How does the food change in consistency and form?
The changes are as follows:
Food - Bolus - Chyme - Feces
1. Mouth (Ingestion and Initial Breakdown):
- Food is mechanically broken down by chewing, which grinds it into smaller pieces.
- Saliva contains enzymes (e.g., amylase) that begin the chemical breakdown of carbohydrates.
- The food becomes a moist, partially digested mass known as bolus.
2. Esophagus (Transport):
- The bolus is pushed down the esophagus by a series of muscular contractions called peristalsis.
- There’s no significant change in the food’s form, but it’s moved from the mouth to the stomach.
3. Stomach (Further Breakdown):
- In the stomach, food mixes with gastric juices, including hydrochloric acid and digestive enzymes (e.g., pepsin) that help break down proteins.
- The food is churned and becomes a semi-liquid substance called chyme.
- The stomach acid also helps to sterilize the food by killing harmful bacteria.
4. Small Intestine (Nutrient Absorption):
- Chyme enters the small intestine, where it’s further digested by bile (produced by the liver) and pancreatic enzymes.
- Nutrients like sugars, amino acids, and fatty acids are absorbed through the walls of the small intestine.
- The consistency changes from liquid chyme to a more nutrient-rich solution, as the body absorbs vitamins, minerals, and other nutrients.
5. Large Intestine (Water Absorption and Waste Formation):
- In the large intestine, excess water is absorbed, and the remaining material becomes more solid.
- The food, now in the form of waste, is turned into feces as the body absorbs water, salts, and other nutrients, leaving indigestible fibers and waste products.
6. Rectum and Anus (Excretion):
- Feces are stored in the rectum until they are excreted through the anus during a bowel movement.
How could the body absorb the nutrients from the foods we eat?
1. Digestion:
- When food enters the stomach, it’s broken down into simpler components by mechanical (i.e., chewing and churning) and chemical (i.e., enzymes and stomach acids) processes.
- Carbohydrates are broken down into simple sugars (i.e., glucose), proteins into amino acids, and fats into fatty acids and glycerol.
2. Absorption in the Small Intestine (Primary site for nutrient absorption):
- Villi and Microvilli: The inner surface of the small intestine is lined with tiny finger-like projections called villi, and the cells on these villi have even smaller projections called microvilli. These structures greatly increase the surface area available for absorption.
- Transport Mechanisms:
- Active Transport: Some nutrients, like glucose and amino acids, are absorbed via active transport, which requires energy. This process moves nutrients against their concentration gradient (from lower concentration to higher concentration).
- Facilitated Diffusion: Nutrients like some vitamins and minerals pass through the cell membrane with the help of specific transport proteins, but without using energy.
- Simple Diffusion: Lipid-soluble nutrients (like fatty acids and fat-soluble vitamins such as A, D, E, and K) can move across the cell membrane without the need for energy, following the concentration gradient (from high to low concentration).
3. Nutrient Transport:
- Once the nutrients pass through the epithelial cells of the small intestine, they enter the bloodstream or lymphatic system:
- Water-soluble nutrients (e.g., glucose, amino acids, and most vitamins) enter the capillaries within the villi and are carried away by the bloodstream.
- Fat-soluble nutrients (e.g., fatty acids and fat-soluble vitamins) are absorbed into the lymphatic system. They are packaged into chylomicrons, which are transported through lymphatic vessels before eventually entering the bloodstream.
4. Circulation and Distribution:
- Once in the bloodstream, nutrients are transported to various organs and tissues throughout the body, where they are used for energy, growth, and repair.
- Glucose is carried to cells for energy.
- Amino acids are used to build proteins.
- Fatty acids are used for energy storage or incorporated into cell membranes.
5. Storage:
- Some nutrients are stored for later use.
- Glucose is stored as glycogen in the liver and muscles.
- Fat is stored in adipose tissue for energy reserves.