Article by Dr. Thomas Burnell and Dr. Bethany Turner
Next Lesson - Heart Failure
Core
The needs of different systems vary 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 is covered in capillaries, providing a large surface area over which gas exchange can take place.
The pulmonary circulation has a low pressure and a low resistance. This is because the pulmonary arteries have thin walls and are highly compliant (they stretch easily), the vessels are short and wide, and the capillaries run in parallel (side by side), so blood spreads through many routes at once. Pressure needs to be low so that the thin-walled capillaries are not damaged, and so that little fluid is pushed out of them into the lungs, where it would impair gas exchange.
Ventilation-Perfusion Ratio (V:Q Ratio) is the ratio between the volume of air reaching the alveoli each minute (alveolar ventilation) and the volume of blood flowing through the pulmonary capillaries around them each minute (perfusion). 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 because if it falls too low, the blood will be inadequately oxygenated. The patient then becomes hypoxic (short of oxygen), 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 (high blood pressure in the pulmonary arteries), causing an increased afterload on the right ventricle (the resistance it has to pump against).
- The right ventricle has to pump harder against the increased resistance. This leads to hypertrophy (thickening of the muscle), which can then develop into right-sided heart failure (cor pulmonale: a change in the structure and function of the right ventricle caused by lung disease and pulmonary hypertension).
Usually little tissue fluid (fluid that leaks out of the capillaries into the surrounding tissue) forms in the lungs, because the capillary hydrostatic pressure (the pressure that pushes fluid out of the capillary) is low. Only a small amount of fluid leaves the capillaries, and nearly all of it is reabsorbed, drawn back in by the oncotic pressure (the pull of the plasma proteins, a type of 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 (narrowing of the mitral valve) 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 the breathlessness.
- Breathlessness on lying flat is known as orthopnoea.
Diagram - The pressures involved in the formation of tissue fluid
SimpleMed original by Dr. 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 (the flow at rest) 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 helps to keep the cerebral blood flow steady when the pressure rises or falls within a certain range. It begins to fail when mean arterial pressure (the average pressure in the arteries) falls below about 50 mmHg, as the vessels cannot dilate any further.
- 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 partial pressure of carbon dioxide, pCO2) in the brain vasculature results in vasodilation to increase the blood flow to restore the normal pCO2. This is because a high CO2 level usually means that the brain tissue is working harder or is not receiving enough blood.
- 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 all signal that the brain tissue is active or short of oxygen, so the vessels widen to bring in more blood. They include:
- CO2
- Hyperkalaemia - rise in potassium
- Increased adenosine (a chemical released by active or oxygen-starved cells)
- 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 (pressure inside the skull) which impairs the cerebral blood flow by compressing the blood vessels. The reflex is the body’s response to this.
- Impaired blood flow to the vasomotor control regions of the brainstem (the areas that control how wide the blood vessels are) will increase the sympathetic vasomotor activity. This increases the arterial BP and maintains the cerebral blood flow.
- The raised BP is detected by baroreceptors (pressure sensors in the walls of the aortic arch and carotid sinus). These increase the activity of the parasympathetic nervous system through the vagus nerve, which slows the heart rate (bradycardia).
- The result is Cushing’s triad: high BP (the systolic pressure rises, so the gap between the systolic and diastolic pressures widens), a slow heart rate (bradycardia) and irregular breathing, as the raised pressure also compresses the brainstem. It is a sign of raised intracranial pressure.
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 provided by the right and left coronary arteries. The left coronary artery supplies the majority of the left ventricular myocardium, which has the highest oxygen demand. Blood flow through the coronary arteries to the left ventricle primarily occurs during diastole (when the heart relaxes and fills). This is because it is harder for blood to flow through the coronary circulation during systole (when the ventricles contract), as the contracting muscle squeezes the small coronary vessels that run through it. Flow in the right coronary artery, which mainly supplies the right ventricle, is more even across the cycle because the right ventricle generates a lower pressure.
The skeletal muscle circulation must have capacity to increase oxygen and nutrient delivery and metabolite removal during exercise. It also helps to control blood pressure: skeletal muscle receives about 20% of the cardiac output at rest, so narrowing or widening its arterioles changes the total peripheral resistance (the resistance to blood flow in all the arterioles of the body added together).
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 because these muscles have to be active for long periods of time.
Many capillaries are poorly perfused at rest, because the precapillary sphincters (rings of smooth muscle at the entrance to capillaries) are contracted. This allows for increased recruitment of capillaries (opening more of them so that more blood reaches the muscle) during times of increased requirement, e.g. exercise. This is achieved by control of small resistance vessels (small arteries that control how much blood flows on), particularly arterioles, which can limit or increase flow to groups of capillaries. Sympathetic nerves, adrenaline and local metabolites (chemicals released by working muscle, such as adenosine) all promote the vasodilation (widening of the vessels) that increases flow during exercise.
The cutaneous circulation is mainly used to regulate temperature. The skin is the main surface the body loses heat from (its heat-dissipating surface); temperature loss is regulated by cutaneous blood flow. The cutaneous circulation also has a role in maintaining BP by vasoconstricting.
Apical/acral skin (mainly the hairless skin of the hands and feet; the nose and ears may have some too) has arteriovenous anastomoses (AVAs) (direct links between small arteries and veins that bypass the capillaries) 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 (a large skin surface compared with the amount of tissue), 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, the blood vessels also widen actively when the body is hot (active vasodilation), which increases heat loss from the body. The exact mechanism is not fully settled. One proposal is that sweat glands release bradykinin (a chemical that widens blood vessels), which acts as a local vasodilator. Other studies suggest that cholinergic nerves (nerves that release acetylcholine) cause the widening, working together with nitric oxide (a chemical that relaxes the walls of blood vessels).
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Marcus Judge
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