Blood Pressure
Blood pressure refers to systemic arterial blood pressure - the force bllod exerts on vessel walls as a result of the pumping of the heart. Systolic blood pressure is defined as the peak pressure during the cardiac cycle, while diastolic blood pressure measured during the resting phase.
Accurately measuring blood pressure is an important life skill for health care professionals, and needs to be understood, at the very least, by everyone else. Blood pressure values are universally given in mmHg.
Blood pressure is equal to (cardiac output) x (total peripheral resistance), and is regulated by the autonomic nervous system.
Blood Pressure Regulation
Blood pressure is the most important regulated variable in the body.
Blood pressure is constant throughout the body, with regional blood flowadjusted to provide more or less blood to given tissues.
There are various sensors that continuously measure blood volume (via pressure) and osmolarity.
- Kidney Regulation
- Vascular Regulation
Kidney Receptors
The juxtaglomerular apparatus contains its own baroreceptors that mediate renin release. The macula densa also measures NaCl concentration, a measure of blood volume.
Renin-Angiotensin-Aldosterone System (RAAS): The afferent arterioles and the juxtaglomerular apparatus of the kidney produce renin, the initiation of the renin-angiotensin-aldosterone system, to cause blood vessel constriction and blood volume expansion. This system affects long-term blood pressure.
Angiotensin II is a potent vasoconstrictor. It works on the adrenal cortex to induce aldosterone release, and directly on the kidney to increase Na+ resorption. While it slightly increases glomerular filtration rate via afferent arteriole constriction, its overall effect is to increase GFR via increasing renal perfusion pressure via efferetn vessel constriction.
Aldosterone affects sodium retention and thereby water retention in the kidney.
Vascular Receptors
Baroreceptor reflex: Pressure sensors located in the aortic arch and carotid sinus detect drops in blood pressure and adjust mean arterial pressure via disinhibition of the SNS. This leads to increased heart rate and contractility, as well as total peripheral resistance.
Atrial natriuretic peptide (ANP) is released by atrial cells in response to atrial stretch, leading to vasodilation, natriuresis, and diuresis. B-type, or brain natriuretic peptide (BNP) is produced by the ventricles in response to ventricular pressure and volume.
While epinephrine and norepinephrine of the autonomic nervous system.
CNS Integration
The reticular formation of the medulla and lower 1/3 of the pons produce a coordinated neural and endocrine response to regulate cardiac output and renal sodium excretion.
Intravascular Pressure
Both the highest systolic pressure, and the biggest differences between systolic and diastolic pressure, are found in the LV.
As blood courses through the arterial system, pressure drops off in a sigmoidal curve, and the difference between systolic and diastolic disappears in the arterioles.
A good picture would be nice here.
Measuring Intravascular Pressure
The pressures of the heart's chambers can be measured by inserting a catheter into a systemic vein and advancing into the right side of the heart. Measuring pressure in the left atrium can be approximated pulmonary wedge pressure, whereby the catheter tip is positioned so that it occludes a pulmonary artery. The pressure measured will be almost directly the left atrium pressure.
Peripheral Resistance
Pouiseuille's Law states that resistance is inversely proportional to the 4th power of the radius.
The site of greatest resistance is the arterioles, as the steepest pressure drop occurs there.
Increasing arteriolar constriction increases arterial pressure, but decreases venous pressure. The converse is also true - decreasing arterial constriction increases venous pressure.
Baroreceptor Reflex
Pressure receptors located in the aortic arch and carotid bodies communicate with control centres in the medulla, which in turn send efferent neuronal signals to the heart and vasculature.
Mechanical stimulation is transduced into a frequency-modulated signal, with firing rate causing changes in sympathetic/parasympathetic response.
High pressure induces stretch which causes vasodilation and bradycardia.
Decreased stretch decreases firing rate, reducing parasympathetic activity to the heart and increasing sympathetic innervation of the heart and blood vessels. This combines to increase cardiac output and return arterial pressure toward normal.
Sympathetic activity increases:
- heart rate and contractility
- arteriolar vascoconstriction - inc. TPR
- venous vascoconstriction - inc. venous return
- epinephrine release
- Na and water retention in kidney
- renin release
- ADH release
Low Pressure Receptors
Atrial receptors exist as A- and B- type fibres that join the vagus nerve - CN X - to signal low volume. A fibres fire during atrial systole, while B fires fire during ventricular systole.
Decreased firing of these fibres signal the hypothalamus to secrete ADH.
Increased firing also causes secretion of atrial naturetic paptide - ANP. ANP is a powerful vasodilator that also promotes diuresis, lowering blood pressure.
Estimating Mean Arterial Pressure
Flow = <>P / R. In the CV system, CO ~ Q and <>P ~ Part as Pven is small and R ~ TPR. Therefore,
CO = Part / TPR and Mean Part = CO / TPR
or, mean pressure = 1/3 pulse pressure + diastolic pressure.
Mean pressure is related to CO and TPR, while pulse pressure is related to stroke volume and arterial compliance.
Vessel Compliance
Vessel compliance, the change in volume given a change in pressure, decreases with age and smooth muscle contraction.
A decrease in compliance will result in an increase in pulse pressure.
Normally compiant arteries will store blood during systole and eject it during diastole, providing pretty constant flow trhough the capillaries. Hardened arteries will experience increased systolic pressure and decreased diastolic pressure, causing decreased capillary flow.
Arteries are not very compliant, with only modest increases in volume following increases in pressure. This gives them the name resistance vessels.
Veins, on the other hand, hava high compliance, changing their volume dramatically with changes in pressure. This gives them the name capacitance vessels. Veins can transport blood at low blood volumes due their ability to alter their shape.
This is only to a certain point, however. Due to their inelasticity, veins will only stretch so far.
Vessel Tension
Tension is related to pressure x radius
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