The next stage of digestion happens in the stomach. For our cheese sandwich this is where the protein from the cheese gets digested. A human stomach is a muscular bag which is around the size of a coke can. In the lining of the stomach there are gastric pits which is where oxyntic cells are found. These cells secrete hydrochloric acid which kills bacteria on the food. Chief cells secrete pepsinogen which is an inactive form of pepsin. The hydrochloric acid produced by the oxyntic cells cuts off some amino acids from pepsinogen: this then changes the shape to expose the active site so it becomes active pepsin. It starts to digest proteins by braking the long protein into shorter chains. The saliva amylase which was produced in the mouth stage stops working and breaking down food after about an hour as the pH becomes too low. The lingual lipase however continues to work.
When the stomach churns fats melt and proteins begin to digest. This creates an acid mix of semi-digested food called chime. After about 4 hours the food leaves the stomach due to the sphincter muscle, between the stomach and duodenum relaxes.
In the small intestine stage is where the bread is broken down by amylase and the protein is broken down by protease.
Chime enters the duodenum this triggers the pancreas to release pancreatic juice and the gall bladder to release bile, due to the hormones that travel in the blood. Bile is made in the liver but stored in the gall bladder. Bile doesn't contain enzymes but it does have salts that emulsify fats, increasing their surface area for lipase enzymes to work on.
Pancreatic juice which is produced from the pancreas contains enzymes such as, trypsinogen which is activated by enterokinase in the small intestine to trypsin, that digests the partly digested proteins into smaller polypeptides. Amylase digests starch to maltose and lipase digests lipids to fatty acids and monoglycerides.
The ileum has epithelial cells called enterocytes which contain enzymes embedded in their cell membranes. These enzymes protrude into the intestine so foods comes into contact. In the ileum, maltose digests to glucose by maltase and polypeptides are broken down to amino acids by peptidases. After 6-12 hours of the food being in the ileum the nutrients which are useful have been digested into smaller molecules. Carbohydrates have been broken down to simple sugars such as glucose, lipids have been broken down to fatty acids and monoglycerides and proteins have been broken down to amino acids.
Food that is not absorbed into the blood at the ileum is passed into the large intestine. The main function of the large intestine is to push the contents along to the rectum for the faeces to be egested. The colon part of the large intestine is where water and minerals are absorbed.
Absorption
The wall of the ileum has a large surface area for absorption due to it being highly folded and having numerous villi. The epithelial cells which line the villus have micro villi on their plasma membranes, again increasing the surface area.
Glucose and amino acids are absorbed by a type of active transport involving co-transport of sodium ions. ATP is used to actively transport sodium ions out of the epithelial cells, this lower the concentration of sodium ions in the cell. Sodium ions enter the from the ileum, y facilitated diffusion down their gradient, through a co-transporter protein channel in the plasma membrane. They bring with them either glucose or amino acid molecules. The resulting high concentration of amino acids and glucose in the cells leads to them passing out of the other side of the cell by facilitated diffusion. They then diffuse into the bloodstream to be transported around the body.
Fatty acids and monoglycerides are fat soluble and diffuse through plasma membranes into the epithelial cells. They pass to the smooth endoplasmic reticulum and are converted back to triglycerides. They then pass to the Golgi apparatus and are the wrapped in protein coats to become chylomicrons. The chylomicrons pass into the lacteal and pass through the lymph fluid and enter the bloodstream, when the thoratic lymph duct drains into the subclavian vein.
Transportation
Fats are absorbed through lacteals into the thoratic duct and into the sub clavicle vein to then be carried to the heart by the hepatic vein, then enters the vena cava into the blood through the pulmonary vein into the lungs.
When it is in the blood it goes into the capillaries and into plasma. Plasma carries the nutrients into the veins through the hepatic vein which goes to the liver. It then gets carried into the heart through the vena cava into the right ventricle which then pushes it up through the pulmonary artery. It is then carried to the lungs by the pulmonary artery, it then moves into the lungs to add oxygen. When oxygen enters the body it is done by breathing. Inspiration is when the muscles between the ribs contract and raise the ribcage up and out. He diaphragm muscles contract and flatten, the thoracic cavities volume increases which reduces the air pressure inside the thorax. Air moves down the pressure gradient from the atmosphere into the alveoli of the lungs this is where oxygen and carbon dioxide are exchanged.
The lungs are adapted for gaseous exchange as they have a large surface area for diffusion of gases due to the fact they have millions of alveoli. In the alveoli the oxygen and nutrients are absorbed into the blood which is then moved into the aorta after passing through the heart to then be pumped out around the body.
The transportation of blood throughout the body is down to blood vessels. The different types of blood vessels have different properties ad functions. Arteries are elastic and transport blood away from the heart. Whereas veins are again elastic but transport blood to the heart. Capillaries are small vessels that are found in tissues which transport blood from arteries to veins.
