HUMAN NUTRITION PART 10: TRIBUTE TO THE LIVER

FUNCTIONS OF THE LIVER
A. METABOLISM OF GLUCOSE

• Excess glucose is converted with the help of insulin into glycogen and stored in the liver cells and the muscle tissues.
• When there is a decrease in the blood sugar level, the glycogen is converted back to glucose with the help of a hormone, glucagon and released into the bloodstream.
• With the help of the hormones, insulin and glucagon, the liver actually helps to regulate the amount of glucose in the blood.
  

B. METABOLISM OF AMINO ACIDS

• Amino acids are transported to all parts of the body for the synthesis of proteins, enzymes, hormones and the replacement of damaged tissues.
• Since excess amino acids cannot be stored by the body, they are DEAMINATED into two parts: the amino part and the carbohydrate part. The carbohydrate part is converted into glycogen and stored in the liver cells and muscle tissues whereas the amino part is converted into urea and transported to the kidneys for excretion.
  

C. PRODUCTION OF BILE

• The liver produces bile which helps to emulsify fats globules into tiny fat droplets.
• Note: Emulsification occurs in the duodenum.
  

D. STORAGE OF IRON

• The iron from the breakdown of haemoglobin is stored in the liver.
• It is used for the synthesis of new haemoglobin molecules.
  

E. EXCRETION OF BILE PIGMENTS

• The breakdown of haemoglobin produces bile pigments (called bilirubin and biliverdin).
• Both pigments are excreted into the duodenum as bile pigments (The colour of the faeces is actually due to these bile pigments).
  

F. SYNTHESIS OF PLASMA PROTEINS

• Plasma proteins such a s fibrinogen, serum globulin and serum albumin are synthesized by the liver from amino acids.
• Fibrinogen together with other proteins plays a very important role in blood clotting.
  

G. DETOXIFICATION

• The liver removes 95% of the alcohol from the blood. The other 5% of the alcohol in the blood leaves the body in perspiration, in the urine and in the breath.
• The alcohol contains sugar and this sugar can be oxidised in the liver to release energy.
• Overconsumption of alcohol may lead to obesity since the surplus sugar in the alcoholic drinks cannot be oxidised. So, instead the surplus is converted into fats and stored in the body.
  

H. STORAGE OF VITAMINS

• Vitamins A and D are stored in the liver.
  

I. RESERVOIR FOR BLOOD

• The blood spaces and network of blood capillaries in the liver hold a large volume of blood.
  

J. SOURCE OF HEAT ENERGY

• The liver is metabolically active. Hence it produces a large amount of heat energy.
• The heat energy is distributed to all parts of the body by the bloodstream and helps in maintaining the body temperature.
  

HUMAN NUTRITION PART 9: ASSIMILATION AND EGESTION

ASSIMILATION
Blood transports the digested food products round the body after they are being absorbed by the villi in the ileum. They are then taken up by cells. The UPTAKE and USE of the products of digestion is called ASSIMILATION.

A. ASSIMILATION OF GLUCOSE

• Glucose is used as fuel by all respiring cells. Energy is produced as a result of respiration and used in various cellular activities.
• Excess glucose is converted into glycogen with the help of Insulin and stored in the liver cells and muscle tissues in the form of glycogen granules. When the body needs energy or when the blood sugar level decreases, the glycogen in the liver will be converted back to glucose and released into the blood circulatory system.
• If the stored glycogen is not used for more than 6 hours, it may be converted to fats and stored in the adipose tissues.
  

B. ASSIMILATION OF FATS

• A portion of the absorbed fats are stored in the adipose tissues as fat droplets.
• Some are used in the synthesis of cell membrane and the rest are used for energy (only if the body is in short of carbohydrate as a source of energy).
  

C. ASSIMILATION OF AMINO ACIDS

• Amino acids serve as the basic units in the synthesis of large protein molecules. The proteins may be used for growth and development.
• Enzymes and hormones are also synthesized from amino acids.
• Excess amino acids cannot be stored. They are broken down in the liver into the amino part and the carbohydrate part. The amino part will then be transported to the kidney in the form of UREA for excretion whereas the carbohydrate part will be converted into glycogen and stored in the liver cells and muscle tissues.
• The process in which EXCESS AMINO ACIDS are broken down in the liver is called DEAMINATION.
  

EGESTION
The residues of food that cannot be digested or absorbed leave the small intestine and pass into the large intestine. There, water and vitamins (synthesised by bacteria) are absorbed and waste material is compacted into faeces. The faeces is propelled along the colon and rectum. When the anal sphincter is relaxed, the faeces is expelled. This process is called defaecation or egestion.

HUMAN NUTRITION PART 8: ABSORPTION IN THE ILEUM

Remember in the duodenum? The food is fully digested in the duodenum. The molecules (SIMPLE SUGARS (LIKE GLUCOSE, FRUCTOSE and GALACTOSE), AMINO ACIDS, and FATTY ACIDS and GLYCEROL) are small enough already to be absorbed in the ileum. Lets focus first on the structure of the ileum. In what ways is the ileum efficient in absorbing the products of the digested food?

The following figure shows the structure of some parts of the ileum:
ADAPTATIONS TO FUNCTION:

• It is fairly long and presents a large absorbing surface to the digested food.
• Its internal surface is greatly increased by circular folds bearing thousands of tiny projections called VILLI (singular = VILLUS).
• The lining epithelium is very thin and the fluids can pass rapidly through it. The outer membrane of each epithelial cell has microvilli which increase the exposed surface of the cell.
• There is a dense network of blood capillaries in each villus.

ABSORPTION!!!

• Simple sugars (glucose mainly, galactose and fructose) and amino acids pass through the walls of the villi into the blood capillaries by ACTIVE TRANSPORT.
• Mineral salts and vitamins also pass into the blood capillaries of the villi by active transport.
• Fatty acids and glycerol diffuse into the epithelium and RECOMBINE to form minute fat globules which later diffuse into the lacteal.
• The absorbed simple sugars, amino acids, mineral salts and vitamins are then transported to the liver by the HEPATIC PORTAL VEINS.
• The minute fat globules which absorbed into the lacteal are then transported into the lymphatic vessels that eventually empty into the blood circulation in the neck.
  

HUMAN NUTRITION PART 7: DIGESTION IN THE DUODENUM

The duodenum is the first few feet of the small intestine. We can say that it is the most active part of the digestive system since all types of food are digested here. The liver and the pancreas are playing very important role here. Before we discuss digestion in the duodenum, lets discuss the liver and the pancreas briefly.
THE LIVER

• The role of the liver in digestion is to manufacture BILE.
• Bile is a green watery fluid containing NO ENZYMES. It contains BILE PIGMENTS (hence the green colour) formed from the breakdown of haemoglobin in the liver. It also contains BILE SALTS which helps to emulsify fats.
• What is EMULSIFICATION? The breakdown of fat globules into smaller fat droplets (to increase the surface area so as to facilitate enzyme action). Note that, emulsification only breaks the fats physically but it does not change the fats chemically.
• After the bile is manufactured it is then stored in the GALL BLADDER. The presence of food in the duodenum triggers the secretion of bile from the gall bladder into the duodenum through the bile duct.
• EMULSIFICATION IS JUST LIKE CUTTING A BIG CHUNK OF BUTTER INTO SMALLER CUBES. THEY ARE ONLY DIFFERENT PHYSICALLY BUT STILL THE SAME CHEMICALLY. AND THINK! WHICH WILL MELT DOWN FASTER, THE CHUNK OR THE CUBES? OF COURSE THE CUBES RIGHT? SIMILAR TO FAT GLOBULES AND FAT DROPLETS, ENZYME REACTION WILL BE FASTER ON THE DROPLETS RATHER THAN ON THE GLOBULES.
  

THE PANCREAS

• The important role of the pancreas in digestion is to produce PANCREATIC JUICE.
• Pancreatic juice contains SODIUM HYDROGEN CARBONATE, PANCREATIC AMYLASE, LIPASE and TRYPSINOGEN.
• The sodium hydrogen carbonate helps to neutralise the acidic chyme once it enters the duodenum
  

WHAT HAPPENS IN THE DUODENUM?

• As mentioned earlier, the duodenum is very active since all types of food are digested here.
  
• Apart from the BILE and the PANCREATIC JUICE, the duodenum itself secretes the INTESTINAL JUICE produced by the intestinal glands in the wall of the duodenum itself.
• The intestinal juice contains ENTEROKINASE, EREPSIN, MALTASE, SUCRASE, LACTASE and LIPASE.
  
• So all three juices are actually working together in the duodenum to digest all types of food present in the chyme. To make our life easier, lets discuss the roles of the contents of the juices by the types of food they work on.
  

CARBOHYDRATE DIGESTION IN THE DUODENUM

• The remaining undigested starch from the mouth is digested by the pancreatic amylase into maltose and the maltose is further digested into glucose by maltase.
• Lactose is digested into glucose and galactose by lactase.
• Sucrose is digested into glucose and fructose by sucrase.
  

PROTEIN DIGESTION IN THE DUODENUM

• Trypsinogen is first activated into trypsin by enterokinase.
• Trypsin then digests proteins into polypeptides.
• Polypeptides are then digested further by erepsin into amino acids.
  

FATS DIGESTION IN THE DUODENUM

• Bile first emulsify fat globules into fat droplets (SURFACE AREA!!!).
• The fat droplets are then digested chemically by lipase into FATTY ACIDS and GLYCEROL.
  

END PRODUCTS OF DIGESTION

• Carbohydrates --> GLUCOSE, FRUCTOSE and GALACTOSE
  
• Proteins --> AMINO ACIDS
• Fats --> FATTY ACIDS and GLYCEROL

NOW, THE MOLECULES ARE SMALL ENOUGH TO BE ABSORBED BY THE VILLI IN THE ILEUM!!!

HUMAN NUTRITION PART 6: DIGESTION IN THE STOMACH

• The stomach has a strong muscular walls. The muscles contract and relax to churn the food an mix it with the enzymes and mucus. The mixture is called CHYME.
• The presence of food in the stomach, triggers the gastric glands (in the wall of the stomach) to secrete GASTRIC JUICE.

• The gastric juice contains PEPSINOGEN (inactive form of pepsin), HYDROCHLORIC ACID and MUCUS.
• The function of the hydrochloric acid is to provide acidic medium for the pepsin to function well (Pepsin works well at pH 2). Apart from this, it also kills any bacteria coming along with the food and activates the inactive pepsinogen into PEPSIN (REMEMBER these three functions of the hydrochloric acid). (Note: The salivary amylase from the mouth is denatured in the stomach? WONDER WHY?)
  
• The mucus protects the stomach wall from being attacked by the acid (REMEMBER the nature of an acid? CORROSIVE!!!)
• Any proteins present in the chyme is digested by the pepsin into POLYPEPTIDES.

NOTE: IN THE STOMACH, ONLY PROTEINS ARE DIGESTED CHEMICALLY (ALSO PHYSICALLY DUE TO THE STOMACH MUSCLE CONTRACTION). STARCH AND FATS ARE NOT DIGESTED CHEMICALLY IN THE STOMACH AS THE STOMACH DO NOT PRODUCE ENZYMES THAT CAN WORK ON STARCH AND FATS!!!

HUMAN NUTRITION PART 5: DIGESTION IN THE MOUTH AND OESOPHAGUS

DIGESTION IN THE MOUTH

• Once ingested, the food is first broken down into smaller pieces (physical digestion - chewing action with the help of the teeth) so as to increase the surface area and this in turn facilitates chemical digestion.
• The presence of food in the mouth triggers the salivary glands to secrete saliva.
• Saliva contains water, mucus and the enzyme, amylase.
• The water helps to soften the food whereas the mucus lubricates the food (bolus) so that the bolus can be swallowed into the oesophagus.
• The enzyme, amylase digests starch in the food into maltose. Since the bolus is not kept in the mouth for a very long time, some of the starch is still remain undigested.
• REMEMBER: Only starch is digested in the mouth (though not all) whereas proteins and fats are not, since there are no enzymes in the saliva that can digest them.
• The bolus is then swallowed into the oesophagus through the pharynx.
• Swallowing action causes the epiglottis to close the mouth of the trachea. This prevents food from entering the trachea during swallowing.
  

DIGESTION IN THE OESOPHAGUS

• No chemical digestion occurs in the oesophagus because there are no enzymes produced by the oesophagus. However starch digestion from the mouth continues as the bolus moves down the oesophagus. But not all the starch is digested as the bolus only stays in the oesophagus very briefly.
  
• Physical digestion continues as the circular and longitudinal muscles contract and relax alternately. This breaks the bolus further physically and at the same time helps to move the bolus down into the stomach.
  

HUMAN NUTRITION PART 4: THE ALIMENTARY CANAL

Before discussing chemical digestion along the digestive tract, it is better to know the parts in your digestive system first (Actually it helps, if you can memorize the figure and draw it freely).

From the following figure, it can be seen clearly that your digestive system consists of two main parts:

• The ALIMENTARY CANAL. This is the muscular tube (consisting of the circular and longitudinal muscles) in which digestion takes place. It is about 8 metres long, running from your mouth to your anus and has several distinct parts each one fulfilling an important role in digestion.
• The ASSOCIATED ORGANS. These associated organs consist of the salivary glands, liver, gall bladder, gastric glands (in the stomach), and pancreas. No digestion takes place in these associated organs except the stomach. These organs however play a very important role in chemical digestion since they produce enzymes and chemicals which help to facilitate chemical digestion.

HUMAN NUTRITION PART 3: PHYSICAL DIGESTION

WHY PHYSICAL (MECHANICAL) DIGESTION?
To break down food into smaller pieces. This increases the surface area of the food, hence facilitates chemical digestion by the enzymes.

HOW DOES IT OCCUR IN HUMAN?
It occurs in two ways:

• CHEWING action by the mouth with the help of the teeth.
• PERISTALSIS - This is the alternate contraction and relaxation of the circular muscles and the longitudinal muscles along the digestive tract. Apart from breaking down food physically, peristalsis also helps to move food down the digestive tract.

Note: Not only the oesophagus has these antagonistic set of muscles. The same set of muscles are also present in the stomach, and the intestines.

The following figure shows these antagonistic muscles as found in the oesophagus.

REMEMBER AGAIN: At the end of any physical digestion, the food is only changed physically but not chemically. For example, protein will still be protein, and starch will still be starch at the end of physical digestion.

HUMAN NUTRITION PART 2: THE TEETH

MAIN FUNCTION OF THE TEETH
To break down food into smaller pieces so as to increase the surface area so that chemical digestion can be facilitated.

THE STRUCTURE OF A TOOTH

• Enamel is the hardest substance which is very difficult to break or chip. However it can be dissolved by acids produced by bacteria as a result of feeding on sweet foods left on the teeth.
  
• Dentine is rather like bone. This is also hard but not as hard as the enamel. It has channels in it which contain living cytoplasm.
• Pulp cavity contains nerves and blood vessels. The blood vessels supply the cytoplasm in the dentine with food and oxygen.
• Cement has fibres growing out of it. These attach the tooth to the jawbone but allow it to move slightly when biting or chewing.

TYPES OF TEETH
MAMMALS HAVE TWO SETS OF TEETH IN THEIR LIFE

• The first set is called the milk teeth consisting of 20 teeth (8 incisors, 4 canines and 8 premolars)
• The second set is called the permanent teeth consisting of 32 teeth (8 incisors, 4 canines, 8 premolars and 12 molars)

GUM DISEASE


TOOTH DECAY

PREVENTION OF GUM DISEASE AND TOOTH DECAY

• Stop or reduce taking too much food containing or made of sugar. Brush teeth or at least rinse mouth immediately after taking food containing sugar.
• Use a fluoride toothpaste regularly. Fluoride make your teeth more resistant to decay.
• Visit the dentist to have a dental check up at least twice a year.