Flashcards in Medical Physiology Block 3 Week 3 Deck (50):
Which of the four mechanisms of controlling regional blood flow is responsible for auto-regulation?
myogenic response: increased pressure and the accompanying stretch of VSMCs elicit vasoconstriction
Describe the vascular anatomy of the cerebrum. What are characteristics specific to the brain circulation?
the common carotid artery bifurcates into the internal and external carotid arteries; the internal carotid artery anastomoses into the anterior cerebral artery and middle cerebral artery (the collateral artery in the anterior circulation is the anterior communicating artery; the posterior circulation arises from the two vertebral arteries that join together to become the basilar artery. The basilar artery then anastomoses into the posterior cerebral artery and posterior communicating artery; together the anterior and posterior circulation form the "Circle of Willis"; does not tolerate ischemia; limited total vascular volume due to being encased in the boney cranium
T/F Veins in the brain are devoid of smooth muscle and valves?
Describe how the major cerebral arteries (middle cerebral artery) branch into arterioles and capillary beds
the middle cerebral artery courses the lateral surface of the brain giving rise to pial arteries; pial arteries anasastomose into arterioles which enter at right angles to the parenchyma (branch centripetally to give rise to capillaries
Does the brain have lymphatics?
What is unique about the blood brain barrier? Is flux different in this circulation?
continuous endothelial cells (lots of mitochondria); no solute is excluded from the brain, but its flow is significantly attenuated by the BBB
How is cerebral blood flow controlled?
Neurovascular coupling: neural activity stimulates vasodilation by release of neurotransmitters that increase calcium in neurons leading to upregulation of NO and other vasodilators (signal upstream to arterioles ); Neural activity leads to release of adenosine into BECF, lowers arterial oxygen, increases arterial carbon dioxide, and lowers BECF pH → vasodilation (also signal to pial arteries); increased transmural pressure leads to vasoconstriction (myogenic); ANS stimulation is weak in the cerebrum
What is the reflex that occurs when intracranial pressure rises due to a mass or obstruction?
Cushing reflex: increased arterial pressure, decreased heart rate, and shallow breathing
What is normal cerebral perfusion pressure (definition and value)?
Cerebral perfusion pressure is the difference between the systemic arterial pressure and intracranial venous pressure, which is nearly equal to intracranial pressure (about 10 mm Hg); normally 70 mm Hg;
What happens to the cerebral circulation when MAP falls to 60 with normal intracranial pressures? What happens when intracranial pressue increases to over 25 mm Hg for patients with a normal MAP?
In both situations, the brain will become ischemic because the cerebral perfusion pressure has dropped below the levels for which the brain's circulation can auto-regulate flow
Describe the vascular anatomy supplying the left ventricle.
the left coronary artery (supplying both the LA & LV) divides into two branches: left circumflex artery and left anterior descending artery ("widowmaker" supplies apex and interventricular septum); Very high capillary density (ability to recruit more capillaries during exercise); drain into epicardial veins
Describe blood flow to the left ventricle. Is flow increased in diastole?
Myocardial contraction effectively compresses its own vascular supply (lower wall tension in right ventricle thus most flow occurs during systole; LV: diastole); Tachycardia (decrease in diastole reduces time for left ventricle perfusion; More blood flow to the epicardium compared to endocardium; Increase in myocardial blood flow during exercise
What is a mechanism to increase blood flow corresponding to increased oxygen demand?
The heart can meet large increases in oxygen demand only by increasing coronary blood flow (vasodilation; adenosine = effector by lower intracellular calcium);
What are the two myocardial energy sources? Which are more prominent?
Glucose and fatty acids (ketones, lactate, and pyruvate); fatty acids are more prominent
What metabolite is most important for regulating coronary blood flow?
adenosine (breakdown product of ATP) causes vasodilation
What is coronary steal?
administration of a vasodilator to counteract ischemia may backfire as downstream vasculature may already maximally dilated (the drug only dilates other vessels and lowers the CBF to the ischemic region)
Which blood vessels are directly attached to skeletal muscle? What is a microvascular unit?
Transverse arterioles and terminal arterioles (last vessel with VSMCs) are attached to skeletal muscle (1st and 2nd order arterioles are in the periphery); The group of capillaries supplied by a terminal arteriole represents one microvascular unit
What is the mechanism for rapid vasodilation of skeletal muscle?
The rapid vasodilation that occurs with skeletal muscle activation is initiation by K-mediated hyperpolarization of VSMCs
Since most skeletal muscles are controlled by sympathetic tone, what counteracts vasoconstriction during exercise?
1. redistribution of blood supply 2. metabolic demand counteracts sympathetic tone 3. flow to splanchnic system is reduced 4. venous return increases due to the muscle pump in the lower extremities and abdomen
Is the muscle pump activated during isometric contraction?
Describe the vascular anatomy of the splanchnic circulation.
most of the system is supplied by the celiac artery (superior mesenteric artery supplies intestines; the distal 2/3 of the colon are supplied by the inferior mesenteric artery); the liver receives blood from the portal vein and hepatic artery and drains into hepatic vein; Arterioles run along the muscularis layer (branch into arterioles at submucosal layer and branch into capillaries at the villus)
An increase in portal venous pressure results in what?
ascites (edema); remember that blood flows from high to low pressure
Describe counter-current exchange.
movement of oxygen from the arterioles to the venules before the arteriovenous villi tip (predisposition from ischemia)
What are causes for increased splanchnic blood flow?
Active transport of nutrients across epithelium increases metabolic activity(decrease oxygen, increase CO2 and adenosine), causing vasodilation; the uptake of nutrients in the villus vessels results in hyperosmolality and increases blood flow; During digestion, locally produced hormones and kinins cause vasodiliation (cholecystokinin and neurotensin) (increase sequentially during a meal and blood flow is sustained for a few hours)
How does the splanchnic system remain viable with decreased blood flow during exercise?
Preservation of intestinal viability in response to decreased blood flow requires increased oxygen extraction. If this is insufficient, necrosis & sloughing of tips of villi occurs (sepsis)
What uptakes lipophilic molecules in the splanchnic circulation?
What is apical skin?
glomus bodies (arteriovenous anastomoses) in the nose, lips, ears, hands, and feet
does the epidermis have a direct blood supply?
What happens to cutaneous blood vessels when the skin is directly heated? when body temperature rises?
dilate; also dilate
what is the mechanism of sympathetic tone in response to a decrease in core body temperature?
vasoconstriction of the glomus bodies (noepinephrine; no active vasodilation in glomus bodies)
What is the proposed mechanism of vasodilation of nonapical tissue?
sympathetic acetylcholine stimulates sweat glands, causing secretion of sweat and release of kinins (bradykinin) which cause vasodilation; alternate method: sympathetic acetlycholine stimulates vasoactive intestinal peptide and calcitonin gene-related peptide
What happens to the cutaneous vasculature following scratching of skin?
1. activation of histamine 2. reinforced by neural parasympathetic vasodilation (wheal: local edema from increased capillary permeability)
Of the endocrine factors, which drives a decrease in blood pressure when released? increase in blood pressure?
atrial natriuretic peptide & NO; endothelin, angiotensin, NE, E, and antidiuretic hormone
An elderly patient of generally good health has complained of feeling faint upon getting out of bed in the morning. Which would not be on your list of things to try at home?
sleep with head below the legs (sleeping with head raised above legs can induce a compensatory response)
The fight-or-flight response and vasovagal syncope vary in their physiological outcomes, but share what?
vasopressin release and cortical stimulation
A healthy patient undergoes a non-cardiac procedure, but has sedentary for several weeks. The patient's rehabilitation includes an exercise regimen. Upon exercising, cardiac output will increase rapidly. Why?
increase in heart rate (and contractility)
During irreversible hemorrhage, the failure of the capillary refill directly results in what?
What is orthostasis?
response to a rapid change in blood pressure as a result of getting up from a recumbent position
What is orthostasis? Describe it. What does an individual not faint or what might induced fainting?
response to a rapid change in blood pressure as a result of getting up from a recumbent position; Move to upright position → fluid shift from thorax to legs → decreased venous return → decreased right atrial pressure → decrease in stroke volume → decrease in cardiac output; LE vessels are stiffer (also non-uniform distribution of blood); increased body temperature results in vasodilation to extremities and thus a greater effect associated with orthostasis
What is the body's response to orthostasis?
high pressure baroreceptor reflex → increase in sympathetic output → increase in vascular tone, heart rate, and contractility → MAP returns to normal; plus skeletal muscle pump
What happens during the fight-or-flight response?
skeltal: activation of sympathetic cholinergic neurons → increase blood flow; release of epinephrine → beta2 receptor → vasodilation → increase in blood flow; muscle metabolites → vasodilation → increase in blood flow; reduced splanchnic blood flow; venous constriction (increase MAP and cardiac output);
What is vasovagal syncope? Describe the Bezold-Jarisch reflex.
neurally-mediated response to psychological (or pharmacological) stimulus to cause transient drop in cerebral perfusion; Vagal afferents carry signals to higher CNS centers, which act through autonomic nuclei in the medulla to massively stimulate parasympathetic system & abolition of sympathetic tone. (bradycardia, hypotension or decrease MAP, and hypoventilation; also inefficient high pressure baroreceptor response)
Describe the early response to exercise.
(mediated by CNS) Increase in heart rate and contractility; decreased blood flow to inactive muscle splanchnic regions; transient decrease in blood flow to skin; cholinergic sympathetic vasodilator fibers innervate vascular smooth muscle cells & skeletal muscle & peripheral vasodilation (TPR) in active skeletal muscle
Describe the late response to exercise.
exercise pressor reflex: In peripheral skeletal muscles, chemically- and mechanically sensitive receptors monitor the concentration of waste products and the amount of tension within muscle tissue. Thus, the receptors detect the intensity of exercise in a working muscle. Type III and IV afferent fibres carry this message from the muscles to the dorsal horn of the spinal cord, where ascending neurons relay the message to autonomic control centres in the medulla. Subsequently, autonomic nerves travel down from the brainstem and elicit coordinated changes that increase the oxygen supply to working muscles; arterial baroreflexes: resetting of high pressure baroreceptors; increaesed venous return; histamine increases capillary filtration; increase in sweat production and blood flow to cutaneous circulation; epinephrine increases cardiac output
What reflexes are active to increase MAP during hemorrhage?
high pressure baroreceptor; low pressure baroreceptors (decreased blood flow to kidney); peripheral chemoreceptors; Central chemoreceptors (in the brain sense low pH): brain ischemia (from hypovolemic shock) lowers pH increasing sympathetic output; capillary refill
What are signs of irreversible blood loss? Why does this happen?
MAP below 70; Faintness, weak pulse, cold & moist skin, oliguria; failure of vasoconstrictor response, failure of capillary refill response, and failure of either or both the cardiac response and sympathetic tone (CNS)
What other organs function to stabilize blood flow during hemorrhage?
brain (thirst); liver (albumin production); kidney (salt and water conservation)
What is the result of myocardial acidosis?
reduces calcium and contractility
Which three organs' actions can reduce blood volume?
breathing (expiration); skin (sweat); kidney (urine)