Blood circulation & its short term regulation Dr. Wasif Haq

Circulation transports nutrients, hormones & waste products throughout body, ensuring optimal function & thus survival of cells. Classified as (a) Systemic circulation/Greater circulation. (b) Pulmonary circulation. 84% systemic circulation (64% veins, 13% arteries, 7% arterioles & capillaries) 16% heart & lungs Overview

Arteries; Strong vascular walls, transport blood under high pressure & velocity. Arterioles: Smallest branches of arterial system, can dilate and constrict. Capillaries: Thin walls with pores hence exchange of substances takes place Venules: Unite to form larger veins Veins: Transport blood back to heart, reservoir for blood, pressure is low. Components of circulation

1. Blood supply to active tissues: Highly active tissue has more O2 demand. This is met by increased cardiac output & largely by CO2 & waste metabolites accumulation that dilates blood vessels. 2. Increased blood return to heart increases cardiac output: More venous return to heart, the more rapid heart sends blood to arteries. 3.Arterial pressure regulation mechanisms independent of local blood flow & cardiac output Principles of circulatory function

Very rapid response acting in seconds. A. Systemic circulation arterioles constriction increases peripheral resistance & blood pressure. B. Constriction of veins & larger vessels: increased blood return to heart, results in greater heart rate & force of contraction. C. Control by autonomic nervous system (symphathetic & parasymphathetic) Nervous control mechanism

Baroreceptors/pressoreceptors :stretch receptors located in walls of large systemic arteries. Whenever arterial pressure rises, the stretch on baroreceptors convey signals to vasomotor center of brain to cause dilatation or constrction of blood vessels. Carotid baroreceptors; above the bifurcation of carotid artery, area known as Carotid sinus. Aortic baroreceptors: wall of aortic arch A. Baroreceptors

Carotid baroreceptors Aortic baroreceptors Hering nerves Vagus nerve Glossopharyngeal nerve Baroreceptors Pathway Tractus Solitarius in medulla

No stimulation between 0-50 &60mm Hg by Carotid baroreceptors. Pressures higher will lead to rapid response, maximum pressure reached 180mm Hg. Aortic baroreceptors response similar to carotid baroreceptors except that response occur at 30mm higher levels than Carotid. Baroreceptors response

If arterial pressure is very high,impulses transmitted to tractus solitarius will inhibit the vasoconstrictor center in medulla & stimuate parasymphathetic center to cause vasodilatation and hence lowering of blood pressure. Hence arterial pressure and cardiac output decrease leading to decrease in arterial pressure. Low arterial blood pressure will have opposite effects. Baroreceptors Mechanism

Baroreceptors respond more to rapidly changing pressure than stationary pressure. Posture changes induced fluctuation in arterial pressure can lead to fainting, baroreceptors prevent dropping of arterial pressure. Importance in long term blood pressure regulation: not much as baroreceptors reset in 1-2 days. Baroreceptors & Posture

Chemosensitive cells( carotid & aortic) detect changes in concentration of blood O2, CO2 and H2. These have extensive blood flow, whenever arterial pressure falls, chemoreceptors excite vasomotor center (Sympathetic nervous system excitation & Parasympathetic nervous system inhibition). Response occurs if blood pressure falls below 80mm Hg. B.Chemoreceptors

Stretch receptors in atrial and pulmonary artery walls. Minimize arterial pressure changes in response to change in blood volume. Atrial stretch causes increase fluid loss by kidney through increase in glomerular filtration, inhibition of Antidiuretic hormone & release of atrial natriuretic peptide. C. Atrial & pulmonary artery reflex

Whenever atrial pressure rises, simultaneous rise in heart rate also occurs. Stretch receptors in atria transmit afferet signals through vagus to medulla, efferent signals from vagus & symphathetic nerves act to increase heart rate & force of contraction. Hence damping of blood in veins, atria and pulmonary circulation prevented. D. Bainbridge reflex

Arterial pressure elevation in response to cerebral ischemia. Also known as ‘last ditch stand’. Blood flow to vasomotor center becomes depressed enough to remove excess CO 2, lactic acid & other metabolities. The accumulation of these cause strong symphathetic stimulation that can raise arterial pressure to 250mm Hg. Doesn't become activated till pressure falls below 60mm Hg, only emergency pressure control. E. C.N.S. ischemic response

If cerebrospinal fluid pressure equals the cerebral arterial pressure in brain, the blood supply to brain is cut off resulting in ischemia. Blood supply is restored when arterial pressure will rise above cerebrospinal fluid pressure. It is type of C.N.S. ischemic response and protects vital centers of brain to develop against loss of nutrition. Cushing reaction

Upon baroreceptors and chemoreceptors stimulation, nerve signals are conveyed to skeletal muscles in abdomen causing compression of venous reservoirs of abdomen that leads to movement of blood out of abdominal vessels towards the heart. Results in increase in cardiac output and arterial pressure. F. Abdominal compression reflex