CHAPTER 3: Homeostasis

homeostasis definition

• maintenance of stable, balanced internal environment in response to changes in both the interna and external environments

• achieved through range of mechanisms inc:

  • structural - physical features assist in tolerating changes

  • physiological - internal processes detect and respond to changes

  • behavioral - behaviors or actions helping organisms survive environment

• variable factors must be kept at optimum levels, changes in environment cause variables to become electaed or lowered - homeostasis maintains optimum

tolerance limits general

• max and min range of factors individuals can tolerate before life processes cant be maintained - limit is different depedning on organism and environment

• Liebig’s Law of the Minimum - if variables are outside of tolerance levels, poor functioning of organism will result, irrespective of other variables

tolerance limits - body temperature

• mammals body temp is 36-38C

• enzymes work best around 37C and are responsible for all cell’s metabolic activity

• internal temp rises much above 40C → hyperthermia

• internal temp drops few degrees below 37C → hypothermia

tolerance limits - water

• water/solute concentration balance in extra- and/or intracellular environment

• shift from normal level leads to swelling or shrivelling of cells due to osmosis

• solute concentration of 0.9% NaCL is isotonic with blood plasma

tolerance limits - blood carbon dioxide concentration

• normally between 5-6% in humans

• non-toxic gas at normal levels, needed to stimulate and control rate and depth of breathing

• high levels of CO2 dissolving in blood cause ph to lower (acidosis)

• levels greater than 10% cause highly toxic levels - lead to unconciousness and death

tolerance limits - blood glucose levels

• normally around 75-95mg/dL - sustained deviations cause serious health issues

• hyperglycaemia (too high blood glucose) and hypoglycaemia (too low blood glucose - if untreated both lead to symptoms inc. loss of consciousness

• diabetes - high level of blood glucose - lack of insulin hormone or tissue responses to insulin

how variables in internal environment work in practice and how responses control them

• variable in internal environment oscillates around set-point in practice

• factors in environment cause inc or dec in variables and homeostatic responses work in the opposite way to reverse changes

stimulus response model elements

• stimulus - variable factor in internal or external environment can be detected by organism

• receptor - cells of tissues detecting stimulus and send messages along nerve cells

• transmission - relay of information via nerves and/or hormones to an effector

• effector - usually a gland or muscle bringing about response after receiving information

• response - action occuring due to initial stimulus

• feedback - impact of response on initial stimulus (positive or negative)

negative feedback

• response diminishes or reverses original stimulus

• Homeostasis is characterised by it as it returns the original variable factor that was changed back to steady state

• e.g. inc. in exercise raises metabolic activity, inc blood temp. blood temp inc detected by receptors in tissues - homeostatic responses by effectors reverse the original stimulus and lower the temperature

positive feedback (e.g. babty)

• response reinforces or brings about an increase in the initial stimulus

• less common

• e.g. newborn baby suckles on mother - causes release of hormone in mother, stimulating further release of milk (initial response).

  • releases oxytocin in mothers brain - promotes affection and bonding between the two

  • oxytocin passes through breast milk to baby and promotes bond

  • more breastfeeding → more oxytocin released → stronger bond between baby and mum

nervous system

• involved when more rapid and direct communications are required

• made up of the central nervous system (CNS) and the peripheral nervous system (PNS)

• nerves in PNS relay information from the sensory receptors to CNS and from the CNS to the effectors

central nervous system (CNS)

• consists of brain and spinal cord

• primary role - storing, arranging, managing information

peripheral nervous system (PNS)

• mainly involved in transmission of information to and from CNS - nerves in PNS relay information from the sensory receptors to the CNS and from the CNS to the effectors

• subdivided into somatic (voluntary) and autonomic (involuntary) types

• Voluntary nerves are under conscious control (e.g. influencing skeletal muscles)

• autonomic system consists of nerves involved in range of unconcious responses (e.g. changes in heart rate, activity of body systems, gland activity)

  • further divided into sympathetic and parasympathetic systems

neuron and how they transmit messages

• receptors detect stimuli and send messages along nerve cells (neurons) to CNS, muscles, and glands to bring about a response

• 3 types of neurons: sensory, interneuron (relay), motor

• transmit messages through electrochemical ipulses

  • involves changing conc grandients of sodium and potassium ions - requires movement across cell membranes through facilitated diffusion and active transport

parts of neuron

• dendrite - receives impulses from sensory receptors or other neurons and transmits info towards the cell body

• cell body - contains many cell organelles, ER, mitochondria

• axons - long extensions of a nerve cell, necessary to transmit info to another cell or effector. Microscopic but very long

  • end of axon has axon branches - have tiny swellings at ends, release neurotransmitter chemicals which transmit info between neurons or between neuron and effector

  • some have fatty covering called myelin sheath - formed by Schwann cells, wrap around axons. nerve impulses travel faster along neurons with sheaths as impulses jump from node to node

sensory neuron (inc structure, location of cell body)

• transmit information from receptors (e.g. cells detecting sound, light, temperature) to the CNS

• has cell body joined togehter by 2 long processes - dendron and axon

• extensions of dendrons (called dendrites) act as receptors - transmit info towards cell body

interneuron (relay) (inc structure, location of cell body)

• located in the CNS, transmits information from sensory. neurons to motor neurons

• have short axons as they enable connections between sensory and motor neuron

motor neuron (inc structure, location of cell body)

• transmit messages from CNS to effectors (typically muscles or glands)

• usually have short dendrites and one long axon

synapses and neurotramsitters

• synapse - junction between neurons or between neuron and receptor/effector

• The end of the axon is found close to the membrane of dendrite or the cell body of the next neuron

• An electrical message cant cross the synaptic cleft (gap between neurons)

• nerve impulse causes release of chemical transmitter substance into synaptic cleft called neurotransmitter - diffuses across space, binds to receptors on receiving membrane

• activates ion channels in the membrane, leading to nervous impulses being transmitted further

• neurotransmitter can be broken down and deactivated by enzymes