Special Circulations (8) Flashcards
What adaptions are there in the pulmonary circulation?
- Very high density of capillaries for large capillary surface area
- Short diffusion distance, very thin layer of tissue separating gas phase from plasma
What are the two main blood supplies to the lungs?
- Bronchial circulation: meets metabolic requirement of the lungs
- Pulmonary circulation: blood supply to the alveoli, required for gas exchange
What do the adaptions of the pulmonary circulation allow?
- Increased O2 and CO2 transport capacity
What are the ideal conditions for the pulmonary circulation?
- Low pressure
- Low resistance: arterioles have relatively little smooth muscle, lots of capillaries, short and wide vessels
What is meant by V:Q ratio and what is the optimal value?
- Ventilation:Perfusion ratio
- 0.8
What is the role of the V:Q ratio?
- Need to match ventilation of alveoli with perfusion of alveoli for efficient oxygenation
How is the V:Q maintained?
- Diverting blood from alveoli which aren’t well ventilated
- Hypoxic pulmonary vasoconstriction
Why may chronic hypoxia occur?
- At altitude
- Consequence of lung disease
What may occur due to chronic hypoxia?
- Chronic increased vascular resistance -> chronic pulmonary hypertension
- High afterload on RV -> RV heart failure
What effect does the position of the capillaries in the lungs have?
- In upright position (orthostasis) there’s greater hydrostatic pressure on vessels in lower part of lung.
- Vessels higher up collapse during diastole
- Vessels in centre continuously patent
- Vessels lower down distended by gravity (hydrostatic pressure)
How does tissue fluid form?
- Starling’s law determines fluid formation
- Hydrostatic pressure of blood within capillary pushes fluid out
- Colloid osmotic pressure- pressure exerted by large molecules normally brings fluid back into capillaries
What is hydrostatic pressure influenced by?
- Venous pressure
Why is low capillary pressure important in the pulmonary circulation?
- Minimises lung lymph formation
- Increased capillary pressure causes more fluid to filter out -> oedema
What effect may a pulmonary oedema have and how is it treated?
- Impairs gas exchange
- Use diuretics to decrease pressure
How does cerebral circulation meet the high O2 demand?
- High capillary density: large surface area for gas exchange, reduced diffusion distance
- High basal flow rate (10 times higher than rest of body)
- High O2 extraction
How is the cerebral blood supply secure?
- Structurally: anastomoses between basilar and internal carotid arteries
- Functionally: brain stem regulates other circulation
myogenic auto regulation maintains perfusion during hypotension
metabolic factors control blood flow
What is myogenic auto regulation?
- Response to changes in transmural pressure
- Increase in blood pressure -> vasoconstriction - prevents high bp in cerebral circulation by acting as a blocker
- Decrease in blood pressure -> vasodilation
What metabolic regulation is there in the cerebral circulation?
- Cerebral vessels are very sensitive to arterial PCO2 changes
- Hypercapnia: Increased PCO2 -> vasodilation
- Hypocapnia: Decreased PCO2 -> vasoconstriction
Why may hyperventilation lead to fainting?
- Hyperventilation -> Hypocapnia -> fainting due to vasoconstriction
What role does adenosine have in the cerebral circulation?
- Powerful vasodilator of cerebral arterioles
What is Cushing’s reflex?
- If cerebral blood flow is impaired vasomotor control regions of brain stem increases sympathetic vasomotor activity
- Increasing arterial blood pressure
- Helps maintain cerebral blood flow
What forms the blood brain barrier and what does it do?
- Cerebral capillaries form tight blood-brain barrier
- Lipid soluble molecules such as O2/CO2 can diffuse freely
- Lipid insoluble can’t
What helps maintain a high basal flow in the coronary circulation?
- Continuous production of NO
What are the requirements of the skeletal muscle circulation?
- Must increase O2 and nutrient delivery
- Must increase removal of metabolites during exercise
- Regulate arterial blood pressure
What does the capillary density depend on?
- Muscle type
What are vasodilators of skeletal muscle blood vessels?
- Increased K+ conc
- Increased osmolarity
- Inorganic phosphates
- Adenosine
- Increased H+ conc
- Adrenaline at arterioles
What is cutaneous circulation responsible for?
- Temperature regulation
- Skin’s main heat dissipating surface
What is meant by AVA and it’s primary role?
- Artereovenous anastomoses
- Regulate heat loss from apical skin
How is AVA regulated?
- Neural control
- Sympathetic vasoconstrictor fibres
If the core temperature decreases what happens in the AVA?
- Increases sympathetic tone in AVAs -> decreased blood flow to apical skin
How do AVAs dilate?
- Reduced vasomotor drive to AVAs
