Homeostasis

I. Regulation of Body Temperature A. thermoregulation - the maintenance of body temperature within a range that enables cells to function efficiently. 1. ectotherms - animals who warm their body mainly by absorbing heat from their surroundings. i.e.) most invertebrates, fish, amphibians & reptiles 2. endotherms - animals who derive most or all of their body heat from their own metabolism i.e.) mammals, birds, some fish, and numerous insects are endothermic.
A) Endothermic RegulationAdvantages: the animal can remain active at a wide range of environmental temperatures.Disadvantages:1) It takes a great deal of energy2) Birds and mammals can't tolerate much change in their core temperature.3) Body size restrictions - the animal can't be too small.
Thermoregulation in mammals and birds involves generating heat, retaining heat, &cooling mechanisms.
1. thermiogenesis - active generation of heat
a. oxidative metabolismb. shivering - rapid contraction of opposing musclec. nonshivering thermiogenesis - occurs in some mammals and a few birds; increase in metabolism triggered by hormonesd. utilization of brown fat - fat with numerous mitochondria (thus brown color). These mitochondria release heat from metabolism (no ATP generation). Brown fat is found in human infants, bats & hibernating mammals.
2. regulating heat exchange - slowing heat loss
a. Vasoconstriction of vessels close to surface of body prevents heat loss from blood flowing close to the surface. This also helps to keep core temperature stable. Costly- the limbs cool down and the muscles don't work as well.b. Insulation - fat, boy hair, feathers
3. regulating heat exchange - cooling
a. vasodilation of vessels close to surface of body allows heat escape by radiation from blood vessels.b. convection - heat lost by the movement of air across the surface of the body; evaporative coolingc. conduction - the direct transfer of heat by contact to a cooler solid; such as an animal sitting in a pool of cold water or on a cool rock.
Endotherms also can help to regulate their temperature by behavioral mechanisms:ceasing activity & finding a cooler environment.
4. controlling thermoregulationIn humans, the hypothalamus, the body's thermostat, monitors the temperature of the blood flowing through it & also receives information from sensory receptors in the skin. In response to temperatures below the normal range, the thermostat activates thermiogenesis & heat-saving mechanisms. In response to warmer temperatures, the thermostat activates body cooling mechanisms such as vasodilation, sweating, or panting.B) Ectothermic RegulationAdvantage: takes little energyDisadvantage: in cool weather, activities slow downEctotherms can tolerate a wider range of temperatures than endotherms.Ectotherms generate their body temperature by behavior- absorbing sun or contact with a warmer surface.Ectotherms use many of the same mechanisms for regulating body temperature such as endotherms: vasodilation, vasoconstriction, conduction, convection, and evaporative cooling.
II. Water Balance & Waste Disposal A. Osmoregulation Process of maintaining proper water & ion balance. Some animals remain isotonic to their environment (osmoconformers) - they change their osmolarity as that of the environment changes. Other animals are osmoregulators - and must regulate water and salt balance (They stay fairly constant inside in spite of the environment outside.). A. Marine animals 1. osmoconformers - change with the environment. Most marine animals are osmoconformers & can tolerate some change in salinity (some more than others). These animals must regulate kinds of ions. stenohaline. euryhaline. 2. osmoregulators a. sharks and rays - maintain body fluids isotonic to cytoplasm of cells by pumping out salt (thru rectal glands into anus); retain an osmolarity close to seawater b. bony fish - pump out salt with salt screening cells in gills c. marine birds and reptiles - secrete salt thru special glands located near eye, drain thru duct into nose d. marine mammals - use kidney as salt-excreting gland
B. Freshwater animals Pump out water; also have to expend energy to pump ions back in. a. planarians (flatworms) - flame cell system b. fish - kidneys; gills pump salt back in. C. Terrestrial animals Drying out problem. Only arthropods and vertebrates are successful on land. a. earthworm - nephridia; used for osmoregulation and excretion of nitrogenous wastes b. arthropods - water resistant cuticle; protected respiratory surface; elimination of nitrogenous wastes as uric acid; Malpighian tubules (in insects) for osmoregulation and excretion of nitrogenous wastes. c. vertebrates - water tight skin; specialized kidney
III. Excretory Systems A. Vertebrate Kidney Used for osmoregulation and excretion of metabolic wastes. A) Structure and Function of Kidneys 1. Structure Kidneys: cortex - outer portion consisting of nephrons and blood vessels medulla - inner portion consisting of loops of nephrons; pyramids - joined collecting ducts renal pelvis - large collecting duct
Urine - exits kidney through a duct called the ureterà urinary bladder à urethraKidneys are made of secretory tubules called nephrons (functional units of the kidney).
Nephrons remove nitrogenous wastes and osmoregulate.~1,000,000 nephrons per human kidney
Nephron structure:Bowman's capsule - surrounds capillary bed (glomerulus)proximal tubuledescending tubule - of loop of Henleascending tubule - of loop of Henledistal tubulecollecting duct
Each part has a special function in secretion or absorption of certain molecules.Nephrons oriented perpendicular to kidney surface. Loop of Henle oriented toward medulla; capsules, proximal & distal tubules - in cortex.Two kinds of nephrons:
1) cortical - found only in the cortex; short loop of Henle; 80% of nephrons in humans2) juxtamedullary - extend from cortex to medulla; long loop of Henle; 20% of nephrons in humans; important for water conservation
Blood vessels to kidneys:Blood enters kidneys via renal artery and exits via renal vein.
the pathway of the flow of blood throughout the kidneys. afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries, vasa recta. 2. Function- Physiology of the Nephron Bowman's capsule Force filtration occurs here. Filtration is nonselective. The fluid from the blood is forced through the porous capillaries of the glomerulus. The Bowman's capsule has specialized cells called podocytes. Together they filter the blood, allowing water and small solutes, but not blood cells or large molecules such as plasma proteins to pass through. Proximal tubule Much of the reabsorption & some secretion occur here. The transport epithelium which lines this tube have a brush border (like in the small intestine). Reabsorption: (Remember that reabsorption is the returning of the substances that were force filtered into the Bowman's capsule back to the blood.)
- ~70% of the water is reabsorbed - 75% of the NaCl is reabsorbed, by active transport. (Actually Na+ is actively transported out and Cl- follows passively.)- Glucose & amino acids are reabsorbed by Na+ cotransport.
- K+ and HCO3- are reabsorbed by passive transport.
Secretion:(Remember that secretion is the 'transport' of substances from the blood to the kidney tubule.)- drugs and poisons that have been processed by the liver- H+ is actively transported into the proximal tubule. H+ secretion helps to maintain a constant pH in the body.- Ammonia (NH3+) which is actually synthesized by the cells of the transport epithelium is secreted into the tubule. The ammonia neutralizes the acidity of the filtrate. Loop of Henle - descending limbReabsorption of H2O continues here. The transport epithelium of the descending limb is permeable to water, but not very permable to NaCl and other small solutes. In addition, the medulla is increasingly saltier (hyperosmotic) than the cortex.Loop of Henle - ascending limbHere the transport epithelium is permeable to NaCl but not to water. Initially this part of the tubule is thin. NaCl diffuses out. The tubule becomes thick. NaCl is actively transported out of the thick segment. Thus the filtrate becomes more dilute.Distal tubuleAnother region of reabsorption & secretionReabsorption:- Na+ is actively transported out of the tubule (Cl- follows).- H2O follows NaCl (controlled by ADH - antidiuretic hormone)- HCO3 - is reabsorbed; contributes to regulation of pH
Secretion:- H+; contributes to regulation of pH- K+; this is where the concentration of K+ in the body is controlled- Some drugs are secreted here, i.e. penicillin Now the filtrate in the tubules is isotonic to the blood in the nearby capillaries.Collecting ductThis epithelium is permeable to water (due to action of ADH) but not to NaCl.Filtrate becomes more and more concentrated as water is lost.
The bottom of the collecting duct is permeable to urea. Some urea diffuses out. This contributes to the high osmolarity of the medulla. The urea diffuses back into the tubule at the bottom of the loop of Henle. The point is that by keeping the medulla high in osmolarity, the kidney can conserve water while secreting a hyperosmotic urine. This is a key adaptation to terrestrial life- it enables mammals to get rid of nitrogenous wastes without a great water loss.
3. Regulation of KidneysSo, now you understand that the kidneys can secrete a urine that is very hyperosmotic to the blood & body fluids, thus conserving water. Sometimes, however, it may be important to be able to secrete urine that is not only not hyperosmotic, but actually hypoosmotic.There are hormonal and nervous system controls that make it possible for the kidney to have this kind of verastility.
a. ADH - antidiurectic hormoneThe hypothalamus (part of the brain) & the walls of the circulatory system & kidney have sensory cells that measure the osmotic pressure of the blood as it passes through.If the osmolarity of the blood drops, the osmoreceptor cells stimulate the release of ADH (made by the hypothalamus, but stored and released from the pituitary.)
ADH acts on the distal tubules & the collecting duct to increase their permeability to water. Thus more water is reabsorbed.ADH is inhibited by caffeine and alcohol. Consumption of these liquids leads to large volumes of dilute urine - diuresis.
b. JGA - juxtaglomerular apparatusThe JGA is a specialized tissue that crosses the afferent arteriole. The JGA regulates blood pressure and volume. If the blood pressure or volume in the afferent arteriole drops, the enzyme renin is released.Renin catalyzes the reaction that converts angiotensiogen (a blood protein) to angiotensin II.Angiotensin II increases blood pressure and volume by :
1) constricting arterioles2) stimulates proximal tubules to reabsorb more NaCl and water3) stimulates adrenal glands to release aldosterone, which acts on distal tubules causing them to reabsorb more Na+ and water.
This entire system is called the RAAS - renin, angiotensin, aldosterone system
c. ANF - atrial natriuretic factorThis is another hormone - a peptide - that opposes RAAS.Walls of the atria of heart release ANF in response to an increase in blood volume and pressure.ANF inhibits release of renin and aldosterone.
4. Excretion of Nitrogenous WastesMetabolism produces toxic waste products. i.e.) nitrogenous waste from the metabolism of proteins and nucleic acids. Some animals excrete ammonia(toxic) directly. Others convert it to urea or uric acid.