muscles

• Muscles of inspiration: 

  • Diaphragm 
  • External intercostals 
  • Accessory muscles of inspiration 

    - scalenes 

    - sternocleidomastoid  
• Muscles of forced expiration:  

  • Abdominals  
  • Internal intercostals 
• Diaphragm: 

  • Musculotendinous sheet separating the thorax and abdomen  
  • Large dome shaped muscle  
  • Lower posteriorly than anteriorly  
  • Main muscle of inspiration 
  • 3 sets of fibres  

    - Sternal  

    - Costal  

    - Lumbar (Crural)  

  • Fibres are named according to their origin – all insert into the same place 
  • All fibres converge into central trefoil-shaped tendon 
• Right hemidiaphragm is higher than left - Liver sits under the right diaphragm and stomach sits under left which is why the left side is slightly bigger 

  Superior surface of diaphragm is covered with the parietal pleura 

  Pericardium of heart is attached to the central tendon 

  Innervated by the phrenic nerve   

  C3,4,5 keeps the diaphragm alive 
• Sternal fibres: 

  • Arise from the posterior surface of the xiphoid process of the sternum 
  • Fibres run upwards and medially  
  • Insert into the anterior border of the central tendon 
• Costal fibres: 

  • Arise from the inner surface of the lower 6 ribs and their CCs (interdigitates with transversus abdominis) 
  • Inserts into the anterolateral part of the central tendon  
  • Make up majority of muscle fibres of the diaphragm 
• Lumbar fibres: Crural Fibres 

  • Right crus - larger - from the anterolateral aspects of the bodies and intervening discs of L1 – L3 
  • Left crus - from the bodies and discs of L1 and 2 
• • The remainder of the lumbar part of the diaphragm arises from the medial and lateral arcuate ligaments
• Medial arcuate ligament - from side of body of L2 to tip of transverse process of L1
• Lateral arcuate ligament - from transverse process of L1 to tip of 12th rib
• Median arcuate ligament Tendinous band connecting the two crura
• The Diaphragm – Actions: 

  • Main muscle of inspiration  
  • At rest central tendon is opposite T8  
  • During quiet inspiration it descends to the level of T9 increasing the vertical diameter of the thorax  
  • Deep inspiration increases vertical, lateral and AP diameters, cause ribs to move upwards and outwards in an AP direction 
• The intercostals: 

  •  Pass between adjacent ribs  
  • 11 pairs between ribs 1 to 12 in the intercostal space  

   

  • Arranged in 3 layers;  
  • Outer layer – external intercostals* 
  • Middle layer – internal intercostals* 
  • Deep layer – innermost intercostal, stabilise chest wall  
• The External Intercostals: 

  • From the inferior border of the rib above to the superior border of the rib below 
  • Fibres run obliquely downwards and forwards  
  • Action – inspiration 

   
• The Internal Intercostals: 

  • Deep to external intercostals  
  • From inferior border of rib above to superior border of rib below 
  • Fibres pass obliquely downwards and backwards  
  • Action – forced expiration 
• accessory muscles - Scalenes and Sternocleidomastoid, are used when we are breathless, usually a sign of respiratory distress 
• Forced Expiration: 

  • Forced expiration is brought about by the internal intercostals and the abdominal muscles 
  • Contraction of the abdominal muscles causes the abdominal viscera to push up against the diaphragm reducing the vertical diameter of the thorax 
• Atmospheric Pressure: 

  • Respiratory pressures are ALWAYS described relative to atmospheric pressure  
  • It is the pressure exerted by the air (gases) surrounding the body 
  • Atmospheric pressure (760mmHg) is the sum of all the partial pressures of the different gases within air 
  • Therefore % of O2 in air is 21%  
  • Rest of air made up mostly of nitrogen – 78% 
  • Negative respiratory pressures means less than atmospheric pressure  
  • Positive respiratory pressure means more than atmospheric pressure  
  • Zero respiratory pressure is equal to atmospheric pressure 
• Intrapulmonary Pressure: 

  • Intrapulmonary pressure (Ppul) 
  • May also be called intra-alveolar pressure  
  • The pressure within the alveoli  
  • Rises and falls with the different phases of breathing  
  • Negative on inspiration, positivie on expiration  
  • ALWAYS eventually equalises with atmospheric pressure 
• Intrapleural Pressure: 

  •  Intrapleural pressure (Pip) 
  • Pressure within the pleural cavity  
  • Also fluctuates within breathing phases  
  • ALWAYS negative relative to intrapulmonary pressure (Ppul)  
  • Approx 4mmHg less than Ppul 
• Why is Intrapleural Pressure Negative?  

  • Negative intrapleural pressure is created by 2 opposing forces  
  • Tendency of the chest wall to expand
  • Tendency of the lungs to recoil  
  • These 2 forces act to pull the visceral pleura away from the parietal pleura  
  • Pleural fluid ensures the 2 layers remain locked together 
• Transpulmonary Pressure: 

  • The difference between the intrapulmonary and intrapleural pressures (Ppul – Pip) 
  • If intrapulmonary pressure is 760mmHg and intrapleural pressure is 756mmHg then the transpulmonary pressure is 4mmHg  
  • If the transpulmonary pressure is 0mmHg then the lungs will collapse 
• Ventilation – the movement of air in and out of the lungs  

  Inspiration – air flows into the lungs  

  Expiration – gases exit the lungs  

  Ventilation is a mechanical process that depends on volume changes within the thoracic cavity  

  Volume changes lead to pressure changes which leads to flow of gases 
• Respiratory Mechanics: 

  • If the volume of a container increases then the pressure inside it will decrease  
  • If the volume of a container decreases then the gas molecules will be forced closer together and therefore the pressure will increase  
  • Gases always flow down pressure gradients (from high to low) 
• Quiet Inspiration: 

  • Respiratory muscles contract – diaphram and external intercostal muscles 
  • Increase in thoracic and lung volume  
  • Decrease in intrapulmonary pressure  
  • Air flows into the lungs down the pressure gradient  
  • Airflow stops when intrapulmonary pressure is equal to atmospheric pressure 
• Quiet Expiration: 

  •  Passive process  
  • Inspiratory muscles relax  
  • Lungs recoil  
  • Reduced thoracic and lung volume  
  • Increased intrapulmonary pressures  
  • Gas flows out of the lungs until intrapulmonary pressure is 0