Special Circulations

Question 1

What is special about the coronary arteries ?

Answer 1

Fill during diastole, as:
-Aortic valve is closed
-Not compressed by ventricular systole (causes extravascular compression)

Question 2

What effect does increased heart rate have on the durations of systole and diastole ?

Answer 2

When heart rate increases diastole shortens
-less myocardial perfusion when HR increases e.g. exercise

Systole barely decreases

Question 3

Which factors regulate autoregulation and thus control of coronary heart flow ?

Answer 3

1. Physical
   -Aortic pressure
2. Neural
   -Sympathetic and parasympathetic
3. Metabolic
   -Decreased ratio of supply to demand of oxygen induces release of vasodilators like:
   Adenosine
   Nitric oxide (NO)
   K+

Question 4

Describe the main physical factor affecting coronary blood flow

Answer 4

Aortic pressure
-Generated by the heart itself
-Changes in aortic pressure = changes in coronary blood flow
-Autoregulation – steady coronary blood flow despite alteration in perfusion pressure (within limits)

Red band is isovolumetric contraction

Question 5

How do sympathetic neural factors affect coronary blood flow ?

Answer 5

Sympathetic stimulation (‘fight or flight’)
-Increases heart rate (diastole shortens)
-Increases force of contraction (more extravascular compression)

Despire this there is an increase in coronary blood flow as autoregulation causes coronary artery dilatation so above are offset

Question 6

How do parasympathetic neural factors affect coronary blood flow ?

Answer 6

Parasympathetic stimulation (‘rest and digest’)
-Vagus nerve stimulation; slight vasodilatation
-Acetylcholine release; nitric oxide (vascular endothelial cells)

Question 7

How do Metabolic factors affect coronary blood flow ?

Answer 7

Myocardial blood flow = myocardial metabolic demand
Decreased Supply and increased Demand of oxygen causes release of vasodilators:

-Adenosine; lowers intracellular Calcium
-Nitric oxide; induces production of cGMP; activates protein kinase G; which activates MLCP and induces relaxation
-K+; released by contracting myocytes to increase coronary perfusion

Question 8

What does autoregulation do ?

Answer 8

Maintains steady coronary blood flow

Question 9

How does skeletal muscle affect blood flow and how does the flow to it change ?

Answer 9

Low resting blood flow; sympathetic vasoconstrictor tone so high resistance
-Skeletal muscle vascular bed resistance has large impact on blood pressure

Metabolic hyperaemia overcomes sympathetic tone during exercise’ Blood flow to skeletal muscle increases
-Skeletal muscle circulation increases up to 50x during exercise
(Rest 5-10 mL/min/100g, Exercise 250mL/min/100g)

Question 10

What is the skeletal muscle pump ?

Answer 10

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 11

What occurs if venous valves become incompetent ?

Answer 11

Blood pools in lower limb veins if venous valves become incompetent e.g. due to:
-Pregnancy (more blood)
-Obesity (greater leg venous pressure)
-Thrombophlebitis (direct damage)

Leasd to varicose veins

Most leg swelling in old people is due to valve failing and poor venous return; check for varicose veins, not always heart failure

Question 12

Where does the entire cardiac output flow from

Answer 12

Entire cardiac output flows from right ventricle into pulmonary circulation – same as for the systemic circulation

Pulmonary circulation CO = systemic circulation CO

Question 13

How are the Metabolic needs of airways met ?

Answer 13

systemic bronchial circulation

Question 14

Compare pulmonary and systemic resistance and BP

Answer 14

Pulmonary resistance ~10% of that of the systemic circulation ; Low resistance circulation

Pulmonary artery BP typically 20/25 mmHg- 6/12mmHg

Question 15

Where are pulmonary capillaries located and why ?

Answer 15

Pulmonary capillaries are located between alveoli to allow
for efficient gas exchange

Question 16

Describe three adaptations of the pulonary circulation

Answer 16

1) Pulmonary capillary pressure is low (~ 8-11 mmHg)
-Systemic capillary pressure (~ 17-25 mmHg)

2) Absorptive forces exceed filtration forces
-Protects against pulmonary oedema

3) Hypoxia causes vasoconstriction of pulmonary arterioles.
-Completely opposite to effect of hypoxia on systemic arterioles.
-Divert blood from poorly ventilated areas of lung to maintain optimal ventilation perfusion (V-P) ratio

Hypoxia thing moves flow from trash alveoli to good ones; PE can do this