Nutrition and Digestion Flashcards

Question

What are general anatomical features of the liver?

Answer 1

Large, lobulated exocrine and blood-processing gland, with vessels and ducts entering and leaving at the porta. Enclosed by a thin Collagen Tissue capsule, mostly covered by mesothelium. Collagen tissue of the branching vascular system provides gross support. Parenchymal cells are supported by fine reticular fibres.

Answer 2

Portal vein: bringing food-rich blood from the gut. Hepatic artery: bringing arterial blood. Hepatic veins: taking away processed blood into the vena cava. Lymphatics: taking away some lymph. Hepatic ducts: removing bile to the gallbladder and gut.

Answer 3

25% from the hepatic artery (which comes off the aorta), supplies arterial blood. 75% from the portal vein, brings food-rich blood from the gut. Hepatic veins take blood away to vena cava after processing. Very little nerve supply at sinusoidal level but some sympathetic and parasympathetic of perivascular structures.

Answer 4

Microanatomy of liver shows large glandular cells throughout the liver substance. The cells are arranged in perforated plates, one cell wide. Between the plates are sinusoidal blood channels 9-l2 μm wide, lined by endothelial cells. Scattered in the glandular mass are blood vessels, alone and accompanied by other vessels. The distribution of these vessels defines the classic hepatic lobules.

Answer 5

Central vein/terminal hepatic venule - very thin wall, lies in the centre of a lobule, with sinusoids converging towards and opening into it. Sublobular/intercalated vein - thicker wall, lies alone at the periphery of the lobule. Branch of portal vein - again at the periphery of the lobule, but accompanied by one or more small hepatic arteries/arterioles, one or more bile ducts/ductules lined by cuboidal epithelium, and lymphatics. Portal vein, artery, and bile duct constitute a portal triad; the area in which they lie is a portal area.(The lymphatics are ignored for this naming).

Answer 6

Portal vein, artery, and bile duct constitute a portal triad; the area in which they lie is a portal area.

Answer 7

Comes from branches of the portal vein and hepatic artery; from the periphery towards the centre. It is in the sinusoids, between the cell plates. Blood collected in central veins goes to sublobular veins, then to collecting veins, and then hepatic veins leaving the liver. Intralobular bile flow is from the lobule's centre towards the peripheral bile ducts, and runs, within any one cell plate, between the liver cells in bile canaliculi.

Answer 8

A functional unit in the liver of three or so lobules. Explains differences in exposure to blood supply among various parts of lobules. These differences are reflected in varied functional activities and degrees of susceptibility to toxic agents - metabolic zonation. Acinus territory has one final branch of the portal vein as it's axis and is subdivided into: 1. periportal (closest to portal triads), 2. intermediate, 3. perivenous (close to the central vein) zones, with the initial periportal zone being roughly spheroid, and isolated from periportal zones of adjacent acini.

Answer 9

Sinusoids are low pressure vascular channels that receive blood from terminal branches of the hepatic artery and portal vein at the periphery of lobules and deliver it into central veins.

Answer 10

Sinusoids are low pressure vascular channels that receive blood from terminal branches of the hepatic artery and portal vein at the periphery of lobules and deliver it into central veins. They are lined by fenestrated endothelial cells, loosely attached, and hold phagocytic Kupffer cells (larger, stellate, with a pale oval nucleus). Fenestrated lining cells are not tightly attached and rest on microvilli of underlying hepatic cells, without a basal lamina intervening. Plasma can thus pass through the sieve plate, formed by the lining cells, out into the perisinusoidal space of Disse to interact with the hepatocytes. Some of this fluid may pass to the periphery of the lobule to be collected as lymph. Disse's 'space' contains ECM materials, but not a visible basal lamina. Scarce, fat-storing, stellate cells of Ito lie outside the endothelial cells. They store vitamin A. They respond to a variety of insults by making collagen and causing cirrhosis (fibrosis).

Answer 11

1. Blood cleansing (e.g. of gut bacterial toxins) 2. Haemopoiesis in the embryo (RBC production) 3. Bringing plasma into intimate contact with the hepatic cell for its many metabolic functions of storage, transformations, syntheses, regulation of plasma concentrations, detoxifications, the production of bile, and assisting defence by producing acute-phase proteins.

Answer 12

Phagocytotic cells (immunologically active) that are held within the fenestrated endothelial cells lining liver sinusoids.

Answer 13

Perisinusoidal space between hepatocytes and liver sinusoids, containing ECM but no visible basal lamina. Plasma can pass through into it to interact with the hepatocytes so some of the fluid can be passed to the periphery of the lobule to be collected as lymph.

Answer 14

Lie outside the endothelial cells lining the liver sinusoid. Scarce but store fat and vitamin A. Respond to a variety of insults by making collagen and causing cirrhosis (fibrosis).

Answer 15

Main functional cells of the liver. Account for 80% of mass of the liver. In three dimensions, hepatocytes are arranged in plates that anastomose with one another. The cells are polygonal in shape and their sides are in contact either with sinusoids (sinusoidal face) or neighbouring hepatocytes (lateral faces). A portion of the lateral faces of hepatocytes is modified to form bile canaliculi. Microvilli are present abundantly on the sinusoidal face and project sparsely into bile canaliculi. Hepatocyte nuclei are distinctly round, with one or two prominent nucleoli. A majority of cells have a single nucleus, but binucleate cells are common.

Answer 16

System of canaliculi between the hepatic cells leads to Canals of Hering/cholangioles, with both hepatocytes and bile duct cells in their walls. Next there are Bile ductules, in the portal areas, with only small, cuboidal cells, firmly held by membrane interdigitations and junctional complexes, and having a few luminal microvilli. Bile ducts' epithelium changes to columnar mucous cells and, extrahepatically, the ducts acquire smooth muscle as well as Collagen tissue. Cystic duct allows reflux into the gallbladder, when sphincter of Oddi at the duodenal outlet of the common bile duct is closed.

Answer 17

Lymph is formed by filtration of plasma into the spaces of Disse as blood flows through the sinusoids. Then lymph percolates between the space of Disse and portal tracts. Then, lymphatics are formed that run along portal vessels and biliary ducts.

Answer 18

Glucose and bilirubin (death of RBCs makes Hb bilirubin).

Answer 19

Bilirubin (anion transport) - assesses transport Aminotransferases (Alanine ALT and Aspartate AST) - assesses hepatocyte damage Gamma glutamyl transferase (GGT) - assesses impaired bile flow (cholestasis) Alkaline phosphatase (Alk Phos) (liver but also isoform in bone) - assesses impaired bile flow choestasis Albumin - assesses protein synthesis Prothrombin time ratio - assesses protein synthesis

Answer 20

Parietal cells (which have powerful proton pumps) in the stomach produce HCl. 2.5L of secretions a day. Oesophageal and duodenal sphincters prevent acid from leaving stomach. Mucus barrier on the stomach produces bicarbonate so it doesn't get digested. It is important to break up food to prepare for small bowel.

Answer 21

Parietal cells - have powerful proton pump action, make HCl Neuroendocrine cells: Enterochromaffin cells - make histamine; G cells - make gastrin; D cells - make somatostatin (inhibits gastric secretion) Other cells: Mucus cells - make mucus (barrier); Chief cells - make pepsinogen (protein digestion)

Answer 22

Enzyme produced in mouth and pancreas that breaks down carbohydrates. Converts starch and glycogen into glucose. Glucose absorbed in stomach and small intestine (with help of insulin).

Answer 23

Enzymes that break down proteins. Pepsinogen (made by chief cells) - breaks down pepsin; Trypsinogen (made by pancreas, activated by enteropeptidase) - breaks down trypsin; Chymotrypsinogen (made by pancreas, activated by trypsin) - breaks down chymotrypsin Allows absorption in small intestine.

Answer 24

Enzyme to break down fats. Pancreatic lipase, and, Pancreatic lipase related protein 2 Bile salts emulsify fats and allow lipases to act. Enterohepatic circulation is the movement of bile acid molecules from the liver to the small intestine and back to the liver.

Answer 25

Liberated from protein binding by acid and pepsin in stomach. Binds to R factors (cobalamin-binding proteins-haptocorrin secreted by saliva, bile and pancreas). Pancreatic proteases release this complex in duodenum where it is then bound to intrinsic factor (produced by gastric parietal cells). IF-B12 complex absorbed in terminal ileum.

Answer 26

Animal products and leafy green vegetables in polyglutamate form. Cleaved to monoglutamate formin jejunum. Key role in DNA synthesis and repair, increased use in pregnancy.

Answer 27

2 forms of iron: Ferrous/Heme iron (Fe2+) - soluble, well absorbed, in meat, complexed to heme; Ferric/Non-Heme iron (Fe3+) - insoluble, in veg/cereals/etc. not as well absorbed. Gastric acid needed to abdorb Fe3+, as it reduces it to Fe2+ form and promotes formation of chelates. Vitamin C also reduces Fe3+ to Fe2+ to promote absorption. Absorbed in duodenum by enterocytes. Transported around the blood by transferrin.

Answer 28

Energy and substrates for growth are derived entirely from dietary fuels: fat, carbohydrate and protein. These are digested and the products absorbed in the intestine and transported through the blood to tissues, where they are metabolised – degraded or stored. They can by degraded completely, to CO2, or partially, to intermediates that are then used to make macromolecules that are assembled into cell structures. Catabolic (breakdown) pathways yield energy, conserved as ATP, and reduced coenzymes (NADPH) and both of these are required for anabolic (biosynthetic) pathways.

Answer 29

Used as an energy currency within cells - formed directly by some catabolic pathways such as glycolysis, but mostly produced from TCA cycle which reduces coenzymes (NAD, ubiquinone) that are then reoxidized by the mitochondrial electron-transport chain, coupled to ATP synthesis. Hydrolysis of ATP to ADP and inorganic phosphate (Pi) liberates energy that can be used to drive biosynthetic reactions, for ion-translocation by membrane pumps or to do mechanical work in muscle. Some energy is also lost to heat.

Answer 30

A coenzyme, derived from the vitamin nicotinamide, it’s reduced in several catabolic pathways; the most important is the pentose phosphate pathway, a pathway of glucose breakdown that is active in liver, adipose tissue and elsewhere, but there are others. NADP effectively carries 2 hydrogens (actually 2 electrons and 1 H+). NADPH is used as the reductant in the synthesis of fatty acids, cholesterol, deoxyribonucleotides and elsewhere.

Answer 31

It contains alpha-amylase, from the paratoid gland, meaning starch digestion begins in the mouth.

Answer 32

Gastric cells secrete pepsin, a proteinase, so protein digestion begins in the stomach. The low pH of the stomach helps to denature dietary protein, making it more susceptible to enzymic attack, and it stops bacterial growth; but no other digestive enzymes are active in the stomach.

Answer 33

The pancreas secretes enzymes that act on starch (alpha-amylase), fat (TAG lipase) and proteins (trypsin, chymotrypsin, elastase, carboxypeptidase), and these are all active in the small intestine. The small intestine also has its own resident enzymes that complete the digestion of starch and proteins and some that act on disaccharides (maltose, isomaltose, lactose and sucrose).

Answer 34

Bile salts, made in the liver and secreted into the small intestine via the bile duct, are essential for the digestion of dietary fat by pancreatic TAG lipase.

Answer 35

Digestion in the large intestine is carried out by resident bacteria (the microbiome); degradation and fermentation of complex dietary carbohydrates may contribute significantly (6-10%) to overall energy production.

Answer 36

Digestion of polysaccharides requires enzymes that hydrolyse glycosidic bonds (i.e. the links between sugar monomers) and these are specific for the type of link. Thus starch requires alpha-amylase (secreted by the parotid and the pancreas) to hydrolyse the alpha(1-4) links in the long chains; the digestion products, maltose (two glucose units linked alpha(1-4)) and isomaltose, with alpha(1-4) links, from the branches, requires other enzymes, glucoamylase and isomaltase, which are expressed in the small intestine. The major disaccharides sucrose and lactose are alsohydrolysed by specific intestinal enzymes (sucrase and lactase). The digestion products of the dietary carbohydrates are taken up by intestinal cells, then enter the blood and are taken up by other cells, where they are metabolised.

Answer 37

All cells in the body can metabolise glucose; the energy supply to the brain (20-25% of total energy expenditure) is mostly met by glucose; and some cell types (e.g. erythrocytes) can only use glucose for energy production. In the major breakdown pathway, glycolysis, glucose is oxidised to pyruvate, with the production of 2 ATPs; pyruvate is then either reduced to lactate, which enters the blood (anaerobic glycolysis), or it enters mitochondria, where it is oxidized to CO2 in the TCA cycle, with a much greater yield of ATP. The other major pathway of glucose breakdown, the pentose phosphate pathway, does not produce ATP, and its purpose is the production of NADPH and of other sugars, such as ribose.

Answer 38

Fat is stored in the body as triacylglycerol (TAG), esters of fatty acids with glycerol. The fatty acids are unbranched, of variable length, and may contain double bonds. Dietary fat (mostly TAG) is solubilized by bile salts digested (secreted by the liver, via the gallbladder) and hydrolysed to monoacyl glycerol (MAG) by pancreatic lipase. The digestion products are taken up by intestinal cells, which secrete them into the blood as chylomicrons, the largest of the various classes of plasma lipoproteins; these have a core of TAG and acylcholesterol, surrounded by a shell of phospholipids, cholesterol and proteins. Lipoprotein lipase, on the plasma side of several cell types, hydrolyses the TAG in chylomicrons, and the products are taken up and either metabolised or re-converted to TAG for storage. Chylomicrons have a very short half-live (~5 min) in the plasma; when most of the TAG has been removedthe chylomicron remnants are taken up by the liver. The fat-soluble vitamins A,D,E and K reach the liver by this route.

Answer 39

The requirement for dietary protein arises because the proteins in the body are all turned over (although with half-lives that vary widely, from minutes to months). The breakdown products may be used for protein synthesis, energy production or synthesis of other compounds. The recommended intake of nitrogen declines as the rate of growth decreases, with a plateau of 0.8g/kg/day around age 18 (with some exceptions).

Answer 40

Protein digestion begins with pepsin in the stomach, is continued by pancreatic proteinases in the small intestine and is completed by resident enzymes there. The products (mostly amino acids, but a few peptides) are taken up by intestinal cells and then enter the plasma. Proteinases are secreted as inactive precursors called zymogens. Thus pepsin is secreted as pepsinogen, and the low pH in the stomach causes its activation. In the small intestine, trypsinogen is activated by enteropeptidase, and trypsin then activates the zymogens for chymotrypsin, elastase and carboxypeptidase. The proteinases are relatively specific for cleavage of the peptide bonds from particular types of amino acid, hence the need for a repertoire of different enzymes. During their turnover, cellular proteins are digested by proteinases within lysosomes.

Answer 41

Aminoacids are classified as essential/non-essential; dietary protein intake needs to include allthe essential aminoacids, irrespective of its N-content. They are also classified asglucogenic/ketogenic, according to their metabolic outcomes; a few large aminoacids are both glucogenic and ketogenic.

Answer 42

Aminoacid breakdown occurs in many tissues. Each of the 20 aminoacids has its own breakdown pathway, and most of these begin with transamination, the transfer of the amino-group to oxaloacetate to form glutamate, leaving a ketoacid that is then degraded. Most nitrogen excretion is as urea, which is formed from glutamate in the liver; the urea enters the plasma and is excreted by the kidneys. Glutamate formed in other tissues is transaminated to a different aminoacid, alanine, which is then taken up by the liver to form glutamate, and then urea. Another route of N-excretion is the formation of glutamine, which is taken up by kidneys and deamidated to ammonia, which is excreted directly. Other N-containing excretory products are uric acid (from purine breakdown) and creatinine, derived from creatine in muscle.

Answer 43

Glucose is broken down in the glycolytic pathway to pyruvate, which is taken up and oxidized to acetyl-coenzyme A. Fatty acids enter mitochondria and are broken down to acetyl-CoA in the beta-oxidation pathway. Aminoacids are degraded in their individual pathways, some yielding acetyl-CoA, others being converted to glucose. Acetyl-CoA is a major fuel of the TCA cycle, although other intermediates, from aminoacids or other fuels, may also enter the cycle. Acetyl-CoA is completely oxidized to CO2; the TCA cycle itself does not consume oxygen, but generates reduced coenzymes (NAD and ubiquinone) that are reoxidized by the mitochondrial electron-transport chain, which reduces O2 to water. The energy liberated is conserved first as a transmembrane electrochemical H+-potential, which isused to drive ATP synthesis (the chemiosmotic mechanism of oxidative phosphorylation).

Answer 44

TAG is not hydrated and has a large energy yield (KJ/g) but unlike glycogen cannot supply energy anaerobically (so needs oxygen). The mobilised form of fat, non-esterified fatty acids (NEFA), cannot supply energy to the brain, because it is not taken up (cannot cross blood-brain barrier). Around 10-20kg stored in adipose and muscle.

Answer 45

Free glucose in the plasma is not a significant energy store (~3g), its concentration being strictly controlled because of its reactivity, as it’s used by all cells in the body. The glycogen stores in liver (~100g) and muscle (~300g) have whole-body functions as stores. Neither is very large because the hydration of glycogen adds a large weight penalty so they are highly hydrated. Glycogen stores in liver are used for plasma glucose homeostasis. Glycogen stores in muscle are only used only within muscle. Can be rapidly mobilised and provide ATP anaerobically.

Answer 46

Protein is potentially a large store of fuel in the body (around 6-12kg), but it has only a moderate energy yield. It is convertible to glucose and ketone bodies. There is no ‘storage protein’ as it is all functional (muscle, plasma proteins, ECM etc) so its use as a fuel always has adverse consequences.

Answer 47

Breakdown of liver glycogen is first defence against hypoglycaemia. If it continues gluconeogenesis (in liver and kidney) becomes important, the precusors of glucose being aminoacids from protein breakdown and also glycerol (from lipolysis) as well as lactate (from anaerobic glycolysis). TAG breakdown occurs, liberating NEFA (non-esterified fatty acids), which is animportant fuel for muscle, kidney, etc, but not for brain. NEFA is oxidised in liver to acetyl-CoA, which is mostly diverted from the TCA cycle to form ‘ketone bodies’ (acetoacetate and 3-hydroxybutyrate) which can fuel many tissues, including brain.

Answer 48

After a meal, starch digestion raises glucose concentration from normal resting value (4-5 mmol/L) to a peak which then declines rapidly as tissues remove the glucose from plasma. The glucose stimulates release of insulin from beta-cells in pancreas, promoting glucose uptake and utilisation. Subsequent decrease in glucose concentration causes less insulin secretion and plasma concentration decreases since short half-life.

Answer 49

Liver: increase glycogen synthesis, decrease glycogenolysis and gluconeogenesis. Muscle: increase glucose uptake, glycogen synthesis, protein synthesis and decrease glycogenolysis. Adipose: increase glucose uptake, TAG synthesis and decrease lipolysis.

Answer 50

NEFA concentration increases in a fasted state, insulin secretion (caused by increase glucose plasma) causes NEFA concentration to decrease. This is because fat digestion results in chylomicron production in the intestine - NEFA comes from breakdown of TAG in adipose tissue, which is inhibited by insulin.

Answer 51

Insulin-dependent, from failure to produce insulin due to auto-immune destruction of pancreatic beta-cells. 10-20% of cases. Decreased glycogen synthesis and glucose uptake and increased glycogenolysis and gluconeogenesis produce hyperglycaemia. There is also increased lipolysis which results in weight loss and ketonaemia. Early and rapid onset; polyuria, metabolic acidaemia. Acute ketoacidaemia is a dangerous outcome of untreated type 1 diabetes.

Answer 52

Resistance to insulin, on down-regulation of insulin receptors in target tissues. 80-90% of cases. Decreased glycogen synthesis and glucose uptake and increased glycogenolysis and gluconeogenesis produce hyperglycaemia. There is decreased lipolysis so unlike type 1, no weight loss or ketonaemia. Slow onset, usually >35 yrs, associated with obesity. Retinopathy, nephropathy, neuropathy heart disease, circulatory problem can occur.

Answer 53

Drug adapted from a natural product, galegin, to treat type 2 DM. It inhibits gluconeogenesis in liver by its effects on the oxidation state of NAD, and also by increasing the activity of a regulatory enzyme, AMP-activated protein kinase – note that the regulator of this enzyme is AMP, not cyclic AMP!

Answer 54

Affects 20-30% of population, males > females. Characterised by obesity (particularly increased abdominal fat), high plasma TAG concentrations and low high-density lipoprotein (dyslipidaemia), insulin resistance and consequent glucose intolerance; the long-term effects are increased blood pressure, cardiovascular disease and risk of stroke.

Answer 55

Congenital leptin deficiency causes obesity. Can be treated with daily leptin injections but only with those with abnormal leptin expression as plasma concentration correlated to BMI. Other peptide hormones secreted into the bloodstream by different parts of the digestive tract control appetite and digestion. Growth rate and body weight are tightly controlled: if they are increased or decreased by changes in diet they revert to a set point on resumption of a normal diet - weight loss produced by dieting is largely reversed on resumption of a normal diet. Gastric bypass surgery is more successful in treating obesity, presumably because of changes in the secretion of hormones that affect digestion.

Answer 56

Leptin: released by adipoytes, circulating levels correlate with body’s fat content, leptin receptors in the brain, including hypothalamus, linked to suppression of food intake causing motivation to feed, rewards for feeding and satiety, increases metabolic rate; Ghrelin: secreted by gastric cells, orexigenic - increases hunger, levels fall after feeding, inhibits insulin secretion; Cholecystekinin: secreted by duodenum, stimulates secretion of digestive enzymes and bile, slows gastric emptying, anorexigenic; Gastric inhibitory peptide: secreted by duodenum and jejunum, inhibits gastric acidification, slows digestion, stimulates insulin secretion; Peptide YY: secreted by ileum and colon, anorexigenic

Nutrition and Digestion Flashcards

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