Detailed Explanation of the Nine Major Systems of the Human Body and Basic Concepts of Metabolism

6. The endocrine system is an important regulatory system outside the nervous system. Its functions include transmitting information, participating in the regulation of metabolism, growth and development, and reproductive activities, and maintaining the homeostasis of the internal environment. Common problems associated with the endocrine system include obesity, diabetes, parathyroid disease, hypothyroidism, and hyperthyroidism.

7. The most important defense system for the human body against pathogens is the immune system, which consists of immune organs, immune cells, and immune molecules. The immune system is divided into innate immunity and adaptive immunity, with adaptive immunity further divided into humoral immunity and cellular immunity. Common problems with the immune system include bone marrow and blood diseases.

8. The brain, spinal cord, and the nerves surrounding them constitute the nervous system. Composed of nerve cells, it controls and regulates the activities of other systems and maintains the unity of the body's external environment. It is the most structurally and functionally complex system in the human body. It includes the central nervous system (composed of the brain and spinal cord) and the peripheral nervous system (composed of cranial nerves, spinal nerves, and visceral nerves). Common problems with the central nervous system include: low IQ, neurodegeneration, epilepsy, ADHD, and Alzheimer's disease.

9. The circulatory system consists of extracellular fluids such as plasma, lymph, and tissue fluid, and the pathways through which they circulate, including the heart and blood vessels. Its main tasks are: 1) to transport nutrients absorbed from the digestive tract to various tissues and organs; 2) to transport and excrete metabolic waste products from various tissues and organs; 3) to transport heat to all parts of the body to maintain body temperature; and 4) to transport hormones to target organs to regulate their function.

Here we need to focus on the small intestine, which belongs to the digestive system, as it is the main site for nutrient absorption. Food is broken down into absorbable small molecules through digestion in the small intestine. Food stays in the small intestine for a relatively long time, generally 3-8 hours, providing ample time for nutrient absorption. The small intestine is the longest part of the digestive tract and the main absorptive organ; the villi of the small intestine are the primary sites for nutrient absorption. The small intestine is very long and coiled within the abdominal cavity. It is typically 5-6 meters long. The intestinal mucosa forms many circular folds and numerous villi protruding into the intestinal lumen. The surface of each villi is covered by a layer of columnar epithelial cells, and the cell membrane at the apex of these columnar epithelial cells forms many tiny projections called microvilli. The circular folds, villi, and 1000-3000 microvilli on the free surface of each villi cell of the small intestine increase the surface area of ​​the small intestinal mucosa by 600 times. The inner surface area of ​​the small intestine in an adult can reach 200 square meters, with the villi epithelial cells facing the intestinal lumen. A larger inner surface area facilitates nutrient absorption. Furthermore, the villi contain capillaries, and both the villi and capillary walls are very thin, consisting of only one layer of epithelial cells. These structural features make it easy for nutrients to be absorbed and enter the bloodstream. The large absorptive surface area of ​​the small intestine further enhances absorption efficiency. The villi contain capillary networks, lymphatic capillaries, smooth muscle fibers, and nerve networks. The relaxation and contraction of the smooth muscle fibers allow the villi to contract and swing, accelerating blood and lymph flow and aiding absorption.

Metabolism is one of the most fundamental characteristics of life, encompassing both material metabolism and energy metabolism. The body obtains nutrients from the external environment, including carbohydrates, fats, proteins, trace elements, dietary fiber, water, and vitamins. Among these, carbohydrates, fats, and proteins are the body's primary energy sources. Through metabolism, the body absorbs nutrients from the external environment, breaks them down, and releases the chemical energy stored within, converting it into energy that tissues and cells can utilize. The body uses this energy to maintain life activities. The process of oxidizing energy substrates within the body to produce energy is called energy consumption, such as the energy conversion of proteins, carbohydrates, and lipids. The release, transfer, storage, and utilization of energy accompanying material metabolism are generally referred to as energy metabolism. The body's daily total energy consumption (also known as energy metabolism) is divided into three parts: basal energy consumption (basal metabolism), physical activity energy consumption (exercise metabolism, also called behavioral metabolism), and the thermic effect of food (energy heat exchange).

The above explanation has given us a general understanding of our bodies. To further explain the relationship between obesity and body fat percentage, we need to understand the significance and function of fat and muscle.