Digestion System Flashcards
Describe the major functions of the digestive system.
The digestive system is primarily devoted to the intake, digestion, and absorption of nutrients, and the elimination of waste materials taken in with the nutrients. In addition, the digestive system has a number of other important functions. Intestinal bacteria produce several vitamins, including vitamin B12 and vitamin K. The collection of numerous intestinal microorganisms (often called the “gut microbiome”), plays an important role in assisting in immune system functioning, and contribute to a variety of other aspects of health.
Describe the relationship between the following processes in the gastrointestinal system: ingestion, digestion, absorption, defecation.
Ingestion:
Eating, or the taking of food into the digestive tract.
Digestion:
There are two forms of digestion: chemical and mechanical. Mechanical digestion is the grinding, chewing and thorough mixing of food by anatomical structures such as teeth, muscles and the tongue. Mechanical digestion aids in chemical digestion.
Absorption:
The passage of food molecules from the digestive tract into the circulatory or the lymphatic system for transport to the body cells.
Defaecation:
The elimination or the passing of faeces from the rectum out of the body.
Distinguish between extracellular digestion and intracellular digestion.
Extracellular digestion is digestion that takes place outside of cells. In humans this occurs inside the digestive tract, which consists of a tube that is open at both ends, the mouth at one end and the anus at the other.
intracellular digestion occurs inside cells. An example of intracellular digestion is the digestion of a bacterial cell by a human white blood cell in a process known as phagocytosis.
Describe the anatomy of the buccal cavity and explain its functions in digestion.
Buccal cavity (oral cavity)
- Boundaries are the lips in front, cheeks at sides and roof (formed by hard and soft palate)
Soft palate
- Behind hard palate
- Separates nasal passage from food passage
Uvula
- Hanging from middle of posterior edge of soft palate
- Finger-like process like a little tongue
Hard palate
- Tissue covering part of the maxilla in the anterior roof of the mouth.
Functions:
(i) Chewing (mastication) of food to reduce the particle size
(ii)Mixing of food with saliva
- With the aid of the tongue, food is mixed with saliva to form a soft and
flexible round ball (bolus) that can be easily swallowed
(iii) A little digestion of starch occurs in the mouth
- Salivary amylase enzyme in saliva begins the digestion of starch and glycogen in the mouth where it is optimally active (inactivation of the enzyme by acidity of the stomach within 1 hour)
Describe the anatomy and functions of the esophagus.
Anatomy:
- collapsible muscular tube about 25-cm long - extends from the pharynx to the stomach
Location:
- behind the trachea and in front of the vertebral column
- runs through the neck, thorax and diaphragm into the stomach
Function:
Movement of food from the buccal cavity to the stomach:
- food is pushed down the oesophagus by muscular movements called
Peristalsis
- like the stomach and the small and large intestines, the oesophagus has two types of smooth muscles: circular and longitudinal
- when circular muscles contract, the lumen (internal diameter of the digestive tract) narrows
- when longitudinal muscles contract, the length of the digestive tract decreases and the lumen widens
- peristalsis involves alternating contractions of the circular and longitudinal muscles:
- the circular muscles above the bolus contract and squeeze the bolus downwards
- to receive the bolus the longitudinal muscles around and below the bolus contract, widening the lumen
- peristalsis continues throughout other parts of the digestive tract
Describe the anatomy and functions of the stomach.
General anatomy:
- Fairly large, J-shaped organ with 2 curvatures (a lesser and a greater curvature)
- An enlargement of the digestive tract
- As the digestive tract is a continuous tube, various organs are simply enlargements of that tube.
The stomach has four major parts or areas:
(i) Cardia - the lower portion of the oesophagus opens into this area (ii) Fundus - a rounded portion of the stomach immediately to the left of
and slightly above the cardia
(iii) Body - the larger central part of the stomach, below the fundus (iv)Pylorus - narrower, inferior portion of the stomach
- lies to the right of the body of the stomach
- leads into the duodenum (the first part of the small intestine)
Functions:
(i) Acts as a holding sac (bag) for food
- storing food for a period of time and controlling its release allows the stomach to
help regulate the rate of food digestion in the intestine
(ii) Produces a hormone called gastrin which stimulates release of gastric juice
(iii) Produces highly acidic gastric juice which mixes with the food (due to gentle peristaltic movements of the stomach wall). This changes bolus into a thin liquid called chime.
(iv)A small amount of chemical digestion
- the stomach is not a major digestive organ
- a little bit of digestion of protein due to the enzyme pepsin in gastric juice
- a little bit of digestion of lipids due to lingual lipase enzyme (made by glands in the tongue and activated by the acidity of the stomach) and also due to gastric lipase enzyme
Describe the liver with reference to: anatomy, function, connection to the duodenum and gallbladder, blood supply.
a) Anatomy
- the heaviest organ in the human body (about 1.4 kg in the average adult) and the
second largest (after the skin)
- divided into two principle lobes: a very large right lobe and a smaller left lobe
- the lobes are separated by the falciform ligament which also attaches the liver to the diaphragm (fold of peritoneum which will be explained later)
- the liver has two other small lobes (beside the 2 main lobes); these are really parts of the right lobe and can only be seen from a posterior view
b) Functions
(i) Produces bile salts from cholesterol
- bile salts emulsify fats and assist in fat and cholesterol absorption
- bile has no enzymes but it is essential in digestion of fats; bile emulsifies
fats, that is, bile converts large fat droplets into smaller droplets which
increases the surface area of the fats, thus allowing lipase to work faster
(ii) Controls the amount of glucose in the blood
- sugars absorbed from the small intestine are taken by the blood to the liver (see hepatic portal system below)
- if there is too much glucose in blood, the liver changes excess glucose into glycogen or fat for storage
- if there is too little glucose in blood, the liver changes glycogen into glucose, releasing it into the blood
(iii) Converts poisonous substances into less harmful compounds (especially when proteins are burned for energy)
- when proteins and amino acids are burned for energy, toxic substances containing nitrogen are produced (e.g. NH3); the liver changes these into urea which is MUCH less harmful
(iv) Produces many plasma proteins (e.g. fibrinogen)
(v) Stores fat soluble vitamins (A, D, E, & K)
(vi) Stores some elements (e.g. copper and iron)
- worn-out blood cells are broken down by the spleen
- some of the iron taken from haemoglobin in this process is stored in the
liver and is later used to produce new molecules of haemoglobin (vii) Involved in lipid metabolism
- converts fatty acids into forms that can be stored or transported (viii) Degrades hormones
(ix) Detoxifies various substances (e.g. alcohol and many drugs)
c) Connection to the duodenum and gall bladder:
Transport of bile from the liver to the gallbladder and the duodenum:
- several small bile ducts in the right lobe of the liver join to form the right
hepatic duct; similarly, several ducts in the left lobe merge to form the left hepatic duct
- the left and right hepatic ducts then join to form a common hepatic duct
- the common hepatic duct comes together with a duct from the gallbladder called the cystic duct to form the common bile duct
- just before it reaches the duodenum, the common bile duct is joined by the pancreatic duct (from the pancreas) to form a common duct called the ampulla of Vater which projects slightly into the duodenum
d) Blood Supply: The Hepatic Portal System
- veins arising from blood capillary networks of the stomach, the small intestine and the large intestine join up to form the hepatic portal vein
- instead of taking blood to the inferior vena cava or to the heart (like other veins of the lower body), the hepatic portal vein takes blood to the liver
- although the liver receives some arterial blood through the hepatic artery, it receives the majority of its blood from the hepatic portal vein
- in the liver, both the hepatic artery and the hepatic portal vein break up into blood capillaries and release blood (which mixes together)
- eventually the blood drains into two hepatic veins and enters the inferior vena cava
Importance of the hepatic portal system:
- many of the nutrients released and absorbed during digestion enter capillary networks and are transported by the hepatic portal system to the liver
- several important metabolic functions of the liver involve the regulation and modification of these nutrients (e.g. regulation of blood glucose)
- the liver also removes some toxins from the blood and adds plasma proteins - the hepatic portal system thus allows the liver to modify, store and control various nutrients in the blood and detoxify blood BEFORE it enters the general circulation
Describe the anatomy and functions of the pancreas.
Anatomy:
- a soft gland about 12-15 cm long and 2.5 cm wide - divided into three parts: (i) head, (ii) body, (iii) tail
Location:
- lies just below the greater curvature of the stomach
- the head of the pancreas is near the C-shaped curve of the duodenum - the body is the central portion and the tail tapers to the left
Functions:
(i) Produces pancreatic juice which:
- is transported to the duodenum through the pancreatic duct (which merges with the common bile duct) and through the duct of Santorini
- contains digestive enzymes for digestion of carbohydrates, lipids and
proteins
- contains sodium bicarbonate which makes pancreatic juice alkaline, neutralising the acidic chyme and providing the pH required for digestion in the small intestine
(ii) Produces glucagon and insulin, which are the primary hormones that regulate glucose levels in the blood
Describe the anatomy and functions of the small intestine.
Anatomy & Location:
- from the pyloric sphincter to the cecum (first part of the large intestine) - longer (3 m), but narrower (2.5 cm) than the large intestine
- divided into 3 parts:
(i) duodenum - first about 25 cm from pyloric sphincter (ii)jejunum - extends about 1 m from duodenum to ileum (iii) ileum – extends about 2 m from ilium to cecum
Functions:
(i) Produces a hormone called secretin which stimulates the release of bicarbonate-rich pancreatic juice and increases the secretion of bile from the liver
(ii)Produces enzymes involved in digestion of carbohydrates and proteins (ii) Most chemical digestion occurs here
(iii) Most absorption of nutrients and water out of the digestive tract
- the internal lining has many folds (villi) to increase the surface area available
for absorption out of the lumen and for secretion into the lumen
Describe the anatomy and functions of the large intestine.
Anatomy:
- about 1.5 m long and 6.5 cm in diameter
- extends from the small intestine to the anus - is divided into 3 parts:
i) Caecum - 6-cm pouch below where the ileum empties into the large intestine - the appendix arises from the caecum
ii) Colon - main part, about 1.3 m long
- can be further divided into ascending colon, transverse colon, descending colon and sigmoid colon
iii) Rectum - the final about 20 cm of the gastrointestinal tract
- the last 2-3 cm or so of the rectum is also called the anal canal - the opening of the anal canal to the exterior is called the anus
Functions:
- small but significant amount of absorption of water, mineral salts and vitamins - formation of faeces
Describe the process of defecation
Defaecation occurs in three steps:
(i) Mass peristaltic movements (involuntary) move the faeces into the rectum
(ii) Pressure-sensitive receptors in rectum are stimulated; this causes a reflex order to the longitudinal muscles of the rectum to contract and put pressure on the internal sphincter, causing the internal sphincter to open
(iii) Voluntary contraction of abdominal muscles puts pressure on the rectum trying to force the faeces out, causing the external sphincter to open; having the external sphincter under voluntary control allows us to control the emptying of the rectum
