Human Physiology: Nutrition, Respiration, and Excretion
Nutrients and Cellular Function
Nutrients are the raw materials from which cells produce their own components. They allow the body to replenish lost components, grow, and produce new cells. Furthermore, nutrients provide the fuel cells require for energy to perform their essential functions.
Balanced Diet Composition
A balanced diet should ideally consist of:
- Carbohydrates: 55% of energy intake
- Lipids: 30% of energy intake
- Proteins: 15% of energy intake
Basal Metabolism and Energy Needs
Energy requirements depend on activity levels, age, sex, and environmental temperature. Basal metabolic formulas are as follows:
- Women: (7.4 x weight in kg) + (428 x height in meters) + 572
- Men: (16.6 x weight in kg) + (77 x height in meters) + 572
An adult with an appropriate weight for their sex and age has their energy needs met if their weight remains stable. Note that malnutrition refers to inadequate nutrition resulting from either a poor diet or excessive food consumption.
The Respiratory System
The respiratory system consists of the airways and lungs located within the thoracic cavity. Changes in the composition of air and blood in the lungs occur due to gas exchange:
- Oxygen: Passes from air to blood via simple diffusion in the alveoli.
- Carbon Dioxide: Moves in reverse via the same mechanism.
The entry and exit of air in the lungs, known as pulmonary ventilation, is driven by changes in the volume of the thoracic cavity.
Excretion and Homeostasis
Excretion is the elimination of waste substances resulting from cellular activity. The kidneys perform a dual function: cleansing the blood of wastes and regulating water and salt levels.
The Role of Nephrons
Each nephron functions as a small factory for urine production. Urine is essentially filtered blood; it differs from blood plasma by being more concentrated, lacking useful organic nutrients like glucose or proteins, and containing high levels of waste products such as urea.
Maintaining Homeostasis
Homeostasis refers to the set of mechanisms by which all living things maintain stability in the properties of their internal environment.