The Vascular Tree

sophietevans
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From the 01/10/13 Human Physiology lecture.

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Question Answer
What is the function of the cardiovascular system? To perfuse tissues with sufficient blood to meet their differing and changing needs. This includes the delivery of oxygen and nutrients, and the removal of CO2 and other cellular wastes.
How is the pressure to counteract inertia and promote fluid transport generated in the cardiovascular system? The heart provides the kinetic energy through systole, while the blood vessels provide the resistance against blood flow. This interaction generates pressure.
Once blood starts moving through the cardiovascular system, what form of energy does the pressure energy dissipate as? Heat/friction.
Which factors does blood flow depend on? A pressure gradient, the resistance of the vessels, blood viscosity, vessel length, and vessel radius.
Which factors that influence blood flow can be considered constant? Vessel length and blood viscosity.
What is a pressure gradient? A pressure gradient is developed along a tube that provides resistance to the pump at the origin, as energy is lost as heat through friction with the tube/vessel walls. It is both a product of, and required by, blood flow.
What is the calculation for vessel resistance? Resistance = 1 / radius^4
Halving the diameter of a blood vessel would increase its resistance by what factor? What does this mean for pressure? It would increase resistance by 16x, meaning that a pressure increase of 15x would be necessary to maintain flow - a large increase.
Where is the greatest drop in pressure in the cardiovascular system? What is the benefit of this? The greatest drop of pressure occurs in the arterioles as they offer the most resistance. They are a physiological bottleneck, which is significant for control (i.e. of perfusion of different tissues).
Which structural layers do arteries possess that veins don't? Why is this? Arterial vessels possess internal and external elastic laminae, unlike veins, because they are exposed to large pressures generated in the ventricles which they must be protected against, and because their role is in transport of blood to tissues so they must be able to maintain a pressure gradient via elastic recoil between systoles.
What does the vascular shunt consists of? A metarteriole and a venous thoroughfare channel.
What is the point of precapillary sphincters? Precapillary sphincters are sensitive to the metabolic needs of the tissues downstream from them. They can respond by opening (smooth muscle dilation) and allowing blood to flow through an arterial bed, or by closing (smooth muscle contraction) and forcing blood to only move through the vascular shunt.
Which arteriole does an arterial bed branch from? The terminal arteriole.
What proportion of vessels are closed at rest? Up to 95%.
Which factors to precapillary sphincters respond to in order to influence flow through the microcirculation? Autonomic nervous stimulation, blood pressure, local metabolities (CO2, K+), and circulating hormones.
Describe the structure of a continuous capillary, and suggest where one might be found. A continuous capillary has very tight endothelial cell junctions and an intact endothelial lining. This is the most common capillary type, and is found in the blood-brain barrier as the tight endothelial junctions do not allow most substances through. They are also found in skeletal muscle.
Describe the structure of a fenestrated capillary, and suggest where you might find one. Fenestrated capillaries contain fenestrations - small pores which allow for more rapid exchange by sacrificing a degree of control over exchange. There are less tight inter-cell junctions between endothelial cells. They are found in the kidneys and intestines as they allow for exchange (e.g. absorption, filtration, reabsorption etc).
Describe the structure of a sinusoid capillary, and suggest where you might find one. Sinusoid capillaries retain the least control over exchange, in order for movement of cellular components and even cells to be possible. They have an incomplete endothelial cell lining and basement membrane, and are present in the bone marrow and liver for haematopoiesis and processing of blood, respectively.
Describe the forces influencing fluid exchange in capillaries. Pressure on blood will push it forward, but also hydrostatic pressure will push it towards the periphery of the capillary, while interstitial forces will push it back, and osmotic pressure force fluid in and out of capillaries depending on composition of fluids.
What is oedema? Build up of fluid in the interstitial space.
Why does oedema not occur more often, given the tendency for fluid to move out of capillaries due to the net forces? The lymphatic system returns lost fluid to the circulation via the subclavian veins, at a rate of ~2L/day.
Write an equation for the net outflow into the extracellular fluid. Net outflow = net filtration - net absorption
What is the pressure gradient in the venous circulation? ~20 mmHg.
Which force is the venous pressure gradient not large enough to oppose? Hydrostatic forces.
What is the muscular pump? Given that the pressure gradient in the venous circulation alone is not enough to return blood to the heart, the skeletal muscles, particularly those of the legs, aid the process by squeezing blood up the veins during contraction. The blood only travels towards the heart because of the presence of valves, which ensure unidirectional flow.
What is the respiratory pump? The venous pressure gradient is not sufficient alone to overcome hydrostatic forces in order to return blood to the heart. The respiratory pump aids blood return (inadvertently) as when the intrathoracic volume increases during inspiration, the great veins are expanded. This accommodates a larger volume of blood to fill them, and then reduction of the thoracic volume during expiration compresses the vessels and aids return to the right atrium.
List the 4 main factors that mean arterial pressure is determined by. Blood volume, cardiac output, vessel resistance, and distribution of blood between the arterial and venous aspects of the circulation.
Which long-term mechanism is the blood volume controlled by? The renin-angiotensin system, which also recruits aldosterone and antidiuretic hormone in order to conserve Na+ and therefore H2O and raise blood pressure.
What is the equation for the factors that cardiac output is controlled by? Cardiac output = heart rate x stroke volume
What might heart rate be altered by? Autonomic stimulation, and/or the presence of epinephrine.
What is Starling's Law? Starling's Law states that the heart will always try to pump what is returned to it. So, a greater preload (set by the end-diastolic volume), increased contractility (e.g. caused by epinephrine), and increased afterload (the pressure against which ventricles pump), all increase blood pressure.
What is the preload? The volume of blood in the ventricles which helps to determine the contractility of the muscle. It is set by the end-diastolic volume.
What is afterload? The pressure developed in the ventricles in order to eject blood against the pressure of the arteries, predominately the aorta, so that the aortic valve will open.
What factor is vessel resistance determined by? Vessel radius, so the activity of the smooth muscle cells in the walls of arterioles.
Which factors is smooth muscle cell activity in the arteriole walls affected by? Direct response to stretch, response to local metabolites and circulating hormones (e.g. adrenaline, ADH, renin-angiotensin II, and endothelium-derived factors), and autonomic activity (e.g. the baroreceptor reflex).
Which are the capacitance vessels? The great veins, such as the femoral veins and vena cavae.
What is the distribution of blood between venous and arterial vessels predominately determined by? Dilation or constriction of the capacitance vessels, which hold ~65% of circulating blood.
Constricting the great veins will shift a significant volume of blood to the arterial side of the circulation. What other ratio determines the distribution of blood? The plasma:interstitial fluid distribution, determined by osmotic/hydrostatic forces.
What is hypertension defined as? Systolic/diastolic pressures >140/90 mmHg.
Pathogenesis of hypertension is multi-factorial. What does this mean? There are both genetic and environmental predispositions to its pathogenesis.
What type of disease is hypertension? Chronic.
List some other conditions that hypertension is involved in. Atherosclerosis, ischaemic heart disease, stroke, and diabetes.
Which produces a sharper increase in mortality: an increase in systolic pressure or an increase in diastolic pressure? An increase in systolic pressure.
Consumption of which mineral increases blood pressure? Salt.
What factor does blood pressure 'naturally' rise with? Age!
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