With each rhythmic pump, blood is pushed under high pressure and velocity away from the heart, initially along the main artery, the aorta. As blood moves into the arteries, arterioles, and ultimately to the capillary beds, the rate of movement slows dramatically to about 0.
While the diameter of each individual arteriole and capillary is far narrower than the diameter of the aorta, and according to the law of continuity, fluid should travel faster through a narrower diameter tube, the rate is actually slower due to the overall diameter of all the combined capillaries being far greater than the diameter of the individual aorta. The slow rate of travel through the capillary beds, which reach almost every cell in the body, assists with gas and nutrient exchange and also promotes the diffusion of fluid into the interstitial space.
After the blood has passed through the capillary beds to the venules, veins, and finally to the main venae cavae, the rate of flow increases again but is still much slower than the initial rate in the aorta. Blood primarily moves in the veins by the rhythmic movement of smooth muscle in the vessel wall and by the action of the skeletal muscle as the body moves.
Because most veins must move blood against the pull of gravity, blood is prevented from flowing backward in the veins by one-way valves. Because skeletal muscle contraction aids in venous blood flow, it is important to get up and move frequently after long periods of sitting so that blood will not pool in the extremities. For example, after a large meal, most of the blood is diverted to the stomach by vasodilation of vessels of the digestive system and vasoconstriction of other vessels.
During exercise, blood is diverted to the skeletal muscles through vasodilation while blood to the digestive system would be lessened through vasoconstriction. The blood entering some capillary beds is controlled by small muscles, called precapillary sphincters, illustrated in Figure. If the sphincters are open, the blood will flow into the associated branches of the capillary blood. If all of the sphincters are closed, then the blood will flow directly from the arteriole to the venule through the thoroughfare channel see Figure.
These muscles allow the body to precisely control when capillary beds receive blood flow. Varicose veins are veins that become enlarged because the valves no longer close properly, allowing blood to flow backward. Varicose veins are often most prominent on the legs. Why do you think this is the case? Proteins and other large solutes cannot leave the capillaries.
The loss of the watery plasma creates a hyperosmotic solution within the capillaries, especially near the venules. The fluid in the lymph is similar in composition to the interstitial fluid. The lymph fluid passes through lymph nodes before it returns to the heart via the vena cava. Lymph nodes are specialized organs that filter the lymph by percolation through a maze of connective tissue filled with white blood cells. Measuring pressure invasively by penetrating the arterial wall to take the measurement is much less common and usually restricted to a hospital setting.
The noninvasive auscultatory and oscillometric measurements are simpler and faster than invasive measurements, require less expertise, have virtually no complications, are less unpleasant and painful for the patient.
However, noninvasive methods may yield somewhat lower accuracy and small systematic differences in numerical results. Noninvasive measurement methods are more commonly used for routine examinations and monitoring. Typical Tools of Auscultatory Measurement : Shown here are a stethoscope and a sphygmomanometer, which are used for ascultatory measurement.
The auscultatory method uses a stethoscope and a sphygmomanometer. This comprises an inflatable cuff placed around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The mercury manometer, considered the gold standard, measures the height of a column of mercury, giving an absolute result without need for calibration.
A cuff of appropriate size is fitted smoothly and snugly, then inflated manually by repeatedly squeezing a rubber bulb until the artery is completely occluded. Listening with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. The pressure at which this sound is first heard is the systolic blood pressure. The cuff pressure is further released until no sound can be heard fifth Korotkoff sound , at the diastolic arterial pressure.
The auscultatory method is the predominant method of clinical measurement. Pulse is a measurement of heart rate by touching and counting beats at several body locations, typically at the wrist radial artery. The pulse is the physical expansion of an artery generated by the increase in pressure associated with systole of the heart. Pulse is often used as an equivalent of heart rate due to the relative ease of measurement; heart rate can be measured by listening to the heart directly through the chest, traditionally using a stethoscope.
Measurement of the pulse : Measurement of the pulse can occur at several locations, including the radial artery shown here. Pulse rate or velocity is usually measured either at the wrist from the radial artery and is recorded as beats per minute bpm. Other common measurement locations include the carotid artery in the neck and popliteal artery behind the knee. Pulse varies with age; a newborn or infant can have a heart rate of about bpm.
The heart rate may be greater or less than the pulse rate depending upon physiologic demand. In this case, the heart rate is determined by auscultation or audible sounds at the heart apex, not the pulse.
The pulse deficit difference between heartbeats and pulsations at the periphery is determined by simultaneous palpation at the radial artery and auscultation at the heart apex. While a simple measurement of pulse rate is achievable by anyone, trained medical staff are capable of much more accurate measurements.
Radial pulse is commonly measured using three fingers: the finger closest to the heart used to occlude the pulse pressure, the middle finger used get a crude estimate of blood pressure, and the finger most distal to the heart used to nullify the effect of the ulnar pulse as the two arteries are connected via the palmar arches.
Where more accurate or long-term measurements are required, pulse rate, pulse deficits, and more physiologic data are readily visualized by the use of one or more arterial catheters connected to a transducer and oscilloscope.
The rate of the pulse is observed and measured by tactile or visual means on the outside of an artery and is recorded as beats per minute.
The pulse may be further indirectly observed under light absorbencies of varying wavelengths with assigned and inexpensively reproduced mathematical ratios.
Measurement of blood pressure includes systolic pressure during cardiac contraction and diastolic pressure during cardiac relaxation. Blood pressure is the pressure blood exerts on the arterial walls.
It is recorded as two readings: the systolic blood pressure the top number occurs during cardiac contraction, and the diastolic blood pressure or resting pressure the bottom number , occurs between heartbeats when the heart is not actively contracting. A spygmomanometer : A blood pressure cuff and associated monitor used for determining systolic and diastolic pressures within an artery.
A normal blood pressure is about mmHg systolic over 80 mmHg diastolic. Usually the blood pressure is read from the left arm, although blood pressures are also taken at other locations along the extremities. These pressures, called segmental blood pressures, are used to evaluate blockage or arterial occlusion in a limb for example, the ankle brachial pressure index.
The difference between the systolic and diastolic pressure is called the pulse pressure. The measurement of these pressures is usually performed with an aneroid or electronic sphygmomanometer. The classic measurement device is a mercury sphygmomanometer, using a column of mercury measured off in millimeters. There is no natural or normal value for blood pressure, but rather a range of values that are associated with increased risks for disease and health:. The guidelines for acceptable readings also take into account other cofactors for disease, such as pre-existing health factors.
Therefore, hypertension is indicated when the systolic number is persistently over — mmHg. Low blood pressure, or hypotension, is indicated when the systolic number is persistently below 90 mmHg. Chronically elevated blood pressure is called hypertension, while chronically low blood pressure is called hypotension. In healthy adults, physiological blood pressure should fall between the range of mmHg systolic and mmHg diastolic. Blood pressures above this are classed as hypertension and those below are hypotension, both considered medical conditions.
Arterial Blood Pressure over the Cardiac Cycle : Graph showing changes in blood pressure during a single contraction-relaxation cycle of the heart. Hypertension is a major risk factor for stroke, myocardial infarction heart attacks , heart failure, aneurysms of the arteries e. Even moderate elevation of arterial blood pressure is associated with a shortened life expectancy. Dietary and lifestyle changes can improve blood pressure control and decrease the risk of associated health complications, although drug treatment is often necessary in people for whom lifestyle changes prove ineffective or insufficient.
Hypotension is best understood as a physiological state rather than a disease and is often associated with shock, though not necessarily indicative of it. However, blood pressure is considered too low only if noticeable symptoms are present. For some people who exercise and are in top physical condition, hypotension is a sign of good health and fitness. For many people, low blood pressure can cause dizziness and fainting or indicate serious heart, endocrine, or neurological disorders.
Severely low blood pressure can deprive the brain and other vital organs of oxygen and nutrients, leading to a life-threatening condition called shock. Privacy Policy. There is a more gradual response in HR to sympathetic activity as opposed to parasympathetic, and this is mediated by two main factors. Firstly, the former is reliant on adenylyl cyclase producing cAMP as a secondary messenger in the pacemaker cells, as opposed to direct coupling.
Secondly, release of the neurotransmitter NA at postganglionic nerve endings is slower than Ach. Peripheral circulation: As alluded to, the SNS has the greater importance in regulation of vascular tone. The distribution of parasympathetic nerves is relatively limited and PNS effects mediate dilatation mainly via endothelial mechanisms. The vasculature of the skin, kidney, spleen and mesentery has extensive sympathetic innervation although vascular beds of the heart, brain and skeletal muscle have less [18].
Intrinsic: Arterial baroreceptors are specialised pressure-responsive nerve endings situated in the walls of the aortic arch and internal carotid artery just above the sinus bifurcation [19].
Afferent fibres relay with the CCC. There is basal discharge from baroreceptor afferents at physiological arterial pressures. When receptor endings are stretched, AP are generated and transmitted at a frequency roughly proportional to the pressure change. Afferent input results in negative chronotropic and inotropic effects, in addition to a reduction in vasoconstrictory tone of arterioles and venules.
Hence, increased BP provides a reflex negative feedback loop to maintain homeostasis, with responses greatest to changes in blood pressure in the physiological range mmHg. Clinically, this reflex is evident in the acute setting such as when standing from a sitting position with the kidneys playing a more prominent role in mediation of long-term pressure regulation [20].
A reduction in responsiveness can occur with age, hypertension and coronary disease. Baroreceptors are also present to a lesser extent in the atria, vena cavae and ventricles. The aortic and carotid bodies also contain chemoreceptors, which respond to reductions in the arterial partial pressure of oxygen PaO2 and increases in arterial partial pressure of carbon dioxide PaCO2. Afferent pathways are located in the same nerves as adjacent baroreceptors. Their primary function is to increase respiratory minute volume, but sympathetic vasoconstriction occurs as a secondary effect [21].
Extrinsic: Extrinsic influences play a smaller and less consistent role in circulatory regulation. Nonetheless, they become of increased relevance in states of stress, including pain, central nervous system CNS ischaemia and the Cushing reflex.
Pain can produce variable responses. Mild-moderate severity may generate a tachycardia and increases in arterial BP mediated by the somatosympathetic reflex [22].
Severe pain, however, may elicit bradycardia, hypotension and symptoms of shock. The CNS ischaemic response occurs when severe hypotension mean BP The adrenal medulla is unique in that the gland is innervated by preganglionic SNS fibres which originate directly from the spinal cord [25].
The adrenal medulla secretes adrenaline and NA in response to stimulation and function as hormones by entering the bloodstream and exerting distant effects on target organs. In view of this, activity is prolonged in comparison to NA release as a neurotransmitter. The RAA system does not play a major role in health, but is rather of increased relevance in BP maintenance during periods of hypovolaemia or impaired cardiac output when renal perfusion is compromised [26].
The enzyme renin initiates the cascade and is secreted by juxtaglomerular cells, which are modified VSMCs located in the media of the afferent arteriole immediately proximal to the glomerulus. Renin cleaves angiotensinogen, synthesised in the liver, to angiotensin I. This is physiologically inactive but rapidly hydrolysed by angiotensin-converting enzyme ACE , found in high concentrations in pulmonary vascular endothelium, to form angiotensin II.
It also stimulates transmission in the SNS. Additionally, it stimulates the zona glomerulosa of the adrenal cortex to synthesise and secrete aldosterone which targets the sodium-potassium exchanger in the distal collecting tubule and collecting duct of nephrons to cause sodium and water retention. This results in an increase in circulatory volume [27].
Angiotensin II also activates secretion of antidiuretic hormone ADH , otherwise known as vasopressin. This peptide is synthesised in the brainstem and transported for storage in the posterior lobe of the pituitary gland [28]. In addition to angiotensin II, secretion is also triggered by increased plasma osmolality detected by receptors in the hypothalamus and decreased plasma volume detected by receptors in the atria.
ADH induces translocation of aquaporin-2 channels in collecting ducts to enhance free water permeability and resorption anti-diuresis. ADH also has direct vasoconstrictory effects which are generalised and affect most regional circulations. This is a less potent vasoconstrictor but has comparable activity in stimulating aldosterone secretion. NO is deemed to be one of the most important mediators of vascular health. In general, tissue factors are more concerned with regulating organ blood flow than systemic arterial pressure; however, any change in vessel tone will affect both organ blood flow and systemic arterial pressure.
Finally, neurohumoral mechanisms play a very important role in regulating systemic vascular resistance and arterial pressure, particularly in certain forms of secondary hypertension.
Neurohumoral mechanisms are regulated principally by arterial baroreceptors and to a lesser extent by chemoreceptors. Many of the therapies used for reducing arterial pressure involve inhibiting the action of neurohumoral mechanisms. Cardiovascular Physiology Concepts Richard E. Klabunde, PhD. Klabunde, all rights reserved Web Design by Jimp Studio.
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