Blood flow, blood pressure, andresistance Blood flow is the actual volume of blood flowing through a vessel, and organ, or the entire circulation in a given period of time. Blood pressure is the force per unit area exerted on the wall of a blood vessel by its contained blood.
Resistance is opposition to flow and is ameasure of the amount of friction bloodencounters as its passes through the vessels.Peripheral resistance is resistanceencountered in the peripheral circulation.
Blood viscosity is the internal resistance toflow and is related to the thickness of afluid.The longer the total vessel length, thegreater the resistance encountered.The smaller the vessel diameter, the greaterthe resistance encountered.
Arterial blood pressure Systolic pressure is the pressure exerted by ventricular contraction, and is measured in the aortic arch at about 120mm Hg. Diastolic pressure is the pressure exerted by the arteries when the ventricles are in diastole.
Pulse pressure is the difference betweensystolic and diastolic pressure.Mean arterial pressure (MAP) is responsiblefor actually propelling the blood to thetissues: = diastolic + pulse pressure/3.
Venous blood pressure The respiratory pump is a lessening of pressure created by pressure changes occurring the in the body cavity during breathing. The muscular pump is pressure created by the contraction of skeletal muscles.
Neural controls The neural center that oversees changes in the diameter of blood vessels is the vasomotor center, which is located in the medulla.
The vasomotor center transmits impulses ina fairly steady stream along sympatheticefferents called vasomotor fibers.Vasomotor tone is a state of moderateconstriction of arterioles as a result ofinervation by vasomotor fibers.
The carotid sinus reflex protects the bloodsupply to the brain due to acute changes inblood pressure.The aortic reflex maintains adequate bloodpressure in the systemic circuit as a whole.
Chemical controls Adrenal medulla hormones enhance the sympathetic response, either by increasing cardiac output (epinephrine), or by vasoconstriction (NE). Atrial natriuretic peptide causes blood volume and blood pressure to decline by promoting excretion of sodium and water by the kidneys.
Antidiuretic hormone stimulates the kidneysto conserve water, and also causesvasoconstriction in order to increase bloodpressure.Angiotensin II causes intensevasoconstriction, and is generated inresponse to renin release by the kidneys.
Nitric oxide promotes systemic andlocalized vasodilation.Alcohol causes blood pressure to drop byinhibiting ADH release.
Renal regulation The renin-angiotensin system is an indirect renal mechanism used to raise blood pressure due to a decline in arterial blood pressure. Angiotensin II is a vasoconstrictor produced by the enzyme renin, which increases renal perfusion, and stimulates the adrenal cortex to secrete aldosterone.
Monitoring circulatory efficiency Vital signs are measurements of pulse and blood pressure. A pulse is the alternating expansion and recoil of elastic arteries during each cardiac cycle. Pressure points are areas where blood flow can be compressed due to trauma, or to take a pulse measurement.
Systemic arterial blood pressure ismeasured indirectly by the auscultatorymethod, which measures blood pressure inthe brachial artery using asphygmomanometer.
Alterations in blood pressure Hypotension is low blood pressure, and is generally considered to be a systolic pressure of < 100 mm Hg. Hypertension is high blood pressure, and is generally considered to be > 140/90. Elevated diastolic pressure is more significant clinically, because it indicates progressive occlusion or arteriosclerosis.
Primary hypertension accounts for 90% ofall cases, and has no underlying cause.Secondary hypertension accounts for 10%of cases, and is due to identifiable disorders.
Blood flow through body tissues Tissue perfusion is blood flow through the body. Autoregulation is the automatic adjustment of blood flow to each tissue in proportion to its requirements at any point in time.
Myogenic responses are stimulation ofvascular smooth muscle due to changes inarteriolar blood pressure, and keep tissueperfusion fairly constant.Reactive hyperemia is the dramatic increasein blood flow into a tissue that occurs afterthe blood supply to an area has beentemporarily blocked.
When blood flow increases to skeletalmuscles in direct proportion to their greatermetabolic activity, it is called activehyperemia.
Blood flow through capillaries Vasomotion is the slow and intermittent flow of blood through capillaries, and is due to the opening and closing of precapillary sphincters. Gases, most chemical nutrients, and metabolic wastes pass between the blood and interstitial fluid by diffusion.
Hydrostatic pressure is the force exerted bya fluid pressing against a wall.Capillary hydrostatic pressure tends to forcefluids through the capillary walls.Interstitial fluid hydrostatic pressure is thepressure acting outside the capillaries,which opposes blood pressure.
Capillary colloid osmotic pressure is due toplasma proteins in capillaries, such asalbumin, that draw water into thecapillaries.The net filtration pressure is a result of allforces acting at the capillary bed.NFP = (HPc-HPif) - (OPc-OPif)
Circulatory shock Circulatory shock is any condition in which blood vessels are inadequately filled and blood cannot circulate normally.
Types of circulatory shock Hypovolemic shock results from large-scale loss of blood, as might follow acute hemorrhage. Vascular shock is due to a huge drop in peripheral resistance due to extreme vasodilation.
Cardiogenic shock occurs when the heart isso inefficient that it cannot sustain adequatecirculation.