Thermoregulation: Another homeostatic system Review: Negative Feedback loops:
A stimulus produces a reaction that ultimately reduces the stimulus. For example, a baby is hungry and cries. When baby gets fed, it stops crying. The stimulus for the baby is hunger. The baby's response is to cry. The parental stimulus is the baby's cry. The parent response is to feed the baby.
Now assume that the baby cried because of a deep rash. If the parents got up and tried to feed it, the baby would cry louder and the parent would try harder to feed. This is a positive feedback loop. The stimulus (crying) evokes a response that further increases the stimulus.
Negative feedback is very stable in nature.
Positive feedback is unstable and uncommon in nature. It is often associated with illness.
Another non-biological example of Negative Feedback is the home thermostat. Biological homeostatic systems have limits. Many different organisms can regulate their body temperatures but over a wider range than others.
Thermoregulation:
Chemical reactions are influenced by temperature. Animals must develop a life style that permits survival over a range of internal temperatures or must develop ways to keep the temperature constant.
Ectotherms are cold blooded (poikilotherms) organisms that absorb heat from the surroundings. Endotherms are warm blooded (Homeotherms) organisms that maintain a consistent internal temperature. They derive heat from the metabolism.
Why Thermoregulation?
Animals are subjected to changing environmental temperatures. There are two ways to deal with this:
1) Some organisms go into a metabolic torpor where biochemical processes slow down and they become sluggish or immobile.
2) Organisms can take measures to conserve metabolic heat and retain it for the parts that need it the most.
In Ectotherms: the rates of most enzyme reactions increase by a factor of about 2 for every 10oC, until the protein denatures. This is the Q10 effect, e.g. the rate of enzyme reactions is usually twice as fast at 33oC than at 23oC.
Thermoregulation involves mechanisms that cause heat loss, as well as heat gain. In fact,
physiological damage created by too much excessive heat is often permanent. Damage caused by low temperatures is often temporary, except in extreme cases (frostbite). Below 36oC (97oF) or above 40oC (104oF) can cause disorientation, and above 42oC (108oF) can cause convulsions and permanent cell damage.
A) Conduction is a direct transfer of heat between the environment and body surfaces. Heat is always conducted from a body of higher temperature to one of lower temperature.
B) Convection is the flow of air or liquid past the surface of a body.
C) Radiation is the transfer of heat between objects that are not in direct contact. Heat is transferred as electromagnetic waves.
D) Evaporation is the loss of heat from the surface of a liquid that is losing some of its molecules as gas.
Thermoregulation In Animals: Invertebrates:
Invertebrates have little control over body temperature. They adjust their temperature through behavioral or physiological mechanisms. However, some larger flying insects can generate internal heat, and the Winters moth have countercurrent exchange.
Honeybees maintain a special social organization that keep them warm in the winter and cool in the summer.
Fishes:
How tuna swim fast:
Tuna have a specialized circulatory arrangement.
In a slow moving fish, major vessels are centrally located, with branches serving the propulsion muscles and outer portions of the body. This kind of system tends to cool the blood, preventing heat from accumulating in the swimming muscles.
In a tuna, major arteries and veins are always paired and parallel, as are peripheral branches. This fosters counter current exchange.
Counter current exchange: When arteries and veins run closely parallel carrying blood in opposite directions, the blood is of different temperatures. Venous blood is cold, while arterial blood is warm. The arterial blood gives up heat to the returning venous blood. Also in some swimming muscles there is a rich supply of blood, this network of blood vessels is called the rete mirabile (wonderful net). Counter current heat exchanger helps keep the swimming muscles warm.
Regulating Behaviorally:
Fish move to a temperature zone that is appropriate for the species.
Amphibians:
The optimal temperature for amphibians varies from 7-25oC. They produce little heat and lose heat rapidly through evaporation. Behavioral adaptations enable them to maintain body temperature.
Reptiles can also improve behavioral thermoregulation by changing color. The horny toad is actually a horned lizard. When the body temperature is low, the skin darkens, increasing the absorption of sunlight.
Marine Mammals:
The optimal body temperature for a marine mammal is around 36 - 38oC. Mammals and birds are now Endotherms. When it is cold, they can increase ATP production (increase metabolism). The ATP can be used as heat. As the ATP production increases, the oxygen consumption, and
circulation also increase. The metabolism of marine mammal is not much higher than land mammals that conserve heat more effectively. Marine mammals have insulation in the form of blubber with countercurrent exchange in tail and flippers. When they migrate to a warmer environment, the blood vessels of the skin dilate.
Birds:
Birds have an average body temperature of 40oC. They have no sweat glands, but they can pant to promote evaporative heat loss. Feathers provide great insulation, but birds must warm their legs. Through the legs they lose large amount of heat. Warm blood from the body core must flow to cells of the extremities.
Countercurrent heat exchange reduces heat loss. Arteries carrying warm blood down the legs are in close contact with veins carrying blood in the opposite direction. The heat from the arterial blood is transferred to the vein. Blood in the veins becomes warmer and warmer as it moves up the legs, as the veins come into contact with warmer arteries.
Birds and Mammals conserve heat by a number of methods:
In winter, birds puff up and mammal's hair stands up on end (piloerection). These actions trap insulating air under the hair and feathers. Shivering is a method of heat production. Other animals store layers of fat or may huddle together. Some use Brown Fat, special fat, which provides energy for quick energy production. Basically, they control the amount of heat lost to the environment they can change their behavior and the rate at which ATP is produced.
Terrestrial Mammals: Thermoregulation: Production of Heat:
Metabolism generates heat that warms the body. Fur and fat help retain the heat. The rate of heat production or heat loss to the environment can be controlled in the following ways. This is our ‘heating center’:
a) Increase contraction of muscles by moving and shivering
b) Action of certain hormones such as epinephrine and thyroxine may increase cellular respiration rates. The adrenal medulla is stimulated and releases epinephrine. There is an energy increase in liver and skeletal muscles. This will increase the metabolic rate, there is an increase in ATP usage and this increases heat. This is an immediate response.
metabolism, increase ATP production and increase heat. This takes days to weeks to occur.
This brings up the discussion of metabolism, the amount of energy your body uses. The metabolic rate can change with activity or season of the year. To standardize the rate of metabolism, we will use Basal Metabolic Rate (BMR).
A way to monitor BMR is to monitor the respiratory activity. The amount of energy used is directly proportional to the amount of oxygen taken in. An average person has a BMR of 70 kcal/hour or 1,680 kcal/day. There is still some variation according to age, gender, body weight, physical condition, genetic influences, etc.
Sometimes we can measure the amount of circulating thyroxine to measure BMR.
If your daily intake of calories exceeds the amount needed to survive, the excess energy is stored as lipids in adipose cells.
To control appetite, there are a variety of mechanisms:
Stretch receptors in the digestive system, especially the stomach, tell you when you’re full.
Cholecystokinin (CCK) and adrenocorticotrophic hormone (ACTH) can
suppress appetite.
Adipose cells produce leptin. Leptin will bind to the CNS and controls appetite. If the adipose cells lose lipids, then leptin production decreases, and you want to eat.
c) Some mammals have BROWN FAT which is a tissue located in the neck and between the shoulders. Brown fat is specialized for rapid heat production. Human babies are born with brown fat and use it to produce energy for the first few days of life (until the mother starts producing milk fat—the mother produces colustrum for the first three days). In fact, it is normal for the baby to lose 10% of its weight in the first 3 days of life—all in brown fat. At the end of the first year, all the brown fat is gone.
d) Adjust the rate of heat exchange with the environment: Vasoconstriction occurs when the superficial blood vessels constrict and the blood flow decreases. The body temperature increases.
e) Behavior: Many animals bask in the sun when it is cool and hide/migrate when it is hot
Cooling Off: the following methods are used by mammals to cool themselves. This is their cooling center.
a) Change the rate of metabolic heat production. Their thyroid stops producing thyroxine, so the cells decrease the ATP production. Hypothalamus will decrease TRH, which doesn’t stimulate the thyroid to release TSH, which doesn’t stimulate the thyroid to release
thyroxine. There will be a decrease in metabolism by the cells. The adrenal medulla will not be stimulated to release epinephrine, the liver and skeletal muscles will not increase heat production.
superficial blood vessels dilate; this increases the blood flow to the body surface. The heat is transferred to the environment by conduction, convection and radiation. Body
temperature decreases.
c) For mammals, large ears are a way to get rid of heat. There are a lot of blood vessels that lose the heat to the environment.
d) In the summer, mammals shed.
e) Evaporative heat loss is loss of water from the respiratory tract surface and across the skin. Panting and sweating increase evaporation.
f) Behavior: Many animals bask in the sun when it is cool and hide/migrate when it is hot.
The Thermostat:
How the body measures the temperature.
1) The brain is constantly monitoring the temperature of the blood flowing through it. Temperature monitoring of the brain is done by the HYPOTHALAMUS, which behaves like an ordinary
thermostat. It can distinguish differences as small as 0.01oC. The hypothalamus has two thermoregulatory areas:
a) The heating center controls vasoconstriction of superficial vessels, erection of fur, and shivering.
b) The cooling center controls vasodilation, sweating, and panting.
2) Sensory neurons on the skin are sensitive to temperature. Some of the temperature receptors are called Ruffin Organs (warm receptors); increase the activity when temperature increases. Others are called Bulbs of Krause (cold receptors) and increase their activity when their temperature decreases. Warm receptors stimulate the cooling center of the hypothalamus and the cold receptors stimulate the heating center.
If the temperature is too cold, The Bulbs of Krause sends a message to the brain. The hypothalamus also picks up the temperature by the blood. The hypothalamus will stimulate the warming center, which increases heat production. The Hypothalamus will release a hormone that stimulates anterior pituitary to release TSH (thyroid stimulating hormone). This causes the thyroid to increase
thyroxine production, increasing metabolic activity to increase body temperature. The
hypothalamus also stimulates the adrenal medulla to release norepinephrine, which also increases the metabolic activity of cells thus creating heat. The heating center also causes blood vessels to vascocontrict. The organism may use brown fat to produce energy, start to shiver, move to a warmer location, or huddle with other animals.
If too hot the Ruffin Organs send a message to the brain. The Hypothalamus also picks up the temperature change by monitoring the blood. The hypothalamus stimulates the cooling center. The organism will start to pant or sweat. The blood vessels will vasodilate. The hypothalamus will stop stimulating the anterior pituitary, which will stop stimulating the thyroid, which will decrease the cell metabolism. The hypothalamus will also stop stimulating the adrenal medulla, which will also decrease cellular metabolism. The animal might start to shed or move to a cooler location.
Temperature Acclimation:
Physiological acclimation to a new temperature has several aspects: 1) Cells may increase the production of certain enzymes.
2) They may produce the same enzyme but at a different temperature optima.
3) The cell membrane may change the properties of saturated and unsaturated lipids. 4) Cells can make rapid adjustments to temperature changes. In mammalian cells with a rapid increase in temperature (37 – 43oC), heat shock proteins are produced, which help maintain the shape of other proteins.
Torpor:
A state in which metabolism decreases and the heart and respiratory system slows down.
HIBERNATION allows an animal to withstand long periods of cold temperatures and decreased food supply.
