Blood Pressure Physiology

Introduction

Arterial blood pressure is created by the combined forces, and complex interactions, of cardiac output, systemic vascular resistance (resistance produced mainly in the arterioles), and viscosity of the blood - Blood Pressure (BP) = Cardiac Output (CO) x Systemic Vascular Resistance (SVR). The arterial blood pressure is the force causing blood to flow through the arteries, into the capillaries, then back to the heart via the veins.

The left ventricle contracts (systole), ejecting blood into the aorta, creating a pressure pulse. This can be measured as the systolic blood pressure. Part of this blood that is ejected during systole remains within the distended arteries, and rebounds during diastole (relaxation of the ventricle), creating the diastolic blood pressure. An adequate blood pressure is required for normal blood flow, however, this does not mean that the presence of a normal arterial blood pressure guarantees that the blood flow is normal.

Physiological Mechanisms to Maintain Normal Blood Pressure

There are both short-term and long-term physiological mechanisms which react in order to maintain blood pressure within normal limits. These are:

1) Autonomic nervous system (ANS) responses

2) Capillary fluid shift mechanism

3) Hormonal responses

4) Kidney and fluid balance mechanisms

These mechanisms all work in combination to ensure that blood pressure is maintained within normal limits. They do this by adapting their responses in both the short-term and the long-term.

ANS Responses

Normal circulatory homeostasis depends upon there being a level of basal activity of both the sympathetic and parasympathetic nervous systems - the balance of the ANS is governed by the medulla oblongata in cardioinhibitory and vasomotor centres. The ANS is the most rapidly responding regulator of blood pressure.

Stimulus for activation

A drop in cardiac output results in a drop in arterial blood pressure, which is sensed by baroreceptors (pressure-sensitive nerve endings) located in the carotid sinus and aortic arch. This information is relayed to the vasomotor centre (VMC) of the brainstem. A drop in signals from the baroreceptors in response to perceived hypoperfusion leads to a central increase in activity of the sympathetic nervous system and an increased release of adrenaline from the adrenal medulla.

Response to stimulus

Adrenaline released from the adrenal medulla into the circulation, and noradrenaline released at nerve terminals, stimulate cardiac and vascular beta and alpha-receptors, resulting in increased contractility of the heart (beta-receptors) and vasoconstriction of both the arterial and venous side of the circulation (alpha-receptors).

Beta Effects:

Stimulation of beta-receptors leads to stimulation of adenylate cyclase, which generates intracellular cyclic AMP. This in turn has many intracellular effects, including increasing the rate and magnitude of intracellular calcium fibres. Positive inotrope, positive chronotrope, positive luisitrope (improved relaxation); stimulates renin release from the Juxtaglomerular apparatus (JGA).

advantages:

Increased strength of cardiac contraction; increased rate of cardiac contraction and improved myocardial relaxation.

disadvantages:

Increased myocardial oxygen consumption (increased heart rate and increased work due to inotropic stimulation); increased intracellular calcium can lead to calcium overload that in turn can result in cardiac rhythm disturbances and cell death. Chronic stimulation leads to down-regulation of the system.

Alpha Effects:

advantages:

Increases arterial blood pressure and protects perfusion of essential vascular beds (cerebral and coronary).

disadvantages:

Increased systemic vascular resistance leads to increased afterload and decreased cardiac output, with increased myocardial oxygen demand. It is likely to exacerbate hypoperfusion of non-essential vascular beds.

NOTE There is a strong correlation between the severity of heart failure and the degree of stimulation of the adrenergic system. Veterinary studies have confirmed greater stimulation of the adrenergic system in patients with more advanced heart disease. Human studies have demonstrated a worse prognosis in those patients with higher levels of circulating noradrenaline.

Capillary Fluid Shift Mechanism

The maintenance of normal pressures within the arterial and venous circulations is essential for the maintenance of normal fluid homeostasis. The Starling hypothesis describes the state whereby the equilibrium of fluid exchange across the capillary wall (between the blood and the interstitial fluid) is determined by the hydrostatic pressures and oncotic pressures that exist across the capillary wall. This fluid exchange is controlled by the capillary blood pressure, the interstitial fluid pressure and the colloid osmotic pressure of the plasma. Normally there is a net loss of fluid from the capillary at the arteriolar end, and a net gain at the venous end, resulting in almost perfect fluid balance being maintained. Any net fluid movement from the intravascular to the extracellular space can be compensated for by lymphatic drainage. Low blood pressure results in fluid moving from the interstitial space into the circulation, helping to restore blood volume and blood pressure.

Hormonal Responses

Hormonal responses exist for the purpose of both lowering and raising blood pressure. They act in various ways, including vasoconstriction, vasodilation and alteration of blood volume.

Renin-Angiotensin-Aldosterone System (RAAS):

The juxtaglomerular apparatus of the kidneys plays an important role in the control of blood volume and blood pressure - renin is released from this area. The stimulus for renin release into the circulation includes; local baroreceptors in afferent renal arteriole - a drop in renal blood flow stimulates renin release; cardiac and arterial baroreceptors (in response to reduced local perfusion) - beta-1 mediated effect via innervation of the juxtaglomerular apparatus (JGA); decreased chloride delivery to the macula densa (detected by chemoreceptors) - a drop in glomerular filtration rate (GFR) leads to increased reabsorption in the proximal convoluted tubule, that is sensed as a drop in chloride presented at the macula densa. Renin then leads to the cleavage of circulating Angiotensinogen (produced by the liver) to angiotensin I, and then angiontensin converting enzyme (ACE) catalyses the production of angiotensin II from angiotensin I - this occurs mainly in the lungs. ACE is bound to endothelial cells throughout the vascular system. Angiotensin II acts as a stimulus to aldosterone release from the zona glomerulosa of the adrenal cortex, which in turn leads to sodium and water retention in the cortical collecting duct of the renal tubule.

NOTE This system is responsible for the long-term maintenance of blood pressure, but is also activated very rapidly in the presence of hypotension.

Effects of Hormones:

Angiotensin II is a potent vasoconstrictor (causing an increase in mean arterial pressure), which also causes the direct stimulation of sodium retention in the proximal convoluted tubule of the kidney, via its increased synthesis and release of aldosterone. Aldosterone stimulates reabsorption of sodium and chloride, and secretion of potassium and protons. Initially, effects are advantageous by protecting perfusion to essential vascular beds and expanding the circulation fluid volume, and therefore increasing contractility by the Starling mechanism. Disadvantages include increased systemic vascular resistance, therefore increased myocardial work and increased myocardial oxygen demand. Expanded circulation fluid volume ultimately results in congestion of vascular beds when the Starling mechanism is not effective in the failing heart. Angiotensin II and aldosterone have effects at the level of the gene involving altered expression, which may lead to a progression of the myocardial dysfunction present. They therefore play a role in the regulation of hypertrophy and fibrosis.

Kidney and Fluid Balance Mechanisms

The kidneys help to regulate the blood pressure by increasing (when blood pressure falls) or decreasing (when blood pressure rises) the blood volume, and also by the renin-angiotensin system described above. The kidney-fluid system is the main method of the long-term control of blood pressure.

Local Regulators of Blood Flow

Individual vascular beds have the ability to regulate blood flow according to the demands of the organ being supplied. Alterations in vasculature at this level probably contribute to the pathophysiology of heart failure, but are more difficult to establish because levels of regulators are regionally variable and cannot therefore be assessed systemically.

Endothelin:

A polypeptide factor involved in local regulation of blood flow. Manufactured in the vascular endothelium and active locally.

Prostaglandin/Prostacyclin:

Locally produced vasodilatory substances.

Nitric Oxide (EDRF - endothelin derived relaxation factor):

A locally produced vasodilator. Myocardial dysfunction is associated with dysfunction of this system.

Methods of Blood Pressure Measurement

Arterial blood pressure can be measured either directly or indirectly. The two main sites for measuring blood pressure in animals are the limbs and tail - the area used depends upon the animal and the situation, and also the choice of the operator:

• • dogs: forearm or at the base of the tail
  • cats: upper arm
  • horses: base of the tail.

It tends to be difficult to obtain an accurate blood pressure reading in the conscious animal due to the effects of the situation. Some cats, for example, may suffer from the "white coat effect". That is their stress levels have already increased due to being in an unusual environment, and when the vet attempts to take the blood pressure, this will stress them out even more. This has the effect of increasing the blood pressure. That said however, there are still ranges of normal blood pressure that take this into account. It is also difficult to obtain an accurate reading in a sedated or anaesthetised animal because most drugs have an effect on blood pressure. However, measurements can still be taken during surgery, for example, to compare throughout the procedure.

Indirect Methods

The width of the cuff that is used in indirect methods of blood pressure measurement is also important in getting accurate readings, in relation to the cross-sectional area of the limb or tail. The width of the cuff should be 40% of the circumference of the area the cuff is being placed. The cuff should be applied so that only a small finger can be inserted between the cuff and the leg, to ensure that it is neither too tight nor too loose.

1) Doppler Ultrasound:

An inflatable cuff is attached to a manometer. As the cuff is inflated, it occludes the artery, and a piezoelectric crystal placed over the artery distal to the cuff detects blood flow. The cuff is slowly released until blood flow returns to the artery - this causes a frequency change in sound waves, and is detected by the piezoelectric crystal, which converts it into sound detected by the operator. As the sound is detected, the cuff pressure is equal to systolic pressure.

2) Oscillometric Techniques:

An inflatable cuff is used to occlude the artery, and it detects the oscillations of blood flow while the artery is partially occluded. Systolic pressure, mean pressure and diastolic pressure can all be calculated. The cuff is attached to a control unit which continually detects arterial pressure. The cuff inflates until the artery is occluded, and then automatically begins to deflate. The first oscillation of blood flow detected is the systolic pressure, the largest oscillation is equal to the mean pressure, and the oscillations subside at diastolic pressure.