【Biology Basic】Maintaining the Internal Environment of the Human Body: The Perfect Master Guide
Hello everyone! In this chapter, we’re going to explore the amazing mechanisms our bodies use to stay healthy. Why doesn't our body temperature fluctuate wildly, even when it's scorching hot or freezing cold outside? Why does our blood sugar remain constant even after we eat a lot of sweets?
This process of "maintaining the internal environment" (homeostasis) is a crucial topic that appears very frequently in the Common Test. It might feel like there's a lot to memorize at first, but once you understand how the mechanisms fit together, it’s just like solving a fun puzzle. Let’s take it one step at a time!
1. The Internal Environment and Body Fluids
The cells in our bodies are not directly exposed to the outside air; they are bathed in fluid. The state of this fluid is called the internal environment. In contrast, the world outside our body is called the external environment.
Types of Body Fluids
There are mainly three types of fluids that make up the internal environment (body fluids). By circulating through the body, they transport oxygen and nutrients while collecting waste products.
- Plasma: The liquid component of blood. It flows through the blood vessels.
- Tissue fluid: Fluid that seeps out of the capillaries and fills the spaces around cells.
- Lymph: A portion of the tissue fluid that enters the lymphatic vessels.
【Pro Tip】
These don’t exist in isolation; they are constantly being exchanged and circulated. Keep the following flow in mind: Plasma → Tissue fluid → Lymph → Vein (Plasma)!
Homeostasis
The property of maintaining the internal environment within a certain range, even when the external environment changes, is called homeostasis. The two main pillars that support this are the autonomic nervous system and hormones.
💡 Fun Fact:
The term "homeostasis" comes from the Greek words "homeo" (same) and "stasis" (state/standing still).
★ Summary of this section:
"Homeostasis" is the mechanism that keeps the internal environment (body fluids) constant, and the autonomic nervous system and hormones act as the command centers.
2. Composition and Function of Blood
Let’s take a closer look at blood, the "delivery service" that supports homeostasis.
Formed Elements of Blood (Cellular Components)
- Red blood cells (Erythrocytes): Contain hemoglobin and transport oxygen. They do not have a nucleus.
- White blood cells (Leukocytes): Involved in immunity, eliminating foreign substances that enter the body. They have a nucleus.
- Platelets: Involved in blood clotting (forming scabs). They do not have a nucleus.
Oxygen-Hemoglobin Dissociation Curve (Crucial!)
Hemoglobin in red blood cells has the property of binding to oxygen in areas with high oxygen concentration (the lungs) and releasing it in areas with low oxygen concentration (tissues). A graph showing this relationship is the oxygen-hemoglobin dissociation curve.
It might seem difficult at first, but just remember this rule: "In tissues where carbon dioxide is high, hemoglobin releases oxygen more easily (= delivering more oxygen to the cells)."
★ Summary of this section:
Blood is composed of red blood cells (oxygen transport), white blood cells (defense), platelets (hemostasis), and plasma (component transport).
3. Kidneys and Liver: Regulation of Body Fluids
To keep the composition of body fluids appropriate, two major organs are at work. This is the peak of calculation problems for the Common Test!
Kidney (Urine Formation)
The kidneys act as a "precision filter" that filters blood to remove waste and reabsorb necessary substances.
- Filtration: Substances other than blood cells and proteins are pushed from the glomerulus into Bowman's capsule (forming primary urine).
- Reabsorption: Necessary substances like water, glucose, and inorganic salts are returned to the blood in the renal tubules and collecting ducts.
【Formula for Calculation】
The amount excreted as urine can be expressed by the following formula:
\( \text{Excretion amount} = \text{Filtration amount} - \text{Reabsorption amount} \)
【Common Mistake】
"Proteins" and "blood cells" are too large to pass through the filter during the initial filtration stage. If these are found in urine, it is a sign that the filter is damaged.
Liver (The Chemical Factory)
The liver is a multi-functional organ said to have over 500 different roles.
① Blood glucose regulation: Synthesis and breakdown of glycogen.
② Urea formation: Converts highly toxic ammonia into less toxic urea (ornithine cycle).
③ Bile production: Creates bile, which aids in fat digestion.
④ Detoxification: Breaks down substances like alcohol.
⑤ Body temperature maintenance: Generates heat because it is metabolically active.
★ Summary of this section:
The kidney "cleans body fluids by making urine," and the liver is a place for "converting substances and detoxifying."
4. Autonomic Nervous System and Hormones
Let's learn about the command centers of homeostasis: the autonomic nervous system and hormones. These are controlled by the hypothalamus in the diencephalon.
Autonomic Nervous System: The Accelerator and Brake
These are nerves that operate independently of your conscious will.
- Sympathetic nervous system: The "fight or flight" nerve. Increases heart rate, dilates pupils. (Active mode)
- Parasympathetic nervous system: The "rest and digest" nerve. Decreases heart rate, promotes digestion. (Relaxation mode)
Hormones: Messages Carried by Blood
Chemical substances produced by specific organs (endocrine glands), transported by the blood, and acting on specific cells (target cells).
【List of Important Hormones】
1. Vasopressin (Posterior pituitary): Promotes water reabsorption in the kidney (reduces urine volume).
2. Thyroxine (Thyroid gland): Promotes metabolism.
3. Adrenaline (Adrenal medulla): Increases blood glucose levels.
4. Glucocorticoids (Adrenal cortex): Increases blood glucose levels (makes sugar from proteins).
5. Insulin (Pancreas, Islets of Langerhans B cells): Decreases blood glucose levels (the ONLY hormone that does this!)
💡 Tip for memorization:
Insulin is the only hormone that lowers blood sugar. Everything else (adrenaline, glucagon, glucocorticoids) works to raise it. For humans, starvation (low blood sugar) is a life-threatening crisis, so we have many mechanisms to raise it!
★ Summary of this section:
The autonomic nervous system controls the body via "electrical nerve signals," while hormones act via "chemical signals in the blood."
5. Regulation Processes for Blood Glucose and Body Temperature
Finally, let’s look at how these work together step-by-step.
When Blood Glucose Increases (e.g., after a meal)
- Hypothalamus detects the increase.
- Through the parasympathetic nervous system, insulin is secreted from the B cells of the pancreas.
- Cells take up glucose, and glucose is converted into glycogen in the liver.
- Result: Blood glucose level decreases.
When Body Temperature Decreases (e.g., when cold)
- Hypothalamus detects the decrease.
- Activates the sympathetic nervous system to constrict blood vessels and prevent heat loss.
- Increases secretion of adrenaline and thyroxine to boost metabolism and generate heat.
- Result: Body temperature increases.
【Key takeaway!】
The body constantly uses a mechanism called "feedback." By having the result (e.g., blood sugar rising) loop back to affect the cause (e.g., hormone secretion), the body prevents over-regulation.
★ Summary of this section:
The hypothalamus acts as a sensor and uses both the autonomic nervous system and hormones to return the body to its optimal state.
Great job! "Maintaining the internal environment of the human body" is a team sport that keeps us alive. There are many characters (organs and hormones) involved, but once you organize what "role" each one plays, your understanding will deepen significantly.
Start by locking in easy-to-remember images like "Insulin = the only friend that lowers blood sugar" and "Kidney = filter." I'm rooting for you!