VASCULAR: SPECIAL CIRCULATION Flashcards
What is the myogenic response that assist with autoregulation
Increased blood flow leads to increased stretch and reflexive constriction
What is the metabolic response that assist with autoregulation
Decreased blood flow leads to metabolic build up and a decrease in O2 pressure → leads to local vasodilation
How does autonomic nervous control assist with autoregulation
Sympathetic stimulation → vasoconstriction
parasympathetic stimulation → vasodilation
Where can you find the B1 adrenergic receptors and how do they assist with autoregulation?
located at cardiac muscle; greater affinity for norepinephrine than epinephrine; increases contractility
Where can you find the B2 adrenergic receptors and how do they assist with autoregulation?
located at bronchioles and has a much greater affinity for epinephrine than norepinephrine; results in bronchodilation
located at the large skeletal muscle vasculature and coronary arteries; also greater affinity for epinephrine; results in vasodilation and increased blood supply during exercise
Where can you find the A1 adrenergic receptors and how do they assist with autoregulation?
located at vascular smooth muscle; greater affinity for norepinephrine than epinephrine; results in profound vasoconstriction and increase in blood pressure
In the skeletal muscle during rest, what is the response for autoregulation?
Sympathetic tone predominantes
Norepinephrine binds to a1 and a2 adrenoceptors → vasoconstriction
Adrenaline levels are so low so there is little B2 adrenergic stimulation
In the skeletal muscle during exercise, what is the response for autoregulation?
Vasodilator metabolites (especially adenosine) → dilation
Dilation predominates despite increased sympathetic tone
Adrenaline levels rise → B2 adrenergic stimulation → dilation
Define: reactive hyperemia
Reactive hyperemia: increased flow in response to ischemia; K+ release → hyperpolarizes VSM → vasodilation
Define: active hyperemia
Active hyperemia: metabolite induced vasodilation and therefore increased blood flow; can take a few seconds to initiate
How does % blood flow change from rest to exercise?
- brain
- heart
- muscle
- skin
- kidney
- abdominal organs
- others
At rest vs during exercise % blood flow
Brain: 13% → 4%
Heart: 4% → 4%
Muscle: 20% → 73%
Skin: 9% → 11%
Kidney: 20% → 3%
Abdominal organs: 24% → 3%
Other: 10% → 2%
What type of dilation occurs in the muscle, heart, and skin during exercise?
Metabolic dilation predominates in exercising muscle and heart
Thermoregulatory dilation occurs in skin vasculature
What decreases flow to non-essential regions during exercise?
Adrenergic constriction
Describe the skeletal muscle pump action that enhances venous return
Blood enters the vein, the upper valve opens further when muscle contracts but the lower valve closes so that contracting muscle pushes blood upwards. After muscle relaxes, upper valve closes to prevent backflow and lower valve opens allowing veins to fill
define: apical skin
Apical skin: skin to the face, hands, and feet; typically poorly insulated
Describe how apical skin blood flow occurs
arteriovenous anastomoses: direct connections between small arteries and veins; arteriole to venule shunting
This is a low resistance pathway which allows a high flow of blood
Sympathetic regulated
Insensitive to metabolic vasodilators
What is apical skin important for?
- Important area for heat dissipation
- Increased core temperature causes decreased sympathetic output, less vasoconstriction, more flow through anastomoses, and heat dissipation
Describe how non apical skin blood flow differs from apical skin
- fewer A-V anastomoses
How does the non apical skin assist with returning the body to normal core temp after hyperthermia
Hyperthermia (elevated core temp): hypothalamic cholinergic fibre activates and sweat glands release bradykinin and other local mediators which lead to vasodilation and increased heat dissipation
How much of the cardiac output goes to the brain? How much does it weigh?
10-15% cardiac output yet only 2% body weight
Why is the brain considered intolerant of ischemia (reduced blood flow)
Interruption of flow for just a few seconds causes loss of consciousness (syncope)
depends on oxidative metabolism of glucose
What is cerebral blood flow dependent on?
Autoregulation is critical (pressure and metabolic regulation > neural control)
Describe the blood brain barrier
- what is it gated by
- what types of molecules can pass through
- Gated by continuous tight junctions and limited transcytosis
- few select breaches e.g. posterior pituitary
- highly lipid soluble molecules can pass e.g. CO2, alcohol, caffeine, nicotine, heroine, methadone (not H+, not adrenaline)
- permits water through endothelial water channels
What arteries supply the brain?
Internal carotid arteries (2)
Vertebral arteries (2)
What is the circle of Willis
Receives blood through internal carotid and vertebral arteries and distributes blood to the rest of the brain.
Circle shape blood vessel which maintains effective flow even if flow in one artery is reduced
Why are some people more prone to syncope (fainting)? (4)
- Their cerebral autoregulation is less effective; when they stand up, they have more pronounced reduction in flow as cerebral pressure falls
- Their cerebral circulation is abnormally sensitive to carbon dioxide
When you stand up there is increased ventilation and decreased carbon dioxide which leads to vasoconstriction. In fainters, this is more pronounced - Baroreflex blood pressure is impaired; decreased systemic pressures with standing further reduce cerebral pressure
- Upon standing, decreases in blood pressure and CO2 cause marked decreases in cerebral blood flow and loss of consciousness
Describe Ficks principle applied to whole body O2 consumption
O2 consumption = Q* (arterial-venous O2 difference)
If assuming a cardiac output of 5 l/min, O2 consumption is 200 mL O2/min in the whole body.
Describe the a-vO2 difference of the whole body when the body is at rest
At rest (whole body)
Arterial O2 content = 20 ml O2 per 100 ml of blood
Venous O2 content = 16 ml O2 per 100 ml of blood
↳ a-v O2 difference = 4 ml O2 / 100 ml
Describe the a-vO2 difference of the coronary circulation when the body is at rest
Cardiac arterial O2 content = 20 ml O2 per 100 ml blood
Cardiac venous O2 content = 4 ml O2 per 100 ml blood
↳ a-v O2 difference = 16 ml O2 / 100 ml
→ near maximal oxygen extraction at rest
How does coronary blood flow differ in terms of O2 extraction?
Cardiac muscle cannot increase O2 extraction to meet increased demand thus flow must increase to meet demand.
What type of dependence does myocardial O2 consumption have on coronary blood flow (i.e. delivery)
linear
How does a-vO2 difference differ between the whole body and the coronary circulation
coronary circulation extracts near maximal oxygen at rest
(16 ml O2/ 100 ml vs 4 ml O2 / 100 ml)
During exercise, how does the heart increase O2 consumption if there is already maximal oxygen extraction in the coronary circulation at rest?
since oxygen extraction can’t increase, blood flow must increase
Describe coronary blood flow starting from the aorta
- Aorta splits into two branches which are the left and right conary artery
- Right coronary artery supplies the right ventricle, atrioventricular node and sino-atrial node
- Left coronary artery supplies the left anterior descending artery and circumflex artery
- Left anterior descending artery supplies the left ventricle, bundle of his, and conducting tissue
- Coronary circulation drains into right atrium via the coronary sinus
Describe the mechanical regulation (ventricular compression) of coronary blood flow
Ventricular compression caused by the cardiac muscles contracting that squeeze the arteries and prevent their flow. Decreases arteriole radius and increases their radius. Results in decreased flow during systole
There is more flow during diastole
What type of myocardial tissue is most susceptible to myocardial infarction?
Endocardium tissue is most susceptible to myocardial infarction as the arteries must travel through the epicardium and myocardium first (more road ahead that can get blocked)
Describe the neurological regulation of coronary blood flow
High intrinsic vasomotor tone: there is always resting sympathetic vasoconstriction
Vasodilator reserve: describes the reserved capacity to dilate due to high intrinsic vasomotor tone
Describe the metabolic regulation of coronary blood flow
Metabolic dilators in the coronary circulation:
- decreased O2
- increased Co2
- increased adenosine
- decreased pH
- Increased K+
- Decreased ATP (opens KATP channels)
- NO is a potent coronary artery dilater
- Medications: GTN and nitroprusside are used in the treatment of angina
Describe the myogenic regulation of coronary blood flow
sense stretch –> constriction to maintain pressure
Describe coronary artery disease
- Collateral vessels will develop with slow onset ischemia (reduced blood flow)
– vascular endothelial growth factor and hypoxia inducible factor are upregulated and induce angiogenesis/proliferation.
