Lecture 23 : Blood Vessels - Part 2 Flashcards
What mechanisms is vascular tone controlled by?
Intrinsic and extrinsic mechanisms
What is the general location of vasodilator nerves?
In organs where brain needs to control and command a profound increase in blood flow
What 2 transmitters are commonly released by parasympathetic fibres?
- Acetyl choline (ACh)
- Vasoactive intestinal peptide (VIP)
What do parasympathetic vasodilator nerves innervate?
- Salivary glands (ACh and VIP)
- Pancreas and intestinal mucosa (VIP)
- External genitals (NO and VIP)
What do sympathetic vasodilator nerves innervate?
Skin - sweating (ACh and VIP)
What hormone does the adrenal medulla secrete and what is the role of this hormone?
Epinephrine
- Increases plasma epinephrine
- Acts on α (vasoconstrictor) and β (vasodilator) receptors
- Acts on smooth muscle in skeletal muscle arterioles
- Alters arteriolar radius
What hormone is released from sympathetic neurons to skeletal muscle arterioles and what is the role of this hormone?
Norepinephrine
- Increases norepinephrine in ECF
- Acts on α (vasoconstrictor) receptors
- Acts on smooth muscle in skeletal muscle arterioles
- Alters arteriolar radius
What do β receptors cause?
Vasoconstriction
What do α receptors cause?
Vasodilation
What is adrenaline?
Main catecholamine secreted by adrenal medulla
What is angiotensin ll?
Vasoconstrictor
- Responds to hypovolaemia (haemorrhage) and cardaic failure
- Raised in some hypertensives
What is vasopressin (ADH)?
Vasoconstrictor
- Responds to hypovoleamia
What is atrial natriuretic peptide?
Moderate vasodilator
What is autoregulation?
When arterial pressure is altered blood flow in many vascular beds remains constant
What is myogenic autoregulation?
Resistance vessels respond directly to an increase in pressure by vasoconstriction or vasodilation
Why is myogenic autoregulation important?
Protects organ perfusion against minute-minute fluctuation in arterial blood pressure
What 3 places is myogenic autoregulation most developed in the body?
- Brain - hypotension during spinal anaesthesia
- Myocardium - blood flow downstream stenosed artery
- Kidney - stabilises GFR
What is metabolic regulation (active hyperemia)?
Intrinsic mechanism by which blood flow to organs is adjusted to match metabolic activity
What are the vasodilators of metabolic regulation?
- CO2
- Lactate
- H+
- Adenosine
What is the vasoconstrictor of metabolic regulation?
Oxygen
What vasoconstrictors do endothelium produce?
- Endothelin
- Angiotensin ll
What vasodilators do endothelium produce?
- Nitric oxide
- Prostacyclin (PGI2)
Endothelial derived hyperpolarizing factor (EDHF)
Describe the structure of capillaries:
- Varies from organ to organ
- Thin walled tube of endothelial cells without smooth muscle cells
- Covered by basement membrane
- 500-1000µm long and 4-8µm wide
- Total thickness ~0.5µm
What are the 3 types of capillaries?
- Continuous capillary: have tight junctions
- Fenestrated capillary: more permeable
- Sinusoidal capillary: incomplete basement membrane
What are the 2 functions of capillaries?
- Fluid exchange
- regulation of plasma & interstitial fluid volumes - Solute exchange
- nutrition of tissue
- hormone & drug delivery
Describe the cross-sectional area and velocity of blood flow of capillaries:
Very large CSA
Low velocity of blood flow
How do small non-lipid soluble substances get across endothelial cells?
- Fenestrations
- Intercellular cleft
How do lipid soluble substances get across endothelial cells?
Diffusion - O2, CO2
How do large non-lipid soluble proteins get across endothelial cells?
- Big gaps in inflammation
- Transcellular channels
- Vesicle transport
What is the mechanism behind fluid exchange?
Bulk flow
- due to pressure
gradients across wall
- obeys Starling principle
How does size affect the diffusion of a substance across capillaries?
Low molecular weight = high permeability e.g. water, NaCl
High molecular weight = low permeability e.g. albumin
What is the mechanism behind solute exchange?
Diffusion (98% nutrients)
- due to concentration
gradients across wall
What is the starling principle of fluid exchange?
Capillary blood flow is affected by plasma osmotic pressure (colloid osmotic pressure, COP) and hydrostatic pressure
What 4 pressure determine filtration rate?
- Capillary hydrostatic pressure (Pc) - outwards
- Interstitial fluid hydrostatic pressure (PIF) - inward
- Osmotic force due to plasma protein concentration (πc) - inward
- Osmotic force due to interstitial fluid protein concentration (πIF) - Outward
What is the value of interstitial fluid hydrostatic pressure?
0 mmHg
What is the value of osmotic force due to interstitial fluid protein concentration?
3 mmHg
What is the value of osmotic force due to plasma protein concentration?
28 mmHg
How is net filtration pressure calculated?
Outward:
Pc + π IF - P IF - πc
What is favoured when net filtration is negative?
Absorption
What is favoured when net filtration is positive?
Filtration
What drives the movement of water across the capillary wall?
Sum of hydrostatic and colloid osmotic pressures of the blood and interstitial fluid
What is colloid osmotic pressure of the blood generated by?
Oncotic pressure is driven by plasma proteins dissolved in the plasma
What effect do hydrostatic forces have on water?
Move water out of the capillary - filtration
What effect do osmotic forces have on water?
Draw water in - reabsorption
What makes up the lymph system?
Lymph capillaries
Lymphatic vessel
Lymph node
Valve
What is the result of an imbalance in absorption and filtration?
- Deep vein thrombosis: Pc raised
- Cardiac failure, starvation, malabsorption, liver failure: πc reduced
= Oedema: excess collection of fluid
Describe the structure of veins:
- Walls are thinner than
arteries - Less smooth muscle and elastin than arteries
- Highly distensible; capacitance vessels that act as blood reservoirs
What 4 factors increase venous pressure?
- Sympathetic innervation
- Skeletal muscle pump
- Blood volume
- Respiratory pump
What are varicose veins?
Veins that have become dilated and tortuous because of incompetent (leaky) valves
