Human Body: Nutrient Absorption and Respiratory System
Nutrient Absorption
Absorption is the process by which nutrients, obtained through digestion, cross the intestinal wall into the circulatory system. From there, they are distributed to all of the body’s cells.
How Absorption is Carried Out
- Most organic nutrients are absorbed in the small intestine. They cross the plasma membrane of the cells in the intestinal mucosa and travel towards the intestinal villi, which are made up of a network of capillaries. Nutrients then pass through these capillaries and into the circulatory system.
- In the large intestine, water and minerals continue to be absorbed, and faeces are formed. A number of bacteria, known as intestinal flora, live in this portion of the intestine. Their function is to decompose food that has not been digested or absorbed by the small intestine. They synthesize amino acids and vitamins, which are absorbed by the blood capillaries.
Defecation
Faeces formed in the large intestine are waste products that the body cannot digest and, therefore, cannot absorb. Faeces accumulate in the rectum until the defecation reflex is triggered. That is when the faeces are expelled through the anus.
Respiratory Tract
The respiratory tract is a network of tubes that carry air to the lungs. It is made up of:
- The nasal passages
- The pharynx
- The larynx
- The trachea, which splits into two bronchi, one for each lung
These divide into small bronchi, which branch out into even smaller passageways called the bronchioles. The bronchioles end in sacs called the pulmonary alveoli. These have very thin walls made up of a single layer of cells, which are surrounded by a network of capillaries.
Lungs
The lungs are two spongy organs located in the rib cage and separated from the abdomen by a muscle called the diaphragm. They are protected by the ribs, as well as a membrane called the pleural membrane, which is full of pleural fluid.
Inhalation
Inhalation takes place when the diaphragm and the muscles that raise the ribs (intercostal muscles) contract. The combined action of these muscles causes the rib cage to expand. As a result, air enters through the nose, where it warms up, becomes moist, and travels through the respiratory tract towards the lungs.
Gas Exchange
Gas exchange takes place in the pulmonary alveoli and in tissues. It is carried out by diffusion. That is, gases move naturally across cell membranes from where they are most concentrated to areas where there is a lower concentration.
For example, the air in the pulmonary alveoli contains a greater amount of oxygen than the blood in the surrounding capillaries. As a result, oxygen tends to move towards the inside of the capillaries. In a similar way, there is a greater concentration of carbon dioxide in the blood within the pulmonary capillaries than in the air within the alveoli. It tends to move from the blood to the alveoli and, from there, is expelled.
Exhalation
During exhalation, the diaphragm relaxes, and the lungs reduce in volume. This means that the air they contain, which is loaded with CO2, is expelled through the respiratory tract.