Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure

Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 1 Unit 12 Study Guide Excluded from Study: Blood flow and the pressure gradient – calculation formula on pages 338 – 339 Page 348, figure 8-12 Page 349, table 8-2 Blood is the liquid that the heart pumps through vessels to supply nutrients and oxygen, and remove the wastes from all the cells of the body. Blood vessels carrying blood away from the heart are called arteries, while those returning blood to the heart are called veins. Contraction of the ventricles forces blood through the largest of the arteries. These large vessels branch into smaller arteries that carry blood to the various organs, where the arteries branch into the smaller arterioles. Arterioles supply the capillary beds, which are intermeshed with all of the body’s tissues. The capillary beds are where the exchange between blood and cells takes place. After the nutrient/waste exchange in the tissues, the capillaries become tiny veins called venules, which act as tributaries to ever larger veins. In systemic circulation, arteries always carry oxygenated blood, and veins carry deoxygenated blood; in pulmonary circulation, arteries carry deoxygenated blood, and veins carry oxygenated blood. Blood Pressure I. Baroreceptors in the aortic arch and carotid sinuses affect, via the sympathetic nervous system, the cardiac rate and the total peripheral resistance. o The baroreceptor reflex causes pressure to be maintained when an upright posture is assumed. This reflex can cause a lowered pressure when the carotid sinuses are massaged. o Other mechanisms that affect blood volume help to regulate blood pressure. II. Blood pressure is commonly measured indirectly by auscultation of the brachial artery when a pressure cuff is inflated and deflated. Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 2 o The first sound of Korotkoff, caused by turbulent flow of blood through a constriction in the artery, occurs when the cuff pressure equals the systolic pressure. o The last sound of Korotkoff is heard when the cuff pressure equals the diastolic blood pressure. III. The mean arterial pressure represents the driving force for blood flow through the arterial system. Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 3 Blood Flow • The rate of blood flow through a vessel (volume of blood passing through per unit of time) is directly proportional to the pressure gradient and inversely proportional to vascular resistance F= ∆P R » F = flow rate of blood through a vessel » ΔP = pressure gradient » R = resistance of blood vessels Blood Volume I. Tissue fluid is formed from and returns to the blood. o The hydrostatic pressure of the blood forces fluid from the arteriolar ends of capillaries into the interstitial spaces of the tissues. o Since the colloid osmotic pressure of plasma is greater than tissue fluid, water returns by osmosis to the venular ends of capillaries. o Excess tissue fluid is returned to the venous system by lymphatic vessels. o Edema occurs when there is an accumulation of tissue fluid. II. The kidneys control the blood volume by regulating the amount of filtered fluid that will be reabsorbed. o Antidiuretic hormone (ADH) stimulates reabsorption of water from the kidney filtrate and thus acts to maintain the blood volume. o A decrease in blood flow through the kidneys activates the reninangiotensin system. o Angiotensin II stimulates vasoconstriction and the secretion of aldosterone by the adrenal cortex. o Aldosterone acts on the kidneys to promote the retention of salt and water. Pressure Gradient • • The pressure gradient is the pressure difference between beginning and end of a vessel – Blood flows from area of higher pressure to area of lower pressure Resistance is measure of opposition of blood flow through a vessel Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure • 4 Resistance is dependent on blood viscosity, vessel length, vessel radium Neural Influences on Blood Pressure and Heart Rate Sympathetic Neurotransmitter   Norepinephrine: released from nerve terminals, minimal levels in the circulation Epinephrine: released from the adrenal medula into the circulation (during fight or flight conditions) Receptors     α1 adrenergic receptor: expressed by vascular smooth muscle near sympathetic nerve terminals; elicits vasoconstriction α2 adrenergic receptor: expressed by vascular smooth muscle distal from sympathetic nerve terminals; elicits vasoconstriction β1 adrenergic receptor: expressed by heart muscle; elicits increased heart rate, contratility and AV node conduction velocity β2 adrenergic receptor: expressed by vascular smooth muscle in skeletal muscle & coronary circulation; elicits vasodilation. These receptors are not innervated, and endogenously are primarily stimulated by circulating epinephrine. Primary Effects     Increased heart rate Increased heart contractility Increased vasoconstriction (increased TPR) Increased mean arterial blood pressure Parasympathetic Neurotransmitter  Acetylcholine: released by vagal nerve terminals (and all sympathetic and parasympathetic ganglia). Negligible (zero) levels in the circulation due to acetylcholinesterase expressed by red blood cells in the blood, which rapidly metabolizes acetylcholine Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 5 Receptors  muscarinic receptors: expressed by heart muscle; elicits decreased heart rate, decreased AV conduction, reduced contractile force in the atrium (but not in the ventricle). o NOTE: Five muscarinic receptor subtypes have been identified & cloned (wikipedia). Cardiac muscarinic receptors are primarily M2.  muscarinic receptors: expressed by the vascular endothelium; elicits vasodilation “primarily” by via release of nitric oxide (mediated primarily by M 3 receptors). Primary Effects    decreased heart rate vasodilation decreased mean arterial blood pressure The Renin-Angiotensin-Aldosterone System (RAAS) Renin is a substance that is released from the juxtaglomerular cells of the kidneys. There is an increased release of renin when the pressure in the renal artery falls. Renin converts angiotensinogen (which is produced by the liver) into angiotensin I (angiotensin I has little or no biological activity). Angiotensin I is then converted in the lungs by angiotensin converting enzyme (ACE) into the active form, angiotensin II. Angiotensin II has three major effects on the circulation: 1. it is a potent vasoconstrictor (40 times more potent than norepinephrine on a molar basis), increasing TPR and thus BP 2. angiotensin II increases the synthesis of aldosterone, a hormone produced in the adrenal glands that then acts on the kidneys to promote retention of salt and water, thus increasing blood volume and hence BP 3. angiotensin II acts on the pituitary gland in the brain to stimulate secretion of vasopressin or anti-diuretic hormone (ADH), which, as its name suggests, promotes retention of water by the kidneys, increasing blood volume, venous return, and thus eventually BP The renin-angiotensin-aldosterone system (RAAS) is involved in several forms of hypertension. For example, stenosis of a renal artery produces a fall in renal perfusion pressure and so activates the RAAS, inducing renal hypertension. A tumor of the adrenal gland can also result in hypersecretion of aldosterone, resulting in hypertension. Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 6 Arteries • Specialized to two functions – Serve as rapid-transit passageways for blood from heart to organs • Due to large radius, arteries offer little resistance to blood flow – Act as pressure reservoir to provide driving force for blood when heart is relaxing • Arterial connective tissue contains – Collagen fibres – provide ensile strength – Elastin fibres – provide elasticity to arterial walls Arterial Blood Pressure • • • Force exerted by blood against a vessel wall – Depends on • Volume of blood contained within vessel • Compliance of vessel walls Systolic pressure – Peak pressure exerted by ejected blood against vessel walls during cardiac systole – Average - 120 mm Hg Diastolic pressure – Minimum pressure in arteries when blood is draining off into vessels downstream – Average - 80 mm Hg Pulse Pressure • • The pressure difference between systole and diastole • a measure of the strength of the pressure wave • older people – high pulse pressure – indicates hardening of the arteries The pulse can be felt close to the surface of the skin is due to pulse pressure Mean arterial pressure (MAP) • Average pressure responsible for driving blood forward into the tissues throughout the cardiac cycle Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure • Mean arterial pressure= diastolic pressure + 1/3 pulse pressure • If BP = 120/80 = 80+1/3 of 40 = 93 mmHg 7 The systolic pressure is the peak pressure exerted in the arteries when blood is pumped into them during ventricular systole. The diastolic pressure is the lowest pressure exerted in the arteries when blood is draining off into the vessels downstream during ventricular diastole. The pulse pressure is the difference between systolic and diastolic pressure. The mean pressure is the average pressure throughout the cardiac cycle. Arterioles • Major resistance vessels • Radius supplying individual organs can be adjusted independently to – Distribute cardiac output among systemic organs, depending on body’s momentary needs – Help regulate arterial blood pressure • Mechanisms involved in adjusting arteriolar resistance – Vasoconstriction • Refers to narrowing of a vessel – Vasodilation Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure • Refers to enlargement in circumference and radius of vessel • Results from relaxation of smooth muscle layer • Leads to decreased resistance and increased flow through that vessel Factors regulating arterial pressure and heart rate Intrinsic factors:     Stroke volume (Frank-Starling Mechanism) SA node (heart rate) AV node (AV conduction block can reduce ventricular rate) Conducting system abnormalities (loss of synchronized ventricular contraction can reduce SV) Extrinsic factors: Nervous control:   Sympathetic nerve activity Parasympathetic nerve activity Reflex control: 8 Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure    9 Baroreceptor reflex Chemoreceptor reflex Atrial Receptor reflex (Bainbridge reflex) Humoral factors:       Renin-Angiotensin-Aldosterone System (RAAS) Vasopressin (antidiuretic hormone)(ADH) Atrial natriuretic peptide (ANP) Epinephrine – released from the adrenal gland o stimulates adrenergic (α1,α2,β1 & β2) receptors o Low concentrations are β2-selective, producing vasodilation o High concentrations stimulate also stimulate α1, α2 & β1 receptors, producing vasoconstriction (α1 & α2 mediated), increased heart rate & contractility β1 mediated) Thyroxin – released by the thyroid gland (increased release in hyperthyroidism, reduced in hypothyroid states) o upregulates β1 receptors, increasing heart rate Glucagon – released by pancreatic α cells o increases heart rate (glucagon receptors stimulate the same 2nd messanger system as β1 receptors) Some of these regulating factors act very quickly (e.g. the baroreceptors act within one heartbeat), while others act much more slowly (e.g. control of blood volume by the kidneys acts on a time scale of days). The strength (“gain”) of the reflex response also varies widely from system to system. Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure Blood Pressure Abnormalities • Hypertension Hypertension is high blood pressure. Blood pressure is the force of blood pushing against the walls of arteries as it flows through them. – Blood pressure above 140/90 mm Hg – Two types of hypertension:  Primary hypertension • Blood pressure elevated by unknown causes rather than by a single disease entity • Potential causes being investigated » Defects in salt management by the kidneys » Excessive salt intake » Family history » Lifestyle behavior  Secondary hypertension • Accounts for about 10 percent of hypertension cases • Occurs secondary to another known primary problem • Examples of secondary hypertension » Renal hypertension » Endocrine hypertension • Hypotension Hypotension is the medical term for low blood pressure – Blood pressure below 100/60 mm Hg 10 Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 11 Venous System • Blood reservoir – Resting state – contain >60% of the total blood volume – Blood passageways back to heart – Venous return • Pressure at the beginning of venous system = 17 mmHg • Left atrial pressure is near 0 • Pressure gradient – (17-0) mmHg Venous Return the return of blood to the heart via the superior and inferior vena cava and the coronary sinus. • Can be affected by – Left and right atrial pressure (velocity of blood flow) Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure – Action of the skeletal muscle contraction • No muscle contraction in bedridden patients, paralysed patients • Predisposed to get edema, venous thrombosis 12 Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 13 Breathing and respiratory activity will promote venous return Increased sympathetic vasoconstriction promotes venous return Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure Capillaries • Thin-walled, small-radius, extensively branched • Sites of exchange between blood and surrounding tissue cells – Maximized surface area and minimized diffusion distance – Velocity of blood flow through capillaries is relatively slow • Provides adequate exchange time 14 Lesson 1 Chapter 8 – Blood Vessels and Blood Pressure 15 Hypertension, Shock, and Congestive Heart Failure I. Hypertension, or high blood pressure, is classified as either primary or secondary. o Primary hypertension, also called essential hypertension, may be the result of the interaction of many mechanisms that raise the blood volume, cardiac output, and/or peripheral resistance. o Secondary hypertension is the direct result of known, specific diseases. II. Circulatory shock occurs when there is inadequate delivery of oxygen to the organs of the body. o In hypovolemic shock, low blood volume causes low blood pressure that may progress to an irreversible state. o The fall in blood volume and pressure stimulates various reflexes that produce a rise in cardiac rate, shift of fluid from the tissues into the vascular system, decrease in urine volume, and vasoconstriction. III. Congestive heart failure occurs when the cardiac output is insufficient to supply the blood flow required by the body. The term congestive is used to describe the increased venous volume and pressure that results. 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