arteries, veins, and control of vascular resistance Flashcards
arteries
High pressure
Thick muscle layer
Highly elastic
Contains 17% of blood
Dampens pulsatile flow
veins
Low pressure
Thin muscle layer
Distensible: high volume
Contains 70% of blood
Valves prevent backflow
artetioles
a variable resistance system that distributes blood between vascular beds by dilating and constricting
Important in determining BP
Diameter < 100 µm
capillaries
large network of very small vessels comprising single layer of overlapping endothelial cells
Exchange vessels: gases, fluid, and biomolecules are exchanged with tissues
venules
similar to arterioles they can constrict to alter blood flow. The balance of effects influences pressure in the capillaries and therefore fluid exchange
endothelial cells
Contiguous layer lining ALL vessels and the inside of the heart chambers
Regulate contraction and relaxation of underlying smooth muscle cells
Prevent platelet aggregation and blood clot formation
Permeability barrier for nutrients / fluid between plasma and interstitial fluid
- angiogenesis and vessel remodelling
endothelial function and dysfunction
Constitutive release of nitric oxide: SMC relaxation; inhibits platelet aggregation
Releases constrictors: endothelin, thromboxane; & dilators: prostacyclin
Can influence proliferative state of smooth muscle cells, e.g. in hypertension
Dysfunctional ECs can release free radicals which oxidise LDL (pro-atherosclerosis)
Can express molecules which tether inflammatory cells
vascular smooth muscle cells
Present in all vessels with the exception of the smallest capillaries
Exhibit intrinsic contractile tone
Determine vessel radius by contracting and relaxing when stimulated by nerves, hormones, local signalling molecules
Secrete an extracellular matrix which gives the vessels their elastic properties
Can proliferate in some diseases, e.g. hypertension, atherosclerosis
arterial elasticity
Compliance is important to allow large arteries to act as a pressure reservoir and maintain a high diastolic pressure
This ensures organ perfusion during diastole (particularly important for coronary vessels)
Compliance declines with age and in hypertension
blood pressure
Blood is forcefully ejected into the arteries only during systole and so is pulsatile
Windkessel effect in large arteries keeps diastolic pressure high
This is necessary to overcome the resistance to flow from the entire downstream vascular system, i.e. ‘total peripheral resistance’ (TPR)
Thereby maintaining perfusion of organs to meet metabolic demands
blood pressure control- sympathetic innervation
Release of norepinephrine from post-ganglionic nerve varicosities
Activate α1-adrenoceptors on smooth muscle cells to cause constriction and increase in TPR
Innervation density: Arterioles > large arteries > veins
1-2 impulses / sec at rest results in mild tonic vasoconstriction
Selective α1-adrenoceptor antagonists (e.g. prazosin) decrease vasoconstrictor tone to ↓ TPR
N.b. parasympathetic system affects CO, but not TPR directly
-parasympathetic system only innervates a few vascular beds, erectile tissue in genitalia, pancreas and salivary glands
local bp control and flow autoregulation
Autoregulation of blood flow ensures consistent supply despite changing metabolic demands
Hyperaemia = increased blood flow
active hyperaemia
Active hyperaemia: changes in gases & metabolites can dilate arterioles
hypoxia (low O2) relaxes vascular SMCs
High CO2 causes vasodilation via NO release
Increased metabolic demand: ATP → adenosine, which acts on A2-receptors to relax SMCs
reactive hyperaemia
Reactive hyperaemia: blocking blood flow temporarily induces vasorelaxation (can be used to test vascular function in humans)
capillaries function
Contain ~5% of total blood volume
Network consists of ~25,000 miles in total
Perform the ultimate function of the CV system, i.e. exchange of nutrients / waste products
One cell thick to allow rapid exchange with tissues (semi-permeable membrane)
Blood flow depends on supply from arterioles and contraction of venules
factors influencing transport
The blood in the capillaries exerts a pressure on the capillary wall (hydrostatic pressure)
This tends to favour movement of water out of the capillary (filtration)
The plasma has an intrinsic osmotic pressure due to plasma proteins (colloid pressure)
This tends to favour movement of water into the capillary
The sum of these 2 pressures determines the net water flow across the capillary wall
hydrostatic and colloid pressure
Hydrostatic pressure will decrease as blood pressure drops from arterial - venous
Colloid pressure will remain the same as abundant plasma proteins (e.g. albumin) are too big to leave the capillary
- fluid will move out at the arterial end and move in at the venous end
pulmonary capillaries
Pulmonary Hydrostatic pressures are lower than in systemic capillaries (~8 mmHg)
This is due to larger diameter capillaries within a low resistance parallel capillary network
Osmotic pressure is also lower, since more permeable to proteins
Net effect slightly favours filtration
congestive heart failure
Congestive heart failure: impaired LV output results in elevated EDP. Pressure ‘backs-up’ into pulmonary circulation to increase hydrostatic pressure and cause fluid accumulation in the lungs (pulmonary oedema) resulting in breathlessness
right sided failure
greatly increases venous pressure observed as jugular distension and increased capillary hydrostatic pressure in peripheral tissues, particularly the extremities
- symptoms: jugular distension and pitting oedema
lymphatic system
~ 8L / day is filtered from the capillaries into the tissue spaces
Returned to the blood via the Lymphatic System
Lymphatic capillaries have closed bulbous endings consisting of a single layer of ECs
Highly permeable to interstitial fluid, proteins, and bacteria
Drains into collecting lymphatics, where SMCs contract to move lymph fluid and valves prevent back-flow
Eventually flow into lymph nodes, where bacteria are phagocytosed and activate immune response
Lymphatics also imp in the absorption of lipids from the intestines
lymph fluid
Lymph fluid is absorbed from lymph node by vascular capillaries
Remaining lymph drains into thoracic duct
