Neural and chemical control

• At rest, body cells use

  Around 200mls of oxygen each minute
• Strenuous exercise

  Typically increases 15-20-fold in normal healthy adults
• Elite endurance trained athletes

  Typically increases 30-fold
• The challenge for the body

  • To match respiratory effort to metabolic demand
• Rhythm of respiration

  1. Controlled by inspiratory area

  2. Brain stem (Medulla oblongata)

  3. Normal cycle = 5-6 seconds (2/3 or 2/4)

  4. Progressive increase in strength of excitatory signals to the inspiratory muscles

  5. Contraction of the inspiratory muscles

  6. Thorax increases in size, internal pressure drops, and air enters passively

  7. Suddenly the respiratory signal ceases

  8. Relaxation

  9. Expiration - a passive process (normally)

  10. Does not require muscle effort (therefore energy-efficient)

  11. Relaxation of inspiratory muscles (switch-off)

  12. Thorax decreases in size, pressure rises and air leaves the lungs

  
• Controlled by 2 separate interacting neural mechanisms

  • Voluntary system - cerebral cortex
  • Autonomic system - medulla and pons

  
• Basic rhythm of respiration

  1. Controlled by two specialised groups of neurons in the brain stem

  2. Medulla oblongata - controls the basic rhythm of respiration (respiration rhythmicity centre - RRC). Dorsal group - inspiration. Ventral group - expiration.

  3. Pons - adjusts the activity of the RRC in response to input from other areas of the brain - rate and depth of inspiration

  
• Pneumotaxic area

  • Helps turn off the inspiratory area "prematurely"
  • Shortens the duration of inhalation
  • Increases breathing rate

  
• Apneustic area

  • Sends excitatory impulses to inspiratory area to "keep going"
  • Facilitates respiration
  • Prolongs inhalation - long deep inhalation

  
• Controlled mechanically to prevent over-inflation and Barotrauma (Hering-Breuer Reflex)
• Basic rhythm set and co-ordinated by respiratory area in Medulla Oblongata
• Factors modifying respiration rate

  • Chemical Stimuli
  • Non-chemical stimuli
  • Other factors (medication, limbic system)

  
• Chemoreceptor regulation

  • Certain chemical stimuli determine how quickly and deeply we breath
  • Chemicals regulating activity are: O2, CO2, H+ Ions
  • Raised PCO2/H+ ions or lowered O2 levels = increase respiratory centre activity
  • Lowered PCO2/H+ ions or raised O2 levels = decrease respiratory centre activity

  
• Central chemoreceptors

  • Found on the ventral surface of the medulla near, but not part of the respiratory centre
  • Superficial enough to be bathed in CSF, as well as being surrounded by the brain's extra cellular fluid
  • Respond to a change in concentration of H+ ions and levels of CO2 in these fluids

  
• Location of the peripheral chemoreceptors

  • Two carotid bodies - near the bifurcation of the carotid artery at each side
  • Two or more aortic bodies- near the aortic arch
• Carotid bodies

  More sensitive than aortic bodies (consider brain)
• CO2 hydration

  1. Increased CO2 - PCO2 = 5.3 KPa (40 mmHg)

  2. Increase in PCO2 (hypercapnia) - increases H+ ions - stimulates central and to a lesser extent peripheral chemoreceptors

  3. Highly activated inspiratory area - rate and depth of breathing increases - hyperventilation

  4. Continues or increases as output of PCO2 and decreased pH is produced

  5. Allows exhalation of more CO2 until PCO2 and H+ ions are lowered to normal as stimulation decreases

  6. Below 5.3 KPa (40mmHg) - hypocapnia

  7. Central and peripheral receptors not stimulated - lack of stimulus to inspiratory area

  8. Inspiratory area sets a slow pace (rate/depth) until CO2 accumulates and rises to 5.3 KPa (40mmHg)
• Severe deficiency of O2

  • Depresses the central chemoreceptor activity and inspiratory area
  • Stops responding effectively to any inputs and sends fewer impulses to respiratory muscles
  • Decreases breathing rate - worsening situation
  • Hypoxaemia can kill in minutes
  • If PaO2 drops to 8KPa (60mmHg) the peripheral chemoreceptors in carotid bodies will stimulate the inspiratory centre
  • Patients with chronic respiratory illness may rely on this "back-up" system

  
• Other factors affecting respiration

  • Limbic system in brain - emotional anxiety /exercise anticipation/performance stress
  • Temperature increases (fever and vigorous muscular exercise) - increases rate
  • Temperature decreases (hypothermia) - decreases rate
  • Proprioceptors - exercise, your rate and depth of breathing increases even before changes in chemical - proprioceptors that monitor joint and muscle movement send signals to medulla

  
• Terms to become familiar with


  • Hypoventilation
  • Hyperventilation
  • Tachypnoea
  • Dyspnoea
  • Apnoea
• Lungs and heart work together
• One can help other (to a certain extent)
• Both fulfil the same overall role in homeostasis
• The heart also has specific and specialist mechanisms for ensuring cardiovascular demand meets supply
• Regulation of the heart

  Adjustments to the heart rate are important in the short term control of cardiac output and blood pressure
• CO

  Cardiac Output = Stroke Volume x Heart Rate
• BP

  Blood Pressure = Cardiac Output x Systemic Vascular Resistance
• Left to self - SA node - sets rate at a constant e.g., 100 bpm
• Different volumes of blood required by tissues under different conditions
• Exercise - cardiac output increases to supply working muscles with more O2 and nutrients
• Systems of regulation

  • ANS
  • Hormones released by adrenal glands
• ANS

  1. Nervous system regulation of heart originates in CV centre of medulla

  2. Receives input from a variety of sensory receptors and higher centres in brain - cerebral cortex and limbic system

  3. CV centre - directs appropriate output by increasing and decreasing frequency of nerve impulses sent out to ANS

  4. Both sympathetic and parasympathetic branches
• Sympathetic system

  • CV centre excites sympathetic neurons which up-regulate HR
  • Via cardiac accelerator nerve
  • Innervates conducting system - atria and ventricles of heart
  • Cardiac accelerator nerve releases nor-epinephrine (nor-adrenaline) to increase HR
• Parasympathetic system

  • CV centre excites parasympathetic neurons which down-regulate HR
  • Via vagus nerve - (cranial nerve X)
  • Innervates conducting system - atria
  • Neurotransmitter they release is - acetylcholine - slowing activity of SA node

  
• Normal resting HR

  • Balance between SNS and PNS
  • To increase HR = increase SNS activity, decrease PNS activity
  • To slow HR = decrease SNS activity, increase PNS activity
• Stroke volume control

  1. Increase SNS activity enhances myocardial contractility

  2. Leads to better emptying of the ventricles

  3. Increase venous return = greater stretch of ventricular walls prior to contraction

  4. Further increases SV

  5. Known as Frank-Starling Law of the Heart

  6. Ensures demand and supply meet
• Sensory receptors

  • Baroreceptors (pressure sensors)
  • Chemoreceptors
• Baroreceptors

  • Neurones sensitive to changes in BP
  • Located in the Arch of the Aorta and Carotid Arteries
  • Increased BP - baroreceptors send impulses along sensory neurons to CV centre in medulla
  • CV centre responds - more impulses along parasympathetic (motor) nerve and decrease in accelerator output
  • Decrease Heart rate, decrease CO, and thus decrease BP
  • Decrease in BP - Baroreceptors do not stimulate CV centre - Lack of stimulus - Increase HR, increase CO and this leads to increase BP to normal levels
• Chemoreceptors

  • Sensitive to O2, CO2, H+
  • Located in Carotid Arteries and Arch of Aorta
  • Hypoxia, Acidity (pH), Hypercapnia
  • Stimulate the chemoreceptors to send information to CV centre in medulla
  • Sympathetic stimulation to peripheral arterioles and veins, producing vasoconstriction and BP
  • Also increase HR and increase force of contraction
  • Supply matched to demand

  
• Chemical regulation

  • Hormones: epinephrine and norepinephrine - adrenal medulla; enhance the hearts effectiveness as a pump; increase HR and force of contraction
  • Excitement, stress, exercise - adrenal medulla - produce these hormones
  • Longer-term changes (Revising for exams)