DIGESTIVE SYSTEM

Question 1

INGESTION

Answer 1

Selective in take of food and drink

Question 2

DIGESTION

Answer 2

Mechanical:
Physical breakdown of food into smaller pieces

Chemical:
Breakdown of food by digestive enzymes and acids
• CHO → Sugars
• Proteins → Amino acids
• Fats → Fatty acids
• Nucleic acids → Nucleotides

Question 3

ABSORPTION

Answer 3

Up take of nutrients into digestive cells, blood and lymph.

Question 4

SECRETION

Answer 4

H2O, HCl, salts, buffers, digestive enzymes

Question 5

PROPULSION

Answer 5

Propulsion: Moving food along the digestive tract from one part to the next

Question 6

DEFAECATION

Answer 6

Elimination of undigested material

Question 7

MUCOSA

Answer 7

Part of the structure of the alimentary canal. (GI tract)

• Innermost layer of the GI epithelium
• Surrounds the lumen
• Epithelium type varies depending on the organ function it lines
• Stomach mucosa→Secretion • Intestine→Absorption
• Major functions are:
• Secrete mucus/digestive enzymes/hormones
• Absorb the end products of digestion to blood
• Protect against infectious disease

Question 8

SUBMUCOSA

Answer 8

Part of the structure of the alimentary canal. (GI tract)

• Areolar connective tissue layer
• Highly Vascular: contains blood vessels
• Contains most of the GI Nerves:
• Regulating the movement of stomach/intestine/blood vessels vasoconstriction
• Attaches the mucosa layer to the muscularis layer

Question 9

MUSCULARIS

Answer 9

Part of the structure of the alimentary canal. (GI tract)

• Mainly smooth muscle except:
• Mouth + pharynx + upper region of the oesophagus
[Skeletal muscle → Voluntary swallowing]
• Anus (External anal sphincter)
[Skeletal muscle → Defecation] • Consists (generally) of 2 layers of
muscles:
• Inner circular layer
• Outer longitudinal layer
• Contraction of muscularis moves food along the digestive tract

Question 10

SEROSA/ADVENTITIA

Answer 10

Part of the structure of the alimentary canal. (GI tract)

The outermost tissue layer
• If present:
• It attaches the organ to its surrounding structures
• Secretes serous fluid (reduces friction as it moves over other organs in the abdominal cavity)

Those portions of the GI tract that are suspended in the abdominal cavity have a superficial layer called the serosa. As its name implies, the serosa is a serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium). The serosa is also called the visceral peritoneum because it forms a portion of the peritoneum, which we examine in detail shortly. The oesophagus lacks a serosa; instead, only a single layer of areolar connective tissue called the adventitia forms the superficial layer of this organ.

adventitia (ad-ven-TISH-a) The outermost covering of a structure or organ; the superficial coat of the ureters and the posterior and inferior surfaces of the urinary bladder.

Question 11

GI TRACT

Answer 11

gastrointestinal (gas-trō-in-TES-tin-al) (GI) tract A continuous tube running through the ventral body cavity extending from the mouth to the anus. Also called the alimentary (al′-i-MEN-tar-ē) -canal.

Question 12

TONGUE

Answer 12

Composed of skeletal muscle
• During chewing(mastication):
• Tongue grips food
• Moves food around the mouth
• Mixes food with saliva
• Forms a bolus
• Swallowing process:
• Tongue collects food
• Pushes it to the back of the mouth
• Down to the pharynx
• Speech

Manoeuvres food for mastication, shapes food into a bolus, manoeuvres food for deglutition, detects sensations for taste, and initiates digestion of triglycerides.

Question 13

PHARYNX

Answer 13

Common pathway of both the
• Respiratory system
• Digestive tract
• Connects the mouth to the oesophagus and trachea
Consists of Nasopharynx (superior), Oropharynx (middle) Layrngopharynx (inferior)

Question 14

OESOPHAGUS

Answer 14

• 25–30cm straight muscular tube
• Lined with stratified squamous epithelium
• Skeletal muscle in upper part and smooth muscle in lower part
• Extends from pharynx downward to the stomach, penetrating the diaphragm en route

The oesophagus secretes mucous and transports food into the stomach. It does not produce digestive enzymes, and it does not carry on absorption

Question 15

MASTICATION

Answer 15

To chew and grind up food with teeth.

Question 16

BOLUS

Answer 16

A soft, rounded mass, usually food, that is swallowed

Question 17

CHYME

Answer 17

The semifluid mixture of partly digested food and digestive secretions found in the stomach and small intestine during digestion of a meal.

Question 18

TEETH

Answer 18

Accessory structures of digestion, composed of calcified connective tissue and embedded in bony sockets of the mandible and maxilla, that cut, shred, crush, and grind food. Also called dentes (DEN-tēz).

Question 19

PEPSIN

Answer 19

Protein-digesting enzyme secreted by chief cells of the stomach in the inactive form pepsinogen, which is converted to active pepsin by hydrochloric acid.

The strongly acidic fluid of the stomach kills many microbes in food. HCl partially denatures (unfolds) proteins in food and stimulates the secretion of hormones that promote the flow of bile and pancreatic juice. Enzymatic digestion of proteins also begins in the stomach. The only proteolytic (protein-digesting) enzyme in the stomach is pepsin, which is secreted by chief cells. Pepsin severs certain peptide bonds between amino acids, breaking down a protein chain of many amino acids into smaller peptide fragments. Pepsin is most effective in the very acidic environment of the stomach (pH 2); it becomes inactive at a higher pH.

What keeps pepsin from digesting the protein in stomach cells along with the food? First, pepsin is secreted in an inactive form called pepsinogen; in this form, it cannot digest the proteins in the chief cells that produce it. Pepsinogen is not converted into active pepsin until it comes in contact with hydrochloric acid secreted by parietal cells or active pepsin molecules. Second, the stomach epithelial cells are protected from gastric juices by a layer 1–3 mm thick of alkaline mucous secreted by surface mucous cells and mucous neck cells.

Question 20

BILE

Answer 20

The liver synthesizes bile salts…
These are used in the small intestine for emulsification and absorption of lipids, cholesterol, phospholipids and lipoproteins

• ~800 – 1000ml produced per day → pH 7.6 – 8.6
• Bile is an emulsifying agent→physical breakdown of lipids
that exposes a greater surface area to lipases
• Bile is a yellow-green, alkaline fluid containing minerals, bile salts, cholesterol, triglycerides, bile pigments (bilirubin, biliverdin), phospholipids (lecithin) and electrolytes (Na, Cl, HCO3)
• Abnormal increase in bile salts in blood results in extraction by sweat glands, intense itching of the skin (pruritus) and interferes with absorption of water and salt

Question 21

LIVER

Answer 21

Digestive function is the production of bile (a fat emulsifier).

Located under the diaphragm in the upper right quadrant
• Consists of 4 lobes, which are made up of thousands of lobules
• These lobules are the functional units of the liver
• The liver can lose 3/4 of its cells (hepatocytes) before it stops functioning - the only organ in the body that can regenerate itself
• The associated gallbladder sits in a pocket on the ventral surface

Liver Blood Supply
There are two distinct sources of blood supply TO the liver:
• oxygenated blood flows in via the hepatic artery
• venous blood flows in via the hepatic portal vein carrying nutrients and toxic materials from the intestine, blood cells and their breakdown products from the spleen, and insulin and glucagon from the pancreas
• Sinusoids are vascular channels allowing exchange between blood and hepatocytes
• Blood exits each lobule by small veins which combine to carry blood AWAY from the liver via the hepatic vein

Liver Structure
• Bile produced in hepatocytes drains into canaliculi → exits the lobules via bile ducts
→ exits the liver via the hepatic duct
→ enters the duodenum via the common bile duct

Other functions of the liver
• Carbohydrate metabolism – maintains blood glucose via glycogenesis, glycogenolysis and gluconeogenesis
• Protein metabolism – synthesizes amino acids, transamination and deamination
• Lipid metabolism – produces triglycerides via lipogenesis, breakdown of fat to produce glycerol (for gluconeogenesis), synthesizes cholesterol
• Metabolism of toxic substances – medications, drugs, alcohol
• Storage – vitamins (A, D B12) and minerals (iron, copper, zinc)
• Hormone metabolism – breakdown of steroid hormones eg: oestrogen, testosterone, aldosterone, cortisol

Question 22

SALIVARY GLANDS

Answer 22

Small intrinsic glands found under mucous membrane of the mouth, lips, cheeks and tongue
• Parotid (near the ear)
• Submandibular (inferior and to the back of the sublingual gland)
• Sublingual (under the tounge)
• 1st secretion of GI tract (Saliva):
- 99.5% H2O
- 0.5% Solutes

Accessory Structure – Salivary Glands
Functions
• Moisten (mucus): aids in swallowing
• Begin carbohydrate digestion (amylase)
• Cleanse teeth; inhibit bacteria (lysozyme = enzyme that kills bacteria and immunoglobulin A = inhibits bacterial growth)
• Bind food together into a bolus
• Neutralise acidic foods: phosphate and bicarbonate ions (saliva pH of 6.8 - 7.0)
• Dissolve food: taste receptors

Saliva produced by these glands softens, moistens, and dissolves foods; cleanses mouth and teeth; initiates the digestion of starch.

Question 23

DUODENUM

Answer 23

Part of the small intestine
• Mostly retroperitoneal, first 25 cm direct from the stomach
• Curves around the head of the pancreas
• Receiving bile and pancreatic juice

Question 24

JEJUNUM

Answer 24

Part of the small intestine.

Next ~2.5m → between duodenum and ileum
• Covered with serosa and suspended by mesentery

Question 25

ILEUM

Answer 25

Part of the small intestine
• Last ~3.6m → joins large intestine at ileocecal valve
• Covered with serosa and suspended by mesentery

Question 26

LOBULES

Answer 26

Functional units of the liver.

Question 27

DIGLUTITION (SWALLOWING)

Answer 27

The movement of food from the mouth into the stomach is achieved by the act of swallowing, or deglutition (dē-gloo-TISH-un) Deglutition is facilitated by the secretion of saliva and mucous and involves the mouth, pharynx, and oesophagus. Swallowing occurs in three stages: (1) the voluntary stage, in which the bolus is passed into the oropharynx; (2) the pharyngeal stage, the involuntary passage of the bolus through the pharynx into the oesophagus; and (3) the oesophageal stage, the involuntary passage of the bolus through the oesophagus into the stomach.

Swallowing starts when the bolus is forced to the back of the oral cavity and into the oropharynx by the movement of the tongue upward and backward against the palate; these actions constitute the voluntary stage of swallowing. With the passage of the bolus into the oropharynx, the involuntary pharyngeal stage of swallowing begins (figure 24.10b). The bolus stimulates receptors in the oropharynx, which send impulses to the deglutition centre in the medulla oblongata and lower pons of the brain stem. The returning impulses cause the soft palate and uvula to move upward to close off the nasopharynx, which prevents swallowed foods and liquids from entering the nasal cavity. In addition, the epiglottis closes off the opening to the larynx, which prevents the bolus from entering the rest of the respiratory tract. The bolus moves through the oropharynx and the laryngopharynx. Once the upper oesophageal sphincter relaxes, the bolus moves into the oesophagus.

Deglutition (swallowing). During the pharyngeal stage (b) the tongue rises against the palate, thenasopharynx is closed off, the larynx rises, the epiglottis seals off the larynx, and the bolus ispassed into the oesophagus. During the oesophageal stage (c), food moves through the oesophagus into the stomach via peristalsis.
Deglutition is a mechanism that moves food from the mouth into the stomach.

Question 28

STOMACH

Answer 28

Main functions of the stomach:

• Mixes saliva, food and gastric juice to form chyme
• Serves as a reservoir for food before release into the small intestine
• Secretes gastric juice, which contains HCI (kills bacteria and denatures proteins), pepsin (begins the digestion of proteins), intrinsic factor (aids absorption of vitamin B12) and gastric lipase (aids digestion of triglycerides).
• Secretes gastric into blood.

The stomach is a J-shaped enlargement of the GI tract directly inferior to the diaphragm in the abdomen. The stomach connects the oesophagus to the duodenum, the first part of the small intestine (figure 24.11). Because a meal can be eaten much more quickly than the intestines can digest and absorb it, one of the functions of the stomach is to serve as a mixing chamber and holding reservoir. At appropriate intervals after food is ingested, the stomach forces a small quantity of material into the first portion of the small intestine. The position and size of the stomach vary continually; the diaphragm pushes it inferiorly with each inhalation and pulls it superiorly with each exhalation. Empty, it is about the size of a large sausage, but it is the most distensible part of the GI tract and can accommodate a large quantity of food. In the stomach, digestion of starch and triglycerides continues, digestion of proteins begins, the semisolid bolus is converted to a liquid, and certain substances are absorbed.

The stomach has four main regions: the cardia, fundus, body, and pyloric part (figure 24.11). The cardia (KAR-dē-a) surrounds the opening of the oesophagus into the stomach. The rounded portion superior to and to the left of the cardia is the fundus (FUN-dus). Inferior to the fundus is the large central portion of the stomach, the body. The pyloric part is divisible into three regions. The first region, the pyloric antrum, connects to the body of the stomach. The second region, the pyloric canal, leads to the third region, the pylorus (pī-LOR-us; pyl- = gate; -orus = guard), which in turn connects to the duodenum. When the stomach is empty, the mucosa lies in large folds, or rugae (ROO-gē = wrinkles), that can be seen with the unaided eye. The pylorus communicates with the duodenum of the small intestine via a smooth muscle sphincter called the pyloric sphincter. The concave medial border of the stomach is called the lesser curvature; the convex lateral border is called the greater curvature.

The stomach wall is composed of the same basic layers as the rest of the GI tract, with certain modifications. The surface of the mucosa is a layer of simple columnar epithelial cells called surface mucous cells (figure 24.12). The mucosa contains a lamina propria (areolar connective tissue) and a muscularis mucosae (smooth muscle) (figure 24.12). Epithelial cells extend down into the lamina propria, where they form columns of secretory cells called gastric glands. Several gastric glands open into the bottom of narrow channels called gastric pits. Secretions from several gastric glands flow into each gastric pit and then into the lumen of the stomach.

Question 29

CARDIA

Answer 29

The cardia (KAR-dē-a) surrounds the opening of the oesophagus into the stomach

Question 30

FUNDUS

Answer 30

The rounded portion superior to and to the left of the cardia is the fundus (PART OF THE STOMACH)

Question 31

PYLORUS

Answer 31

Inferior to the fundus is the large central portion of the stomach, the body. The pyloric part is divisible into three regions. The first region, the pyloric antrum, connects to the body of the stomach. The second region, the pyloric canal, leads to the third region, the pylorus (pī-LOR-us; pyl- = gate; -orus = guard), which in turn connects to the duodenum.

pylorus communicates with the duodenum of the small intestine via a smooth muscle sphincter called the pyloric sphincter. The concave medial border of the stomach is called the lesser curvature; the convex lateral border is called the greater curvature.

Question 32

PANCREAS

Answer 32

The pancreas (pan- = all; -creas = flesh), a retroperitoneal gland that is about 12–15cm (5–6in.) long and 2.5cm (1in.) thick, lies posterior to the greater curvature of the stomach. The pancreas consists of a head, a body, and a tail and is usually connected to the duodenum by two ducts (figure 24.15a). The head is the expanded portion of the organ near the curve of the duodenum; superior to and to the left of the head are the central body and the tapering tail.

Pancreatic enzymes digest starches (polysaccharides), proteins, triglycerides, and nucleic acids.

Pancreatic juices are secreted by exocrine cells into small ducts that ultimately unite to form two larger ducts, the pancreatic duct and the accessory duct. These in turn convey the secretions into the small intestine. The pancreatic duct, or duct of Wirsung (VĒR-sung), is the larger of the two ducts. In most people, the pancreatic duct joins the common bile duct from the liver and gall bladder and enters the duodenum as a dilated common duct called the hepatopancreatic ampulla (hep′-a-tō-pan-krē-A-tik), or ampulla of Vater (VA-ter). The ampulla opens on an elevation of the duodenal mucosa known as the major duodenal papilla, which lies about 10cm (4in.) inferior to the pyloric sphincter of the stomach. The passage of pancreatic juice and bile through the hepatopancreatic ampulla into the duodenum of the small intestine is regulated by a mass of smooth muscle surrounding the ampulla known as the sphincter of the hepatopancreatic ampulla, or sphincter of Oddi (OD-ē). The other major duct of the pancreas, the accessory duct (duct of Santorini), leads from the pancreas and empties into the duodenum about 2.5cm (1in.) superior to the hepatopancreatic ampulla.

Histology of the pancreas
The pancreas is made up of small clusters of glandular epithelial cells. About 99% of the clusters, called acini (AS-i-nī), constitute the exocrine portion of the organ (see figure 18.18b, c). The cells within acini secrete a mixture of fluid and digestive enzymes called pancreatic juice. The remaining 1% of the clusters, called pancreatic islets (islets of Langerhans) (Ī-lets), form the endocrine portion of the pancreas. These cells secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide. The functions of these hormones are discussed in chapter 18.

Question 33

GALL BLADDER

Answer 33

Stores, concentrates, and delivers bile into duodenum via common bile duct.

The gall bladder (gall- = bile) is a pear-shaped sac that is located in a depression of the posterior surface of the liver.Itis7–10cm(3–4in.)long and typically hangs from the anterior inferior margin of the liver

The parts of the gall bladder include the broad fundus, which projects inferiorly beyond the inferior border of the liver; the body, the central portion; and the neck, the tapered portion. The body and neck project superiorly.

The mucosa of the gall bladder consists of simple columnar epithelium arranged in rugae resembling those of the stomach. The wall of the gall bladder lacks a submucosa. The middle, muscular coat of the wall consists of smooth muscle fibres. Contraction of the smooth muscle fibres ejects the contents of the gall bladder into the cystic duct. The gall bladder’s outer coat is the visceral peritoneum. The functions of the gall bladder are to store and concentrate the bile produced by the liver (up to tenfold) until it is needed in the small intestine. In the concentration process, water and ions are absorbed by the gall bladder mucosa.

Stores and concentrates bile and releases it into the small intestine.

Question 34

CIRCULAR FOLDS

Answer 34

Located on the walls of the small intestines.

Circular folds or plicae circulares are folds of the mucosa and submucosa (see figures 24.18b and 24.19a). These permanent ridges, which are about 10 mm (0.4in.) long, begin near the proximal portion of the duodenum and end at about the midportion of the ileum. Some extend all the way around the circumference of the intestine; others extend only part of the way around. Circular folds enhance absorption by increasing surface area and causing the chyme to spiral, rather than move in a straight line, as it passes through the small intestine.

Question 35

MICRO VILLI/VILLI

Answer 35

Villi
Fingerlike projections of mucosa that are sites of absorption of digested food and increase surface area for digestion and absorption.

Microvilli
Microscopic, membrane-covered projections of absorptive epithelial cells that contain brush-border enzymes (listed in table 24.5) and that increase surface area for digestion and absorption.

Besides circular folds and villi, the small intestine also has microvilli (mī-krō-VIL-ī; micro- = small), which are projections of the apical (free) membrane of the absorptive cells. Each microvillus is a 1-μm-long cylindrical, membrane-covered projection that contains a bundle of 20–30 actin filaments. When viewed through a light microscope, the microvilli are too small to be seen individually; instead they form a fuzzy line, called the brush border, extending into the lumen of the small intestine (figure 24.20d). There are an estimated 200 million microvilli per square millimetre of small intestine. Because the microvilli greatly increase the surface area of the plasma membrane, larger amounts of digested nutrients can diffuse into absorptive cells in a given period. The brush border also contains several brush-border enzymes that have digestive functions (discussed shortly)

Also present in the small intestine are villi (= tufts of hair), which are fingerlike projections of the mucosa that are 0.5–1 mm long (see figure 24.19b, c). The large number of villi (20–40 per square millimetre) vastly increases the surface area of the epithelium available for absorption and digestion and gives the intestinal mucosa a velvety appearance. Each villus (singular form) is covered by epithelium and has a core of lamina propria; embedded in the connective tissue of the lamina propria are an arteriole, a venule, a blood capillary network, and a lacteal (LAK-tē-al = milky), which is a lymphatic capillary (see figure 24.19c). Nutrients absorbed by the epithelial cells covering the villus pass through the wall of a capillary or a lacteal to enter blood or lymph, respectively.

Question 36

ANUS

Answer 36

The distal end and outlet of the rectum.

Question 37

CAECUM

Answer 37

A blind pouch at the proximal end of the large intestine that attaches to the ileum.

The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine.[1] It is typically located on the right side of the body (the same side of the body as the appendix, to which it is joined).

It receives chyme from the ileum, and connects to the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve (ICV) or Bauhin’s valve. It is also separated from the colon by the cecocolic junction. While the cecum is usually intraperitoneal, the ascending colon is retroperitoneal.[2]

The opening from the ileum into the large intestine is guarded by a fold of mucous membrane called the ileocecal sphincter (valve), which allows materials from the small intestine to pass into the large intestine. Hanging inferior to the ileocecal valve is the caecum, a small pouch about 6cm (2.4in.) long. Attached to the caecum is a twisted, coiled tube, measuring about 8cm (3in.) in length, called the appendix or vermiform appendix (VER-mi-form; vermiform = worm-shaped; appendix = appendage). The mesentery of the appendix, called the mesoappendix (mez-ō-a-PEN-diks), attaches the appendix to the inferior part of the mesentery of the ileum.

Question 38

COLON/LARGE INTESTINE

Answer 38

The wall of the large intestine contains the typical four layers found in the rest of the GI tract: mucosa, submucosa, muscularis, and serosa. The mucosa consists of simple columnar epithelium, lamina propria (areolar connective tissue), and muscularis mucosae (smooth muscle) (figure 24.24a). The epithelium contains mostly absorptive and goblet cells (figure 24.24b, d). The absorptive cells function primarily in water absorption; the goblet cells secrete mucous that lubricates the passage of the colonic contents. Both absorptive and goblet cells are located in long, straight, tubular intestinal glands (crypts of Lieberkühn) that extend the full thickness of the mucosa. Solitary lymphatic nodules are also found in the lamina propria of the mucosa and may extend through the muscularis mucosae into the submucosa. Compared to the small intestine, the mucosa of the large intestine does not have as many structural adaptations that increase surface area. There are no circular folds or villi; however, microvilli are present on the absorptive cells. Consequently, much more absorption occurs in the small intestine than in the large intestine.

The portion of the large intestine consisting of ascending, transverse, descending, and sigmoid portions.

ascending colon (KŌ-lon) The part of the large intestine that passes superiorly from the caecum to the inferior border of the liver, where it bends at the right colic (hepatic) flexure to become the transverse colon.

transverse colon (trans-VERS KŌ-lon) The portion of the large intestine extending across the abdomen from the right colic (hepatic) flexure to the left colic (splenic) flexure.

descending colon (KŌ-lon) The part of the large intestine descending from the left colic (splenic) flexure to the level of the left iliac crest.

sigmoid colon (SIG-moyd KŌ-lon) The S-shaped part of the large intestine that begins at the level of the left iliac crest, projects medially, and terminates at the rectum at about the level of the third sacral vertebra.

Chemical digestion in the large intestine
The final stage of digestion occurs in the colon through the activity of bacteria that inhabit the lumen. Mucous is secreted by the glands of the large intestine, but no enzymes are secreted. Chyme is prepared for elimination by the action of bacteria, which ferment any remaining carbohydrates and release hydrogen, carbon dioxide, and methane gases. These gases contribute to flatus (gas) in the colon, termed flatulence when it is excessive. Bacteria also convert any remaining proteins to amino acids and break down the amino acids into simpler substances: indole, skatole, hydrogen sulphide, and fatty acids. Some of the indole and skatole is eliminated in the faeces and contributes to their odour; the rest is absorbed and transported to the liver, where these compounds are converted to less toxic compounds and excreted in the urine. Bacteria also decompose bilirubin to simpler pigments, including stercobilin, which gives faeces their brown colour. Bacterial products that are absorbed in the colon include several vitamins needed for normal metabolism, among them some B vitamins and vitamin K.

Absorption and faeces formation in the large intestine
By the time chyme has remained in the large intestine 3–10 hours, it has become solid or semisolid because of water absorption and is now called faeces. Chemically, faeces consist of water, inorganic salts, sloughed-off epithelial cells from the mucosa of the gastrointestinal tract, bacteria, products of bacterial decomposition, unabsorbed digested materials, and indigestible parts of food.

Although 90% of all water absorption occurs in the small intestine, the large intestine absorbs enough to make it an important organ in maintaining the body’s water balance. Of the 0.5–1.0 litre of water that enters the large intestine, all but about 100–200 mL is normally absorbed via osmosis. The large intestine also absorbs ions, including sodium and chloride, and some vitamins

Question 39

CRYPTS OF LIEBERKUHN

Answer 39

Located in the small intestine.

The small intestinal mucosa contains many deep crevices lined with glandular epithelium. Cells lining the crevices form the intestinal glands, or crypts of Lieberkühn (LĒ-ber-kūn), and secrete intestinal juice (to be discussed shortly). Besides absorptive cells and goblet cells, the intestinal glands also contain paneth cells and enteroendocrine cells. Paneth cells secrete lysozyme, a bactericidal enzyme, and are capable of phagocytosis. Paneth cells may have a role in regulating the microbial population in the small intestine. Three types of enteroendocrine cells are found in the intestinal glands of the small intestine: Scells, CCK cells, and K cells, which secrete the hormones secretin (se-KRĒ-tin), cholecystokinin (CCK) (kō-lē-sis′-tō-KĪN-in), and glucose-dependent insulinotropic peptide (GIP) (in-soo-lin′-ō-TRŌ-pik), respectively.

Question 40

MUCUS

Answer 40

In the human digestive system, mucus is used as a lubricant for materials that must pass over membranes, e.g., food passing down the esophagus. Mucus is extremely important in the gastrointestinal tract. It forms an essential layer in the colon and in the small intestine that helps reduce intestinal inflammation by decreasing bacterial interaction with intestinal epithelial cells.[21] The layer of mucus of the gastric mucosa lining the stomach is vital to protect the stomach lining from the highly acidic environment within it.[scientific 1]

Question 41

SINUSOIDS

Answer 41

Large, thin-walled, and leaky type of capillaries, having large intercellular clefts that may allow proteins and blood cells to pass from a tissue into the bloodstream; present in the liver, spleen, anterior pituitary, parathyroid glands, and red bone marrow.

Hepatic sinusoids are highly permeable blood capillaries between rows of hepatocytes that receive oxygenated blood from branches of the hepatic artery and nutrient-rich deoxygenated blood from branches of the hepatic portal vein. Recall that the hepatic portal vein brings venous blood from the gastrointestinal organs and spleen into the liver. Hepatic sinusoids converge and deliver blood into a central vein.

Question 42

SPHINCTER

Answer 42

A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning.

the upper oesophageal sphincter (UES) (e-sof′-a-JĒ-al), which consists of skeletal muscle, and the lower oesophageal (cardiac) sphincter (LES), which consists of smooth muscle and is near the heart. The upper oesophageal sphincter regulates the movement of food from the pharynx into the oesophagus; the lower oesophageal sphincter regulates the movement of food from the oesophagus into the stomach

Pyloric sphincter opens to permit passage of chyme into duodenum.
Regulates passage of chyme from stomach to duodenum; prevents backflow of chyme from duodenum to stomach.

opening from the ileum into the large intestine is guarded by a fold of mucous membrane called the ileocecal sphincter (valve), which allows materials from the small intestine to pass into the large intestine.

The opening of the anal canal to the exterior, called the anus, is guarded by an internal anal sphincter of smooth muscle (involuntary) and an external anal sphincter of skeletal muscle (voluntary). Normally these sphincters keep the anus closed except during the elimination of faeces.

Question 43

RECTUM

Answer 43

The last 20cm (8in.) of the gastrointestinal tract, from the sigmoid colon to the anus.

The rectum is a part of the lower gastrointestinal tract. The rectum is a continuation of the sigmoid colon, and connects to the anus. The rectum follows the shape of the sacrum and ends in an expanded section called an ampulla where feces is stored before its release via the anal canal. An ampulla (from Latin bottle) is a cavity, or the dilated end of a duct, shaped like a Roman ampulla.[4] The rectum joins with the sigmoid colon at the level of S3, and joins with the anal canal as it passes through the pelvic floor muscles.[4]

Unlike other portions of the colon, the rectum does not have distinct taeniae coli.[5] The taeniae blend with one another in the sigmoid colon five centimeters above the rectum, becoming a singular longitudinal muscle that surrounds the rectum on all sides for its entire length.

The rectum acts as a temporary storage site for feces. The rectum receives fecal material from the descending colon, transmitted through regular muscle contractions called peristalsis. [10] 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 pass feces, a process called defecation.[10]

An internal and external anal sphincter, and resting contraction of the puborectalis, prevent leakage of feces (fecal incontinence). As the rectum becomes more distended, the sphincters relax and a reflex expulsion of the contents of the rectum occurs. Expulsion occurs through contractions of the muscles of the rectum.[10]

Question 44

CANACULI

Answer 44

In the liver:
Grooves in the cell membranes between neighbouring hepatocytes provide spaces for canaliculi (described next) into which the hepatocytes secrete bile

canaliculi (kan-a-LIK-ū-li = small canals) are small ducts between hepatocytes that collect bile produced by the hepatocytes. From bile canaliculi, bile passes into bile ductules and then bile ducts.

Question 45

HEPATOCYTES

Answer 45

Hepatocytes (hepat- = liver; -cytes = cells) are the major functional cells of the liver and perform a wide array of metabolic, secretory, and endocrine functions. These are specialised epithelial cells with 5 to 12 sides that make up about 80% of the volume of the liver. Hepatocytes form complex three-dimensional arrangements called hepatic laminae (LAM-i-nē). The hepatic laminae are plates of hepatocytes one cell thick bordered on either side by the endothelial-lined vascular spaces called hepatic sinusoids. The hepatic laminae are highly branched, irregular structures. Grooves in the cell membranes between neighbouring hepatocytes provide spaces for canaliculi (described next) into which the hepatocytes secrete bile. Bile, a yellow, brownish, or olive-green liquid secreted by hepatocytes, serves as both an excretory product and a digestive secretion.

Question 46

LACTEAL

Answer 46

LAK-tē-al) One of many lymphatic vessels in villi of the intestines that absorb triglycerides and other lipids from digested food.

Each villus (singular form) is covered by epithelium and has a core of lamina propria; embedded in the connective tissue of the lamina propria are an arteriole, a venule, a blood capillary network, and a lacteal (LAK-tē-al = milky), which is a lymphatic capillary (see figure 24.19c). Nutrients absorbed by the epithelial cells covering the villus pass through the wall of a capillary or a lacteal to enter blood or lymph, respectively

Question 47

LIPASES

Answer 47

Enzymes that split triglycerides and phospholipids.

The most abundant lipids in the diet are triglycerides, which consist of a molecule of glycerol bonded to three fatty acid molecules (see figure 2.17). Enzymes that split triglycerides and phospholipids are called lipases. Recall that there are three types of lipases that can participate in lipid digestion: lingual lipase, gastric lipase, and pancreatic lipase. Although some lipid digestion occurs in the stomach through the action of lingual and gastric lipases, most occurs in the small intestine through the action of pancreatic lipase. Triglycerides are broken down by pancreatic lipase into fatty acids and monoglycerides. The liberated fatty acids can be either short-chain fatty acids (with fewer than 10–12 carbons) or long-chain fatty acids.

Question 48

PERISTALSIS

Answer 48

The oesophageal stage of swallowing begins once the bolus enters the oesophagus. During this phase, peristalsis (per′-i-STAL-sis; stalsis = constriction), a progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis, pushes the bolus onward. (Peristalsis occurs in other tubular structures, including other parts of the GI tract and the ureters, bile ducts, and uterine tubes; in the oesophagus it is controlled by the medulla oblongata.)

Question 49

EMULSIFICATION

Answer 49

the breakdown of large lipid globules into a suspension of small lipid globules in the small intestine.

Question 50

SEGMENTATION

Answer 50

Type of peristalsis: alternating contractions of circular smooth muscle fibres that produce segmentation and resegmentation of sections of small intestine; mixes chyme with digestive juices and brings food into contact with mucosa for absorption