Principles of Homeostasis

• Endocrine gland: secretes product directly into the bloodstream.
• Homeostasis: maintenance of constant internal conditions 
• Receptor: structure that detects changes in the environment+react to the stimuli
• Effector: structure that responds to a stimulus+brings about a response, e.g. muscles, glands.
• Synapse: junction between two nerve cells.
• Paracrine signalling: occurs between cells that are close together.
• Endocrine signalling: involves signalling over longer distances through the circulatory system.
• There are several things that need to be regulated:The body's temperature. If temperature were allowed to rise out of control, protein and therefore enzyme, structure would be affected, perhaps with disastrous results.
• The amount of water within the body. The levels of water can affect metabolism and osmosis.
• The amount of glucose in the body. This level can also affect osmosis and obviously the rate of respiration as well.
• The amount of nitrogenous waste in the body. Nitrogenous waste can become toxic in the body. It is important that this level does not get too high.

• Negative feedback: normal value of a physcological factor is detected and a response is made that restores the value to the norm.

• Positive feedback: proliferation rather than control:

• Radiation: loss of heat from hot objects into cooler surroundings.
• Convection: movement of warm air upwards, density of air decreases when its warm.
• Conduction: transfer of heat energy from a warmer material to a cooler one.
• Evaporation of water from the skin cools the surface, heat energy needed to change the water into water vapouris extracted.

Temperature control in ectotherms:

•  aquatic ectotherms no problem in maintaining stable body temp= water temps don't fluctuate=water has high SHC.
• On land: make sure they dont expose their bodies to extreme temps+must warm their body up when their cold. Low body temp= sluggish movement due to enzymes not working at their optimum temp= more vunerable to predators in morning.
• Ectotherms can tolerate a larger range of body temps than endotherms.
• However, they are much more restricted by their environment temp than endotherms, cannot easily conolise very hot or cold habitats.
• Endotherms need a lot of energy (higher metabolic rate) to maintain a stable body temp, therefore:
• Ectotherms can survive better when food is limited because they need less energy to maintain a stable body temp.

To control body temp endotherms have mechanisms to increase or decrease their temp depending on stimulus:

Cooling mechanisms: 

• Vasodilation of skin capillaries: Arteriole muscles relax to allow more blood flows through the capillaries=blood will lose more heat to surroundings because heat energy has a shorter distance to diffuse out of the capillaries.
• Sweating: Sweat glands produce sweat in hot conditions, evaporation of the sweat cools the body down because energy is required to convert water into water vapour. This heat energy is extracted from the skin during evaporation.
• Flattening of hairs: Air trapped between hairs forms insulating layer, therefore in hot conditions the hair erector muscles relax, so the hairs lie flat=insulating layer of air on skin is thinner, more heat lost.

• Boosting metabolic rate: Most chemical reactions=endothermic. Therefore, in cold conditions thyroxine is released from the thyroid gland, this hormone boosts metabolic rate, thus increasing heat production.
• Shivering: Reflex action from the nervous system. Effectors=muscles, rapid+regular muscle contractions generates heat by the metabolic reactions taking place=warms the blood.
• Vasoconstriction: Arterioles constrict= less blood flows through capillaries=blood is diverted to shunt vessels which are deeper in the skin=not as much heat lost to surroundings because heat energy has a larger diffusion pathway to diffuse out of the skin.
• Erection of hairs: Erector muscles contract (piloerection)=hairs stand on end= larger layer of air, insulates skin more, less heat lost.

• Endotherms also exibit similar behavioural responses to ectotherms: seek shade when environemnt is hot, and will take measures to increase body temp when its cold.
• E.g. Hippos stay in the water throughout the day and come out of the water to feed at night.
• E.g. penguins huddle together to withstand extreme artic conditions= less heat lost to surroudings.
• In order for these behaviours to take place, animal must have a means of dectecting external temp change: peripheral receptors= thermoreceptors found in the skin detect increase or decrease in external temp and then they communicate with the hypothalamus.

Monitoring temp in endotherms:

• Hypothalamus monitors core body temp and it has two control centres: heat loss and heat gain.
• These centres send out nervous and hormonal signals which bring about actions related to the temp control.
• E.g sweating, shivering, vasoconstriction and dilation controlled by nervous system.
• E.g. Thyroxine released from thyroid gland, controlled by TSH, which is released from the pituitary gland in response to hormones released from the hypothalamus.