Biology – Communication, Homeostasis, EnergyModule 1 – Communication and Homeostasis 1) THE NEED FOR COMMUNICATIONKeeping Cells Active – enzymes need a specific set of conditions in which to workefficiently, and cellular activities rely on the actions of enzymes. Therefore, all livingthings must maintain certain conditions inside their cells, including: - A suitable temperature - A suitable pH - An aqueous environment, containing substrates and products - Freedom from toxins and excess inhibitorsStimulus/ResponseExternal Environments – external environments will change, which will place a stresson the organism, so the organism must monitor these changes, and change itsbehaviour or physiology to remain active and survive.Stimulus – environmental changeResponse – how the organism changes its behaviour/physiologyThe environment may change gradually/slowly or more quickly, but either way, theorganism must respond to the change.Internal Environments – cells in most multi-cellular organisms are bathed in tissuefluid, the internal environment.The internal environment changes due to cellular activities (metabolic reactions)which use up substrate and produce product. If some products build up, they candisrupt the action of enzymes, i.e. by altering the pH. So excess product creates aSTIMULUS and a RESPONSE can be to reduce cellular activity.The tissue fluid’s composition – the internal environment – is maintained by theblood. Any waste/toxins/excess product will be carried away and excreted.Co-ordination – a communication system requires: - A mechanism to detect change - A mechanism to process information about change - A mechanism to react to the changeA good communication system will: - Cover the whole body
- Enable cells to communicate with each other - Enable specific communication - Enable rapid communication - Enable both short-/long- term responsesCell Signalling – messages sent between cells in order to co-ordinate cellularfunctions.There are two major systems for this: - Neuronal: interconnected neurones, signal to each other across synapses - Hormonal: hormone released by organ, stimulates target cells Neuronal Hormonal Signals are sent through electrical Signals through hormones and chemicals impulses Rapid response to stimuli Slower-acting response to stimuli, excluding adrenaline Short-time lift Longer-term responses Not incremental – all or nothing Incremental – can control how much/how little External stimuli from sensory receptors Internal stimuli by binding of chemicals to receptors on cells 1) HOMEOSTASIS AND NEGATIVE FEEDBACKHomeostasis – the maintenance of a constant, internal environment at a set point(norm) regardless of external changes. It involves a series of receptors, effectors andnegative feedback control mechanisms.Stimulus -> Receptor -> Pathway (cell signalling) -> Effector -> ResponsePositive Feedback – this is less common, as it initiates a response to increase theinitial change, therefore, it is usually harmful, i.e. temperature falling leads to furtherfall in temperature.
2) MAINTAINING BODY TEMPERATURE – ECTOTHERMSThe need to maintain body temperature – enzymes are globular proteins, with 3Dshapes specific to their function. Heat can denature proteins, or low temperaturescan lead to fewer collisions of enzyme and substrate, leading to less enzyme-substrate complexes forming, etc. Therefore, the level of metabolic activity is highlydependent on body temperature.Ectotherms – an organism that relies on external sources of heat to regulate its bodytemperature.Advantages: - Use less food in respiration, as they do not require respiration to maintain body heat, so they need to find less food and can survive for up to two weeks without meals. - They can grow larger, as energy used for respiration in endotherms can be redirected towards growth.Disadvantages: - They are less active at lower temperatures, so they need to warm up before becoming active, leaving them at risk of predation - They may require hibernation during winter, so they must be able to store sufficient stores of energyTemperature Regulation – when an ectotherm is cold, it must change its behaviouror physiology to increase the absorption of heat. When an ectotherm is hot, it mustchange its behaviour or physiology to decrease the absorption of heat. 1) MAINTAINING BODY TEMPERATURE – ENDOTHERMSEndotherms – organisms that can use internal sources of heat to maintain their bodytemperatureAdvantages: - Constant body temperature, whatever external temperature - Activity possible when external temperature is cool - Ability to inhabit colder parts of the planetDisadvantages: - Cold temperatures demand more energy from food, in order to create heat through exergonic reactions, i.e. metabolism - More food required in general - Less energy from food can be used for growth
Temperature Regulation:Sweat: secrete more from glands if hot, but less if it is cold. Heat evaporates waterfrom the skin, using latent heat.Lungs, mouth, nose: panting increases evaporation of water from moist surfaces,using latent heat.Hairs on skin: raise when cold, to trap a layer of insulating air, but they lie flat to theskin if cold.Arterioles: dilate if hot, and constrict if cold, in order to control flow of blood to theskin, where heat is lost by convection and radiation.Liver cells: low temperatures stimulate increased rate of metabolism, however, hightemperatures stimulate a decrease in the rate of metabolism.Skeletal muscles: if the temperature is low, there will be spontaneous contractions ofmuscles (shivering) which will generate heat as the muscle cells will have to respiremore. The opposite applies at high temperatures, i.e. there is no shivering, so nomore respiration in muscle cells. 1) SENSORY NERVESSensory Receptors – sensory receptors are specialised cells that can detectchanges to the external environment. They are energy TRANSDUCERS, meaningthey convert energy from one form to another. The changes in the externalenvironment can be: light levels, pressure on the skin, or many others. Changes inenergy levels in the external environment create a STIMULUS. The specialisedreceptors transduce the energy from the stimulus into electrical energy (anIMPULSE).Generating Nerve ImpulsesChanging Membrane Permeability – neurones have specialised channel proteinsacross their membranes, which are specific to either potassium or sodium ions. Also,they possess a “gate,” which can open or close the channel, thus changing thepermeability of the membrane to a particular ion. The channels are usually keptclosed.Polarisation – in addition, nerve cells contain carrier proteins (sodium/potassium ionpumps) that actively transport sodium ions OUT of the cell, and potassium ions INTOthe cell (remember this by Next of Kin). More sodium ions are actively transportedout of the cell than potassium ions are transported into the cell. This creates anegatively charged intracellular environment, thus POLARISING the plasmamembrane.Depolarisation – nerve impulses are created by altering the permeability of themembrane to sodium ions. The sodium ion channels open and move across themembrane down a concentration gradient (diffusion); this creates a change in thecharge across the membrane, and the inside of the cell becomes less negativelycharged. The cell surface membrane becomes DEPOLARISED.
Generator Potentials – receptors cells respond to changes in the environment,opening the gated sodium ion channels, allowing sodium ions to diffuse across themembrane. A small change in potential, for example if one or two sodium channelsopen, creates a GENERATOR POTENTIAL. The larger the change to the externalenvironment (the stimulus), the more gated channels will open. If enough sodiumions enter the cell, the charge will change significantly, initiating an impulse orACTION POTENTIAL.Sensory and Motor Neurones - Sensory neurones carry the action potential from a sensory receptor in the peripheral nervous system (PNS) to the central nervous system (CNS). - Relay neurones connect sensory and motor neurones together in the CNS. - Motor neurones carry an action potential from the CNS, back to the PNS, to an effector such as a muscle or gland.A motor neurone A sensory neuroneThe function of a neurone is to transmit an impulse from one area of the body to theother, so neurones all have a very similar structure and are specialised to theirfunction with certain features: - Very long so that they can transmit over a long distance. - The plasma membrane has gated ion channels that control the entry/exit of sodium, potassium and calcium ions. - They have sodium/potassium ion pumps, allowing them to pump ions in and out of the cell respectively, through using ATP. This maintains a polarised cell membrane. - They are surrounded by fatty, insulating Schwann cells to insulate the neurone from the electrical activity of nearby cells. - Their cell body contains a nucleus, many ribosomes and many mitochondria. - Motor neurones’ cell body is in the CNS, with a long axon that carries an action potential out to the effector.
- Sensory neurones’ long dendrites carry the action potential from a sensory receptor to the cell body, positioned just outside the CNS. 1) RESTING POTENTIALS AND ACTION POTENTIALSResting Neurones – when sodium/potassium ion pumps are pumping ions across themembrane at a ratio of 3:2, using ATP, the interior of the cell is maintained at anegative potential compared to the outside. The resting potential is a potentialdifference across the membrane of -60mV.Action Potential - if some of the sodium ions channels are opened then sodium ionswill quickly diffuse down the concentration gradient into the cell, causingdepolarisation of the membrane. In the GENERATOR REGION of the receptor cells,energy changes from the external environment opens the gated channels, allowingsodium ions to diffuse across the membrane. Gates further along the neurone areopened by the changes in the charge across the membrane. These channels arecalled VOLTAGE-GATED CHANNELS, as they respond to the depolarisations ofthe membrane.RestingPotentialDepolarisationThreshold Potential – if the depolarisation of the membrane is great enough, it willreach THRESHOLD POTENTIAL, and it will begin to have an effect on the nearbyvoltage-gated channels, opening them. This causes a large influx of sodium ionsdiffusing down their concentration gradient. If the depolarisation reaches +40mV, anaction potential is produced, and is then transmit to the very end of the neurone.
Ionic Movements – an action potential isbasically a set of ionic movements. Theions move across the cellmembrane when the correctchannels are opened.The graph above shows thestages: 1) The membrane begins in its resting state – i.e. it is polarised at a potential difference of -60mV 2) Sodium ions channels open and sodium ions diffuse into the cell down a concentration gradient. 3) The membrane, therefore, depolarises, becoming less negative compared to the extracellular environment, eventually reaching a threshold potential of -50mV. 4) Voltage-gated sodium ions channels open and many more sodium ions flood in. As more sodium ions enter, the cell becomes positively charged inside the cell. 5) When the charge across the membrane reaches +40mV, the inside of the cell is positive compared with the outside. 6) At this point, sodium ion channels close and potassium ion channels open. 7) Through potassium ions diffusing out of the cell, the potential difference becomes more negative compared with the outside – REPOLARISATION. 8) The potential difference becomes too negative momentarily, known as HYPERPOLARISATION. 9) The original potential difference is restored through the actions of sodium/potassium ion pumps, returning to its resting potential state.For a short time after an action potential, it is impossible to stimulate the cellmembrane to reach another action potential: a REFRACTORY PERIOD, which alsoallows the cell to recover after an action potential, ensuring that action potentials areonly transmitted in one direction along the axon. 1) TRANSMITTING ACTION POTENTIALS
A stimulus must be intense enough to reverse the polarity of the membrane – the allor none effect, where a stimulus threshold has to be reached before an impulsearises. 1) Depolarisation of an axon membrane following the stimulus opens sodium voltage-gated channels. Na+ enters the axon, and positive feedback occurs, as more Na+ channels opening, allowing more sodium ions to cross the membrane into the neurone. 2) As the sodium ions diffuse across the membrane, the membrane becomes reverse polarised, with the intracellular environment becoming more positively charged than the extracellular. The potential difference is +40mV. 3) The production of an action potential causes the flow of a small, localised electric current, which depolarises the adjacent membrane. This is caused by the Na+ ions flowing away from the positively charged area in the axon. This opens the voltage-gated sodium channels in the adjacent membrane, so more sodium ions diffuse in, creating an action potential in this membrane thus stimulating adjacent membranes, etc. As the process repeats, this is how the action potential is transmitted down the axon.The Myelin SheathThe myelin sheath is an insulating layer of fatty material. Sodium and potassium ionscannot diffuse through it, meaning the ionic movements that transmit an actionpotential along the neurone cannot occur over much of the length of the neurone.This is why there are gaps between the Schwann cells, known as Nodes of Ranvier.Therefore, the ionic movements can only take place at these Nodes of Ranvier. Inmyelinated neurones, the local currents must be elongated and sodium ions diffusealong the axon from one node of Ranvier to the next. This is known as SALTATORYCONDUCTION, as it appears that the impulse “jumps” from one node of Ranvier tothe next.This is advantageous as a non-myelinated neurone cannot transmit an actionpotential as quickly as a myelinated neurone’s rate of 120m/s. Therefore, myelinatedneurones are important in co-ordinating reflex reactions and quickly transmittingimpulses from sensory receptors. 1) NERVE JUNCTIONSA synapse is a junction, around 20nm wide, between two neurones, which allowssubstances known as neurotransmitters to diffuse across the SYNAPTIC CLEFT tocreate a new action potential in the POST-synaptic neurone.The presynaptic neurone ends in a “swelling,” known as the synaptic knob. Itcontains many mitochondria (required as synaptic transmission is an active process,requiring ATP), a large amount of endoplasmic reticulum (as vesicles are required),neurotransmitter (which diffuses across the synaptic cleft) and voltage-gated calciumion channels in the presynaptic membrane.
The process of synaptic transmission begins when an action potential arrives at thesynaptic knob of the PRE-synaptic neurone. This action potential opens voltagegated CALCIUM ion (Ca2+) channels in the membrane to open, allowing thefacilitated diffusion of calcium ions into the synaptic knob across the presynapticmembrane. These CALCIUM ions cause vesicles containing neurotransmitter(usually acetylcholine), to move to and fuse with the presynaptic membrane, wherethey are released by exocytosis.The neurotransmitter diffuses across the SYNAPTIC CLEFT towards receptor siteson the sodium ion channels in the POST-synaptic membrane, causing these sodiumion channels to open.Sodium ions diffuse down a concentration gradient, from an area of higherconcentration to an area of lower concentration into the POST-synaptic neurone.This creates a generator potential, also known as excitatory postsynaptic potential(EPSP) in the POST-synaptic neurone. If the generator potential reaches a thresholdpotential, then an action potential is created in the POST-synaptic neurone.ACETYLCHOLINESTERASE is an enzyme present in the synaptic cleft. Ithydrolyses the neurotransmitter acetylcholine into ethanoic acid and choline. Thisstops the constant transmission of impulses in the postsynaptic neurone, as itprevents acetylcholine from binding to receptor sites on the postsynaptic neurone,and so on. The ethanoic acid and choline diffuse back into the synaptic knob, wherethey are recycled back into acetylcholine using ATP from respiration. The recycledacetylcholine is stored in synaptic vesicles for future use. 2) SIGNALS AND MESSAGESAn action potential is an all or nothing response. Once an action potential begins, itwill be transmitted all the way to the end of the neurone; it does not vary in size orintensity.The Role of SynapsesSeveral presynaptic neurones may converge into one postsynaptic neurone afterneurotransmitters diffuse across the synaptic cleft, creating the same response. Thiscould be useful where several different stimuli are warning us of danger, forexample.One presynaptic neurone might diverge into several postsynaptic neurones, allowingone signal to be transmitted to several parts of the body. This is useful in the reflexarc: one postsynaptic neurone elicits the response while another informs the brain.Synapses ensure that signals are transmitted in the correct direction.Synapses can filter out any unwanted, low-level signals. Even if a low-level stimuluscreates an action potential in the presynaptic neurone, it is unlikely to pass across asynapse to the next neurone because several vesicles containing neurotransmittermust be released to create an action potential in the postsynaptic neurone.
Low-level signals can be amplified by a process called SUMMATION. If a low-levelstimulus is persistent it will generate several successive action potentials in thepresynaptic neurone. The release of many vesicles of acetylcholine over a shortperiod of time will enable the postsynaptic generator potentials to combine togetherto produce an action potential. The same can happen when a little acetylcholine isreleased from many presynaptic neurones converging on one postsynaptic neurone.Acclimatisation – after repeated stimulation, a synapse may run out of vesiclescontaining neurotransmitter. A synapse in this state is said to be fatigued. Themeans the nervous system no longer responds to the stimulus, explaining why webecome acclimatised to a background noise or persistent smell. It may also preventoverstimulation of an effector organ, preventing damage from overstimulation.The Frequency of Transmission – when a stimulus is at higher intensity, thesensory receptor will produce more generator potentials. This causes more actionpotentials in the sensory neurone. When this arrives at a synapse, it causes theexocytosis of more vesicles. Our brain can determine the intensity of the stimulusfrom the frequency of signals arriving. A higher frequency of signals means a moreintense stimulus.Myelinated and Non-Myelinated NeuronesAround one third of peripheral neurones are myelinated. The remainder and theneurones in the CNS are not myelinated. Action potentials in non-myelinatedneurones travel as a wave, rather than jumping from node to node as seen inmyelinated neurones. Myelinated neurones can transmit an action potential muchfaster than a non-myelinated neurone, 120m/s compared to 2 – 20 m/s. Myelinatedneurones tend to carry impulses over a long distance, up to 1m, whereas non-myelinated neurones tend to carry impulses over a short distance. This is becausestimuli must be responded to quickly in comparison to carrying out responses suchas breathing and digestion. 3) THE ENDOCRINE SYSTEMSignalling Using Hormones – the endocrine system uses the bloodstream totransport its signals. Endocrine glands secrete hormones into the blood stream,allowing them to travel to any cell in the body to initiate a response, so long as it hasa complementary receptor. The glands are a ductless group of cells that produce thehormones from organic compounds, usually proteins, in small quantities.Endo-/exo- crine – endocrine glands are ductless, meaning their hormones aresecreted directly into the bloodstream, travelling to a distant organ/tissue effector.Exocrine glands have ducts into which the hormone is secreted. The duct carries thesecretion to another please, i.e. saliva flows along a duct from the saliva glands tothe mouth. Or the tear duct carries tears from the gland to the eye.Targeting the Signal – hormones can only bind to cells with complementaryreceptors to the hormone’s 3D shape. Therefore, certain hormones can only affectTARGET CELLS that are usually grouped together to form TARGET TISSUES. If
every cell has a complementary receptor to a hormone, then the hormone can causea response in every cell in the body. However, if only a certain target tissue hascomplementary receptors, then a very specific response can be initiated.The Action of Adrenaline – adrenaline is unable to enter target cells, as it is anamino acid derivative and therefore not a steroid hormone. The adrenaline moleculebinds to a complementary receptor on the plasma membrane’s extracellular side,which has enzyme (ADENYL CYCLASE) associated with it on the intracellular side.As adrenaline in the blood – the first messenger – binds to the plasma membranereceptor, the enzyme adenyl cyclase is activated. This enzyme is responsible forconverting ATP to cyclic AMP or cAMP. The cAMP is the second messenger in thecell. The effect of cAMP is to activate specific enzyme action inside the cell.The Functions of the Adrenal Glands – there are two adrenal glands, lying abovethe kidneys on either side of the body. Each gland has a MEDULLA REGION and aCORTEX REGION.The Adrenal Medulla – the cells in the medulla specialise in manufacturing thehormone adrenaline, which is released in response to pain, shock or environmentalstresses. Most cells in the human body contain an adrenaline receptor, allowing itseffect to be widespread. Its effects include:- - Relaxing of the smooth muscle in the bronchioles. - Increasing stroke volume of the heart - Increasing heart rate - Raising blood pressure by general vasodilation - Stimulating the conversion of glycogen to glucose - Dilating the pupils in the eyes - Increasing mental awareness an improving perception - Inhibiting the action of the gut, i.e. digestion - Causing body hair to stand erect.The Adrenal Cortex – the adrenal cortex manufactures steroid hormones usingcholesterol, which have various roles in the body.Mineralcorticoids help to control the concentrations of sodium and potassium in theblood.Glucocorticoids, also known as cortisol, help to control the metabolism ofcarbohydrates and proteins in the liver.Advantages of the Endocrine System: 1) Process can occur over a long period of time 2) Hormones can reach a wide range of organs to give a full body response 3) A small amount of chemical can cover a large area 4) A slow response allows gradual increase/decrease in the response 1) THE REGULATION AND HOMEOSTATIC CONTROL OF BLOOD GLUCOSE LEVELS
The Pancreas is the organ of interest in this topic. It lies just below the stomach, andis an organ that can utilise both endocrine and exocrine functions, i.e. is has glandswith ducts and glands without.Secretion of Enzymes – the majority of the cells in the pancreas manufacture andrelease digestive enzymes. The cells responsible for this function are found ingroups surrounding tubules, which then combine to form the pancreatic duct. Thiscarries the pancreatic fluid, which contain the enzymes, to the small intestine. Thefluid contains amylase, trypsinogen (and inactive protease) and lipase, allenzymes necessary for digestion. The fluid is alkaline, as it contains sodiumhydrogencarbonate, in order to neutralise the acidic contents of the small intestinewhich have just left the acidic environment of the stomach. This process is exocrine.Secretion of Hormones – the ISLETS OF LANGERHANS are responsible for thesecretion of hormones from the pancreas. They contain different types of cells,namely α-cells and β-cells. α-cells manufacture and secrete glucagon, when bloodsugar becomes too low, and β-cells manufacture and secrete insulin, when bloodsugar is too high. As the hormones are secreted directly into the numerouscapillaries surrounding the Islets of Langerhans, this process is endocrine.Control of Blood Glucose – this is important, as it is the principle respiratorysubstrate, and also because brain cells cannot metabolise any other metabolites. Aswell as this, glucose exerts an osmotic pressure on the blood and tissue fluids,ensuring the cells do not haemolyse or become crenated.Levels of Glucose Rise Too High:- - The high levels of blood glucose is detected by the beta-cells in the Islets of Langerhans. - The beta-cells then secrete insulin into the surrounding capillaries, which travels to the liver cells – hepatocytes – muscle cells and some other body cells, such as some brain cells. - The cells that the insulin travels to contain receptors complementary to the 3D shape of insulin, where insulin binds as the first messenger. - The binding of insulin results in the activation of the adenyl cyclase enzyme, converting ATP to cAMP which subsequently activates other enzymes in the cell, leading to the following responses: 1) More glucose channels are placed into the cell surface membrane 2) More glucose enters the cell 3) Glucose in the cell is converted to glycogen for storage 4) More glucose is converted to fat 5) More glucose is used in respirationThis, therefore, causes the reduction of blood glucose concentration.
Levels of Glucose Too Low:- - The low concentration of blood sugar is detected by the alpha-cells, which secrete the hormone glucagon in response. - The hormone then travels to the hepatocytes in the blood, after being secreted directly into the bloodstream. - The glucagon then binds to a complementary receptor in the plasma membrane of the hepatocytes, initiating a second messenger which initiates the following responses by activating enzymes: 1) Conversion of glycogen to glucose 2) Use of more fatty acids in respiration 3) The production of glucose by conversion from amino acids and fats 1) REGULATION OF INSULIN LEVELSInsulin brings about the effect of reducing blood sugar concentrations, therefore, it isimportant to be able to control how much insulin in the blood when the blood sugarconcentration returns to its set point, or norm.The control of insulin secretion: - The cell membrane of beta-cells contain both potassium and calcium ion channels. - The potassium ion channels, under normal conditions, are open and the calcium ion channels closed. There is a potential difference across the cell membrane of around -70mV, as potassium ions diffuse out of the cell, making the intracellular space negatively charged. - When glucose concentrations in the surrounding tissue fluid are high, it quickly diffuses across the membrane. - It is metabolised (respired) to produce ATP. - This excess ATP is causes the potassium ion channels to close, causing the potential difference across the membrane to become less negative. - This change in voltage opens the voltage-gated calcium ion channels, allowing calcium ions to diffuse into the cell down a concentration gradient. - The calcium ions cause vesicles containing insulin to move to and fuse with the cell surface membrane, releasing insulin into the tissue fluid and therefore blood stream by exocytosis.
Type I Diabetes Mellitus is also known as insulin-dependent diabetes, as the bodyis no longer able to produce sufficient insulin in the beta-cells of the islets ofLangerhans in the pancreas. It also cannot store excess glucose as glycogen, so theexcess glucose must be excreted in the urine. It is believed to be the result of anauto-immune attack on the body’s own beta-cells, however, it could also be theresult of viral infection.Type II Diabetes Mellitus is also known as non-insulin-dependent diabetes, as thebody can still produce insulin, however, the hepatocytes are no longer as responsiveto it. This is thought to be caused by the specific receptors on the surface of the liverand muscle cells declining, thus losing their ability to respond to insulin in the blood.Certain factors increase your risk of contracting type II diabetes: - Obesity - A diet high in sugars, particularly refined - Being of Asian or Afro-Caribbean descent - Family history of the illnessType II diabetes is generally treatable with close monitoring and control of diet,where carbohydrate intake and use are matched. However, this may eventuallyrequire to be supplemented by insulin injections or drugs that absorb glucose fromthe digestive system.Type I diabetes, generally the more serious form, is treated using insulin injections.The blood glucose levels must be monitored, generally done by using urine tests,and the correct dose of insulin must be administered to ensure blood glucoseconcentrations remain stable and the patient does not become hypo- or hyper-glycemic.Insulin used in insulin injections used to be extracted from the pancreas of pigs,however, bacteria have been genetically engineered (reverse transcriptase) toproduce human insulin. There are certain advantages to this type of insulin: - It matches human insulin exactly, so it is faster acting and thus more effective. - There is less chance of developing a tolerance to human insulin. - There is less chance of rejection due to an immune response. - There is a lower risk of infection. - It is cheaper to manufacture and the process of manufacture is more adaptable to demand. - People will not have moral objections to using humalin compared to insulin from animals. 1) CONTROL OF HEART RATE IN HUMANSThe human heart is involved in pumping blood around the circulatory system inblood vessels, supplying cells with the necessary substrates and substancesrequired for their activities. It also removes the waste products of these cells’
activities, such as urea, CO2, etc. Therefore, the requirements of blood by thesecells vary according to their level of activity. For example, if you are running, youneed more glucose and oxygen due to a higher level of respiration of muscle cells. Itis important, then, for the heart to meet the requirements of the body cells.The heart can adapt to the varying needs of cells by: - An increase in the number of beats per minute, known as an increase in heart rate. - An increase in the strength of its contraction. - An increase in the volume of blood pumped per beat, known as the stroke volume.How does the body control the heart rate? Heart muscle is myogenic, meaning itinitiates its own contraction, through excitation of the atria walls from an actionpotential in the sinoatrial node (SAN). This travels to the atrioventricular node(AVN) and down the Purkyne fibres to initiate contraction in the ventricles. However,the SAN is connected to nerves from the medulla oblongata in the brain. Whilethese nerves do not initiate contraction, they can affect the frequency of contraction.Action potentials sent down the accelerator nerve of the sympathetic nervous systemincrease the heart rate, but action potentials sent down the vagus nerve of theparasympathetic nervous system reduce the heart rate. The cardiac muscle can alsorespond to the presence of adrenaline in the blood stream.Many factors influence heart rate, due to impulses from around the body from theANS arriving at the cardiovascular centre in the medulla oblongata: - Movement of limbs is detected by stretch receptors in the muscles. This sends an impulse to the centre informing it that extra oxygen will be required, tending to increase heart rate. - CO2 is acidic when dissolved in the blood, so this leads to a lower blood pH, which is detected by chemoreceptors in the carotid arteries. These chemoreceptors send impulses to the cardiovascular centre, eventually increasing the heart rate.