CVS S8 - Special Circulations Flashcards Preview

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Flashcards in CVS S8 - Special Circulations Deck (40):
1

What two circulations are present in the lungs?

Pulmonary
Bronchial

2

Describe the bronchial circulation

Part of systemic circulation

Meets the metabolic requirements of the lungs

3

Describe pulmonary circulation

Blood supply to alveoli required for gas exchange

Must accept entire cardiac output (5 - 25L/min)

Low resistance due to short, wide vessels with lots of parallel capillaries and arterioles with relatively little smooth muscle

Low resistance means low pressure in the pulmonary circulation

4

CVS link

What are the average systolic and diastolic pressures in the 4 chambers of the heart, pulmonary artery and aorta?

RA:
0 - 8mmHg

LA:
1-10mmHg

RV:
S = 15 - 30mmHg
D = 4 -12mmHg

LV:
S = 100 - 140mmHg
D = 1 - 10mmHg

Pulm Artery:
S = 15 - 30mmHg
D = 4 -12mmHg

Aorta:
S = 100 - 140mmHg
D = 60 - 90mmHg

5

State the normal pressures in the:

Pulmonary artery
Pulmonary capillaries
Pulmonary veins

PA:
12 - 15mmHg

PC:
9 - 12mmHg

PV:
5mmHg

6

What adaptations do alveoli possess to enable efficient gas exchange?

Very high density capillaries in the alveolar wall:
- Large capillary surface area

Short diffusion distance:
- Thin layer of tissue between gas and plasma
- Endothelium & epithelium distance = 0.3um

These adaptations produce high O2 and CO2 transport capacity

7

What is the perfusion ratio?

The optimal ratio of ventilation/perfusion

Ventilation of alveoli and perfusion of alveoli are matched

0.8 is optimal value

8

How is perfusion ratio maintained in normal and abnormal conditions?

Blood must be diverted from poorly ventilated alveoli

Regulated by pulmonary vascular tone

Hypoxia results in vasoconstriction of pulmonary vessels to ensure perfusion matches ventilation

This optimises gas exchange

9

What problems arise when hypoxic vasoconstriction of pulmonary circulation is chronic?

When might chronic hypoxia occur?

Chronic hypoxia causes chronic increase in vascular resistance

This in turn causes pulmonary hypertension

High afterload on the RV can cause right ventricular heart failure

At altitude or as a consequence of lung disease such as emphysema

10

How does gravity affect the pulmonary circulation?

In the upright position (orthostasis) there is greater hydrostatic pressure on vessels in the lower lung due to gravity

Vessels near the apex collapse during diastole

Vessels in transverse plane with the heart are continuously patent

Vessels in the lower lung distended

11

What is the effect of exercise on the pulmonary circulation?

Increased cardiac output leads to a small increase in pulmonary arterial pressure

This opens apical capillaries

Increased O2 uptake by the lungs

As blood flow increases capillary transit time decreases

At rest about 1s, can fall to 0.3s without compromising gas exchange

12

What forces determine tissue fluid formation in the pulmonary circulation?

Starling forces determine fluid formation

Hydrostatic pressure of blood within the capillary an interstitial oncotic pressure forces/draws plasma out of the capillary

Oncotic (colloid osmotic) pressure exerted by large molecules such as plasma proteins draws fluid into the capillaries

13

Describe how forces determining tissue fluid formation in the pulmonary circulation vary over the course of a capillary?

Capillary hydrostatic pressure is more influenced by the venous pressure in the systemic circulation

Fluid therefore tends to move out of a capillary toward the arterial end and move in at the venous end

14

How are forces that determine tissue fluid formation balanced so as to not cause pulmonary oedema?

This applies to the rest of the circulation as well

Low capillary pressure minimises hydrostatic pressure
(9 - 12mmHg)

This helps to balance the starling forces so only a small amount of fluid flows out (lung lymph)

15

What is the effect of increased pulmonary capillary pressure on interstitial fluid in the lungs?

How is this increase in pressure caused?

Increases hydrostatic pressure causing fluid to flow out of the capillary

This causes oedema

Increased pressure caused by LA pressure raising to 20 - 25mmHg:
- Mitral valve stenosis
- Left ventricular failure

16

What is the effect of pulmonary oedema on the lungs?

How can symptoms of this be relieved?

Impairs gas exchange:
- Affected by posture
- Mostly at the bases when upright
- Throughout the lung when lying down

Use diuretics to relieve symptoms
Treat underlying cause

17

What is the relative oxygen demand of the myocardium in basal conditions?

How does this vary when demand is increased?

Coronary circulation must supply a relatively high basal rate of O2 to satisfy myocardial demand in basal conditions

Myocardial workload can increase 5 fold

Extra O2 supplied by increased blood flow

There is an almost linear relationship between coronary blood flow and myocardial O2 demand (about a 5 fold increase in blood flow)

Until very high O2 demand where there is a small increase in the amount of O2 extracted (gradient is lowered slightly)

18

From where does coronary blood flow arise?

Right and left coronary arteries arise from the left and right aortic sinuses

19

How is high blood flow in the coronary circulation maintained?

Continuous NO production by the coronary endothelium

Metabolic hyperaemia caused by vasodilators:
- Adenosine
- K+
- Lowered pH

20

State fibre diameter and capillary density in skeletal and cardiac muscle

What do the relative capillary densities indicate?

What is the max diffusion distance of O2 through cardiac tissue?

Skeletal muscle:
- Fibre diameter = 50um
- Capillary density = 400/mm2

Cardiac muscle:
- Fibre diameter = 18um
- Capillary density = 3000/mm2

Cardiac muscle more capillary dense, therefore can deliver O2 more efficiently

Max diffusion distance = 9um

21

Explain how coronary arteries are involved in angina and myocardial infarction

Coronary arteries are functional end arteries (few arterio-arterial anastomoses) which are prone to atheroma

Narrowed coronary arteries lead to angina on exercise (increase O2 demand not met)

Sudden obstruction of the end arteries by a thrombus causes myocardial infarction

22

When does a majority of blood flow occur in the heart?

Occurs mainly during diastole:
- Contraction of myocardium compresses coronary vessels during systole
- Aortic sinuses are closed during systole

23

How does increasing heart rate affect rate of coronary blood flow?

Why is this a problem?

As blood flow is mostly during diastole (duration of which is reduced as heart rate increases) the rate of blood flow must rise very high to maintain adequate flow

Therefore minor problems with cardiac circulation might become apparent at higher heart rates

Coronary circulation is therefore much more sensitive to arterial occlusion than the rest of the body

24

Brielfy describe the O2 demand of the cerebral circulation

High O2 demand

Receives 15% of cardiac output (despite only being 2% of body mass)

Grey matter accounts for 20% of O2 consumption at rest

25

How does cerebral circulation meet the high O2 demand?

High capillary density:
- Large exchange area
- reduced diffusion distance (

26

Why is a secure O2 supply to the brain important?

Neurones very sensitive to hypoxia

Loss of consciousness after a few seconds of cerebral ischaemia (syncope)

Irreversible neurone damage after around 4 minutes

Interruptions to the blood supply such as in stroke lead to neuronal death

27

How does the structure of the cerebral circulation help maintain a secure blood supply to the brain?

Anastomoses between basilar and internal carotid arteries

28

How do functional factors contribute to secure blood supply to the brain?

Brainstem regulates other circulations

Myogenic auto-regulation maintains perfusion during hypotension

Metabolic factors control blood flow

29

Describe myogenic auto-regulation of cerebral circulation

Cerebral vessels respond to change in blood pressure

Increased BP = vasoconstriction
Decreased BP = vasodilation

this serves to maintain cerebral blood flow when BP changes

It fails below 50mmHg

30

Describe metabolic regulation of cerebral circulation

Cerebral vessels are sensitive to pCO2

Increased pCO2 (hypercapnia) = vasodilatation
Decreased pCO2 (hypocapnia) = vasoconstriction

In addition, areas of increased neuronal activity have increased blood flow as active neurones release adenosine, a powerful cerebral arteriole vasodilator

31

What is the effect of panic hyperventilation on the brain?

Can cause a rise in pCO2 (hypercapnia) leading to vasoconstriction

This can cause dizziness or fainting (syncope)

32

Describe Cushing's reflex

Rigid cranium doesn't allow volume expansion of the brain

Increased intercranial pressure can therefore result from haemorrhage or tumour formation which can impair cerebral blood flow

Impaired blood flow to the vasomotor control regions of the brainstem increases sympathetic vasomotor activity causing peripheral vasoconstriction

This leads to more blood flow being directed to the brain

33

What are the special roles of cutaneous circulation?

Temperature regulation
Maintaining blood pressure

34

How is the cutaneous circulation involved in maintaining blood pressure?

Vasoconstriction can cutaneous circulation helps maintain BP if necessary (E.g. When in shock)

35

What structures in the cutaneous circulation are involved in temperature regulation?

How is cutaneous circulation controlled?

Most blood flow in the skin isn't nutritive, it flows through arterio-venous anastomoses (AVAs) instead of capillaries

AVAs are under sympathetic control (sympathetic vasoconstrictor fibres in the AVAs)

Not regulated by local metabolites

36

What is the response of cutaneous circulation to increase or decrease in core temperature?

Lowered core temperature:
- Increases sympathetic tone causing AVA constriction
- This decreases blood flow to the apical skin
- Causes heat retention and pallor

Raised core temperature:
- Decreases sympathetic tone causing AVA dilation
- AVA forms a low resistance shunt to venous plexuses
- Increases blood flow to apical skin
- Causes increased heat dissipation

37

What is the function of skeletal muscle circulation during exercise?

What is one other function of skeletal muscle circulation in general?

Increase O2 and nutrient delivery and removal of metabolites

Important role in helping to regulate blood pressure (40% of body mass)

38

Describe the changes that occur in skeletal circulation as you begin to exercise starting from a basal state

Don't worry about how this is controlled, just state the changes

At basal state, 1/2 capillaries closed off by pre-capillary sphincters

Increased blood flow in exercise is then brought about by opening of the pre-capillary sphincters

Vasodilation of vessels (up to 20x)

39

How is skeletal circulation controlled?

Resistance vessels have rich sympathetic innervation controlling vasomotor tone

Metabolic hyperaemia is also involved in control
Vasodilators:
- K+
- Increased Osmolarity
- Inorganic phosphates
- Adenosine
- H+

Adrenaline (B2 adrenoceptor agonist) also acts as a vasodilator of arterioles

40

State the relative basal vasomotor tone of skeletal muscle

Give an explanation as to why

Relatively high

High as it allows up to a 20x increase in blood flow to help meet metabolic demand during exercise