What is Absorption?
- It is the process whereby digested food substances are absorbed into the body cells.
How is the small intestine adapted for absorption?
1)Surface area to volume ratio of small intestine is increased to speed up rate of absorption of nutrients.
a. Inner walls of small intestine highly folded
b. Folds are lined with microscopic, finger like projections called villi
c. Epithelial cells of villi have numerous microvilli
2) The villi have thin walls or membranes to allow faster diffusion.
> Epithelium is only one cell thick.
3)The small intestine is 6m long to provide sufficient time for food absorption.
4)The intestinal wall and villi have many capillaries/dense network of capillaries to continually carry away the absorbed food substances to maintain a steep concentration gradient for faster rate of absorption.
> In each villus: Numerous blood capillaries and a lacteal/lymphatic capillary
How are nutrients absorbed in the small and large intestines?
By diffusion:
- Glucose and amino acids diffuse into the bloodcapillaries.
- Glycerol and fatty acids diffuse into the epithelium and combine to form minute fat globules whichenter the lymphatic capillary / lacteal.
By active transport:
- Glucose and amino acids are absorbed by activetransport.
What happens to the undigested and unabsorbed matter?
- They are stored temporarily in the rectum and discharged as faeces through the anus.
- The discharging of faeces from the body is known as egestion / defecation.
What is the function of the hepatic portal vein?
- The blood capillaries that supply the small intestineunite to form the hepatic portal vein.
- The hepatic portal vein transports sugars andamino acids to the liver.
What happens to the absorbed sugar?
- The hepatic portal vein carries the absorbed sugars to the liver.
In the liver:
- Most of the sugars are converted toglycogen andstored.
- Glucose istransported by thehepatic vein to thedifferent parts of thebody.
What is Assimilation?
- It is the process whereby some of the absorbed food substances are converted into new protoplasm or used to provide energy.
Assimilation: The fate of Glucose
- Glucos is used by cells as a source of energy for all cells. It is oxidized duringtissue respiration to release energy for cellular activities.
- Excess glucose is returned to the liver, the hormone insulinsecreted from the pancreas stimulate the liver cells toconvert excess glucose to glycogen and stored it in liver
- When body needs energy, the hormone glucagon secretedfrom the pancreas stimulates the liver cells to convertstored glycogen back to glucose to be transported by bloodto various parts of body
What happens to the absorbed amino acids?
In the liver:
– Excess amino acids are converted to urea bydeamination.
– Amino acids are transported by thehepatic vein to thedifferent parts of thebody to be utilised.
- Hepatic portalvein carriesabsorbedamino acids tothe liver.
- The hepatic vein distributes amino acids to be used for growth and repair in cells and to form enzymes and hormones. Excess amino acids are deaminated in the liver.
How are fats transported and utilised?
Fats are absorbed into lacteals. Lacteals join to form largerlymphatic capillaries. Fats discharged into bloodstream. Blood carries fats into all parts of body. Fats are converted into forms thatcan either be broken down orstored.
- Excess fats stored in adipose tissues (beneath the skin, around heart etc)
- Used in the synthesis of protoplasm, e.g. cell membrane
- Supply of energy when there is an inadequate supply of glucose