Homeostasis in Biology

Study Notes

Homeostasis is the maintenance of a constant internal environment.
Claude Bernard (1857) stated that a constant internal environment is necessary for life under varying external conditions.
Body functions properly when the fluid surrounding cells is maintained within narrow limits.
Small fluctuations can disrupt biochemical activities; large fluctuations can kill cells.
Homeostasis is Greek for "staying the same."

Chemical composition: Ions, glucose
Blood pH: Range 7.35 to 7.45 (CO2 + H2O = H2CO3; H2CO3 = H+ + HCO3-)
Osmotic pressure: Osmoregulation
Temperature: Regulated in birds and mammals (endotherms); not regulated in most fish and amphibians (ectotherms). Some ectotherms use behavioral mechanisms for temperature regulation.

Temperature regulation & conforming: Some animals (like river otters) regulate their body temperature, while others (like largemouth bass) conform to the surrounding environment.
Regional heterothermy in tuna: Tuna display different temperatures in different parts of their bodies.
Behavioral regulation in ectotherms: Ectotherms can regulate their temperature through behaviors like radiation, evaporation, convection, and conduction.
Pre-flight warm-up in moths: Moths warm specific parts of their bodies before flying.

Stimulus: Temperature change
Receptors: Skin and hypothalamus
Control Center (Set Point): Hypothalamus
Effectors: Skin blood vessels, sweat glands, hair (fur), muscles
Response Mechanisms: Blood vessels constrict or dilate; sweat production; piloerection; shivering thermogenesis.
Homeostasis: Internal body temperature of approximately 36-38°C.

Hyperosmotic: Contains more solutes than another solution and less water.
Hypo-osmotic: Contains less solutes than another solution and more water.
Water moves by osmosis from a high to a low concentration (hyperosmotic to hypo-osmotic).

1. Removal of nitrogenous waste: Products of protein and nucleic acid metabolism.
1. Regulation of water content: Osmoregulation.
1. Regulation of salt balance: Body concentration of Na+, K+, Cl-, Ca²+, Mg²+, etc. (Ionic regulation).

When amino acids are metabolized, the amino group (-NH₂) is removed, forming ammonia (NH₃).
Ammonia can be excreted as: ammonia/ammonium, urea, or uric acid/guano.

Feature	Ammonia	Urea	Uric Acid
Toxicity	High	Low	Very Low
Solubility (ml for 1g N)	500	50	1
Metabolic Cost	None	Some	High
Examples	Fish	Mammals	Birds

Two bean-shaped organs, the size of a clenched fist, located on the back wall of the abdomen.
Receive 25% of cardiac output (1.25 liters per min); 1% body weight.
Control chemical composition of blood.

There are approximately 2 million nephrons in the human body.
Each nephron is 30-60 mm long, totaling ~60 km.
Two kinds of nephrons exist: juxta-medullary (for very concentrated urine) and cortical (for less concentrated urine).

Fluid moves by ultrafiltration.
Hydrostatic pressure forces fluid and solutes through glomerular capillaries into Bowman's capsule.
Passive process (no energy required).

The glomerular ultrafiltrate is similar to blood plasma but lacks plasma proteins.
Glomerular filtrate contains various substances (e.g. urea, uric acid, glucose, amino acids, inorganic salts, protein)

Promoting filtration: Glomerular hydrostatic pressure (55 mm Hg).
Opposing filtration: Capsular hydrostatic pressure (15 mm Hg), glomerular colloid osmotic pressure (30 mm Hg).
Net filtration pressure:10 mm Hg.