Adaptations to a way of life

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

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

ATP

• 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

Haemoglobin

• 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