Physiology-Special Circulations

Question 1

Briefly outline the anatomy of the arterial part of the coronary circulation

Answer 1

The right and left coronary arteries arise from the left and right aortic sinuses within the aorta -the sinuses sit behind the left and right flaps of the aortic valve -when the heart is relaxed the back-flow of blood fills these valve pockets thus allowing blood to enter the coronary arteries The left coronary artery initially branches to yield: -LAD (left anterior descending) which progresses to become the left marginal artery and the left circumflex artery The right coronary artery branches to form the right marginal artery anteriorly.

Question 2

Briefly outline the venous part of the coronary circulation

Answer 2

The coronary sinus is the main vein of the heart located on the posterior surface in the coronary sulcus. The sinus then drains into the right atrium. Within the right atrium the opening of the coronary sinus is located between the atrioventricular orifice and the inferior vena cava orifice. There are 5 tributaries that drain into the coronary sinus

Question 3

What are the adaptions of the coronary circulation?

Answer 3

-high oxygen extraction 75% compared to 25% whole body average -high capillary density -high basal flow

Question 4

What is the major implication of the increased oxygen saturation seen in the coronary circulation?

Answer 4

When more oxygen is required (e.g. exertion) it cannot be supplied by increasing oxygen extraction; it can only be supplied by INCREASING CORONARY BLOOD FLOW

Question 5

Coronary blood flow is controlled by: a) Extrinsic mechanisms b) Intrinsic mechanisms c) Extrinsic and Intrinsic mechanisms Explain answer.

Answer 5

c) Extrinsic and Intrinsic mechanisms Effected by changes in heart itself and the ANS

Question 6

How do intrinsic mechanisms control coronary blood flow?

Answer 6

-decreased ppO2 causes vasodilatation of the coronary arterioles -metabolic hyperaemia causes flow to be matched with demand -adenosine (from ATP) is a potent vasodilator

Question 7

How do extrinsic mechanisms control blood flow?

Answer 7

Coronary arterioles supplied by sympathetic vasoconstrictor nerves BUT… - they are over-ridden by metabolic hyperaemia (increased blood flow) as a result of increased heart rate and stroke volume -:. sympathetic stimulation of the heart results in vasodilation despite direct vasoconstrictor effect (functional sympatholysis) -circulating adrenaline activates Beta2 adrenergic receptors which cause vasodilatation

Question 8

Briefly describe the physiology of the cerebral circulation and why the specialised features are so important

Answer 8

Brain is supplied by:

INTERNAL CAROTIDS AND VERTEBRAL ARTERIES

The CIRCLE OF WILLIS is a special adaption of cerebral circulation

BASILAR (vasculature at base of brain and formed by two vertebral arteries) & CAROTID arteries ANASTOMOSE

Clinically this means:

CEREBRAL PERFUSION should be MAINTAINED even if one carotid artery gets obstructed

BUT an obstruction of a smaller branch of a MAIN ARTERY would DEPRIVE a region of the brain of its blood

This is highly important because…

grey matter is very sensitive to hypoxia

• consciousness lost after a few seconds of ischaemia,
• irreversible cell damage within 3 minutes

Question 9

Does sympathetic stimulation have much effect in overall cerebral blood flow?

Answer 9

NO,

Question 10

Why is AUTOREGULATION of the cerebral blood flow important?

Answer 10

AUTOREGULATION of the cerebral blood flow GUARDS AGAINST CHANGES in CEREBRAL BLOOD FLOW if MAP changes (within range of 60 to 160mmHg)

Question 11

Is the brain affected by baroreceptor reflexes?

Answer 11

BARELY

Participation of the brain in baroreceptor reflexes is neglible which is just as well!

The brain arterioles and so on don’t participate in baroreceptor reflex because don’t want cerebral blood flow to change that much that regularly hence sympathetic activity has little effect.

Question 12

What happens to cerebral blood flow if MAP changes? (within 60mmHg and 160mmHg)

Answer 12

1. If MAP RISES resistance vessels automatically CONSTRICT to LIMIT blood flow
2. If MAP FALLS resistnace vessels automatically DILATE to MAINTAIN blood flow

Question 13

What happens to cerebral blood flow if MAP goes outside of its safe range of 60-160mmHg?

Answer 13

AUTOREGULATION FAILS

• If MAP falls below 60mmHg cerebral blood flow falls
• If MAP rises above 160mmHg cerebral blood flow rises

MABP below 50mmHg results in CONFUSION, FAINTING and BRAIN DAMAGE if not quickly corrected

Question 14

What effect does ppCO2 have on cerebral blood flow?

Answer 14

• Increased ppCO2 causes cerebral vasodilation
• Decreased ppCO2 causes cerebral vasoconstriction

CLINICALLY?

Hyperventilation could lead to fainting

Question 15

What is the effect of intracranial pressure in cerebral blood flow?

Answer 15

Skull is a rigid box filled with approximately: brain 80%, blood 12%, and cerebrospinal fluid 8%.

• Normal intracranial pressure within the skull is about 8-13mmHg
• Cerebral Perfusion Pressure= MAP - ICP
• Increasing ICP (e.g. due to head injury, or brain tumour) decreases CPP and cerebral blood flow
• Some conditions which increase ICP can lead to failure of autoregulation of cerebral blood flow

Question 16

What is the blood brain barrier?

Answer 16

Cerebral CAPILLARIES have VERY TIGHT INTERCELLULAR JUNCTIONS- THE BLOOD BRAIN BARRIER

• Cerebral capillaries are highly permeable to O2 and CO2
• Glucose crosses the BBB by facilitated diffusion using specific carrier molecules
• but other than that the BBB is exceptionally impermeabe to hydrophilic substances such as ions, catecholamines, proteins etc
• this helps PROTECT brain neurones from FLUCTUATING levels of ions etc in blood

Question 17

List and describe/explain the special adaptions of pulmonary circulation.

Answer 17

• pulmonary capillary pressure is low (8-11mmHg) compared to systemic capillary pressure (17-25mmHg)
• absorptive forces exceed filtration forces- protects against pulmonary oedema
• hypoxia causes vasoconstriction of pulmonary arterioles
  
  • completely opposite to effect of hypoxia on systemic arterioles!!!!!!!!!! WHY?
    
    • would help divert blood from poorly ventilated areas of lung

Question 18

What is the effect of exercise on skeletal muscle blood flow?

Answer 18

1. During exercise, local metabolic hyperaemia overcomes sympathetic vasoconstrictor activity
2. Circulating adrenaline causes vasodilatation
   
   1. Beta2 adrenergic receptros
3. Plus increased cardiac output during exercise, these could increase skeletal muscle blood flow many folds

Question 19

What is the skeletal muscle pump?

Answer 19

• Large veins in limbs lie between skeletal muscles
• Contraction of muscles aids venous return
• One-way venous valves allow blood to move forward towards the heart
• Skeletal muscle pump reduces the chance for postural hypotension & fainting

Question 20

1. What happens to the skeletal muscle pump is venous valves become incompetent?
2. How does this present clinically?

Answer 20

1. Blood pools in lower limb veins
2. Presents as VARICOSE VEINS
   
   • varicose veins usually doesn’t lead to reduction of cardiac output but why?
     
     • chronic compensatory increase in blood volume