Blood Vessels (wk7) Flashcards
Anatomy of arteries at different generations away from the heart:
-Adventitia and elastin
-Adventitia -> Provides structural strength and tethers the vessels in place. In large vessels the adventitia contain small blood vessels, the vasa vasorum (the vessels of vessels). Deals with high pressures. Has its own circulation to be able to metabolise itself
-Elastin -> Gives vessels mechanical strength and their elastic properties (expand and recoil). Important in maintaining forward flow, even when the heart is relaxing.
Anatomy of arteries at different generations away from the heart:
-Endothelium, smooth muscle and pre-capillary sphincter
-Endothelium -> The endothelial layer is the filtering interface between the blood and the body. It also secretes various vasoactive products. This is the inner layer. Layer separates blood from tissues at capillary level.
-Smooth muscle -> Supplies the vessels with contractile power and regulates the diameter of the lumen. Is not striated. Involuntary regulated by autonomic nervous system.
-Pre-capillary sphincter -> Controls blood flow to specific capillary beds selectively. Regulating vessel through vasodilation and vasoconstriction of arterioles.
-Layers of arteries are dependent on where in the body it is found
The control of artery diameter
-Mean arterial pressure must be maintained.
-Arterioles are responsible with altering the levels of resistance to blood flow in order to distribute the available amount of blood to where it is needed without disturbing the control of mean arterial pressure
-Arteries are responsible for changing the lumen size. This means the body is better able to survive with the amount of blood it has and the pressure the blood is at.
The control of artery diameter
-Regulation of arterial diameter
-Regulation of arterial diameter -> Arterial smooth muscle is constantly in a ‘tonic state’, which means that it is partially contracted. There is almost no involvement of the parasympathetic system so relaxation of arteries can happen by withdrawal of sympathetic stimulation. Circulating epinephrine can cause vasodilation or vasoconstriction depending on what receptors it binds onto.
Describe the concept of active hyperaemia
-Active hyperaemia is the increase in blood flow according to the metabolic needs of the tissue in question.
-Chemical changes that take place locally in tissue when metabolic rate increases:
* Tissue hypoxia (low o2)
* Increased levels of CO2
* Decrease in pH (more acidic)
* K+, adenosine (ATP), bradykinin, nitric oxide (NO)
Describe the concept of autoregulation
-Flow autoregulation is concerned with the maintenance of blood flow rather than changing it. When the driving pressure drops the vessels dilate, and when it increases the vessels constrict. The main purpose is to maintain flow and there is a reflex response to regulate and maintain the level. Dilation of organs allows the blood flow/ level to restore back to its normal levels.
-Increased arterial pressure causes a rise in blood flow that removes the vasodilators described above. The vessel then constricts and flow is restored to its normal value.
Describe the concept of the myogenic response
-Myogenic regulation of arterial diameter -> Intrinsic property of arterial smooth muscle. An increase in pressure leads to muscle stretch that is followed by constriction, which helps maintain flow relatively constant.
-Increase in vessel pressure – sends a signal to decrease the pressure.
Describe the concept of arterial diameter
-Regulation of arterial diameter -> Arterial smooth muscle is constantly in a ‘tonic state’, which means that it is partially contracted. There is almost no involvement of the parasympathetic system so relaxation of arteries can happen by withdrawal of sympathetic stimulation. Circulating epinephrine can cause vasodilation or vasoconstriction depending on what receptors it binds onto.
* Neural control -> Vasoconstrictors: sympathetic nerves that release norepinephrine, Vasodilators: neurones that release nitric oxide
* Hormonal control -> Vasoconstrictors: epinephrine and vasopressin, Vasodilators: atrial natriuretic peptide
* Local controls -> Vasoconstrictors: internal blood pressure and endothelin-1, Vasodilators: decrease in oxygen and adenosine
-Alpha receptors allow sympathetic neurones to be received into smooth muscle in skeletal muscle arterioles
Characteristics of capillaries
-Capillaries are numerous so that they can reach every part of the body and create a large surface area. They are thin so that they facilitate diffusion.
-We have millions of capillaries – and can be found everywhere
-Fick’s Law of diffusion = flow of gas = area/ thickness X d x (p2-p1). Flow of gas is increased with a bigger surface area, which enhances diffusion. The thinner the layer, the better the rate of diffusion.
-Velocity of flow in capillaries is low -> The total CSA of the capillaries is what dictates the velocity of blood flow, not the size of individual capillaries. The slow blood flow through the capillaries provides enough time during which diffusion can reach equilibrium. Even during peak exercise, red blood cells can be fully loaded.
Characteristics of endothelial arrangements
-Endothelial arrangements -> Continuous: muscle, nerve, fat, lymph nodes (leave very few gaps). Fenestrated: glands, kidneys, intestines (continuous but have gaps that can be regulated as open or closed). Discontinuous: liver, bone marrow, spleen (very loosely formed and leave big gaps between the cells and important in the production/ death of RBC).
-Type of transport -> Diffusion: oxygen, CO2 (gases able to dissolve through the barriers of layers in the body). Vesicle/ active transport: larger molecules (proteins) (substance transported by a ‘bubble’). Bulk flow: water and solutes (more generous transportation where things are transported with fluid through capillaries).
Describe diffusion and exchange at blood vessels
-The rate of diffusion depends on the surface area and thickness of the barrier. Capillaries are perfectly formed to facilitate diffusion. The slow passage of blood through capillaries ensures adequate reserve for diffusion.
Describe fluid balance at capillaries
-Filtration (or fluid movement from the capillaries to the interstitial fluid) -> takes place when hydrostatic pressure exceeds colloid osmotic pressure
-Absorption (or fluid movement from the interstitial fluid to the capillaries) -> takes place when colloid osmotic pressure exceeds hydrostatic pressure
-Hydrostatic pressure forces fluids out of the capillary
-Colloid osmotic pressure of proteins within the capillary pulls fluid into the capillary
-Net pressure = hydrostatic pressure – colloid osmotic pressure
Properties of veins
-Comparison to arteries and valves
-(Veins compared to arteries) -> larger volume capacity, lie closer to skin surface (darker in colour due to low oxygen concentrations), large diameter, have valves, less elastic tissue, less smooth muscle, more distensible, thinner walls
-Veins have valves -> Healthy vein: Flow is only towards the heart. Backflow is prevented by the shut valves. Veins create compartments which allow blood to be pushed into and can only travel in one direction to get out of the compartment. Diseased vein: Leaky valves allow blood to move in the opposite direction of the heart causing blood to accumulate in the extremities (varicose veins). Brain veins do not have valves (gravity).
Properties of veins
-Blood reservoir and pressure
-Veins are a blood reservoir -> Veins contain more than 60% of the total blood volume but the pressure in them is only about 10mmHg. Compared with arteries that contain <15% of the total blood volume at a pressure of about 100mmHg. Veins are affected by hydrostatic pressure which opposes blood flowing towards the heart. Cardiac output and displacement of blood from the veins to the heart are related via the Frank-Starling mechanism.
-Pressure in veins -> Pressure in the veins is low and non-pulsatile. Driving pressure for venous return is difference between central venous pressure (CVP) and right atrial pressure. CVP is determined by 2 factors: blood volume and compliance. Capillary pressure is roughly 35mmHg, Central venous pressure is roughly 5-10mmHg and right atrial pressure is roughly 0mmHg.
Properties of veins
-Changing the compliance of a vein
-Changing the compliance of a vein:
* Neural -> Sympathetic stimulation releases norepinephrine to the smooth muscle of veins in the gastrointestinal (stomach and intestines), renal (kidneys), hepatic (liver) and cutaneous (skin) circulations causes vasoconstriction. This reduces their capacitance and displaces blood towards the heart
* Hormonal -> The diameter of veins is affected by the presence of blood circulating epinephrine
* Endothelial factors -> Paracrine vasoconstrictors and vasodilators (nitric oxide, endoththelin-1, etc) also operate in veins
