Chapter 20.docx

Chapter 20 Blood vessels and circulation Arteries Carry blood away from heart Veins Carry blood back to the heart Capillaries Connect arterioles to venules and complete the circuit Both arteries and veins have three layers called tunics Tunica interna (intima) Lines the blood vessels on the inside Can secrete chemicals to dilate or constrict the vessels Tunica Media Consists of smooth muscle, collagen and elastic tissue Tunica Externa (adventitia) Varo vasorum – literally means vessel of vessels Network of small blood vessels that supply nutrients to the walls of the large blood vessels. Arteries Divided into three classes by size Conducting (elastic or large) arteries – big diameter Biggest arteries Aorta, common carotid, subclavian, pulmonary truck) Expands during systole, recoils during diastole. Distributing (muscular) arteries Distribute blood to specific organs. Brachial, femoral, renal, splenic Thick tunica media Smooth muscle Resistance arteries Arterioles Lumen size is decreased helping to generate the most systemic vascular resistance. Significant determinant of BP Heart has to overcome the resistance. More pressure more heart pumping. Some major arteries above the heart have sensory structures in their walls that monitor blood pressure and composition. There are three kinds. They transmit information to the brain stem to regulate heartbeat, blood vessel diameter and respiration. Carotid sinuses Baroreceptors in walls of internal carotid artery that monitor blood pressure. Transmits information through glossopharyngeal nerve CNIX (9) Carotid bodies Chemoreceptors that monitor blood chemistry. Transmits information through CNIX (9) to brainstem respiratory center Adjusts respiration rate to stabilize pH, CO2, O2 Aortic bodies Chemoreceptors – just like carotid bodies But they transmit information through Vagas nerve CNX (10) Capillaries – exchange vessels between blood and tissue fluid They exchange gases, nutrients, waste, hormones in capillary beds Veins - Capacitance vessels Thin walled Collapse when empty Greater capacity for blood than arteries At rest 64% of blood is in veins Low pressure system Veins do not have much pressure so how do they get blood back to heart? Sympathetic nervous system innervates tunica media to constrict This also pushes blood Muscle milking – when muscles contract and squeeze, pushing blood upwards Valves Respiratory pump – diaphragm pushes blood up as well Varicose veins – blood pooling in veins of lower legs, causes vein to stretch, pulling valves apart allowing blood to leak or back flow. Circulatory routes Simples and most common Heart ---> arteries -----> capillaries -----> veins -----> heart Portal system Blood flows through 2 consecutive capillary networks before returning to heart. Kidneys – connection between hypothalamus and anterior pituitary gland and connect intestines to the liver. Anastomosis There is a point of convergence between 2 blood vessels other than capillaries. Arteriovenous anastomosis (shunt) blood flows from artery directly into a vein bypassing any capillaries. Occurs in fingers, palms, toes and ears. Venous anastomosis Most common. One vein empty directly into another. Arterial anastomoses Two arteries merge Blood supply to a tissue is expressed in terms of flow and perfusion. Flow – amount of blood flowing through an organ, tissue or blood vessel in a given time (ml/min) Perfusion – flow per given volume or mass of tissue in a given time (ml/100g/min) Hemodynamics The physical principles of blood flow based on pressure and resistance The greater the pressure difference between 2 points, the greater the flow. The greater the resistance, the less the flow. Blood pressure and BP vocab Blood pressure Force blood experts against a vessel wall Pulse pressure Difference between systolic and diastolic pressure Mean arterial pressure Diastolic pressure and 1/3 of pulse pressure Hypertension Can weaken arteries Can cause aneurysms, promote atherosclerosis Hypotension Caused by blood loss, dehydration, anemia Blood pressure is determined by three things Cardiac output Blood volume Regulated by kidneys Resistance to flow Three variables Blood viscosity Vessel length Vessel radius Neutral control Central and autonomic nervous system Baroreceptors and chemoreceptors Hormonal control Some hormones influence BP by either constricting or dilating vessels or by adjusting water balance. Angiotensin II Vasoconstrictor to raise blood pressure Hypertension often treated with ACE inhibitors which block production of angiotensin II. This lowers the BP Aldosterone Na+ retention by kidneys Water follows osmotically (water follows salt) and increased BP Natriuretic peptides Secreted by heart Increased Na+ excretion by kidneys, reducing blood volume and BP Antidiuretic hormone (ADH) Water retention increases BP Epinephrine and norepinephrine Stimulate vasoconstriction and raises BP Vasomotion serves 2 purposes Generalized increase or decrease of BP Redirect blood to modify perfusion 1. Generalized increase in BP Important for cerebral profusion when dehydrated or hemorrhage Requires hormones angiotensin II or epinephrine 2. rerouting of blood If a specific artery constricts the pressure downstream drops, pressure upstream rises and blood flow down path of least resistance Rerouting depends on needs of body. EX. After eating a meal, vasocontraction in vessels in lower extremities may be reduced while vasodilation to digestive organs increases. Capillaries Have two-way movement of fluid (in and out) Substances being exchanged in the capillary include: h2o, O2, glucose, other nutrients, antibodies, hormones, cO2, and waste. Chemicals pass via 3 routes Endothial cells cytoplasm Intracellular spaces between endothelial cells Filtration pores of fenestrated capillaries Mechanisms involved Diffusion Transcytosis Filtration Reabsorption Diffusion ----> glucose and o2 diffuse out of blood, Co2 and waste diffuse in Transcytosis ----> vesicle mediated transport Filter and reabsorption ----> fluid filters out of arterial end of capillary and osmotically reenters the venous end which is reabsorption 2 pressures that determine capillary exchange Hydrostatic pressure Drives fluid out of capillary High on arterial end, low on veinous end Colloid osmotic pressure (COP) Draws fluid into capillary The capillary blood pressure is significantly different from the arterial end to the venous end. This is responsible for the shift from filtration to reabsorption. Capillaries absorb about 85% of the fluid they filter, the other 15% is absorbed by lymphatic system Edema The accumulation of excess fluid in tissue. Usually seen in face, hands, feet, ankles or abdomen Can occur in the internal organs as well 3 fundamental causes Increased capillary filtration Due to kidney failure. Water retention and hypertension drives capillary blood pressure up along with increased filtration. Capillaries become more permeable with old age Cases of poor venous return Good venous return depends on muscular activity, therefore edema is common problem for bed ridden or in wheelchair. Reduced capillary reabsorption Reabsorption is dependent on oncotic pressure which is proportional to concentration of albumin in blood (protein) Liver disease leads to hypoproteinemia Hypoproteinemia can be result of severe burns or dietary protein deficiency. Oncotic pressure is the difference between the COP of blood and that of tissue fluid. 3. Obstructed lymphatic drainage Effects of Edema O2 delivery and waste removal are disrupted Tissue begins to die Pulmonary edema Suffocation Cerebral edema Headaches, nausea, seizures and coma If edema is severe enough Circulatory shock Venous Return and Circulatory shock Blood flow back to heart is venous return and is achieved by five ways Pressure gradient Pressure generated by the heart is the most important force. Although it is weaker in the veins than in arteries. Central venous pressure (where the vena cava enters the heart) averages 4.6 mmHg Pressure in the venuoles is about 12-18 mmHg Pressure gradient is about 7-13 mmHg favoring flow towards heart Increased when blood volume up and with vasoconstriction Gravity Blood from your head and neck return downhill by gravity Skeletal pump Contracting muscles squeeze blood out of the compressed part of vein The thoracic (respiratory) pump Inhalation expands thoracic cavity Pressure on inferior vena cava decreases, abdominal pressure on IVC increases forcing blood upward to the heart. Cardiac suction When the ventricles contract, values are pulled downward. Suction draws blood into atria from vena cava and pulmonary veins. Circulatory shock Any state where cardiac output isn’t enough to meet the bodies needs. 2 categories of cardiac shock Cardiogenic shock Inadequate pumping of heart (usually result of myocardial infraction) Low venous return (LVR) Low cardiac output because too little blood returned to heart Three forms of LVR: Hypovolemic shock Most common Loss of blood volume trauma, burns or dehydration Obstructed venous return shock Tumor or aneurysm blocks blood flow Venous pooling shock (dizziness, fainting) Long periods of standing, sitting, or widespread vasodilation Neurogenic shock Loss of vasomotor tone. Other types of shock Septic shock – bacterial toxins triggers vasodilation Anaphylactic shock – severe immune reaction to antigen, histamine release, vasodilation Compensate shock - (response) hemostatic mechanisms cause spontaneous recovery Example – if a person faints and falls. Being in a horizontal position restores blood to brain Decompensated shock - (response) if compensation fails, life threatening, positive feedback loops occur. Condition gets worse causing damage to cardiac and brain tissue. Brain blood flow fluctuates less than in any other organ. Seconds without causes loss of consciousness 4-5 minutes without oxygen causes brain damage. Regulates its own flow. Can shift blood to more active parts of brain as needed. Cerebral arteries dilate and constrict as blood pressure changes The main chemical stimulus is pH PH down triggers vasodilation PH up triggers vasoconstriction The change in pH due to Co2 accumulation or too little Co2 in blood. Baroreceptors – pressure Chemoreceptors – vasodilation/constriction Ischemia – blood flow and O2 restricted or reduced in a part of the body. Stroke (cerebral vascular accident) - sudden death of brain tissue caused by ischemia

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