2324_MD121_CVS_regional_blood_flow.pdf

Physiology School of Medicine Founded in 1845 2023-24 CVS-13&14 REGIONAL BLOOD FLOW Course: MD121 Cardiovascular System Lecturer: Professor AM Wheatley Email: [email protected] Coronary blood flow (CBF) coronary blood supply – variation in CBF during the cardiac cycle Blood supply to vent...

Physiology School of Medicine Founded in 1845 2023-24 CVS-13&14 REGIONAL BLOOD FLOW Course: MD121 Cardiovascular System Lecturer: Professor AM Wheatley Email: [email protected] Coronary blood flow (CBF) coronary blood supply – variation in CBF during the cardiac cycle Blood supply to ventricular muscle Regulation of CBF (O2 requirement, metabolic control) Cutaneous blood flow (SkBF) – cutaneous blood supply (arterioles, capillaries, A-V anastomosis, venous plexus) SkBF, and the sympathetic nervous system SkBF and influence of the SNS Body temperature and SkBF – exposure to heat and cold SkBF and blood pressure Coronary Circulation The two major coronary arteries run from the base of the aorta to the left and right ventricles, respectively, before giving off branches that run down the surface of the heart towards the apex. Right coronary artery supplies the right ventricle parts of the septum the posterior wall of the left ventricle Left coronary artery supplies the rest of the heart its major branch the left anterior descending (LAD) coronary artery which supplies part of the septum Coronary Circulation Coronary sinus – Venous drainage from the heart is primarily through the coronary sinus/cardiac vein Blood drains from these veins directly into the right atrium Variations in coronary blood flow during the cardiac cycle The coronary circulation exhibits marked fluctuations in flow rate which depend on whether the ventricle is in systole or diastole Variations in coronary blood flow during the cardiac cycle The coronary circulation exhibits marked fluctuations in flow rate which depend on whether the ventricle is in systole or diastole The rhythmic pulsations in aortic pressure are partly responsible for these phasic fluctuations The other main contributor is the changing intramural myocardial pressure Variations in coronary blood flow during the cardiac cycle The coronary circulation exhibits marked fluctuations in flow rate which depend on whether the ventricle is in systole or diastole The rhythmic pulsations in aortic pressure are partly responsible for these phasic fluctuations The other main contributor is the changing intramural myocardial pressure Variations in coronary blood flow during the cardiac cycle In the left ventricle: During systole: intramural myocardial pressure rises this compresses the coronary blood vessels compression is primarily in the sub-endocardium There is complete interruption of blood flow into the left ventricle during early systole due to the high pressure development Blood flow in the left ventricle thus happens primarily during diastole Variations in coronary blood flow during the cardiac cycle In the left ventricle: During systole: intramural myocardial pressure rises this compresses the coronary blood vessels compression is primarily in the sub-endocardium There is complete interruption of blood flow into the left ventricle during early systole due to the high pressure development Blood flow in the left ventricle thus happens primarily during diastole Variations in coronary blood flow during the cardiac cycle In right ventricle, intramural pressure is lower right coronary artery blood flow can occur during systole right coronary artery blood flow follows the fluctuations in aortic pressure Coronary Circulation Coronary sinus – Venous drainage from the heart is primarily through the coronary sinus/cardiac vein Blood drains from these veins directly into the right atrium Variations in coronary blood flow during the cardiac cycle During systole: there is a surge of venous blood flow out of the coronary sinus this is due to the compression of the muscular wall of the heart During diastole, coronary venous flow subsides Coronary Circulation The two major coronary arteries run from the base of the aorta to the left and right ventricles, respectively, before giving off branches that run down the surface of the heart towards the apex. Right coronary artery supplies the right ventricle parts of the septum the posterior wall of the left ventricle Left coronary artery supplies the rest of the heart its major branch the left anterior descending (LAD) coronary artery which supplies part of the septum Blood supply to the ventricular muscle Nutrient blood flow reaches the myocardium via vessels that penetrate the wall of the ventricle Consequently, the endocardial regions of the heart, especially in the thick-walled left ventricle, lie at the "end of the line" of coronary artery blood flow This contributes to the vulnerability of the sub-endocardial regions of the left ventricle to decreased blood flow Because the coronary arteries traverse the ventricular wall, delivery of blood to the endocardial regions of the left ventricle is influenced by intramyocardial pressure Cutaneous (skin) blood flow Adult skin -surface area: ~1.8m2 -weight: 2-3 kg -thickness: 1-2mm (epidermis + dermis) Epidermis has no blood vessels (direct diffusion of atmospheric O2 into the epidermis [100μm]) Dermis has many blood vessels Alexander Popov swimmer (4 olympic golds) Skin Forms a waterproof layer to protect the body from the external environment including microorganisms Is involved in temperature regulation Alexander Popov swimmer (4 olympic golds) Skin Forms a waterproof layer to protect the body from the external environment including microorganisms Is involved in temperature regulation -blood flow to the skin plays a central role in temperature regulation Alexander Popov swimmer (4 olympic golds) Skin Forms a waterproof layer to protect the body from the external environment including microorganisms Is involved in temperature regulation -blood flow to the skin plays a central role in temperature regulation Insulation Production of vitamin D Touch Alexander Popov swimmer (4 olympic golds) David Julius: Transient receptor potential cation channel subfamily V member 1 (TrpV1) - pain Ardem Patapoutian: Piezo-type mechanosensitive ion channel component 2 – touch, proprioception Relationship between oxygen consumption and blood flow In some organs/tissues blood flow correlates well with oxygen consumption: -skeletal muscle -liver/gut -brain In other organs/tissues blood flow does not correlates well with oxygen consumption skin: function (thermoregulation) JR Levick, Cardiovascular Physiology 4nd Edition Relationship between oxygen consumption and blood flow In some organs/tissues blood flow correlates well with oxygen consumption: -skeletal muscle -liver/gut -brain In other organs/tissues blood flow does not correlates well with oxygen consumption skin: function (thermoregulation) JR Levick, Cardiovascular Physiology 4nd Edition Cutaneous (skin) blood flow Skin has a low, relatively constant metabolic rate - it only needs low blood flow for its metabolic requirements Temperature is the principal factor regulating skin blood flow Skin blood flow can change > 100 fold during temperature regulation Alexander Popov swimmer (4 olympic golds) Minimum: 1 ml.min-1.100g-1 Normal: 10-20 ml.min-1.100g-1 Maximum: 150-200 ml.min-1.100g-1 Cutaneous (skin) blood flow Skin has a low, relatively constant metabolic rate - it only needs low blood flow for its metabolic requirements Temperature is the principal factor regulating skin blood flow Skin blood flow can change > 100 fold during temperature regulation Alexander Popov swimmer (4 olympic golds) Minimum: 1 ml.min-1.100g-1 Normal: 10-20 ml.min-1.100g-1 Maximum: 150-200 ml.min-1.100g-1 Cutaneous (skin) blood flow Skin has a low, relatively constant metabolic rate - it only needs low blood flow for its metabolic requirements Temperature is the principal factor regulating skin blood flow Skin blood flow can change > 100 fold during temperature regulation Alexander Popov swimmer (4 olympic golds) Minimum: 1 ml.min-1.100g-1 Normal: 10-20 ml.min-1.100g-1 Maximum: 150-200 ml.min-1.100g-1 Cutaneous blood flow must regulate core temperature The temperature of human core (brain, thorax, abdominal organs) is held constant at 37.0-37.5°C Core temperature is adjusted by heat loss by the skin to match core heat production The thermo-neutral environmental temperature is 27-28 °C for a naked human and skin temperature is ~33 °C Skin temperature can vary from 0°C to 45°C for short periods without injury to the skin Alexander Popov swimmer (4 olympic golds) Heat is lost by: 1. Radiation - proportional to temperature difference between skin and ambient temperature - skin temperature is dependent on skin blood flow Heat is lost by: 2. Conduction-convection -warm skin warms up adjacent air which is then moved away by convection (air currents). -Conductive heat loss depends on skin temp and therefore skin blood flow Heat is lost by: 3. Evaporation of sweat (latent heat of evaporation cools the skin). - Both water and heat are delivered by the skin blood flow Cutaneous blood flow (blood flow to the skin) Skin circulation – nutritive blood flow Artery enters the dermis and branches into arterioles (resistance vessels), capillaries (gas and nutrient/waste product exchange), venules and vein which carries the blood out of the skin Blood flow in the skin can take two different routes Skin circulation – nutritive blood flow Artery enters the dermis and branches into arterioles (resistance vessels), capillaries (gas and nutrient/waste product exchange), venules and vein which carries the blood out of the skin Blood flow in the skin can take two different routes Subcutaneous circulation – thermoregulation Blood flows directly from the arteriole via arteriovenous anastomoses (AVAs) into the venous plexus The venous plexus can hold large quantities of blood Heat from blood in the venous plexus is transferred to the surface of the skin for heat loss Blood flow in the AVAs and venous plexus is involved in thermoregulation Blood flow in the skin can take two different routes Subcutaneous circulation – thermoregulation Blood flows directly from the arteriole via arteriovenous anastomoses (AVAs) into the venous plexus The venous plexus can hold large quantities of blood Heat from blood in the venous plexus is transferred to the surface of the skin for heat loss Blood flow in the AVAs and venous plexus is involved in thermoregulation Glabrous versus non-glabrous skin Glabrous skin (smooth and hairless) Glabrous skin is specialized skin with many arteriovenous anastomoses (AVAs) AVAs are coiled, muscular-walled blood vessels Glabrous skin occur in exposed regions with high surface area/volume ratio - fingers - toes - palm of hand - sole of foot - lips - nose - pinna of ear Glabrous skin involved in heat loss and heat retention (radiation and conduction/convection) Glabrous versus non-glabrous skin Non-glabrous skin -trunk -limbs -scalp Non-glabrous skin has some AVAs but not as many as glabrous skin Non-glabrous skin has many sweat glands When heat loss is required, sweating is increased in non-glabrous skin and heat loss occurs by evaporation and conduction-convection Non-glabrous skin sweating (heat loss by evaporation and conduction/convection) Non-glabrous skin When heat loss is required, sweating is increased in non-glabrous skin and heat loss occurs by evaporation and conduction-convection Sweat glands can deliver sweat to the skin surface at rates up to about 30 g/min and the maximum rate of sweat is estimated to comprise 2-4 L/h! Effect of external cold and heat on the AVAs in glabrous skin External heat/cold affects skin blood flow by causing heat/cold related vasodilation or vasoconstriction Effect of external cold and heat on the AVAs in glabrous skin Heat induces direct vasodilation External heat causes vasodilation of arterioles, venules and small veins and increased blood flow to the skin - large volume of well oxygenated blood (increased heat loss) - flushed face Effect of external cold and heat on the AVAs in glabrous skin Cold induces direct vasoconstriction External cold causes a vasoconstriction and reduced blood flow to the skin - reduced volume of blood skin blood vessels - pale skin - pain! Effect of cold atmospheric temperature on glabrous skin blood flow Cold-induced vasoconstriction Paradoxical cold vasodilatation With prolonged external cold the cold-induced vasoconstriction gives way to a paradoxical vasodilation with flushing and pain relief reduced blood flow to the skin Cold vasodilation occurs in Arctic Indians and Norwegian fishermen – prevents skin injury during prolonged cold exposure If cold exposure persist, vasoconstriction will again occur Effect of cold atmospheric temperature on glabrous skin blood flow Cold-induced vasoconstriction Paradoxical cold vasodilatation With prolonged external cold the cold-induced vasoconstriction gives way to a paradoxical vasodilation with flushing and pain relief reduced blood flow to the skin Cold vasodilation occurs in Arctic Indians and Norwegian fishermen – prevents skin injury during prolonged cold exposure If cold exposure persist, vasoconstriction will again occur Facial cold causes peripheral vasoconstriction, aggravates heart ischaemia and may cause a heart attack (more common in cold weather)! Effect of cold atmospheric temperature on glabrous skin blood flow Cold-induced vasoconstriction Paradoxical cold vasodilatation With prolonged external cold the cold-induced vasoconstriction gives way to a paradoxical vasodilation with flushing and pain relief reduced blood flow to the skin Cold vasodilation occurs in Arctic Indians and Norwegian fishermen – prevents skin injury during Frostbite-induced gangrene prolonged cold exposure If cold exposure persist, vasoconstriction will again occur Facial cold causes peripheral vasoconstriction, aggravates heart ischaemia and may cause a heart attack (more common in cold weather)! Extreme exposure to cold Cold stress: - entire skin perfusion can fall to just 1 ml.min-1.100g-1 - subcutaneous fat exerts full insulation to protect core temperature Skin blood flow during exercise Initially, SNS-induced cutaneous vasoconstriction – as core temperature rises, vasodilation occurs Rise in core temperature (exercise) is sensed by warmth receptors in the hypothalamus which is connected to the cardiovascular centre in the brainsteam Nonglabrous skin (scalp, limbs, trunk) vasodilation (sympathetic cholinergic vasodilation - ACh and NO) is associated with increased blood flow and sweating In glabrous skin, reduced SNS stimulation to skin blood vessels occurs – vasodilation and increased blood flow Cutaneous vasodilation contributes to fainting and heat exhaustion in hot weather Heavy exercise in hot environment can lead to heat exhaustion -cutaneous vasodilation for heat loss -muscle vasodilation -reduced TPR (MAP = CO x TPR) -reduced plasma volume due to sweating -reduced venous return -fall in CO -fall in TPR -low MAP -collapse (heat exhaustion) An athlete suffering from heat exhaustion: https://www.youtube.com/watch?v=8mOdzbSBIuU Heat exhaustion: The runner’s story This is my senior year of cross country at Wichita State University, running a 6k race. We were the final race of the day and the temperature when we ran was about 96 degrees F (35.5 C) and 100% humidity. Also, there was virtually no shade on the entire course. I made it to the final 100 meters of the race before my knees started buckling and my vision started going blurry. As soon as I "dove" across the finish line, I blacked out completely and had to be carried off the course. They took my temperature and said it was 106 degrees F (41.1 C), so they put me in a tub of water and started dumping whole coolers of ice on me. I started to cool down and soon started feeling really good. I was pretty messed up for about 3 days after this and had trouble focusing in school, without my head hurting. Could I have prevented this “heat stroke” by drinking more water? The doctor said it wouldn't have necessarily helped me to drink more water beforehand but that I just pushed myself too hard and my body couldn't keep up. Cutaneous blood flow and blood pressure Standing to attention in heat induces cutaneous vasodilation that can cause: large fall in TPR reduction in central venous pressure reduction in cardiac output (Frank-Starling mechanism) hypotension fainting MAP = CO x TPR Triple response If the skin is stroked with a sharp instrument a triple response is elicited: red line : within 15-30 sec a thin red line appears at the site of the stroke flare : within 1 min a red flare develops extending 1 to 2 cm from either side of the red line wheal : in the following 3-5 min an elevation of the skin develops along the red line -referred to as a wheal Triple response The red line -caused by dilation of the vessels because of mechanical stimulation - histamine-mediated vasodilation The flare - caused by dilation of neighbouring arterioles - axon reflex originating at the site of mechanical stimulation - the nerve impulse travels down the sensory nerve back up the small branches of the afferent nerve to the adjacent arterioles - vasodilation caused by local release of substance P - increased blood flow (redness, flare) Triple response The wheal - caused by increased capillary permeability induced by the trauma - mediated by release of histamine and substance P - fluid containing proteins leaks out of the capillaries and produces oedema at the site of injury (swelling from fluid accumulation)