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Article by Dr. Thomas Burnell and Bethany Turner
Questions by Dr. Josh Sturgeon
Next Lesson - Heart Failure
Core
The needs of different systems varies throughout the body. This means some require special circulations to match the specific requirements of that organ.
The pulmonary circulation is required for gas exchange between the alveolar air and the blood. The metabolic requirement of the lungs is supplied by the bronchial circulation which is part of the systemic circulation.
The pulmonary circulation has a high capillary density (high number of capillaries very close together). This means the surface of the alveoli in the lungs are covered in capillaries, providing a large surface area over which gas exchange can take place.
The pulmonary circulation has a low pressure because of this high capillary density. The blood coming into the pulmonary circulation spreads through the many capillaries, reducing the pressure. Pressure needs to be low so that the capillaries do not burst as they have a thin wall to maximise efficiency of gas exchange.
Ventilation-Perfusion Ratio (V:Q Ratio) is the ratio between the volume of air coming in and out of the lungs and the volume of blood flowing past the lungs in the pulmonary circulation. For efficient oxygenation, the ventilation must match the perfusion of the alveoli. The optimal ratio is ~0.8, e.g. 4L/min:5L/min (air flow : blood flow).
The V:Q ratio is important as if it falls too low then the blood will not be inadequately oxygenated leading to the patient becoming hypoxic as there is not enough oxygen in the systemic circulation.
To maintain this ratio, the body must be able to divert blood away from poorly ventilated alveoli and towards better ventilated alveoli. This is done via hypoxic pulmonary vasoconstriction. When alveoli are poorly ventilated, the air in them isn’t being refreshed leading to a low partial pressure of oxygen, and a high partial pressure of carbon dioxide. The body responds by causing vasoconstriction of the pulmonary vessels to direct blood flow away from the poorly ventilated alveoli and towards the better ventilated alveoli to maintain the V:Q.
- Chronic hypoxia can occur due to chronic lung disease or if at high altitude for long periods of time.
- If there is chronic hypoxia, there can be chronic vasoconstriction of the pulmonary circulation.
- This results in pulmonary hypertension, causing an increased afterload on the right ventricle.
- Right sided heart failure can then occur as the ventricle has to pump harder against increased resistance. This leads to hypertrophy which then develops into right sided heart failure (cor pulmonale).
Usually little tissue fluid forms in the lungs as the capillary hydrostatic pressure is very close to the osmotic pressure. However, if the capillary pressure is increased, more fluid will leave the capillary and enter the alveoli causing pulmonary oedema (build up of fluid in the lungs).
- Capillary pressure can be increased if the left atrial pressure rises, e.g. in mitral valve stenosis or left ventricular heart failure.
- Pulmonary oedema impairs gas exchange and is affected by posture. When the patient is standing, the fluid in the lungs pools at the base. However, when the patient is lying down, the fluid spreads out throughout the lungs. This fills more alveoli with fluid, causing impairment of gas exchange, leading to breathlessness. The patient may sleep propped up on pillows to ease this.
- Breathlessness on lying flat is known as orthopnoea.
Diagram - The pressures involved in the formation of tissue fluid
SimpleMed original by Bethany Turner
The brain has a high metabolic requirement, so it has a high oxygen demand. Neurones are also very sensitive to hypoxia, with a patient losing consciousness after a few seconds of reduced blood flow. As a consequence, the brain requires a constant blood flow and has the following features:
- High capillary density to supply all of the neurones with enough oxygen.
- High basal flow rate to maintain a constant blood supply to the brain and supply the high demand of the neurones.
- High oxygen extraction to take as much oxygen from the blood as possible to supply the neurones.
The circulation to the brain has adaptations to maintain a secure blood supply:
- Myogenic autoregulation – cerebral arteries will change their diameter in response to changes in systemic pressure. This ensures the cerebral blood flow remains at the same rate even when there are increases and decreases in pressure.
- If the blood pressure is increased, there is vasoconstriction of the arteries to ensure blood flow to the brain remains constant.
- If the blood pressure is decreased, there is vasodilation of the arteries to maintain perfusion.
- Metabolic factors help to control blood flow:
- Hypercapnia (high pCO2) in the brain vasculature results in vasodilation to increase the blood flow to restore the normal pCO2. This is as the hypercapnia has likely arisen through increased metabolic activity or inadequate perfusion.
- Hypocapnia (low pCO2) in the brain vasculature results in vasoconstriction to decrease the blood flow and increase the pCO2 in the brain.
- There are vasodilators in the brain that help to increase the blood to the cerebral circulation. These vasodilators include:
- CO2
- Hyperkalaemia - rise in potassium
- Increased adenosine
- Decreased oxygen
Cushing’s Reflex – as the skull is a rigid structure, it will not allow for volume expansion of the brain. Any expansion, e.g. due to a tumour or haemorrhage, will result in an increased intracranial pressure which impairs the cerebral blood flow by compressing the blood vessels.
- Impaired blood flow to the vasomotor control regions of the brainstem will increase the sympathetic vasomotor activity. This increases the arterial BP and maintains the cerebral blood flow.
- There is also stimulation of the parasympathetic nervous system, though this takes time to occur.
- If prolonged, Cushing’s Reflex will eventually lead to Cushing’s triad which is characterised by high BP, irregular breathing and bradycardia.
The coronary circulation must deliver oxygen at a high basal rate and must be able to meet any increased demand.
The blood supply to the myocardium is mainly through the left coronary artery. Blood flow through the coronary arteries primarily occurs during diastole. This is as it is harder for blood to flow through the coronary circulation during systole due to the increased pressure on the arteries caused by the contraction of the ventricles.
The skeletal muscle circulation must have capacity to increase oxygen and nutrient delivery and metabolite removal during exercise.
The capillary density of the muscle depends on the muscle type, with postural muscles (e.g. those around the spine) having a high capillary density as they have to be active for long periods of time.
Only around half the capillaries are well perfused at rest allowing for increased recruitment of capillaries during times of increased requirement, e.g. exercise. This is achieved by rich sympathetic innervation acting on precapillary sphincters which contract to restrict blood flow to some capillaries. When precapillary sphincters open, this increases the perfusion of capillaries to increase the blood flow to the muscle.
The cutaneous circulation is mainly used to regulate temperature. The skin is the main dissipating surface of the body; temperature loss is regulated by cutaneous blood flow. The cutaneous circulation also has a role in maintaining BP by vasoconstricting.
Apical/acral skin has arteriovenous anastomoses (AVAs) which open to increase the blood flow to veins where heat can then be lost. AVAs are under neural control and are not regulated by local metabolites.
- Apical skin has a high surface area to volume ratio, and this allows for control of the body temperature.
- If there is a decrease in body temperature, then there will be increased sympathetic tone in the AVAs, causing them to constrict and decrease heat loss.
- If there is an increase in body temperature, then there will be decreased sympathetic tone in the AVAs, causing them to dilate and increase heat loss.
In non-apical skin, sweat glands release bradykinin which acts as a local vasodilator to dilate veins and arterioles to increase heat loss from the body.
Edited by: Dr. Ben Appleby
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