Special Circulations

Question 1

What are the two blood supplies to the lungs?

Answer 1

Bronchial circulation - part of systemic circulation, meets the metabolic requirements of the lungs.

Pulmonary circulations - Blood supply to alveoli, required for gas exchange.

Question 2

What is cardiac output at rest?

Answer 2

5 L/min

Question 3

What is maximum cardiac output?

Answer 3

20-25L/min for non-athelets

Question 4

What is the pressure during systole in the ventricles?

Answer 4

15-30 mmHg in right ventricle

100-140 mmHg in left ventricle.

Question 5

What are the diastolic pressures of the ventricles?

Answer 5

Right ventricle = 0-8mm/Hg

Left ventricle = 60 - 90 mm/Hg

These are the same as the pressures on the atria.

Question 6

Why does pressure in arteries not get very low?

Answer 6

Because of the elastic recoil of artery. This means that the volume always stays similar.

Question 7

What are some features of the pulmonary circulation?

Answer 7

• Low pressure
  
  • Mean arterial pressure - 12-15mmHg
  • Mean capillary pressure 9-12mmHg
  • Mean venous pressue - 5mmHg
• Low resistance
  
  • Short, wide vessels.
  • Lots of capillaries (Many parallel elements)
  • Arterioles habe relatively little smooth muscle

Question 8

What adaptations does the pulmonary circulation have? Why?

Answer 8

The adaptations are to promote efficient gas exchange.

• Very high density of capillaries in alveolar wall
  
  • large capillary surface area.
• Short diffusion distance
  
  • Very think layer of tissue separating gas phase from plasma.
  • Combined endothelium and epithelium thickens is about 0.3um
• Larger surface area and short diffusion distance produce high O₂ and CO₂ transport capacity.

Question 9

What is the ventilation perfusion ratio (V/Q ratio)?

Answer 9

For efficient oxygenation, ventilation (air flow) and perfusion (blood flow) of alveoli need to match

Optimal V/Q = 0.8

Maintaining this means diverting blood from alveoli which are not well ventilated.

Question 10

How does the ventilation perfusion ratio remain well matched?

Answer 10

Hypoxic pulmonary vasoconstriction ensures optimal ventilation / perfusion ratio.

• Most important mechanism regulating pulmonary vascular tine.
• Alveolar hypoxia results in vasoconstriction of pulmonary vessels.
• Ensures that perfusion matches ventilation
• Poorly ventilated alveoli are less well perfused
• Helps optimise gas exchange.

Question 11

What are the disadvantages of chronic hypoxia vasoconstriction?

Answer 11

It can cause right ventricular failure.

• Chronic hypoxia can occur at altitude or as a consequence of lung disease such as emphysema.
  
  • Chronic increase in vascular resistance - chronic pulmonary hypertension
  • High after-load on right ventricles - can lead to right ventricular heart failure.
  • RV heart failure rarely occurs on its own.

Question 12

Why are low pressure pulmonary vessels strongly influenced by gravity?

Answer 12

In the upright position (orthostasis) there is greater hydrostatic pressure in vessel in the lower part of the lung. This increases the transmural pressure within the blood vessles at the base of the lung.

The increase in pressures leads to some filtration of tissue fluid, but will also distend the vessles and increase the flow to those areas.

Question 13

What effect does exercise have on pulmonary blood flow?

Answer 13

• Increased cardiac output
• Small increase in pulmonary arterial pressure
• Open apical capillaries
• Increased uptake of O₂ by lungs
• As blood flow increases capillary transit time is reduced. -
  
  • At rest transit time = 1s.
  • This can fall to 0.3s without compromising gas exchange.

Question 14

How is tissue fluid formed?

Answer 14

• Starling forces determine fluid formation
• Hydrostatic pressure of blood within the capillaries - Pushes fluid out of the capillary.
  
  • This is influenced more by venous pressure in systemic circulation so, hypertension does not usually result in peripheral oedema.
• Oncotic pressure (colloid osmotic pressure) - pressure exerted by large molecules such as plasma proteins - Draws fluid into the capillary.

Question 15

What effect does low capillary pressure have?

Answer 15

It minimises the formation of lung lymph to prevent oedema.

Question 16

What effect does an increased capillary pressure have?

Answer 16

Oedema - if rises above 20-35 mmHg

Question 17

What conditions cause rise in capillary pressure?

Answer 17

• Mitral valve stenosis
• Left ventricular failure

This causes pulmonary oedema.

Upright - ended capillaries at base of lungs (more fluid forms here)

Lying down - Oedema throughout lungs.

Question 18

How do you treat pulmonary oedema?

Answer 18

Use a diuretic to relieve symptoms and treat underlying cause

Question 19

Cerebral circulation

Answer 19

The brain has a high O2 demand

Receives about 15% of cardiac output - but only accounts for 2% of body mass

O₂ consumpton of grey mattter accounts for about 20% of total body consumption at rest

Most provide a secure O₂ supply.

Question 20

How does the cerebral circulation meet the high demand for oxygen?

Answer 20

• High capillary density
  
  • ​Large surface area for gas exchange
  • Reduced diffusion distance (<10um)
• High basal rate
  
  • ​x10 average for whole body
• High O₂ extraction
  
  • ​35% above average

Question 21

Why is a secure O₂ supply to the brain vital?

Answer 21

• Neurones are very sensitive to hypoxia
• Loss of consciousness after a few seconds of cerebral Ischaemia
• Begin to get irreversible damage after about 4 mins
• Interruption to blood supply e.g. a stroke causes neuronal death

Question 22

How is a secure blood supply to the brain ensured?

Answer 22

Structurally:

• Anastomoses between basilar and internal carotid artery (circle of Willis)

Functionally:

• Myotonic autoregulation maintains perfusion during hypotension
• Metabolic factors control blood flow
• Brainstem regulates other circulations

Question 23

Myotonic autoregulation?

Answer 23

• Cerebral resistance vessels have a well developed myogenic (origionates from the smooth muscle itself) response. They respond to changes in transmural pressure (difference in pressure between two sides of a wall or equivalent)
• It serves to maintain cerebral blood flow when BP changes.
• If blood pressure increases the vasoconstriction will occur
• If blood pressure decreases then vasodilation occurs
• These changes keep the cerebral blood flow the same despite the change in mean arterial pressure.
• This response fails below 50 mmHg (fainting)

Question 24

Metabolic regulation

Answer 24

• Cerebral vessels very sensitive to changes in arterial P_CO2
• Panic hyperventilation can cause hypocapnia (less CO₂ in brain) and cerebral vasoconstriction leading to dizziness or fainting
• Hypercapnia is an increase in P_CO2 which causes vasodilation
• Hypocapnia is a decrease in P_CO2 which causes vasoconstriction.

Question 25

How does regional activity produce a local increase in blood low?

Answer 25

* Areas with increased neuronal activity have increased blood flow. * Increases P_CO2, conc. of potassium and adenosine leading to vasodilation. * Decreased P_O2 * Adenosine is a powerful vasodilator of cerebral arterioles.

Question 26

What is Cushing reflex?

Answer 26

Rigid cranium protects the brain but does not allow for volume expansion. Increases intracranial pressure impair cerebral blood flow - caused by cerebral tumour or haemorrhage. Impaired blood flow to vasomotor control regions of the brainstem increase sympathetic vasomotor activity. * Leads to increase in arterial BP, irregular breathing and bradycarida * Helps maintain cerebral blood flow.

Question 27

Coronary circulation?

Answer 27

Must delivery O at a high basal rate. Must meet increased demand - work rate can increase five fold.

Question 28

Why does coronary bloodflow increase with myocardial O2 demand?

Answer 28

* Extra O₂ required at high work load is supplied mainly by increased blood flow * Almost linear relationship until very high O₂ demand * Small increase in amount of O₂ extracted * Vasodilation due to metabolic hyperaemia * Vasodilator - adenosine, increased K+ conc, and decrease pH

Question 29

When does flow in the left coronary artery mainly occur? Why?

Answer 29

Flow in the left coronary artery occurs mainly during **diastole**. This is because, during systole, the pressure in the left ventricular wall increases as the ventricle contracts whereas, the right doesn't increase by as much because it has a thinner wall and contracts with a lower pressure. Increase in heart rate, diastole shortens so less time for left coronary artery to get blood.

Question 30

What is the difference between cardiac muscle and skeletal muscle?

Answer 30

Cardiac mucle has: * Much higher capillary density - facilittes fficient oxygen delivery * Smaller fibre diameter * Capillaries continuously perfused. * Small diffusion distnace * Continuous prodiction of NO by coronary endothelium maitains a high basal flow (means capillaries are always open). All of this leads to veyr efficient gas exchange.

Question 31

What are type of arteries are coronary arteries?

Answer 31

* **Functional end arteries** * Few anterior-arterial anastomoses * Prone to athromas * Narrowed coronary arteries leads to angina on exercise (increased O₂ demand) * Blood flow mostly during diastole - Diastole is rediced as heart rate increases * Stress and cold can also cause sympathetic coronary vasoconstriction and angina * Sudden obstruction by thrombus causes myocardial infarction

Question 32

Skeletal muscle circulation..

Answer 32

Must increase O₂ and nutrient delivery and removal of metabolites during exercise. Important role in helping to regulate arterial blood pressure - 40% of adult body mass. Resistance vessels have rich innervation by sympathetic vasoconstrictor fibres - Baroreceptor reflex maintains BP

Question 33

How does capillary density change in skeletal muscle?

Answer 33

Capillary density depends on muscle type - postural muscles have higher capillary density. Very high vascular tone. - Permits lots of dilation, flow can increase up to 20 times in active muscle. At rest only about half of capillaries are perfused at any one time - allows for increased recruitment Opening of precapillary sphincters allows more capillaries to be perfused. - increases blood flow and reduces diffusion distance.

Question 34

What causes increased flow in skeletal muscle (vasodilation metabolites)?

Answer 34

Various agents are thought to act as vasodilator * Increased conc. of **K+** * Increased **osmolarity** * Inorganic **phosphates** * **Adenosine** * Increased conc. **H+** **Adrenaline** also acts as a vasodilator at arterioles in skeletal muscle. * Acts through B2 receptors * Vasoconstrictor response via NA on a1 receptors.

Question 35

Cutaneous circulation

Answer 35

Special role in **temperature regulation** Core temp is normally maintained around 37 - balance of heat production and heat loss Skin is the main heat dissipating surface - This is regulated by cutaneous blood blood Also has a role in **maintaining BP** Vasoconstriction in cutaneous circulation maintain BP - Shock

Question 36

What do artereoveous anastomoses (AVAs) regulate?

Answer 36

Apical (acral) skin had a high surface area to volume ratio Acral skin has specialised structures called Arteriovenous anastomoses (AVAs) AVAs are under neural control - sympathetic vasoconstrictor fibres Not regulated by local metabolites Decrease core temperature increases sympathetic tone in AVAs - decreases blood flow to apical skin Increased core body temperature opens AVAs Reduced vasomotor drive to AVAs allows then to dilate. Tjis diverts blood to veins near the surface.