Special Circulations (8)

Question 0

What adaptions are there in the pulmonary circulation?

Answer 0

• Very high density of capillaries for large capillary surface area
• Short diffusion distance, very thin layer of tissue separating gas phase from plasma

Question 1

What are the two main blood supplies to the lungs?

Answer 1

• Bronchial circulation: meets metabolic requirement of the lungs
• Pulmonary circulation: blood supply to the alveoli, required for gas exchange

Question 2

What do the adaptions of the pulmonary circulation allow?

Answer 2

• Increased O2 and CO2 transport capacity

Question 3

What are the ideal conditions for the pulmonary circulation?

Answer 3

• Low pressure

- Low resistance: arterioles have relatively little smooth muscle, lots of capillaries, short and wide vessels

Question 4

What is meant by V:Q ratio and what is the optimal value?

Answer 4

• Ventilation:Perfusion ratio

- 0.8

Question 5

What is the role of the V:Q ratio?

Answer 5

• Need to match ventilation of alveoli with perfusion of alveoli for efficient oxygenation

Question 6

How is the V:Q maintained?

Answer 6

• Diverting blood from alveoli which aren’t well ventilated

- Hypoxic pulmonary vasoconstriction

Question 7

Why may chronic hypoxia occur?

Answer 7

• At altitude

- Consequence of lung disease

Question 8

What may occur due to chronic hypoxia?

Answer 8

• Chronic increased vascular resistance -> chronic pulmonary hypertension
• High afterload on RV -> RV heart failure

Question 9

What effect does the position of the capillaries in the lungs have?

Answer 9

• 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)

Question 10

How does tissue fluid form?

Answer 10

• 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

Question 11

What is hydrostatic pressure influenced by?

Answer 11

• Venous pressure

Question 12

Why is low capillary pressure important in the pulmonary circulation?

Answer 12

• Minimises lung lymph formation

- Increased capillary pressure causes more fluid to filter out -> oedema

Question 13

What effect may a pulmonary oedema have and how is it treated?

Answer 13

• Impairs gas exchange

- Use diuretics to decrease pressure

Question 14

How does cerebral circulation meet the high O2 demand?

Answer 14

• 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

Question 15

How is the cerebral blood supply secure?

Answer 15

• 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

Question 16

What is myogenic auto regulation?

Answer 16

• 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

Question 17

What metabolic regulation is there in the cerebral circulation?

Answer 17

• Cerebral vessels are very sensitive to arterial PCO2 changes
• Hypercapnia: Increased PCO2 -> vasodilation
• Hypocapnia: Decreased PCO2 -> vasoconstriction

Question 18

Why may hyperventilation lead to fainting?

Answer 18

• Hyperventilation -> Hypocapnia -> fainting due to vasoconstriction

Question 19

What role does adenosine have in the cerebral circulation?

Answer 19

• Powerful vasodilator of cerebral arterioles

Question 20

What is Cushing’s reflex?

Answer 20

• 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

Question 21

What forms the blood brain barrier and what does it do?

Answer 21

• Cerebral capillaries form tight blood-brain barrier
• Lipid soluble molecules such as O2/CO2 can diffuse freely
• Lipid insoluble can’t

Question 22

What helps maintain a high basal flow in the coronary circulation?

Answer 22

• Continuous production of NO

Question 23

What are the requirements of the skeletal muscle circulation?

Answer 23

• Must increase O2 and nutrient delivery
• Must increase removal of metabolites during exercise
• Regulate arterial blood pressure

Question 24

What does the capillary density depend on?

Answer 24

• Muscle type

Question 25

What are vasodilators of skeletal muscle blood vessels?

Answer 25

• Increased K+ conc
• Increased osmolarity
• Inorganic phosphates
• Adenosine
• Increased H+ conc
• Adrenaline at arterioles

Question 26

What is cutaneous circulation responsible for?

Answer 26

• Temperature regulation

- Skin’s main heat dissipating surface

Question 27

What is meant by AVA and it’s primary role?

Answer 27

• Artereovenous anastomoses

- Regulate heat loss from apical skin

Question 28

How is AVA regulated?

Answer 28

• Neural control

- Sympathetic vasoconstrictor fibres

Question 29

If the core temperature decreases what happens in the AVA?

Answer 29

• Increases sympathetic tone in AVAs -> decreased blood flow to apical skin

Question 30

How do AVAs dilate?

Answer 30

• Reduced vasomotor drive to AVAs