Human Nutrition and Respiratory Systems: Functions

Human Nutrition

Human nutrition requires the coordinated function of the digestive, respiratory, circulatory, and excretory systems.

The Human Digestive System

The human digestive system involves the following processes:

• Ingestion: Intake of food.
• Digestion: Mechanical and chemical breakdown of food.
• Absorption: Transfer of nutrients from the digestive tract into the blood or lymph.
• Egestion: Elimination of substances that can't be absorbed.

Organs of the Digestive System

• Mouth: The entrance to the digestive tract.
• Pharynx: A muscular tube that connects the digestive system to the respiratory system.
• Esophagus: A muscular tube around 25 cm long; it is normally closed but opens when food is ingested.
• Stomach: Connected to the esophagus via a sphincter, it has three layers of muscle that allow it to perform movements to mix food and gastric acid.
• Liver: The largest internal organ in the body. It contains small ducts called hepatocytes that collect the bile secreted by liver cells.
• Gallbladder: A pear-shaped structure located under the liver; its function is to store bile.
• Pancreas: A long, narrow gland with two ducts (main pancreatic and accessory duct) that secrete substances into the duodenum via the ampulla of Vater.
• Small Intestine: Contains digestive enzymes that complete the digestive process.
• Large Intestine: Its glands secrete lubricating mucus to help substances pass through it.

How the Digestive System Works

• Mechanical Digestion: Food is cut, ground, and mixed with digestive juices.
• Chemical Digestion: Food is broken down by hydrolytic enzymes.
• Swallowing: Movement of the food bolus from the mouth towards the pharynx.
• Gastric Digestion: The movement of the stomach mixes food with gastric acids.
• Intestinal Digestion: The chyme moves into the duodenum, where it is mixed with pancreatic juice, bile, and intestinal juice.
• Intestinal Absorption: Nutrients are absorbed by the intestinal villi.
• Egestion: The substances fermented by the intestinal flora are compacted to form feces and temporarily stored.

Human Respiratory System

The main structures include:

• Nasal Cavities: Air flows via the nostrils. The inside cavities are covered by mucus.
• Pharynx
• Larynx
• Trachea
• Bronchi
• Lungs

How the Respiratory System Works

• Inhalation: Air enters during inhalation, which takes place when the intercostal muscles contract and the diaphragm flattens. At the same time, the abdominal muscles relax, and the volume of the chest expands.
• Exhalation: Air is pushed out of the lungs. It is a passive process in which the intercostal muscles and the diaphragm relax while the abdominal muscles contract; the ribs move down, and the diaphragm returns to a relaxed position.
• Gas Exchange: In the lungs, gas exchange takes place between the very thin walls of the blood capillaries and the pulmonary alveoli.