The Internal Environment

• Negative feedback reverses a change and brings a factor back to normal — this is how most homeostasis works.
• Positive feedback pushes a factor further away from normal — useful for quick, one-off events like childbirth or blood clotting, but NOT part of homeostasis.
• Homeostasis keeps internal conditions (temperature, blood glucose, water potential) within narrow limits so enzymes and cells work properly.
• Thermoregulation uses skin structures (arterioles, sweat glands, hair erector muscles) and the hypothalamus to control body temperature via nervous and hormonal responses.
• The kidney is both an osmoregulatory organ (controls water content of blood) and an excretory organ (removes urea and excess salts).
• Urea is made in the liver by deamination of excess amino acids, forming ammonia, which is converted to less-toxic urea via the ornithine cycle.
• Ultrafiltration in the glomerulus/Bowman's capsule forces small molecules out of the blood under high pressure.
• Selective reabsorption in the proximal convoluted tubule reclaims useful substances (glucose, amino acids, salts, water) back into the blood.
• The loop of Henle acts as a countercurrent multiplier, creating a very concentrated medulla so water can be reabsorbed and urine concentrated.
• ADH, released by the posterior pituitary, controls how much water is reabsorbed in the distal convoluted tubule and collecting duct by adjusting aquaporin numbers.
• Hormones control gene expression either directly (steroid/thyroid hormones acting as transcription factors) or indirectly (peptide hormones using second messengers like cAMP).

Think of negative feedback like the thermostat in a house. If the room gets too cold, the heating switches on. Once it's warm enough, the heating switches off again. The system is always trying to pull things back toward a set point — it never lets the temperature drift too far in one direction.

In the body, the same logic applies to things like body temperature, blood glucose, and water balance. Every negative feedback loop has the same basic cast of characters:

• Receptor — detects a stimulus (a change in a physiological factor, e.g. a drop in temperature).
• Coordination system — passes the message along, either via the nervous system (fast, short-lived) or the hormonal system (slower, longer-lasting).
• Effector — a muscle or gland that actually carries out the response.

decrease increase two dynamic equilibrium

amplifies