CH21: Blood Vessels Flashcards
What are the three layers of a blood vessel?
Tunica intima, tunica media, tunica externa
Describe the structure of the tunica intima, its components, and its function
The tunica intima or tunica interna forms the inner lining of a blood vessel as it flows through the interior lining of a blood vessel, called the lumen
The innermost layer is called endothelium, which is continuous with the endocardial lining of the heart. The endothelium is a thin layer of flattened cells that lines the inner surface of the entire cardiovascular system. It is now known that the endothelial cells participate in various vessel-related activities, including physical influences on blood flow, secretion of locally acting chemical mediators etc. Their smooth luminal surface also facilitates efficient blood flow by reducing surface friction
The second component of the tunica intima is a basement membrane deep to the endothelium. Its framework of collagen fibers affords the basement membrane significant tensile strength, yet its properties also provide resilience for stretching and recoil. The basement membrane anchors the endothelium to the underlying connective tissue while also regulating molecular movement.
The outermost part of the tunica intima, which forms the boundary between the tunica intima and tunica media, is the internal elastic membrane. The internal elastic membrane is a thin sheet of elastic fibers with a variable number of window-like openings that give it the look of Swiss cheese. These openings facilitate diffusion of materials through the tunica intima to the thicker tunica media.
Describe the structure of the tunica media and its function
The tunica media is a muscular and connective tissue layer that displays the greatest variation among the different vessel types. In most vessels, it is a relatively thick layer comprising mainly smooth muscle cells and substantiation amounts of elastic fibers. The primary role of the smooth muscle cells, which extend circularly around the lumen, is to regulate the diameter of the lumen
A decrease in the diameter of the lumen is called vasoconstriction, which is caused by smooth muscle contraction and an increase in the lumen diameter is called vasodilation, which is relactation if smooth muscle tissue. Both responses are controlled by sympathetic stimulation. The elastic fibers produced by smooth muscle cells allow the vessels to stretch and recoil under the applied pressure of blood
Describe the structure of tunica externa and its functions
The outer covering of a blood vessel, the tunica externa, consists of elastic and collagen fibers. The tunica externa contains numerous nerves and, especially in larger vessels, tiny blood vessels that supply the tissue of the vessel wall. These small vessels that supply blood to the tissues of the vessel are called vasa vasorum. They are easily seen on large vessels such as the aorta. In addition to the important role of supplying the vessel wall with nerves and self-vessels, the tunica externa helps anchor the vessels to surrounding tissues
Describe the two types of arteries
Elastic arteries are the largest arteries in the body, ranging from the garden hose-sized aorta and pulmonary trunk to the finger-sized branches of the aorta. They have the largest diameter among arteries but relatively thin vessel walls
These vessels are characterized by well-defined internal and external elastic membranes, along with a thick tunica media that is dominated by elastic fibers, called elastic lamellae. Elastic arteries include the aorta and pulmonary trunks.
They help propel blood onward while the ventricles are relaxing. As blood is ejected from the heart into elastic arteries, their walls stretch, easily accommodating the surge of blood. As they stretch, the elastic fibers momentarily store mechanical energy, functioning as a pressure reservoir. Then, the elastic fibers recoil and convert stored (potential) energy in the vessel into kinetic energy of the blood. Thus, blood continues to move through the arteries even while the ventricles are relaxed. Because they conduct blood from the heart to medium-sized, more muscular arteries, elastic arteries also are called conducting arteries.
Medium-sized arteries are called muscular arteries because their tunica media contains more smooth muscle and fewer elastic fibers than elastic arteries. This means that the walls of the arteries are relatively thick and therefore muscular arteries are capable of greater vasoconstriction and dilation to adjust the rate of blood flow.
Because the muscular arteries continue to branch and ultimately distribute blood to each of the various organs, they are called distributing arteries. Examples include the brachial artery in the arm and radial artery in the forearm
Because of the reduced amount of elastic tissue in the walls of muscular arteries, these vessels do not have the ability to recoil and help propel the blood like the elastic arteries. Instead, the thick, muscular tunica media is primarily responsible for the functions of the muscular arteries. The ability of the muscle to contract and maintain a state of partial contraction is referred to as vascular tone. Vascular tone stiffens the vessel wall and is important in maintaining vessel pressure and efficient blood flow.
What are the three types of capillaries? What special structures do they, have, if any?
Most capillaries are continuous capillaries, in which plasma membranes of endothelial cells from a continuous tube interrupted only by intercellular clefts, gaps between neighboring endothelial cells. Continuous capillaries are found in the central nervous system, lungs, muscle tissue and the skin
Other capillaries of the body are fenestrated capillaries. The plasma membranes of the endothelial cells contain many small pores called fenestrations which allow movement of nutrients into the capillary. Fenestrated capillaries are found in the kidneys, intestinal villi, choroid plexuses of the ventricles in the brain, ciliary processes of the eyes, and most endocrine glands.
Sinusoids are wider and more winding than other capillaries. Their endothelial cells may have unusually large fenestrations. They have an incomplete/absent basement membrane and very large intercellular clefts that allow proteins and in some cases, blood cells to pass from a tissue into the bloodstream. For example, newly formed blood cells enter the bloodstream through the sinusoids of red bone marrow. In addition, sinusoids contain specialized lining cells that are adapted to the function of the tissue. Sinusoids in the liver, for example, contain phagocytic cells that remove bacteria and other debris from the blood. The spleen, anterior pituitary, and parathyroid and suprarenal glands also have sinusoids.
Describe the structure of veins and their differences to arteries
While veins do show structural changes as the increase in size from small, medium and large, the structural changes are not as distinct as they are in arteries. Veins generally have very thin walls relative to their total diameter (average thickness less than one-tenth of the vessel diameter). Although veins are composed of essentially the same three layers as arteries, the relative thicknesses of the layers are different.
The tunica intima of veins is thinner than that of arteries; the tunica media of veins is much thinner than in arteries, with relatively little smooth muscle and elastic fibers.
The tunica externa of veins is the thickest layer and consists of collagen and elastic fibers. Veins lack the internal or external elastic membrane found in arteries. They are distensible enough to adapt to variations in the volume and pressure of blood passing through them, but are not designed to withstand high pressure. The lumen of a vein is larger than that of a comparable artery, and veins often appear collapsed (flattened) when sectioned.
Blood pressure is significantly lower in veins than in arteries, hence the structural differences in the two vessels
flaplike cusps which project into the lumen, pointing towards the heart. The low blood pressure in veins allows blood returning to the heart to slow and even back up; the valves aid in venous return by preventing the backflow of blood.
Valves are more crucial in arm veins and leg veins due to the combined effects of gravity, longer vertical distance, reliance on muscle contractions, and the need to overcome greater challenges in returning blood to the heart.
What factors influence the direction of fluid movement into and out of capillary beds
The net filtration pressure indicates the direction of fluid movement. This is defined by hydrostatic pressures and osmotic pressures
What does vascular resistance depend on? Describe how these factors affect blood flow
Lumen size: The smaller the lumen of a blood vessel, the greater its resistance to blood flow. Resistance is inversely proportional to the fourth power of the diameter (d) of the blood vessel’s lumen (R ∝ 1/d4). The smaller the diameter of the blood vessel, the greater the resistance it offers to blood flow
Blood viscosity: The viscosity of blood depends mostly on the ratio of red blood cells to blood plasma (fluid) volume, and to a smaller extent on the concentration of proteins in blood plasma. The higher the blood’s viscosity, the higher the resistance.
Total blood vessel length: Resistance to blood flow through a vessel is directly proportional to the length of the blood vessel. The longer a blood vessel, the greater the resistance. Obese people often have hypertension (elevated blood pressure) because the additional blood vessels in their adipose tissue increase their total blood vessel length.
What affects the velocity of blood flow
The speed or velocity of blood flow (in cm/sec) is inversely related to the cross sectional area. Velocity is slowest where cross-sectional area is greatest. Each time an artery branches, the total cross-sectional area of all of its branches is greater than the cross-sectional area of the original vessel, so blood flow becomes slower and slower as blood moves further away from the heart, and is slowest in the capillaries. Conversely, when venules unite to form veins, the total cross-sectional area becomes smaller and flow becomes faster.
How does the nervous system regulate BP
The nervous system regulates blood pressure via negative feedback loops that occur as two types of reflexes: baroreceptor reflexes and chemoreceptor reflexes
Baroreceptors are pressure sensitive sensory receptors located in the aorta, internal carotid arteries and other large arteries in the neck and chest. They send impulses to the cardiovascular center to help regulate blood pressure. The two most important baroreceptor reflexes are the carotid sinus reflex and the aortic reflex
Baroreceptors in the wall of the carotid sinuses initiate the carotid sinus reflex, which helps regulate blood pressure in the brain. The carotid sinuses are small widenings of the right and left internal carotid arteries just above the point where they branch from the common carotid arteries, seen in the adjacent image. Blood pressure stretches the wall of the carotid sinus, which stimulates the baroreceptors. Nerve impulses propagate from the carotid sinus baroreceptors over sensory axons in the glossopharyngeal (IX) nerves to the cardiovascular center in the medulla oblongata. Baroreceptors in the wall of the ascending aorta and aortic arch initiate the aortic reflex, which regulates systemic blood pressure. Nerve impulses from aortic baroreceptors reach the cardiovascular center via sensory axons of the vagus (X) nerves.
Chemoreceptors are sensory receptors that monitor the chemical composition of blood. They are located close to the baroreceptors of the carotid sinus and aortic arch in small structures called carotid bodies and aortic bodies respectively. These chemoreceptors detect changes in blood level of O2, CO2 and H_. Hypoxia (lowered O2 availability), acidosis (increased H+ concentration) and hypercapnia (excess CO2) stimulates the chemoreceptors to send impulses to the cardiovascular center. In response, the CV center increases sympathetic stimulation to arterioles and veins, producing vasoconstriction and an increase in blood pressure, and they also provide input to the respiratory center in the brainstem to adjust the rate of breathing
Several hormones help regulate blood pressure and blood flow by altering cardiac output, changing systemic vascular resistance, or adjusting the total blood volume:
Renin-angiotensin-aldosterone (RAA) system: Juxtaglomerular cells in the kidney secrete renin into the bloodstream when blood volume falls or blood flow to the kidneys decreases. In sequence, renin and angiotensin-converting enzymes (ACE) act on their substrates to produce the active hormone angiotensin II, which raises blood pressure in two ways. First, angiotensin II is a potent vasoconstrictor; it raises blood pressure by increasing systemic vascular resistance. Second, it stimulates the secretion of aldosterone, which increases reabsorption of sodium ions and water by the kidneys. The water reabsorption increases total blood volume, which increases blood pressure
Epinephrine and norepinephrine: In response to sympathetic stimulation, the suprarenal medulla releases epinephrine and norepinephrine. These hormones increase cardiac output by increasing the rate and force of heart contractions. They also cause vasoconstriction of arterioles and veins in the skin and abdominal organs and vasodilation of arterioles in cardiac and skeletal muscle, which helps increase blood flow to muscle during exercise.
Antidiuretic hormone: ADH is produced by the hypothalamus and released from the posterior pituitary in response to dehydration or decreased blood volume. Among other actions, ADH causes vasoconstriction, which increases blood pressure. For this reason ADH is also called vasopressin. ADH also promotes movement of water from the lumen of kidney tubules into the bloodstream. This results in an increase in blood volume and a decrease in urine output.
Atrial natriuretic peptide (ANP): Released by cells in the atria of the heart, atrial natriuretic peptide lowers blood pressure by causing vasodilation and by promoting the loss of salt and water in the urine, which reduces blood volume.
What are the four types of shock and their causes?
There are four different types of shock
Hypovolemic shock: Shock due to decreased blood volume A common cause of hypovolemic shock is acute (sudden) hemorrhage, which can be external or internal. Sometimes, hypovolemic shock is due to inadequate intake of fluid. Whatever the cause, when the volume of body fluids falls, venous return to the heart declines, filling of the heart lessens, stroke volume decreases, and cardiac output decreases
Cardiogenic shock: Shock due to poor heart function In cardiogenic shock, the heart fails to pump adequately, most often because of a myocardial infarction (heart attack). Other causes of cardiogenic shock include poor perfusion of the heart (ischemia), heart valve problems, excessive preload or afterload, impaired contractility of heart muscle fibers, and arrhythmias.
Vascular shock: Shock due to inappropriate vasodilation. A variety of conditions can cause inappropriate dilation of arterioles or venules. In anaphylactic shock, a severe allergic reaction, for example, to a bee sting, releases histamine and other mediators that cause vasodilation. In neurogenic shock, vasodilation may occur following trauma to the head that causes malfunction of the cardiovascular center in the medulla. Shock stemming from certain bacterial toxins that produce vasodilation is termed septic shock. In the United States, septic shock causes more than 100,000 deaths per year and is the most common cause of death in hospital critical care units.
Obstructive shock: Shock due to obstruction of blood flow. Obstructive shock occurs when blood flow through a portion of the circulation is blocked. The most common cause is pulmonary embolism, a blood clot lodged in a blood vessel of the lungs.
What are the responses to shock
The major mechanisms of compensation in shock are negative feedback systems that work to return cardiac output and arterial blood pressure to normal.
Activation of the renin-angiotensin-aldosterone system: Decreased blood flow to the kidneys causes the kidneys to secrete renin which eventually activates angiotensin II which causes vasoconstriction and stimulates the suprarenal cortex to secrete aldosterone, a hormone that increases reabsorption of Na+ and water by the kidneys. This increases systemic vascular resistance and blood volume
Secretion of ADH: In response to decreased blood pressure, the posterior pituitary releases ADH which enhances water reabsorption by the kidneys which conserves remaining blood volume and causes vasoconstriction to increase systemic vascular resistance
Activation of the sympathetic division of the ANS: As blood pressure decreases, aortic and carotid baroreceptors initiate powerful sympathetic responses throughout the body. One result is marked vasoconstriction of arterioles and veins of the skin, kidneys and other abdominal viscera. Constriction of arterioles increases systemic vascular resistance, and constriction of veins increases venous return. Both effects help maintain an adequate blood pressure. Sympathetic stimulation also increases heart rate and contractility and increases secretion of epinephrine and norepinephrine by the suprarenal medulla. These hormones intensify vasoconstriction and increase heart rate and contractility, all of which help raise blood pressure.
Release of local vasodilators: In response to hypoxia, cells liberate vasodilators like K+, lactating acid, H+ etc and relax precapillary sphincters. This increases blood flow and may restore normal O2 levels in part of the body. However, vasodilation also has the potentially harmful effect of decreasing systemic vascular resistance and thus lowering the blood pressure.
Describe the flow of blood through the systemic and pulmonary circulation?
The systemic circulation includes all arteries and arterioles that carry oxygenated blood from the left ventricle to systemic capillaries, plus the veins and venules that return deoxygenated blood to the right atrium after flowing through body organs. Blood leaving the aorta and flowing through the systemic arteries is a bright red color. As it moves through capillaries, it loses some of its oxygen and picks up carbon dioxide, so that blood in systemic veins is dark red.
When blood returns to the heart from the systemic route, it is pumped out of the right ventricle through the pulmonary circulation to the lungs. In the capillaries of pulmonary alveoli, the blood loses some of its carbon dioxide and takes on oxygen. Bright red again, it returns to the left atrium of the heart and reenters the systemic circulation as it is pumped out be the left ventricle
Describe the various branches of the aorta
The portion of the aorta that emerges from the left ventricle posterior to the pulmonary trunk is the ascending aorta. The beginning of the aorta contains the aortic valve. The ascending aorta gives off two coronary arteries that supply the myocardium. The ascending aorta then forms the aortic arch, which descends and ends between the fourth and fifth intervertebral disc to become the descending aorta. As it continues, it is called the thoracic aorta within the thorax. When the thoracic aorta reaches the bottom of the thorax, it passes through the aortic hiatus of the diaphragm to become the abdominal aorta. he abdominal aorta descends to the level of the fourth lumbar vertebra where it divides into two common iliac arteries, which carry blood to the pelvis and lower limbs
Each division of the aorta gives off arteries that branch into distributing arteries that lead to various organs. Within the organs, the arteries divide into arterioles and then into capillaries that service the systemic tissues (all tissues except the pulmonary alveoli of the lungs)
