Ch. 17 - The Vascular System Flashcards
Distinguish the structures and functions of various blood vessels.
Artery:
- Very strong elastic vessels that are able to carry blood away from the heart under high pressure
- In the systemic circuit arteries carry only oxygenated blood
- In the pulmonary circuit the arteries carry only deoxygenated blood
- 3-layered thick wall(endothelial lining, middle smooth muscle and elastic connective tissue layer, outer connective tissue layer)
- ‘Elastic’ arteries/Conducting Arteries, are located near the heart, including the Aorta and its primary branches, they have largest diameter of the arteries and are the most elastic as they contain more Elastin than other arteries which is found in all 3 tunica layers(most in media), their large lumens means they are low-resistance pathways for blood
- ‘Muscular’ arteries/Distributing Arteries, are smaller, distal arteries delivering blood to the organs of the body, they have thickest tunica media of all the blood vessels which has more smooth muscle than elastic therefore has moe vasoconstrictive actions but is less able to stretch
- Carries relatively high pressure blood from the heart to the arterioles
- Subdivide into thinner tube, giving rise to branched, finer arterioles
Arterioles:
- Smallest of the arteries
- Largest of them have 3 tunic layers, but smallest are basically a single layer of smooth muscle cellsaround an endothelial lining
- The tunica media of larger arterioles is made mostly of smooth muscle, with a small amount of elastic fibers
- Smaller arterioles lead directly onto capillary beds
- Arterioles also referred to as ‘Resistance’ vessels because more pressure is needed to push blood through them than arteries, due to them being more constricted
- Helps control blood flow from arteries to capillaries by vasoconstricting or vasodilating.
Capillary:
- Smallest diameter of the blood vessels
- The wall of capillaries are composed of endothelium and form the semipermeable layer through which substances in blood are exchanged with substances in tissue fluids surrounding cells of the body
- Microscopic vessels with extremely thin walls
- Their walls contain a thin tunica intima and nothing else
- Sometimes a single endothelial cell makes up the entire circumference of the capillary wall
- Capillary walls allow diffusion of blood with high levels of O2 and nutrients, and also allow high levels of CO2 and wastes to move from the tissues into the capillaries
- Most of the body is supplied with continuous capillaries
- Such small lumen they cause RBC’s to move through them 1 at a time
Venule:
- Thinner wall than arterioles(less smooth muscle and elastic connective tissue)
- Larger venules have a thin tunica adventitia and 1-2 layers of smooth muscle cells making up their tunica media
- Microscopic vessels linking Capillaries to Veins
- Smallest/postcapillary venules made up of only endothelium, surrounded by pericytes or contractile cells
- Combine to form veins
Vein:
- Low pressure vessels
- Mostly have 3 tunics, but have thinner walls, the tunica media has little smooth muscle or elastin, even in larger veins, with the heaviest layer being the tunica adventitia
- Larger (low resistance)lumen than arteries
- Some have flap-like Venous Valves, to prevent blood backflow.
- Carries relatively low pressure blood from venules to the heart.
- Serve as blood reservoirs during conditions like arterial haemorrhage, venous constrictions help to maintain blood pressure by returning more blood to the heart, ensuring almost normal blood flow, even when up to a quarter of the blood volume is lost
- Can hold up to 65% of the body’s blood supply at any given moment, but are usually only partially filled
Venous Valves:
- Prevent backflow of blood
- Formed from folds of the Tunica Intima.
- Appear similar to the heart’s Semilunar Valves and have similar functions.
- More Venous Valves exist in veins of the lower limbs, as here, gravity opposes upward blood flow.
- In the abdominal and thoracic cavities, the veins mostly do not contain Venous Valves.
Vascular Anastomoses:
- Interconnections formed by blood vessels.
- Arterial Anastomoses form by the merging of arteries supplying the same body tissues.
- Provide Collateral Channels so blood can reach the tissues.
- If an artery is blocked by a clot/cut, a collateral channel may be able to provide required blood to the region.
- Arterial Anastomoses develop around joints in areas where blood flow through a channel may be slowed or stopped by active movement.
- Also commonly occur in the Brain, Heart and Abdominal regions.
- The arteries of the Kidneys, Retina and Spleen usually do not Anastomose or have littler collateral circulation, meaning blood flow interruption to these areas usually causes cellular death.
Describe the structure of an artery wall.
Conists of 3 layers and a blood-containing space called the lumen.
Tunica Intima(innermost layer):
- layer of simple squamous epithelium known as Endothelium, which rests on a connective tissue membrane with many elastic, collagenous fibres.
- Helps prevent blood clotting and may also help in regulation of blood flow.
- Releases nitric oxide to relax smooth muscle of the vessel.
- In arteries, the outer margin has a thick layer of elastic fibres known as the internal elastic membrane.
- Obtains nutrients directly from the blood.
Tunica Media(middle layer):
- Makes up most of an artery wall, including smooth muscle fibers arranged mostly in circles and a thick elastic connective tissue layer.
- The smooth muscle fiber activity is controlled by many chemicals and the Autonomic nervous system’s sympathetic vasomotor nerve fibers.
- Tunica media is seperated from the next layer, the tunica externa, by a thin band of fibers known as the external elastic membrane.
Tunica Externa(outer layer):
- Also known as Tunica Adventitia.
- Thinner, made mostly of connective tissue with irregular fibres.
- Attached to the surrounding tissues and contains many lymphatic vessels and nerve fibres.
- In larger blood vessels it contains many tiny blood vessels, which comprise a system known as the Vasa Vasorum, which nourishes the outer tissues of blood vessel walls.
In arteries and arterioles, vasomotor fibres recieve impulses to contract and reduce blood vessel diameter, a process known as vasoconstriction.
When inhibited, the muscle fibres relax and the vessel’s diameter increases in a process known as vasodilation.
Changes in artery and arteriole diameters greatly affect blood flow and pressure.
Vessel structure through Sytemic-Pulmonary circuits.
Explain the difference between Pulmonary and Systemic vessels.
Systemic Circuit:
- Supplied by the Left Ventricle
- Arteries carry oxygenated blood
- Veins carry deoxygenated blood
- 5 x more resistance to blood flow than pulmonary circuit
- Supplies whole of the body’s organs, tissues, cells
Pulmonary circuit:
- Supplied by Right Venttricle
- Arteries carry deoxygenated blood
- Veins carry oxygenated blood
- Short, low pressure circulation
- Supplies only the Lungs in order to Oxygenate the blood
Blood vessel structure.
Capillary-Venule structure.
Define blood flow, blood pressure and resistance.
Blood flow:
- the amount or volume of blood that flows through blood vessels, organs, or the systemic circulation, in milliliters per minute(mL/min).
- Throughout the body, blood flow is equivalent to cardiac output. When at rest, this is relatively constant, yet in certain body organs, blood flow may be different from others due to individual requirements.
Blood Pressure:
- The force that blood exerts against the inner walls of blood vessels.
- Most commonly refers to pressure in arteries supplied by the aortic branches, although it actually occurs throughout the body.
- Blood flow is generated by the heart’s pumping action, and blood pressure results from resistance opposing blood flow.
- Expressed in millimeters of mercury(mmHg).
- The maximum pressure reached during ventricular contraction is called the systolic pressure, averaging at 120mmHg in a healthy adult
- the lowest pressure that remains in the arteries before the next ventricular contraction is called the diastolic pressure, averaging at 70-80mmHg in a healthy adult
- therefore systole refers to contraction, while diastole refers to relaxation
Resistance:
- The friction between blood and blood vessel walls.
- Blood pressure must overcome this force for the blood to continue flowing, therefore factors that alter peripheral resistance will cause changes in blood pressure.
- Viscosity is the ease with which a fluid’s molecules flow past one another.
- The higher the viscosity, the greater the resistance to flow.
- Blood viscosity is increased by blood cells and plasma proteins.
- The greater the resistance, the more force needed to move the blood.
- Blood pressure rises as blood viscosity increases, and vice versa.
Describe the effects of the sympathetic and parasympathetic nervous systems on the blood vessels.
Generally, sympathetic stimulation, which releases adrenaline, causes vasoconstriction, which causes an increase in peripheral resistance and therefore increase in blood pressure.
The parasympathetic nervous system has the opposite effect, and generally causes vasodilation, which reduces peripheral resistance and decreases blood pressure.
Blood pressure in relation to distance from the Left Ventricle.
Describe the factors that influence blood pressure and explain how blood pressure is regulated.
The factors that influence blood pressure include homeostatic mechanisms such as cardiac output, peripheral resistance, and lumen size of the arteries and arterioles.
Blood pressure is controlled by the regulation of cardiac ouput with peripheral resistance.
Changes in the diameters of arterioles regulate peripheral resistance, which is controlled by the vasomotor centre of the medulla oblongata.
Peripheral resistance is also influenced by CO2, O2, and H+ ions.
Arterial blood pressure rises and falls according to cardiac cycle phases.
The control of blood vessel size, and therefore blood pressure, is also related to neural activities via reflex arcs, baroreceptors, related afferent fibres, chemoreceptors, high brain centre influences, hormonal controls and renal controls.
Describe Venous Blood Pressure & Venous Return Mechanisms
Venous Blood Pressure is steady and regular and does not pulsate with ventricular contractions like arterial blood pressure
Venous blood pressure is usually too low to cause venous return to be adequate, therefore the following 3 mechanisms are used to increase venous return and stroke volume:
Muscular Pump:
- skeletal muscle activity contracts and relaxes around the veins, moving blood toward the heart
- each vein valve keeps blood that has passed through it from flowing back
Respiratory Pump:
- pressure changes in the body’s ventral cavity during breathing move blood toward the heart
- inhalation increases abdominal pressure, squeezing local veins and forcing blood to the heart
- simultaneously, the chest pressure decreases, the internal and external thoracic veins then expand and increase blood entry into the right atrium
Sympathetic Venoconstriction:
- sympathetic control causes the smooth muscle layer around the veins to constrict, reducing venous volume, blood is therefore pushed toward the heart
