Fix the mistake: Regulation of the Human Body

Body Fluids and Homeostasis: Mechanisms Maintaining the Internal Environment

The cells of multicellular animals live immersed in body fluids. The function of keeping this internal environment constant—homeostasis—is central to sustaining life.

Basic Knowledge

Human body fluids are divided into three types:
① Blood: flows through blood vessels. About 8% of body weight.
② Tissue fluid (interstitial fluid): fills the spaces between cells.
③ Lymph: flows through lymphatic vessels.
Components of blood:
・Plasma (55%): the liquid component. Transports water, proteins, glucose, electrolytes, and waste products.
・Red blood cells (45%): contain hemoglobin and transport oxygen. No nucleus. Lifespan of about 120 days.
・White blood cells: responsible for immunity (including neutrophils and lymphocytes). Have a nucleus.
・Platelets: responsible for blood clotting. No nucleus (cell fragments).
Homeostasis means keeping the internal environment (the properties of body fluids) within a constant range even when the external environment changes. Temperature, pH, osmotic pressure, blood glucose level, and oxygen concentration are examples. The French scientist Bernard proposed the concept of the “internal environment,” and the American scientist Cannon proposed the term “homeostasis.”

Further Study

Hemoglobin is a chemically beautiful molecule. It consists of four subunits (two α chains + two β chains), each containing a heme (a porphyrin ring + Fe²⁺) at its center. Iron ions bind reversibly to oxygen molecules, allowing oxygen to be taken up in the lungs and released in tissues.
Oxygen binding occurs cooperatively, so the oxygen dissociation curve has an S-shape (the Bohr effect). In tissues with high CO₂ concentration and low pH, hemoglobin releases oxygen more easily (a shift to the right). Carbon monoxide binds to heme with a strength more than 200 times that of oxygen, causing carbon monoxide poisoning.
Blood clotting: When a blood vessel is injured, platelets gather at the wound. The blood protein fibrinogen is activated and changes into fibrous fibrin, which traps red blood cells and other components to form a blood clot that closes the wound. This reaction requires Ca²⁺ ions and vitamin K; when they are deficient, bleeding may not stop. Chemical substances such as heparin (an anticoagulant) and nattokinase from natto bacteria are also closely related to medicine.
Negative feedback is the basic principle of maintaining homeostasis. For example, when body temperature rises, sweating lowers it; when it falls, shivering raises it. The farther a value moves from its target, the more strongly the system acts to restore it. This is the same principle as automatic control of chemical reactions.

Points to Note

① Do not confuse red blood cells (no nucleus, oxygen transport), white blood cells (nucleus, immunity), and platelets (no nucleus, hemostasis). ② One hemoglobin molecule has 4 heme groups and binds 4 oxygen molecules. ③ Plasma (the liquid component) and serum (plasma with fibrinogen removed) are different. Serum is used in immunology. ④ Homeostasis means “maintaining the same condition” and comes from Greek.

Practice

1. What protein binds and transports oxygen in red blood cells?
2. Which scientist proposed the term “homeostasis”?
3. What protein becomes fibrous and closes a wound during blood clotting?