Human Biology: Metabolism, Respiration, and Circulation
Metabolism
Metabolism encompasses all the chemical processes within a living organism. It’s broadly divided into two categories:
- Anabolism: The synthesis or construction of matter.
- Catabolism: The degradation of matter.
Gas Exchange
Gas exchange occurs in the lungs, specifically within the alveoli. Oxygen from the air diffuses into the blood, while carbon dioxide diffuses from the blood into the air. This exchange is driven by simple diffusion. Pulmonary ventilation, the process of air entering and exiting the lungs, is caused by changes in the volume of the thoracic cavity.
Digestion
Digestion is the process of breaking down food into smaller molecules that can be absorbed by the body. It begins in the mouth and continues through the esophagus, stomach, small intestine, and large intestine.
In the Mouth
Chewing and saliva begin the digestive process. Saliva contains amylase, an enzyme that breaks down carbohydrates. The tongue pushes the food into the esophagus.
In the Esophagus
Peristalsis, involuntary muscle contractions, moves the food down the esophagus to the stomach.
In the Stomach
The stomach mixes the food with gastric juices, which contain hydrochloric acid and pepsin. Pepsin breaks down proteins. The acidic environment also kills bacteria. Lipids slow down digestion.
In the Small Intestine
The pancreas releases alkaline secretions to neutralize the chyme from the stomach. Pancreatic enzymes, including amylase, lipase, and proteases, further break down carbohydrates, fats, and proteins. Intestinal juice also contributes enzymes.
In the Large Intestine
The large intestine absorbs water, minerals, and some vitamins. Gut flora ferment indigestible material, producing organic acids. The remaining waste is expelled as feces.
Cellular Absorption
Nutrients and oxygen from the blood diffuse through capillary walls into the intercellular fluid, making them available to cells. Inside the cells, intracellular enzymes process and utilize these nutrients.
Blood Circulation
The heart, a muscular organ, pumps blood throughout the body. It’s divided into four chambers: two atria and two ventricles. The right side of the heart handles pulmonary circulation, sending deoxygenated blood to the lungs. The left side handles systemic circulation, sending oxygenated blood to the rest of the body.
- Diastole: Deoxygenated blood enters the right atrium, and oxygenated blood enters the left atrium.
- Atrial Systole: The atria contract, pushing blood into the ventricles.
- Ventricular Systole: The ventricles contract, pumping blood to the lungs and the rest of the body.
Breathing
Air enters through the nose, where it’s warmed, moistened, and filtered. It then passes through the pharynx, larynx, trachea, and bronchi into the lungs. The bronchi branch into smaller bronchioles, ending in alveoli. Gas exchange occurs in the alveoli, where oxygen enters the blood and carbon dioxide exits.