L22. Blood Vessels - Flow and Control Pt.2 Flashcards
(25 cards)
Vasodilator nerves?
Found in organs where brain need to control and command a profound increase in blood flow
- Acetylcholine (ACh) and Vasoactive intestinal peptide (VIP) are common transmitters released by parasympathetic fibres
Parasympathetic vasodilator nerves?
- Salivary glands (ACh and VIP)
- To pancreas and intestinal mucosa (VIP)
- To external genitals (NO and VIP)
Sympathetic vasodilator nerves?
- Skin –> vasodilation associated with sweating (ACh and VIP)
Plasma epinephrine?
Plasma epinephrine can act on both alpha and beta receptors
It can act on the alpha receptors and cause vasoconstriction or it can act on our beta receptors and cause vasodilation
The reason adrenaline can do this is because in different parts of the body, in different tissues or organs, the relative ratio of alpha to beta will differ
So if we increase the amount of adrenaline in the blood, if the smooth muscle surrounding the arterioles primarily have alpha receptors, those organs will see vasoconstriction (opposite for vasodilation)
Adrenaline?
Is the main catecholamine secreted by the human adrenal medulla
Angiotensin II?
Vasoconstrictor important in the response to hypovolaemia (e.g. haemorrhage) and cardiac failure; raised in some (not all) hypertensives
Vasopressin?
(Antidiuretic hormone, ADH) - vascular action is important in response to hypovolaemia (vasoconstriction)
Atrial natriuretic peptide?
Moderate vasodilator
Intrinsic control of blood vessels?
How blood vessels respond to current blood pressure within them
Autoregulation?
When arterial pressure is altered, blood flow in many vascular beds remains constant
Myogenic autoregulation?
Resistance vessels respond directly to an increase in pressure by vasoconstriction or vasodilation
Metabolic regulation (active hyperaemia)?
Intrinsic mechanism by which blood flow to organs such as brain, heart muscle and skeletal muscle is adjusted to match metabolic activity
Endothelial-derived factors?
Control of tissue blood flow by endothelial-derived factors
- Endothelium produces both vasoconstrictors and vasodilators
Vasoconstrictors:
- Endothelin
- Angiotensin II
Vasodilators:
- Nitric oxide (NO)
- Prostacyclin (PGI2)
- Endothelial derived hyperpolarizing factor (EDHF)
Capillary structure?
- Varies considerably from organ to organ
- Thin walled tube of endothelial cells without smooth muscle cells
- Covered by basement membrane
- 500-1000um long and 4-8um wide; total thickness ~0.5um
Three types of capillaries:
1. Continuous capillary = have tight junctions
2. Fenestrated capillary = more permeable
3. Sinusoidal capillary = incomplete basement membrane
Capillary function?
Primary function is exchange - of gas, metabolites, etc.
- Very large total cross-sectional area and very low velocity of blood flow
- Fluid exchange: regulation of plasma and interstitial fluid volumes
Mechanism: bulk flow - due to pressure gradients across wall, obeys Starling’s principle
- Solute exchange: nutrition of tissue, hormone and drug delivery
Mechanism: diffusion - due to concentration gradients across wall
- 98% of nutrient (or drug) transport is by diffusion
- Lipid soluble substances including oxygen and CO2 easily diffuse through endothelial cells
- Ions and polar molecules are poorly soluble and pass through the water filled channels in the endothelial lining - intercellular cleft
- Proteins are not usually allowed to diffuse through the water filled channels (liver is an exception)
Transcapillary diffusion gradient?
Is due to the cellular utilisation or production
Diffusion factors across capillaries?
Permeability and molecular weight
- Low molecular weight = high permeability
- High molecular weight = low permeability
Capillary blood flow?
Capillary blood flow is affected by plasma osmotic pressure (also called colloid osmotic, COP) as well as hydrostatic pressure
Net filtration pressure?
Is equal to the four pressures that determine filtration rate
P = hydrostatic pressure
(pie symbol) = osmotic pressure
c = capillary
IF = interstitial fluid
Answer usually equals 10mmHg favouring filtration
Go on slide 27, lecture 6 of cardiovascular
Fluid exchange at the capillary?
- Water is driven across the capillary wall by the sum of hydrostatic and colloid osmotic pressures of the blood and interstitial fluid
- The colloid osmotic pressure of the blood (oncotic pressure) is generated by the plasma proteins dissolved in the plasma
- The hydrostatic forces tend to move water out of the capillary (filtration); the osmotic forces tend to draw water in (reabsorption)
Normally, the filtration is greater than reabsorption
About 4 litres/day of modified ISF (lymph) is returned in lymph vessels, which drain into the systemic veins at the base of the neck
Imbalance of filtration absorption scale?
If Pc is raised it favours absorption
If osmotic colloid is reduced it favours filtration - causes oedema
Veins?
- The walls are thinner than arteries, so they often appear collapsed in histological slides
- Veins have less smooth muscle and elastin than arteries
- Veins are highly distensible, so they are called capacitance vessels that act as blood reservoirs
- Because veins have a high compliance at normal operating pressures they can release or store blood in response to small changes in pressure
Determinants of venous pressure?
- Sympathetic innervation
- Skeletal muscle pump
- Blood volume
- Respiratory pump
Varicose veins?
- Veins have one-way valves that prevent the backflow of blood
- Varicose veins are veins that have become dilated and tortuous because of incompetent (leaky) valves
- About 15% of adults suffer from this condition, mainly in the lower limbs
