The Cardiovascular System: Blood Vessels:

Transcription

1 The Cardiovascular System: Blood Vessels: Blood Vessels Delivery system of dynamic structures that begins and ends at the heart : carry blood away from the heart; except for pulmonary circulation and umbilical vessels of a fetus : contact tissue cells and directly serve cellular needs : carry blood the heart Structure of Blood Vessel Walls and Tunica, tunica, and tunica Central blood-containing space Capillaries with sparse lamina Tunics Tunica intima lines the of all vessels In vessels larger than mm, a connective tissue basement membrane is present Tunics Tunica media muscle and sheets of elastin nerve fibers control and vasodilation of vessels Tunics Tunica externa (tunica adventitia) fibers protect and reinforce Larger vessels contain to nourish the external layer Elastic (Conducting) Arteries

2 Large thick-walled arteries with in all three tunics and its major branches Large lumen offers --resistance Act as expand and recoil as blood is ejected from the heart Muscular (Distributing) Arteries and Arterioles Distal to elastic arteries; blood to body organs Have thick with more smooth muscle Active in Arterioles Smallest arteries Lead to beds Control flow into capillary beds via and vasoconstriction Capillaries Microscopic blood vessels Walls of thin, one cell thick help stabilize their walls and control permeability Size allows only a single to pass at a time Capillaries In all tissues except for,, and Functions:,,,, etc. Capillaries Three structural types 1. capillaries 2. capillaries 3. capillaries (sinusoids) Continuous Capillaries Abundant in the skin and muscles Tight junctions connect cells allow the passage of fluids and small solutes Continuous capillaries of the

3 Tight junctions are complete, forming the barrier Fenestrated Capillaries Some endothelial cells contain pores ( ) More permeable than capillaries Function in or formation (small intestines, endocrine glands, and kidneys) Sinusoidal Capillaries Fewer tight junctions, larger intercellular clefts, large lumens Usually Allow molecules and blood cells to pass between the blood and surrounding tissues Found in the,, Capillary Beds Interwoven networks of capillaries form the microcirculation between and Consist of types of vessels 1. Vascular (metarteriole thoroughfare channel): Directly connects the terminal and a venule Capillary Beds 2. True capillaries 10 to 100 exchange vessels per capillary bed Branch off the or arteriole Blood Flow Through Capillary Beds sphincters regulate blood flow into true capillaries Regulated by local chemical conditions and nerves Venules Formed when beds unite Very porous; allow fluids and into tissues venules consist of endothelium and a few pericytes Larger venules have one or two layers of smooth muscle cells Veins Formed when converge

4 Have thinner walls, larger compared with corresponding arteries Blood pressure is lower than in arteries Thin tunica media and a thick tunica consisting of collagen fibers and elastic networks Called vessels (blood reservoirs); contain up to 65% of the blood supply Veins Adaptations that ensure return of blood to the heart 1. lumens offer little resistance 2. Valves prevent backflow of blood Most abundant in veins of the limbs Venous : flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain) Vascular Anastomoses of blood vessels Arterial provide alternate pathways ( channels) to a given body region Common at joints, in organs, brain, and heart Vascular shunts of capillaries are examples of anastomoses Venous are common Physiology of Circulation: Definition of Terms Blood flow Volume of blood flowing through a vessel, an, or the entire in a given period Measured as ml/min Equivalent to (CO) for entire vascular system Relatively constant when at rest Varies widely through individual organs, based on needs Physiology of Circulation: Definition of Terms Blood pressure (BP) Force per unit area exerted on the wall of a blood vessel by the blood Expressed in Measured as arterial BP in large arteries near the

5 heart The pressure gradient provides the driving force that keeps blood moving from higher to lower pressure areas Physiology of Circulation: Definition of Terms Resistance ( resistance) to flow Measure of the amount of blood encounters Generally encountered in the systemic circulation Three important sources of resistance viscosity Total vessel length Blood vessel Resistance Factors that remain relatively constant: Blood viscosity The of the blood due to formed elements and plasma proteins Blood vessel length The longer the vessel, the the resistance encountered Resistance Frequent changes alter resistance Varies with the fourth power of vessel radius E.g., if the radius is doubled, the resistance is 1/16 as much Resistance Small-diameter arterioles are the major determinants of resistance Abrupt changes in diameter or plaques from dramatically increase resistance Disrupt laminar flow and cause turbulence Relationship Between Blood Flow, Blood Pressure, and Resistance Blood flow (F) is proportional to the blood (hydrostatic) gradient ( P) If P increases, blood flow

6 Blood flow is proportional to resistance (R) If R increases, blood flow decreases: F = P/R is more important in influencing local blood flow because it is easily changed by altering blood vessel diameter Systemic Blood Pressure The action of the heart generates blood flow Pressure results when flow is opposed by pressure Is highest in the aorta Declines throughout the pathway Is 0 mm Hg in the right atrium The steepest drop occurs in Arterial Blood Pressure Reflects two factors of the arteries close to the heart (compliance or distensibility) of blood forced into them at any time Blood pressure near the heart is Arterial Blood Pressure pressure: pressure exerted during ventricular contraction pressure: lowest level of arterial pressure pressure = difference between and pressure Arterial Blood Pressure : pressure that propels the blood to the tissues MAP = diastolic pressure + 1/3 pulse pressure Pulse pressure and MAP both decline with increasing distance from the heart Capillary Blood Pressure Ranges from capillary pressure is desirable High BP would rupture fragile, thin-walled capillaries Most are very permeable, so low pressure forces filtrate into spaces

7 Venous Blood Pressure Changes little during the cycle pressure gradient, about 15 mm Hg pressure due to cumulative effects of peripheral resistance Factors Aiding Venous Return 1. pump : pressure changes created during breathing move blood toward the heart by squeezing abdominal veins as thoracic veins expand 2. pump : contraction of skeletal muscles milk blood toward the heart and valves prevent backflow 3. of veins under control Maintaining Blood Pressure Requires Cooperation of the, vessels, and Supervision by the brain Maintaining Blood Pressure The main factors influencing blood pressure: output (CO) resistance (PR) Blood volume Maintaining Blood Pressure = P/PR and = P/PR pressure = CO x PR (and CO depends on blood volume) pressure varies directly with CO, PR, and blood volume Changes in one variable are compensated for by changes in the other variables Cardiac Output (CO) Determined by return and neural and hormonal controls Resting heart rate is maintained by the center via the vagus nerves

8 Stroke volume is controlled by Cardiac Output (CO) During stress, the center increases heart rate and stroke volume via sympathetic stimulation decreases and increases Control of Blood Pressure Short-term neural and hormonal controls Counteract fluctuations in blood pressure by altering Long-term renal regulation Counteracts fluctuations in blood pressure by altering Short-Term Mechanisms: Neural Controls controls of peripheral resistance Maintain MAP by altering blood vessel Alter blood in response to specific demands Short-Term Mechanisms: Neural Controls Neural controls operate via that involve Vasomotor and fibers Vascular muscle The Vasomotor Center A cluster of neurons in the medulla that oversee changes in blood vessel diameter Part of the center, along with the cardiac centers Maintains vasomotor tone ( ) Receives inputs from,, and higher brain centers Short-Term Mechanisms: Baroreceptor-Initiated Reflexes are located in Carotid sinuses Aortic arch Walls of arteries of the and Short-Term Mechanisms: Baroreceptor-Initiated Reflexes Increased blood pressure stimulates to

9 input to the vasomotor center the vasomotor center, causing arteriole dilation and venodilation Stimulates the center Short-Term Mechanisms: Baroreceptor-Initiated Reflexes taking part in the carotid sinus reflex protect the blood supply to the brain Baroreceptors taking part in the reflex help maintain adequate blood pressure in the systemic circuit Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors are located in the sinus arch Large arteries of the Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors respond to rise in, drop in or Increase blood pressure via the vasomotor center and the center Are more important in the regulation of respiratory rate (Chapter 22) Influence of Higher Brain Centers Reflexes that regulate BP are integrated in the Higher brain centers ( and ) can modify BP via relays to centers Short-Term Mechanisms: Hormonal Controls Adrenal medulla hormones (NE) and epinephrine cause generalized vasoconstriction and increase cardiac output, generated by kidney release of renin, causes Short-Term Mechanisms: Hormonal Controls

10 peptide causes blood volume and blood pressure to decline, causes generalized hormone (ADH)( ) causes intense in cases of extremely low BP Long-Term Mechanisms: Renal Regulation quickly adapt to chronic or BP Long-term mechanisms step in to control BP by altering blood volume Kidneys act and to regulate arterial blood pressure 1. renal mechanism 2. renal (renin-angiotensin) mechanism Direct Renal Mechanism Alters blood volume of hormones BP or blood volume causes the kidneys to eliminate urine, thus reducing BP BP or blood volume causes the kidneys to water, and BP rises Indirect Mechanism The - mechanism Arterial blood pressure release of renin Renin production of Angiotensin II is a potent vasoconstrictor Angiotensin II aldosterone secretion Aldosterone renal reabsorption of Na + and urine formation Angiotensin II stimulates release Monitoring Circulatory Efficiency Vital signs: and pressure, along with rate and temperature Pulse: pressure wave caused by the expansion and recoil of Radial pulse (taken at the ) routinely used Measuring Blood Pressure

11 arterial BP Measured indirectly by the method using a sphygmomanometer Pressure is increased in the cuff until it exceeds pressure in the brachial artery Measuring Blood Pressure Pressure is released slowly and the examiner listens for sounds of with a stethoscope Sounds first occur as blood starts to through the artery (systolic pressure, normally mm Hg) Sounds when the artery is no longer and blood is flowing (diastolic pressure, normally mm Hg) Variations in Blood Pressure Blood pressure cycles over a period BP peaks in the due to levels of hormones Age, sex, weight, race, mood, and posture may vary BP Alterations in Blood Pressure : low blood pressure Systolic pressure below mm Hg Often associated with long and lack of cardiovascular illness Homeostatic Imbalance: Hypotension hypotension: temporary low BP and dizziness when suddenly rising from a sitting or reclining position Chronic : hint of poor nutrition and warning sign for disease or hypotension: important sign of circulatory shock Alterations in Blood Pressure : high blood pressure Sustained elevated arterial pressure of or higher May be transient adaptations during,, and upset Often persistent in obese people

12 Homeostatic Imbalance: Hypertension Prolonged hypertension is a major cause of, disease, failure, and or hypertension 90% of hypertensive conditions Due to several risk factors including heredity, diet, obesity, age, stress, diabetes mellitus, and smoking Homeostatic Imbalance: Hypertension hypertension is less common Due to identifiable disorders, including disease,, and disorders such as hyperthyroidism and syndrome Blood Flow Through Body Tissues Blood flow (tissue ) is involved in Delivery of O2 and nutrients to, and removal of wastes from, tissue cells Gas exchange (lungs) Absorption of nutrients (digestive tract) Urine formation (kidneys) Rate of flow is precisely the right amount to provide for proper function Velocity of Blood Flow Changes as it travels through the circulation Is related to the total cross-sectional area Is in the aorta, in the capillaries, again in veins capillary flow allows adequate time for exchange between blood and tissues Autoregulation adjustment of blood flow to each tissue in proportion to its requirements at any given point in time Is controlled by modifying the diameter of local arterioles feeding the capillaries Is independent of, which is controlled as needed to maintain constant pressure

13 Autoregulation Two types of autoregulation Metabolic Controls of arterioles and relaxation of precapillary sphincters occur in response to Declining tissue O2 Substances from metabolically active tissues (H +, K +, adenosine, and ) and chemicals Metabolic Controls Effects Relaxation of vascular smooth muscle Release of NO from vascular endothelial cells is the major factor causing vasodilation is due to sympathetic stimulation and endothelins Myogenic Controls responses of vascular smooth muscle keep tissue perfusion constant despite most fluctuations in pressure stretch (increased intravascular pressure) promotes increased tone and vasoconstriction Reduced stretch promotes and increases blood flow to the tissue Long-Term Autoregulation Angiogenesis Occurs when short-term cannot meet tissue nutrient requirements The number of vessels to a region increases and existing vessels enlarge Common in the heart when a coronary vessel is, or throughout the body in people in high-altitude areas Blood Flow: Skeletal Muscles At rest, and general mechanisms predominate During muscle activity

14 Blood flow increases in direct proportion to the metabolic activity (active or exercise ) Local controls override vasoconstriction Muscle blood flow can increase or more during physical activity Blood Flow: Brain Blood flow to the brain is constant, as neurons are intolerant of Metabolic controls Declines in, and increased cause marked vasodilation Myogenic controls in MAP cause cerebral vessels to in MAP cause cerebral vessels to Blood Flow: Brain The brain is vulnerable under extreme pressure changes MAP below mm Hg can cause (fainting) MAP above can result in cerebral Blood Flow: Skin Blood flow through the skin Supplies nutrients to cells ( in response to O2 need) Helps maintain body temperature ( controlled) Provides a blood (neurally controlled) Blood Flow: Skin Blood flow to venous below the skin surface Varies from 50 ml/min to 2500 ml/min, depending on body temperature Is controlled by nervous system reflexes initiated by temperature receptors and the central nervous system Temperature Regulation As temperature rises (e.g., heat exposure, fever, vigorous exercise) signals reduce vasomotor stimulation of the skin vessels radiates from the skin

15 Temperature Regulation Sweat also causes via in perspiration Bradykinin stimulates the release of NO As temperature decreases, blood is shunted to deeper, more vital organs Blood Flow: Lungs circuit is unusual in that The pathway is Arteries/arterioles are more like veins/venules (thin walled, with large lumens) Arterial and are (24/8 mm Hg) Blood Flow: Lungs mechanism is opposite of that in most tissues Low O2 levels cause vasoconstriction; high levels promote Allows for proper O2 loading in the lungs Blood Flow: Heart During ventricular systole vessels are compressed blood flow ceases Stored supplies sufficient oxygen At rest, control is probably Blood Flow: Heart During strenuous exercise vessels in response to local accumulation of vasodilators Blood flow may three to four times Blood Flow Through Capillaries Vasomotion and flow Reflects the on/off opening and closing of sphincters Capillary Exchange of Respiratory Gases and Nutrients Diffusion of O2 and nutrients from the blood to tissues CO2 and metabolic wastes from tissues to the blood

16 -soluble molecules diffuse directly through endothelial membranes -soluble solutes pass through clefts and fenestrations Larger molecules, such as proteins, are actively transported in vesicles or Fluid Movements: Bulk Flow Extremely important in determining relative fluid volumes in the blood and space Direction and amount of fluid flow depends on two opposing forces: and osmotic pressures Hydrostatic Pressures Capillary pressure (HPc) (capillary blood pressure) Tends to force fluids through the walls Is greater at the arterial end ( mm Hg) of a bed than at the venule end ( mm Hg) Interstitial fluid pressure (HPif) Usually assumed to be because of lymphatic vessels Colloid Osmotic Pressures Capillary osmotic pressure ( pressure) (OPc) Created by plasma proteins, which draw water themselves ~26 mm Hg Interstitial fluid pressure (OPif) (~1 mm Hg) due to low protein content Net Filtration Pressure (NFP) comprises all the forces acting on a capillary bed NFP = ( ) ( ) At the arterial end of a bed, hydrostatic forces dominate At the venous end, osmotic forces dominate Excess fluid is returned to the blood via the system Circulatory Shock Any condition in which Blood vessels are inadequately filled Blood cannot circulate normally Results in inadequate blood flow to meet tissue needs

17 Circulatory Shock shock: results from large-scale blood loss shock: results from extreme vasodilation and decreased peripheral resistance shock results when an inefficient heart cannot sustain adequate circulation Circulatory Pathways Two main circulations circulation: short loop that runs from the heart to the lungs and back to the heart circulation: long loop to all parts of the body and back to the heart Differences Between Arteries and Veins Developmental Aspects Endothelial lining arises from mesodermal cells in blood islands Blood islands form rudimentary vascular tubes, guided by cues such as vascular endothelial growth factor The heart pumps blood by the 4th week of development Developmental Aspects Fetal shunts (foramen ovale and ductus arteriosus) bypass nonfunctional lungs Ductus venosus bypasses the liver Umbilical vein and arteries circulate blood to and from the placenta Developmental Aspects Vessel formation occurs To support body growth For wound healing To rebuild vessels lost during menstrual cycles With aging, varicose veins, atherosclerosis, and increased blood pressure may aris