Arteries Flashcards
What is the vascular system?
The conducting pathways of the arterial and venous circulations, which run in parallel and are separated by dense networks of tiny vessels
What are the five composites of the arteries?
Adventitia, smooth muscle, elastic ,endothelium, pre-capillary sphincter
What does the adventitia do?
Provides structural strength and tethers vessels in place; in large vessels contains small blood vessels (vasa vasorum)
What does the smooth muscle do?
Supplies vessels with contractile power, regulating diameter of lumen
What does the elastic tissue do?
Gives mechanical strength and elastic properties
What does the endothelium do?
Filtering interface between blood and body; secretes vasoactive products
What does the pre-capillary sphincter do?
Controls blood flow to specific capillary beds selectively
What is smooth muscle?
Cells form layers and line cavities of hollow organs
What are features of smooth muscle?
Contractions are controlled by ANS; connected to each other electrically and mechanically (but not always); contract slowly but longly, without fatigue; has no sarcomere arrangement, and no NMJ
How is smooth muscle connected mechanically?
By specialised areas on membrane (dense bonds)
How is smooth muscle connected electrically?
By gap junctions on membrane; allows contraction as unit
How are contractile filaments arranged in smooth muscle?
Actin and myosin arranged in form of a lattice around the cell; actin is attached to cell via specialised areas in cytoplasm (dense bodies)
Where are neurotransmitters released from in smooth muscle?
As there are no NMJs, NTs are released non-specifically from varicosities
What is single unit smooth muscle?
Electrically coupled muscle cells by gap junctions; act as unit; blood vessels, intestines, uterus
What is multi-unit smooth muscle?
Not electrically coupled, so each cell needs to activated by a nerve; lung airways, large arteries
What is the implication of the lack of sarcomere arrangement in smooth muscle?
Myosin and actin can slide past each other without encountering ends of sarcomere, so less myosin and actin is needed (often ~50% myosin, can be 200% actin), and force of contraction can vary over a large range of lengths as actin and myosin still overlap
Why is it helpful that force of contraction can be maintained in smooth muscle over a larger range of lengths?
Most smooth muscles surround hollow structures and organs that undergo changes in volume; smooth muscle fibres retain ability to develop tension even with large increases in volume
How is contraction of smooth muscle regulated?
By Ca2+ entering the cell (instead of Ca2+ from SR); can be graded instead of all-or-nothing
What are the two types of action potentials in smooth muscle?
Pacemaker potential and slow waves
What are pacemaker potential action potentials?
Activates at regular intervals to allow regular contraction; intervals can change
What are slow waves?
A state of readiness, with fluctuations in polarisation below threshold; contract only when stimulus pushes slow waves upwards and meet threshold potential, firing action potentials successively as needed
What determines if the process of action potentials forming from slow waves continues?
Depends how much slow wave is pushed up; if all slow wave if within threshold, continuous contractions occurs; if some slow wave is within threshold and some aren’t, contraction is interrupted
How do action potentials stimulate muscular contraction in smooth muscles?
Depolarisation opens v-gated Ca2+ gates on sarcolemma; Ca2+ in; Ca2+ activates calmodulin; activated calmodulin activates myosin light-chain kinase; activated myosin light-chain kinase phosphorylates myosin; phosphorylated myosin is activated and binds to actin. Process only occurs if myosin is being phosphorylated at a higher rate than myosin light chain phosphotase is dephosphorylating myosin (which is always occuring)
Where is the Ca2+ for smooth muscle contraction sourced from?
Extracellular fluid and sarcoplasmic reticulum
How is flow of CV system changed?
MAP must be maintained, and venous pressure is small so can be ignored; arterioles alter levels of resistance to blood flow to distribute available amount of blood to where it is needed
What is hyperemia?
Increase in blood flow according to metabolic needs of tissue in question (e.g. to deliver more oxygen, remove metabolites)
What are chemical changes that take place locally when metabolic rate increases?
Tissue hypoxia; increased CO2, decreased pH, accumulation of K+, ATP, bradykinin, NO
What is flow autoregulation?
Maintenance of blood flow rather than changing it
What does flow autoregulation do when driving pressure increases?
When arterial pressure increases, a transient rise in blood flow occurs; increased flow removed the vasodilators; vessel constricts; flow returns to normal
What is the stretch stimuli to muscle?
A myogenic response, based on metabolic factors
What is the myogenic regulation of arteriole size?
An intrinsic regulation of smooth muscle to regulate flow; transient increase in pressure = increase in diameter of vessel = stretch in smooth muscle, which sends signal to smooth muscle, which constricts in response
How is the arteriole size influenced by neural activity?
Sympathetic innervation; non-adrenergic and non-cholinergic fibres
How does sympathetic innervation affect arterial size?
Release more NE on alpha-adrenergic receptors vasoconstricts (and opposite is true)
How does innervation by non-adrenergic and non-cholinergic fibres affect arterial size?
Release of NO vasodilates (and possibly other unidentified vasoactive substances)
What hormones cause vasodilation?
Adrenaline (on beta-2 receptors); atrial natriuretic peptide
What hormones cause vasoconstriction?
Adrenaline (on alpha); angiotensin II; vasopressin
What role does the endothelium have in controlling arterial size?
Major source of paracrines which regulate vessel size locally, allowing very specific regulation of flow
What vasodilators does the endothelium release?
NO and prostacyclin
What vasocontrictors does the endothelium release?
Endothelin-1
How does shear stress affect the endothelium?
Affects bigger arteries, which respond to increased flow increasing force exerted on walls by dilating; increase NO and PGI2 and decrease ET1 in response to shear stress
What are features of capillaries that enable them to carry out their function?
Numerous to reach each part of body; large SA; thin; low velocity
Why are capillaries thin and create large SA?
Have just a single layer of endothelial cells; flow of gas proportional to SA, inversely proportional to thickness (Fick’s law)
Why is velocity of capillaries so low?
Allows time during which diffusion can reach equilibrium
What determines the flow in capillaries?
Total CSA (increased CSA = slower flow)
What are the three types of capillary endothelial arrangements, and examples?
Continuous: muscle, nerve, fat, lymph nodes; fenestrated: glands, kidneys, intestines; discontinuous (sinusoids): liver, bone marrow, spleen
What are three types of transport which occurs across endothelium of capillaries?
Diffusion, vesicle transport, bulk flow
What is transported across endothelium by diffusion?
O2, Co2, glucose, amino acids, FAs
What is transported across endothelium by vesicle transport?
Large molecules, e.g. proteins
What is transported across endothelium by bulk flow?
Water, solutes
How do you calculate net pressure in a capillary?
Hydrostatic - osmotic pressure
What occurs when hydrostatic pressure > osmotic pressure?
Filtration
What occurs when hydrostatic pressure < osmotic pressure?
Absorption
