Absorption and Control of Digestion SLOs: 2.5 and 2.6

Absorption  The means by which digested nutrients reach the blood  Most absorption takes place through the villi in the mucosa of the small intestine.

Absorption  Within each villus is an arteriole and venule bridged with capillaries arteriole venule capillaries

Absorption through capillaries  Simple sugars  Small proteins (peptides)  Amino acids  Some simple fatty acids  Most water

Transported These materials are then transported through the blood to the liver to be processed, stored or released as needed.

Absorption of Fats  Most fats have another way of entering the blood.  Instead of entering the blood capillaries, they are absorbed by the villi’s more permeable lymphatic capillaries called lacteals.

Lacteals

Absorption of Fats  The absorbed fat droplets give the lymph a milky appearance  The mixture of lymph and fat globules that drains from the small intestine after fat has been digested is called chyle (kile)  Chyle merges with the lymphatic system and eventually enters the blood when the lymph drains into veins near the heart. The absorbed fats then circulate to the liver for further processing

Absorption of Vitamins and Minerals  Minerals and vitamins from ingested food that are dissolved in water are absorbed directly into the blood through the capillaries in the villi  Other vitamins are mixed with fats and are absorbed along with other fats by the lacteals in the villi  Vitamin K and some B vitamins are produced by bacterial action in the colon and are absorbed from the large intestine.

Control of Digestion  The rate of food moving through the digestive system is crucial for good health.  Too slow and the body does not get adequate nutrition  Too quickly and digestion may be incomplete

Control of Digestion  There are two types of control over digestion:  Nervous  Hormonal

Nervous  The nerves that control digestive activity are located in the submucosa and between the muscle layers of the organ walls  Autonomic nervous system –  Parasympathetic stimulation increases activity  Sympathetic stimulation decreases activity  Example: Excess sympathetic stimulation from stress can block food’s movement through the digestive tract and inhibit mucus secretion which is crucial in protecting the lining of the digestive tract

Hormonal and Nervous  Hormones work on a feedback control system  Sight, smell, thought and taste of food triggers (through the nervous system) the secretion of saliva and the release of gastric juice  Food in the stomach, stimulates the release into the blood of the hormone gastrin, which promotes stomach secretions and motility (movement)

Hormonal and Nervous  When chyme enters the duodenum, nerve impulses inhibit stomach motility so that food does not move too quickly into the small intestine  This is an example of negative feedback  Gastric-inhibitory peptide (GIP) is released from the duodenum to reduce activity in the stomach and inhibit the release of gastric juice  It also has the important job of stimulating insulin release from the pancreas when glucose enters the duodenum

Hormonal and Nervous  The hormone secretin stimulates the pancreas to release water and bicarbonate to dilute and neutralize chyme  Cholecystokinin (CCK), stimulates the release of enzymes from the pancreas and causes the gallbladder to release bile

Hormones Active in Digestion (pg 443) HormoneSourceAction Gastrin Gastric-inhibitory peptide Secretin Cholecystokinin