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HBIO2 > Ventilation and Heart Rate
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Regulation of Breathing

Respiratory Centre

  • Medulla oblongata
    • Part of the brain stem
    • Continuous with the upper spinal cord
    • Contains reflex centres for heart rate, respiratory rate, and blood pressure
    • Breathing is controlled by the dorsal and ventral respiratory groups
  • Dorsal Respiratory Group (DRG)
    • Initiates inspiration
  • Ventral Respiratory Group (VRG)
    • Activated during exercise
    • Stimulates internal intercostals and abdominal muscles
    • Both muscles are inactive during normal breathing


  • Inspiration (inhalation)
    • DRG sends impulse via
      • Intercostal nerve → external intercostal muscles contract → pulling ribs upwards
      • Phrenic nerve → diaphragm contracts → moves downwards
    • Atmospheric pressure > Lung pressure
    • Air flows in
    • Lung volume increases
    • Inhalation requires muscular effort, thus burning calories and ATP
  • Normal expiration
    • Stretch receptors are found within smooth muscles of bronchi and bronchioles
    • They are stimulated and send impulses via vagus nerve
    • Vagus nerve inhibits the DRG and stops inspiration
    • Diaphragm moves upwards and external intercostal muscles relax
    • Lung pressure > Atmospheric pressure
    • Air flows out
    • Lung volume decreases
  • Forced expiration (exercise)
    • Stimulation of VRG
    • Abdominal muscles contract, pushing diaphragm upwards
    • Internal intercostal muscles contract, pulling ribs downward
    • Results in more powerful and faster expiration
  • Respiration
    • Maintains pH, oxygen, and carbon dioxide in blood within normal (homeostatic) limits
    • Monitored by brainstem respiratory centres

Control of Breathing

  • Breathing rate is monitored by
    • Blood CO2 levels - increase as more CO2 is produced as a waste product
    • Blood O2 levels - decrease as O2 is used up in respiration to produce ATP
    • Rate is more sensitive to changes in CO2 levels
  • In the blood, carbon dioxide dissolves into hydrogen and bicarbonate ions
    • CO2 + H2O H+ + HCO3-
    • Fall of CO2
      • Equation shifts to the left
      • More CO2 is produced by removing hydrogen ions
      • This increases blood pH (more alkaline)
    • Excess CO2 (exercise)
      • Equation shifts to the right
      • More CO2 dissolves in blood to produce more hydrogen ions
      • This reduces blood pH (more acidic)
  • Chemoreceptors
    • Located in the aorta (aortic bodies) and common carotid arteries (carotid bodies)
    • Monitor pH and CO2 levels
    • Send impulses to the medulla
    • Aortic bodies monitor CO2 and O2 levels, and BP but NOT pH!
    • Carotid bodies monitors CO2 and O2 levels, and pH
  • Exercise
    • Increases CO2 / blood becomes more acidic
    • Chemoreceptors detect low pH and stimulate DRG and VRG in the medulla
    • Respiratory centres send more impulses via phrenic and intercostals nerves
    • Impulses arrive at diaphragm and intercostals muscles
    • Increases breathing rate and depth

Regulation of Heart Rate

Autonomic Nervous System (ANS)

  • Made up of 2 divisions
    • Parasympathetic: stimulates vagus nerve causing ↓heart rate
    • Sympathetic: ↑heart rate
  • Cardiac inhibitory centre
    • Found in medulla oblongata of the brain stem
    • Connected to the heart by parasympathetic fibres found within the vagus nerve
    • Innervate SA and AV nodes
    • When stimulated, they trigger the release of acetylcholine (ACh)
    • This ↓heart rate
  • Cardiac accelerating centre
    • Found in medulla and upper thoracic spinal cord
    • Connected to the heart by sympathetic fibres
    • Innervate SA and AV nodes but also cardiac cells
    • When stimulated, they trigger the release of norepinephrine
    • This ↑heart rate and ↑strength of contractions
    • NB: norepinephrine is the international name for noradrenaline
  • Both centres balance stimulatory and inhibitory effects of the ANS

Hormonal influence

  • Stress releases epinephrine and norepinephrine from the adrenal medulla into the circulation
  • Both hormones ↑heart rate

Electrolyte Balance

  • Excess potassium in the extracellular environment ↓heart rate and ↓strength of contraction
  • Triggers release of excess Ca2+  from cardiac cells
  • Causes spastic contractions of the heart
  • Heart can be defibrillated by applying an electrical current to the chest wall
    • Ceases all contractions and electrical activity
    • Spontaneuous depolarisation of SA node sends out a new impulse
    • Normal cardiac rhythm may be re-established
  • Only a fraction of a KCl infusion is required to kill a patient 

Blood pressure

  • Baroreceptors near aorta and carotid arteries monitor blood pressure
  • Abnormal blood pressure → signal send to medulla
  • Cardiac centre changes heart rate → cardiac output
  • Vasomotor centre changes diameter of blood vessels
  • Shock: blood pressure too low
  • Insufficient nutrients for cells with a high metabolism (heart, brain)
  • Caused by excessive bleeding or extensive vasodilation
  • Treated with vasoconstrictors such as epinephrine (adrenaline), atropine

Cardiac output as a function of stroke volume and heart rate

  • The volume of blood pumped by one ventricle during one beat is called the stroke volume
  • Cardiac Output = Stroke Volume x Heart Rate (number of ventricular contractions/min)
  • ↑Force of contraction → ↑Stroke volume → ↑Cardiac output

Energy sources during exercise

Energy sources

  • Glucose
    • Stored as glycogen in liver
    • Quickly broken down (short-term exercise)
  • Triglycerides
    • Stored in adipose tissue
    • Takes longer to break down (long-term exercise)

Aerobic and anaerobic exercise

  • Aerobic exercise
    • C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
    • Complete oxidation of an organic substrate (glucose, TAG) to CO2 and H2O using free O2
    • Production of CO2 and 38ATP
  • Anaerobic exercise
    • When no oxygen is available (heavy exercise: more O2 required than available)
    • Glucose is converted to lactate (TAG cannot be used)
      • This produces 2ATP
    • Lactate is also called lactic acid
      • Diffuses into blood and lowers its pH
      • Accumulates in muscles causing muscle fatigue and cramps
    • Lactate
      • Most is re-oxidised to CO2 and H2O once oxygen is available again (oxygen debt)
      • Some is converted into glycogen, glucose, and proteins
      • Some is excreted in urine and sweat


  • Adenosine (ribose + adenine) triphosphate (3 phosphate groups)
  • Produced by adding Pi to ADP → phosphorylation
  • Breaks down to ADP (adenosine diphosphate) and Pi (inorganic phosphate ion) by hydrolysis
  • ATP is useful as an immediate energy source/carrier because
    • Energy release only involves a single reaction
    • Energy released in small quantities
    • Easily moved around inside cells, but cannot pass through cell membranes
  • Central molecule in metabolism (ATP hydrolysis)
    • Muscle contraction → changes of position of myosin head relative to actin
    • Protein synthesis → ATP "loads" amino acids onto tRNA
    • Active transport → driven by phosphorylation of membrane-bound proteins
    • Calvin Cycle → cyclic reduction of CO2 to TP
    • Nitrogen fixation → involves ATP-driven reduction of molecular nitrogen
  • ATP in liver is used for active transport / phagocytosis / synthesise of glucose, protein, DNA, RNA, lipid, cholesterol / urea in glycolysis / bile production / cell division


  • Transports Oxygen
  • Lower atm pressure / fewer molecules present / less O2 reaches tissues
  • Body adapts to changes by increasing
    • Heart rate and resting breathing rate
    • Blood plasma
    • Red blood cell production and number of blood capillaries
  • Haemoglobin Hb has 4 subunits, each subunit contains 2 parts
    • Haem → ring of atoms linked to Fe2+
    • Globin → polypeptide chain
    • Sequence of amino acids affects O2 carrying properties
  • Oxyhaemoglobin HbO2 from lungs dissociates in respiring tissues
    • O2 diffuses into body cells while Hb is transported back to lungs
  • Features of red blood cells that allow them to transport O2 more efficiently
    • Biconcave disc → larger surface area to volume ratio for diffusion
    • Absence of nuclei/other organelles → more room for haemoglobin