Cardiorespiratory Physiology

 

• Basic physiological parameters

  Cardiac

  • Cardiac output (CO) = Stroke volume (SV) x HR. Average values are around 70 ml x 70 bpm = 4900 ml/min.
  • BP = CO x Systemic vascular resistance (SVR).
  • Mean arterial pressure (MAP) is ⅓ of the way between DBP and SBP. MAP = ⅔DBP + ⅓SBP, or MAP = {DBP} + {(SBP-DBP)/3}
  • Cardiac index = Cardiac output ÷ Body surface area.

  Respiratory

  Overview:

  • Lung volume (VL) changes throughout respiration, and is represented by various measures
  • 'Volume' is a unit that cannot be broken down; 'capacity' is the sum of two or more volumes.

  Total lung capacity (5-6.5 L):

  • Maximum theoretical capacity of lung.

  Vital capacity (3.5-4.5 L) is the maximum you can actually exhale (and inhale) in one go. Comprises:

  • Tidal volume (0.5 L): volume of normal, quiet breath. Minute volume = Tidal volume x Respiratory rate.
  • Inspiratory (2-2.5 L) and expiratory (1-1.5L) reserve volume: extra you can do if needed. ↓IRV in diseases that ↓compliance, weakens muscles, or cause pain (e.g. broken rib). ↓ERV (obstructive illness) in diseases that ↑compliance.
  • Inspiratory capacity: full amount you can inspire from rest.

  Residual volume (1.5-2 L):

  • Minimum amount always left in lung as chest can't fully compress. Ensures constant O2 for blood, as while respiration is episodic, circulation is continuous.
  • Functional residual capacity (2.5-3.5 L) is the amount actually left in during quiet breathing; the resting point above which we breath.
  • This will decrease during pneumothorax, but cartilage ensures that it never fully collapses as cartilage keeps airways open.

• Autonomic nervous system

  Sympathetic nervous system

  Neuroanatomy:

  • Preganglionic neurons originate in the lateral horn of the spinal cord grey matter and exit the cord via the ventral horn. They originate from T1-L2, but the sympathetic trunk containing the ganglia actually runs alongside the whole spinal cord. Outside of T1-L2, spinal nerves connect to the sympathetic trunks via the gray ramus communicans.
  • Preganglionic neurons (short) synapse with postganglionic neurons (long), with acetylcholine (ACh) as the neurotransmitter. They also synapse with the adrenal gland.
  • Postganglionic neurons synapse with visceral effectors with noradrenaline as the neurotransmitter. The adrenal gland also releases both adrenaline and noradrenaline, and has effects around the body.

  Adrenergic receptors and their effects:

  • β1: cardiac inotropy (↑stroke volume) and chronotropy (↑HR).
  • β2: bronchodilation via airway smooth muscle relaxation.
  • α1: peripheral vasoconstriction.
  • α2: mainly central receptors. Agonists lead to ↓sympathetic output due to a negative feedback mechanism.

  Catecholamines and their affinity:

  • Noradrenaline: α1 > β1. Causes vasoconstriction, ↑HR, ↑stroke volume.
  • Adrenaline: β1 > β2 = α1. Causes ↑HR, ↑stroke volume, bronchodilation, and vasoconstriction.
  • Dobutamine (synthetic): β1 > β2. ↑Stroke volume but can cause tachyarrhythmias.

  Parasympathetic nervous system

  • Involved in 'rest and digest' processes, including: ↑GI motility, sphincter relaxation, ↑secretions, ↓HR, and bronchial smooth muscle contraction.
  • Preganglionic neurons (long) originate from brainstem (nuclei of cranial nerves 3, 7, 9-10) and sacrum (S2-4). They synapse with postganglionic neurons, with ACh as the neurotransmitter.
  • Cell bodies of the postganglionic neurons are located close to or in their target organ, hence postganglionic fibres are generally short. They act on target tissue via ACh on muscarinic (cholinergic) receptors.