Chapter 6: Nutrition in mammals

Holozoic Nutrition:

Is the type of nutrition found in animals. It involves ingestion of complex organic nutrients and its digestion, absorption and assimilation.

Digestive System:

The mouth and the buccal cavity:

• Food enters the body via the mouth which leads into the buccal cavity.

• In mouth, the solid food is broken up into smaller pieces by the chewing action of the teeth.

• Pairs of salivary gland open into the buccal cavity via ducts. They produce saliva which mixes up with food to produce bolus.

• In saliva there is mucin which helps to soften the food while chewing helps to break the food up into smaller pieces.

• Saliva also contains salivary amylase which aids in digestion to digest starch to maltose.

The Pharynx:

• Is the part of gut which leads from mouth to the oesophagus.

• It is the common passage for food and air. Epiglottis prevents food and air from mixing.

Epiglottis is a flap like cartilage opening.

• During swallowing, the larynx moves up so that the larynx is covered by the epiglottis and no food particles can enter the trachea.

The Oesophagus:

• Is a narrow, muscular tube from from the pharynx through the thorax and the diaphragm to the stomach.

• Here peristalsis occur.

  Peristalsis: is the rhythmic wavelike muscle contraction of the walls of oesophagus.

• The two sets of muscles; circular and longitudinal, wok in such a work that one contracts, the other relaxes. When the circular muscle contract the longitudinal muscle relaxes and vice versa. This causes the bolus to squeeze and push forward toward the stomach.

Stomach:

Is a dispensable muscular bag, with thick and well developed muscular walls. It lies immediately beneath the diaphragm to the left side of the abdominal cavity. The food normally stays here for about three to four hours.

• It secretes gastric juice which contains two substances (pepsin+rennin) and hydrochloric acid.

• Function of HCl:

  • stops the action of salivary amylase.

  • changes the inactive forms of of enzymes in the gastric juice to the active form.

  • provides a slightly acidic medium suitable for the action of gastric enzymes.

  • kills germs.

• Pepsin digests proteins to polypeptides or peptones.

• Rennin helps in coagulating milk in young mammals (not humans).

  The partly digested food becomes liquefied, forming chyme which passes in small amounts into the duodenum when the pyloric sphincter relaxes and open.

  Chyme: is the pulpy acidic fluid which passes from the stomach to the small intestine, consisting of gastric juices and partly digested food.

The liver and the Gallbladder:

Liver is the largest gland in the body. It is dark red and is made up of five lobes, three on the right and to two on the left.

• It has three blood vessels connected; hepatic portal vein, hepatic artery and hepatic vein.

• They secrete bile, an alkaline greenish-yellow liquid containing bile salts and bile pigments which give the bile its colour. It has no enzyme, so it cannot digest food but it aids in the digestion of fats. Bile pigments are waste products and are removed with faeces.

• Bile is stored in gallbladder.

• Gall bladder is a greenish yellow attached to the liver.

Functions of liver associated with digestion and assimilation:

• Production of bile:

  • liver plays an important part in the digestion of fats by secreting bile, which is stored temporarily in the gall bladder before use.

• Regulation of blood glucose concentration:

  • liver helps to keep the amount of glucose in the blood constant.

  • as the blood passes through the liver, the excess glucose is converted to insoluble glycogen and is stored. The blood leaving the liver contains a fairly constant amount of glucose. When the tissue cells of the body are in need of glucose they obtain it from the blood. As a result, the glucose in the blood drops. This induces the liver to convert glycogen in it back to glucose which enters the blood.

• Deamination of amino acids:

  • excess amino acids are brought to the liver and are removed and converted to urea.

Pancreas:

Is a gland lying in the loop of duodenum. It is connected to the duodenum by means of the pancreatic duct.

• Pancreas produces pancreatic juice which contains digestive enzyme.

• They also secrete the hormone insulin which plays an important part in the control of the blood sugar level in the body and utilization of carbohydrates.

Large Intestine:

Is shorter but much broader than the small intestine. The small intestine opens into the right side of abdominal cavity.

• The large intestine is about 1.5m long and consists of:

  • the caecum and the appendix.

  • the ascending colon running upwards along the right side of the abdominal cavity.

  • the horizontal transverse colon.

  • descending colon which runs downwards to join the rectum.

  • water is reabsorbed in the blood.

Small Intestine:

Is the longest part of the digestive system.

• Here; the chyme stimulates

  • the intestinal glands to secrete intestinal juice.

  • the pancreas to secrete pancreatic juice.

  • gall bladder to release bile.

• These all substances are alkaline so they neutralize the acidic chyme and provide a suitable alkaline medium for action of pancreatic and intestinal enzymes.

Carbohydrate digestion in the small intestine:

• When the left over undigested starch enter small intestine; they are converted into maltose by pancreatic amylase.

• Maltose is broken down by glucose by maltase, lactose to glucose and galactose by lactase, and sucrose to glucose and fructose by sucrase. Glucose helps in giving energy to the body and for respiration.

  • thus, the end products of carbohydrate digestion are simple sugars (glucose, fructose, galactose).

Fat digestion in the small intestine:

• Bile emulsifies fat.

• Emulsified fats are digested by lipases to fatty acids and glycerol.

  • thus the end products are fatty acids and glycerol.

Protein digestion in the small intestine:

• Protein digestion begins in stomach.

• Undigested proteins are converted to polypeptides to trypsin.

• Polypeptides are digested by trypsin to amino acids.

  • Thus by products are amino acids.

Absorption:

• All digested by products are digested in villi of the small intestine.

Villi: are numerous small finger like projections with large surface area in intestinal cavity.

• Simple sugar amino acids pass through the walls of the villi into the blood capillaries. The blood helps them in transportation.

• Active Transport plays an essential role in the absorption of the digested products, especially glucose,amino acids and mineral salts.

• Water and mineral salts are also absorbed in the large intestine.

• Fatty acids react with bile salts to become soluble, which then helps them diffuse into epithelium. In the epithelium this soluble mixture combines with glycerol to form minute fat globules, which are absorbed by the lymphatic capillaries.

• The undigested and unabsorbed matter is temporarily stored, mainly in the rectum, before it is discharged as faeces through the anus. This removal is called egestion.

Assimilation:

Is the transport and utilization of absorbed foods.

Sugars:

• Hepatic portal vein helps in transportation of sugar to the liver.

• In liver, most of the sugar is converted into glycogen, and then stored. However, some glucose is carried in the blood leaving the liver to be distributed around the body.

  Note:

  • Glycogen is the storage form of glucose (only excess).

  • Glucose is used by all cells for energy production.

• The conversion of glucose to glycogen requires a hormone insulin.

Amino Acids:

• Which enter the the cell are converted into new protoplasm and are used for growth and repair of worn-out parts of the body.

• Also used for formation of some hormones and enzymes.

Fats:

• Are absorbed into lymphatic capillaries, they mix up with the lymph to give a milky a fluid called chyle.

Lymph: is a colorless fluid in lymphatic capillaries.

• Blood carries fat to all parts of the body.

• Fats before being used, are brought to the liver where they are converted to forms that can be oxidized or stored.

• Fats are used to build protoplasm in cell membranes.

• Excess fats are stored in special tissues called adipose.

Adipose: fats storage and insulating tissue.