAPTER 17

THE DIGESTIVE SYSTEM

The Liver• The liver is the largest internal organ

• It is located in the upper-right abdominal quadrant just beneath the diaphragm

The Liver Anatomy

The liver contains two major lobes and two minor lobes:

• The Right Lobe and Left Lobe are separated by a falciform ligament.

• A coronary ligament attaches the liver to the diaphragm

Falciform ligament

The two minor lobes are on the inferior surface of the right lobe:

The Quadrate lobe is located on the anterior inferior surface, next to the gall bladder.

The Liver Anatomy

The Caudate lobe is on the posterior inferior surface, next to the inferior vena cava

The Liver HistologyHepatic Lobules form the functional unit of the liver.

• Plates of Hepatocytes (liver cells) radiating outward from the central vein.

• Hepatic sinusoids - vascular channels through which blood and bile flow.

• Triads– group of three vessels:• A branch of the hepatic portal vein.• A branch of the hepatic artery• A bile duct.

Each lobule contains:

• A central vein

• Kupffer Cells – remain fixed to the inner lining (endothelium) of sinusoidsKupffer cells remove bacteria from blood by phagocytosis

Figure 17.27 (a) cross section of a hepatic lobule.

The Liver Histology

Figure 17.27 (b) Enlarged longitudinal section o f a hepatic lobule.

The Liver Histology

• Branches of the hepatic portal vein bring nutrient rich blood into the sinusoids to be processed by hepatocytes.

• Branches of the hepatic artery supply oxygen rich blood to cells throughout the liver.

• Hepatocytes secrete bile into bile canaliculi, which carry bile to the bile duct.

Figure 17.28 The Paths of blood and bile in a hepatic lobule.

Blood and bile flow through the sinusoids in opposite directions. Blood flows from the triads towards the central vein, while bile flows towards the triads.

Liver Functions• The liver carries on many important metabolic activities, including:

• Glycogenesis: producing glycogen from glucose

• Glycolysis: breakdown of glycogen into glucose

• Gluconeogenesis: converts non-carbohydrates to glucose

• Oxidizes fatty acids

• Synthesizes phospholipids and cholesterol

• Forms urea

• Synthesizes plasma proteins

• Stores glycogen, iron, and vitamins A, D, and B12

• Phagocytosis of worn out RBCs and foreign substances

• Removes toxins such as alcohol and certain drugs from the blood

•Bile contains:• Water

• Bile pigments (bilirubin & biliverdin)• Cholesterol• Electrolytes

•Bile salts: •Emulsify fats (break fat globules into droplets)•Emulsification enhances the digestion and absorption of fats.•Helps absorb fatty acids, cholesterol, and fat-soluble vitamins

Composition of Bile

Gall Bladder

The gall bladder is a pear-shaped sac within a depression on the inferior surface of the liver.

• It stores bile between meals• It concentrates bile by reabsorbing water• The gall bladder itself does not produce bile.

Figure 17.30 Fatty chyme entering the duodenum stimulates the secretion of cholecystokinin (CCK). CCK stimulates the release of bile from the gall bladder, and it stimulates the secretion of pancreatic juices rich in digestive enzymes.

The Small Intestine

The small intestine extends from the pyloric sphincter to the large intestine.

Functions of the small intestine:

In a cadaver, the small intestine measures up to a full 6 meters (20 feet)!Fortunately, muscle tone in life keeps the intestine at about half that length.

• It completes digestion

• Absorbs nutrients

• Transports the remaining residue to the large intestine.

Parts of the Small IntestineThe small intestine consists of three portions:

Duodenum (25cm length)• Short “C” shaped portion• Posterior to the parietal peritoneum (retroperitoneal)

Jejunum (8 feet length)

Ileum (10 feet length)

• Similar to jejunum in structure• Contains Peyer’s patches• Houses many bacteria populations

The jejunum & ileum are suspended from the posterior abdominal wall by mesentery (fold of the peritoneum)

Parts of the Small Intestine

Greater Omentum – fold of the peritoneal membrane that drapes like an apron from the stomach over the small intestine.

Structure of the Intestinal Wall

• The inner wall of the small intestine has a velvety appearance due to many tiny projections of the mucosa, called intestinal villi.

• The lining of the small intestine contains many circular folds, called plicae circulares.

• Simple columnar epithelium that line the mucosa contain fine extensions of the plasma membrane, called microvilli (brush-boarder).

Figure 17.38 Section of the small intestine. Plicae circulares, villi, and microvilli greatly increase the surface area of the small intestine, enhancing absorption of nutrients.

Endoscopic view of the small intestine. The plicae circulares and villi are especially well developed in the duodenum and the jejunum. The amount of folding decreases near the ileum, since most nutrients have already been absorbed in the proximal small intestine.

Each villus contains a blood capillary network and a lymphatic capillary, called a lacteal.

Blood capillary network – nourish the intestine and carry away nutrients.

Lacteals – absorb fats.

Intestinal glands (crypts) – • Secrete various enzymes• Site of new epithelium production (cells turnover every few days)

Villus

Figure 17.35 Structure of a single intestinal villus.

Figure 17.37 Intestinal epithelium. (a) Microvilli increase the surface area of intestinal epithelial cells. (b) Transmission electron micrograph of microvilli (16,000x)

Secretions in the small intestine

Brunner’s glands – located within the duodenum• Secrete alkaline mucus, which neutralizes acidic chyme.

Digestive Enzymes• Peptidases – breakdown peptides into amino acids• Sucrase – splits sucrose (disaccharide) into monosaccharides• Lactase – splits lactose (disaccharide) into monosaccharides• Intestinal lipase – splits lipids into fatty acids and glycerol.• Enterokinase – converts trypsinogen into trypsin

Hormones• Cholecystokinin – Stimulates secretions from the pancreas and the gall bladder

• Somatostatin – inhibits gastric juice secretions• Secretin – stimulates the secretion of alkaline juice from the pancreas

Movements in the small intestine

• The small intestine carries on mixing movements that include:• Peristalsis – pushing movements that propel chyme• Segmentation – ring-like contractions that can move chyme back and forth

• Ileocecal Sphincter – joins the ileum to the large intestine.• Regulates the rate that contents enter the large intestine.

The Large Intestine The large intestine is named because of its diameter

It has four parts:

1. Cecum – blind pouch• Ileocecal sphincter • The appendix projects from the cecum.

2. Colon (ascending, transverse, descending, sigmoid)

3. Rectum - straight

4. Anal canal

• The anus – is guarded by two sphincter muscles.internal anal sphincter – composed of smooth muscle

external anal sphincter – composed of skeletal muscle

Muscle fibers (teniae coli) extend the length of the colon.

Haustra – pouches created by the tension of teniae coli.

Functions of the Large Intestine

The large intestine has little or no digestive function

• It absorbs water and electrolytes• Secretes mucus

Functions Include:

• Houses intestinal flora – bacteria• Breakdown molecules our cells cannot digest (eg. cellulose)• Synthesizes vitamins K and B12

• Forms feces• Carries out defecation

The pungent odor of flatus (intestinal gas) is produced by bacterial compounds including ammonia and hydrogen sulfide.

Feces is composed of materials not digested or absorbed, and include: Water, Electrolytes, Mucus, Bacteria, and Bile pigments

Feces

The color of feces is provided by bile pigments altered by bacteria.

Approximately 1/3 of the dry weight of feces is bacteria.

End of Section 3, Chapter 17