Chapter 21: The Cardiovascular System: Blood Vessels and HemodynamicsBlood VesselsBlood vessels form a closed system of tubes that carry blood away from the heart, transport it to the tissues of the body, and then return it to the heart (Table 21.1 & Fig 21.6)CapillariesFound near every cell in the body but more extensive in highly active tissue (muscles, liver, kidneys & brain) (Fig 21.1)entire capillary bed fills with blood when tissue is activenot found in epithelia, cornea and lens of eye & cartilageCapillary walls are composed of only a single layer of cells (endothelium) and a basement membrane Types of capillaries (Fig 21.4)Continuous capillariesFenestrated capillariesSinusoidsincomplete basement membrane very large fenestrationsCapillary ExchangeMovement of materials in & out of a capillary (Fig 21.7)Diffusion (most important method) Substances such as O2, CO2, glucose, amino acids, hormones, and others diffuse down their concentration gradientsall plasma solutes except large proteins pass freely acrossthrough lipid bilayer, fenestrations or intercellular cleftsblood brain barrier does not allow diffusion of water-soluble materials (nonfenestrated epithelium with tight junctions)Transcytosispassage of material across endothelium in tiny vesicles by endocytosis and exocytosis large, lipid-insoluble molecules such as insulin or maternal antibodies passing through placental circulation to fetusBulk FlowMovement of large amount of dissolved or suspended material in same directionMovement is in response to pressure Hemodynamics – Blood FlowThe velocity of blood flow is inversely related to the total cross-sectional area of all blood vesselsblood flows most slowly where total cross-sectional area is greatest Blood flow velocity decreases from the aorta to arteries to capillaries and increases as it returns to the heart (Fig 21.11)flow in aorta is 40 cm/sec while flow in capillaries is 0.1 cm/secslow rate in capillaries allows for exchangeCirculation time is time it takes a drop of blood to travel from right atrium back to right atriumVolume of blood flowing back to the heart from the systemic veins depends on pressure difference from venules (16 mm Hg) to right atrium (0 mm Hg) – siphon effectRespiratory pumpdecreased thoracic pressure and increased abdominal pressure during inhalation, moves blood into thoracic veins and the right atriumSkeletal muscle pump (Fig 21.9)contraction of muscles & presence of valvesHemodynamics – Blood Pressurecaused by contraction of the ventricleshighest in aorta120 mm Hg during systole & 80 during diastolePressure falls steadily in systemic circulation with distance from left ventricle (Fig 21.8)35 mm Hg entering the capillaries0 mm Hg entering the right atriumFactors that affect blood pressure (Fig 21.10)Cardiac output – heart rate and force of contractionBlood volumeIf decrease in blood volume is over 10%, BP dropsWater retention increases blood pressureElasticity of arteriesVascular resistance - opposition to blood flow as a result of friction between blood and the walls of the blood vesselsaverage blood vessel radiussmaller vessels offer more resistance to blood flowblood viscosity (thickness)ratio of red blood cells to plasma volumeincreases in viscosity increase resistancedehydration or polycythemiatotal blood vessel lengththe longer the vessel, the greater the resistance to flow200 miles of blood vessels for every pound of fatobesity causes high blood pressureSystemic vascular resistance (also known as total peripheral resistance) refers to all of the vascular resistances offered by systemic blood vessels; most resistance is in arterioles, capillaries, and venules due to their small diametersarterioles control BP by changing diameterControl of Blood Pressure & FlowNervous control of blood pressure and flow is regulated by the cardiovascular center (CV) - a group of neurons in the medulla that regulates heart rate, contractility, and blood vessel diameter (Fig 21.12 & 21.13)input from higher brain regions such as cerebral cortex, limbic system & hypothalamusanticipation of competitionincrease in body temperatureProprioceptorsinput during physical activityBaroreceptorschanges in pressure within blood vesselsChemoreceptorsmonitor concentration of chemicals in the bloodoutput from the CV flows along sympathetic and parasympathetic fibersParasympathetic impulses along vagus nerves decrease heart rateSympathetic impulses along cardioaccelerator nerves increase heart rate and contractilityThe sympathetic division also continually sends impulses to smooth muscle in blood vessel walls via vasomotor nerves. The result is a moderate state of tonic contraction or vasoconstriction, called vasomotor toneBaroreceptor reflexes controlled by the CV (Fig 21.14)carotid sinus reflexswellings in internal carotid artery wallglossopharyngeal nerve to cardiovascular center in medullamaintains normal BP in the brainaortic reflexreceptors in wall of ascending aortavagus nerve to cardiovascular centermaintains general systemic BPHormonal Control of Blood Pressure (Table 21.2)Renin-angiotensin-aldosterone system (Fig 18.16)Epinephrine & norepinephrineincreases heart rate & force of contractioncauses vasoconstriction in skin & abdominal organsvasodilation in cardiac & skeletal muscleincreases blood pressureADH (anti diuretic hormone)ANP (atrial natriuretic peptide) lowers BP causes vasodilation ShockShock is the condition in which cardiac output (CO) cannot deliver enough oxygen and nutrients to meet the needs of body cells.There are three stages of shock. Nonprogressive or compensated If the cause is removed the body's negative feedback mechanisms alone can usually restore homeostasis. Progressive or Decompensated Shock becomes steadily worse. Recovery from progressive shock is possible but there will be some damage to tissues. Irreversible shock In this condition body cells have been too long without adequate oxygen and nutrients. All forms of known therapy are unable to save a person's life at this point even though cardiac output and arterial pressure may appear to be restored to normal. There are four main types of shock Hypovolumic - decreased blood volume (Fig 21.16)may be caused by acute hemorrhage (internal or external bleeding) or excessive fluid loss (as occurs in excessive vomiting, diarrhea, sweating, urine production, dehydration and burns) Cardiogenic (cardiac shock, power failure syndrome) - decreased cardiac output may be caused by