Flashcards in Special Circulations Deck (24):
What are the two circulations to the lungs
The lungs have two circulations - the bronchial circulation (part of the systemic circulation) supplies the metabolic requirements of the lungs, and the pulmonary which oxygenates blood.
What volume of blood does the pulmonary circulation receive from the heart?
The pulmonary circulations must accept all the cardiac output of the heart as the amount of blood coming into the heart must be equal to the amount of blood leaving. At rest this is about 5l/min and maximum is 25l/min in a non-athlete.
Describe the characteristics of pulmonary circulation.
Pulmonary circulation is a low pressure low resistance circulation this is due to short wide vessels, lots of capillaries and the arterioles have relatively little smooth muscle.
What are the pressures of the 3 different areas of the pulmonary circulation?
Mean arterial pressure is 12-15mmHg, mean capillary pressure is about 9-12mmHg (lower than in systemic circulation) and mean venous pressure is about 5mmHg.
When the different valves of the heart are open how are the pressures of the corresponding compartments related?
They are the same when the valve is open.
What is the V/Q ratio, what is it's optimum and how is this achieved?
Ventilation/Perfusion ratio (V/Q ratio) V=ventilation, Q=perfusion. You must match the ventilation of alveoli by the perfusion. This means diverting blood away from alveoli which are not ventilated. The optimal V/Q ratio is about 1. Hypoxic vasoconstriction allows this.
How can hypoxic vasoconstriction become pathological?
Hypoxic vasoconstrictioncan become pathological in some cases when it becomes chronic and leads to right ventricular failure due to pulmonary hypertension. This could happen at high altitude due to the lower oxygen concentrations or lung diseases such as emphysema.
How does gravity effect pulmonary blood vessels?
Pulmonary vessels are very susceptible to gravity causing increased hydrostatic pressure at the bottom and low at the top causing these vessels to collapse during diastole. These are called apical capillaries.
What is the effect of exercise on pulmonary blood flow?
Increased cardiac output, increased pulmonary arterial pressure, open apical capillaries increased O2 uptake by lungs, reduced transit time (without any compromising of gas exchange).
What forces form tissue fluid?
Hydrostatic pressure of blood and oncotic pressure (colloid osmotic pressure i.e. pressure exerted by large plasma proteins).
How does the pulmonary circulation prevent pulmonary oedema from occurring under normal circumstances?
In systemic circulation venous pressure influences capillary hydrostatic pressure (you don’t get oedema from hypertension). Low capillary pressure in the lungs minimises the formation of lymph as pulmonary oedema impairs gas exchange.
What causes Pulmonary Oedema?
Can be caused by mitral stenosis and left side failure.
Why cna lying down cause pulmonary oedema if you have an underlying condition?
When lying down all the hydrostatic pressure is spread out equally so the capillary pressure may become too high causing pulmonary oedema. Whereas when standing this only happens at the capillaries at bottom of the lungs, this is only when there is something else wrong.
How does the brain meet it's very high oxygen demand?
The brain has: high capillary density with a high surface area and a reduced diffusion pathway, high basal flow rate, higher than normal oxygen extraction.
How does the brain maintain it's oxygen demand?
There are many anastomoses between basilar and internal arteries.
Myogenic autoregulation, auto detection of flow being too high/low and correction to sort this
Sensitivity to CO2 if there is hypercapnia then they will vasodilate and if there is hypocapnia then they will vasoconstriction
Regional flow control using Adenosine (vasodilator) so that only places requiring large blood flows get it
The brainstem regulates other circulations to prioritise its own circulation
Why is the amount of time spent in systole an issue for the heart?
Issue with sytole squeezing capillaries when higher O2 demand thery are squeezed more often,
How does the heart secure it's blood supply?
There is a very high capillary density, the muscle fibres are thinner allowing for a shorter diffusion pathway. Also there is a constant supply of NO by coronary endothelium to maintain a high basal flow rate.
Why does coronary blood flow increase proportionally as myocardial O2 demand increases until a point?
Due to the loss of flow from decreased time spent in diastole. The extra blood flow is caused by metabolic hyperaemia by K+ and drops in pH.
What is the important characteristics of skeletal muscle circulation? How is this achieved?
Must be able to increase O2 and nutrient supply quickly and by a large amount. At rest only half of the capillaries are perfused at one time (unlike cardiac muscle) there is also a very high vascular tone in skeletal muscle vessels giving a large capacity for vasodilation.
What important function do skeletal vessels have? How is this achieved?
Vessels in the muscles have an important role in maintaining arterial blood pressure, controlled by sympathetic innervation of arterioles. There is a very high vascular tone in skeletal muscle vessels giving a large capacity for vasodilation.
What are the important roles of vasculature in skin?
Role in temperature regulation controlled by sympathetic nervous system also has a role in maintaining blood pressure hence why people in shock will turn pale.
Why does skin usually have low perfusion?
Usually very little perfusion of skin as it does not have a high metabolic rate.
Where are arteriovenous anastomoses (AVA’s) found and what do we call the area of skin that they are found in?
AVA's are found in Specialised areas of skin called Apical skin found in places like your ears, your nose, finger and toes.