CVS 8 - Special Circulations Flashcards
Describe the systemic circulation pathway
- Oxygenated blood is carried away from the heart and to the body
- Deoxygenated blood is carried from the tissues back to the heart
- A parallel system
Where is the output of the systemic circulation?
Left ventricle
Describe the pulmonary circulation pathway
- Deoxygenated blood is carried away from the heart and to the lungs
- Oxygenated blood is carried from the lungs and back to the heart
- A system in series
Where is the output of the pulmonary circulation?
Right ventricle
Describe the pressure and resistance of the pulmonary circulation. Why is it this way?
- Low pressure
- Low resistance
- Vessels are short and wide
- Lots of capillaries
- Low amount of smooth muscle in arterioles
What is the pressure in the pulmonary artery?
- 15-30mmHg (systolic)
- 4-12mmHg (diastolic)
What is the pressure in the pulmonary capillaries?
9-12mmHg
What is the pressure in the pulmonary veins?
5mmHg
What is the pressure range of the right atrium?
0-8mmHg
What is the pressure range of the right ventricle?
- 15-30mmHg (systolic)
- 0-8mmHg (diastolic)
What is the pressure range of the left atrium?
- 1-10mmHg
What is the pressure range of the left ventricle?
- 100-140mmHg (systolic)
- 1-10mmHg (diastolic)
What is the ventilation-perfusion ratio? What is its optimal value?
- The matching of alveolar ventilation and alveolar perfusion with blood to give efficient oxygenation
- 0.8 (no units)
What is the equation for ventilation-perfusion ratio? What is v? What is q?
- v/q
- v = ventilation (amount of air in and out of the lung)
- q = perfusion (cardiac output)
What does a high VQ ratio signify? What does a low VQ ratio signify?
- High = high ventilation, low perfusion
- Low = high ventilation, high perfusion (can’t saturate O2 to match perfusion)
What is VQ mismatch? What can it result in?
- Areas of both high VQ and low VQ
- Leads to different levels of saturation which can lead to a lower overall oxygen saturation
How can VQ mismatch lead to a lower overall oxygen saturation?
- Areas of high perfusion and decreased saturation outweigh other areas due to a higher number of red blood cells
- Can lead to hypoxia
What can cause VQ mismatch?
- Pulmonary embolism (leads to areas of no perfusion = high VQ)
- Pneumonia
What is hypoxic pulmonary vasoconstriction
- Adaptive response to hypoxia
- Causes a decreased perfusion rate (Q) if the ventilation of alveoli is decreased
- Optimises gas exchange
How does the response of the pulmonary circulation to hypoxia differ to that of the systemic circulation?
Response of systemic circulation is to increase perfusion rate due to metabolite build up
What can happen if hypoxic pulmonary vasoconstriction becomes chronic?
- Can increase vascular resistance = chronic pulmonary hypertension
- Increases pressure of right ventricle = failure
What is tissue fluid?
Extracellular fluid that contains neither RBCs nor plasma proteins
What are the starling forces?
- Hydrostatic pressure = pressure exerted by the blood in the capillary (pushes water out)
- Colloid oncotic pressure = pressure exerted by the proteins in the plasma (pulls water in)
What can happen due to an increase in hydrostatic pressure in the pulmonary system? How can this be treated?
- Movement of fluid out into the tissues due to an increase in capillary pressure = oedema
- Impairment of gas exchange
- Treated by diuretics/ underlying problem is treated
What influences capillary pressure in the pulmonary circulation?
- Arterial pressure
- Venous pressure
(increase of either/both = increase of cap. pressure)
What influences capillary pressure in the systemic circulation?
Venous pressure
What is the cerebral circulation? Describe its demand
- The blood supply to the brain in a given period of time
- Very high demand (15% of cardiac output)
How is the cerebral circulation adapted to meet its high demand?
- High capillary density (large SA, short diffusion pathway)
- High basal flow rate
- High oxygen extraction
What are the consequences of ischaemia in the cerebral circulation?
- Loss of consciousness (if for a few seconds)
- Irreversible neuronal damage (if over ~4 minutes)
How is blood supply in the cerebral circulation functionally ensured?
- Myogenic autoregulation = maintenance of perfusion when pressure decreases (increase bp = vasoconstriction, decrease bp = vasodilation)
- Controlled by metabolic factors e.g. more CO2 = More perfusion
- Brain can prioritise e.g. Cushing’s Reflex
What is Cushing’s Reflex?
Increasing of the sympathetic vasomotor activity due to impaired blood flow to the vasomotor control centre of the brain stem
e.g. increased cerebral bp = peripheral vasoconstriction = higher flow to brain
Describe the oxygen demand of coronary circulation. Why is this significant?
- High basal rate for oxygen (high demand)
- Needed for the heart to keep beating
How does cardiac muscle ensure efficient oxygen delivery?
- High capillary density (1 per fibre)
- Smaller fibre diameter
- Continuous production of NO by coronary endothelium
= Constant perfusion at a high basal flow
Describe the filling of the coronary arteries
- Left and right coronary arteries come from left and right aortic sinuses - Fill during diastole
- Systole = contraction means increased pressure so is too high to fill coronary arteries
Describe the relationship between mechanical work and oxygen demand of the myocardium
- High blood flow as high demand
- Nearly linear - very high demand = small increase in o2 extracted due to lower pH and higher metabolites
Why does partial occlusion of the coronary arteries happen? What can it result in?
- Coronary arteries are prone to atheroma (functional end arteries)
- Leads to angina
- Disruption of atheroma can result in a thrombus (MI in coronary artery)
What can also cause angina?
- Stress and cold
- Causes sympathetic vasoconstriction
When does angina occur? Why?
- During exercise (chest pain)
- Increased oxygen demand but decreased duration of diastole due to higher heart rate
- Decreased filling time and blood flow
When does the oxygen and blood supply demand of skeletal muscle increase? Why?
- During exercise
- Increase oxygen and nutrient delivery
- Increase metabolite removal
How are increases in blood flow in skeletal muscle brought about?
- Opening of more capillaries due to vasodilator nervous activity and local metabolites
- Reduces sympathetic vasoconstrictor tone
How is efficient blood flow ensured in skeletal muscle?
- Capillary density is dependent on muscle type
- High vascular tone - lots of vasoconstriction = lots of vasodilation which increases blood pressure
- At rest, only 50% of capillaries are perfused. During exercise, recruitment means that the diffusion distance decreases
- Metabolic hyperaemia - increased perfusion and vasodilation due to an increase in metabolites
Describe the function of the blood flow through the skin
Mostly not nutritive but instead for temperature regulation
How is temperature in the skin regulated? Why is this significant?
- Through the regulation of blood flow via artereo-venous anastamoses (AVAs)
- Leads to rapid bypassing when heat needs to be lost quickly
How is temperature in the skin regulated?
Sympathetically rather than by metabolites
How does a decrease in temperature affect blood flow?
- Increases sympathetic activity which causes vasoconstriction and a decrease in blood flow
How does an increase in temperature affect blood flow?
- Decreases sympathetic activity which causes vasodilation and an increase in blood flow
