1.4.4 Control of blood pressure
Short-term control of blood pressure
The short-term control of blood pressure depends on changes in cardiac output and on vascular resistance and capacitance. Cardiac output is directly affected byarterial blood pressure, because this is the principal component of afterload. Preload and the force of ventricular contraction are also directly affected by changes in vascular tone, particularly by venous tone. The heart rate is affected by blood pressure via the baroreceptor reflex. In the horse, this is particularly apparent when vagal tone is correlated with aortic pressure. Second degree atrioventricular block often occurs when aortic pressure rises above a level determined by the medulla and the baroreceptors. During the long diastolic interval, the aortic pressure falls, and the cycle is then repeated.
Control of vascular tone
The relative differences in vascular resistance in various tissues dictate the proof the cardiac output which perfuses them. This is largely mediated by the tone of the arterioles, which are well supplied by autonomic neurones. Local factors also govern the distribution of the blood flow and, at different times, different organ systems receive more or less blood; however, the brain is always perfused at the expense of other systems. Dramatic changes in muscular and cutaneous blood supply occur during exercise. At this time, blood pressure will fall unless there is an increase in cardiac output. Consequently, despite autocontrol of vascular tone, there is a requirement for a large cardiac reserve in order that demands for an increase in cardiac output can be met. Changes are also mediated by the effects of local vasoactive substances, which act to improve perfusion of hypoxic areas by the systemic circulation. In the lung, however, blood flow to areas of hypoxic tissue is reduced, resulting in a more appropriate distribution of blood in order to maintain gaseous exchange.
In animals with a reduced cardiac output as a result of cardiac disease or hypovolaemia, blood pressure is maintained by an increase in vascular tone. The maintenance of blood pressure under these circumstances requires an increase in systemic vascular resistance (and therefore afterload).
Vascular tone is also important for maintaining preload when there is a demand for an increased cardiac output. Blood pools in the venules and great veins at rest, due to their low muscular tone. At times when blood pressure is in danger of falling, venoconstriction increases venous return and maintains preDuring exercise, skeletal muscular activity increases the return of venous blood from the periphery.
Long-term control of blood pressure
The long-term control of blood pressure is dependent on maintenance of an appropriate blood volume. A complex mechanism involving a number of reflexes and humoral responses centres on adjustments in blood volume mediby the kidney. For a more detailed description of the mechanisms, readers should refer to specialist texts. In an oversimplified form, the changes which occur when blood volume is decreased can be summarised as follows:
- Baroreceptors detect a fall in blood pressure and supply afferent information to the cardiovascular centres.
- Volume receptors detect a fall in atrial volume if preload is inadequate.
- The juxtaglomerular apparatus detects changes in sodium levels and blood pressure within the kidney.
- The thirst centre receives sensory input from osmoreceptors so that water retention can match sodium retention.
- Renal blood supply adjusts to a fall in blood pressure by redistribution within the kidney.
- Angiotensin levels increase, resulting in an increase in aldosterone secretion by the adrenal cortex and in peripheral vasoconstriction.
- Antidiuretic hormone (ADH) levels increase and more water is retained from the collecting ducts by the kidney.
- Atrial naturetic peptide (ANP) levels fall in response to changes within the heart, resulting in increased sodium and water retention.
- Additional complex changes occur in the kidney, including a change in an unknown factor affecting glomerular filtration.
The overall effect of these mechanisms is to reduce sodium excretion by the kidney. This results in increased retention of water, and maintenance of blood volume. The homeostatic mechanism is well adapted to controlling blood volume. However, inappropriate retention of sodium occurs in congestive heart failure (Chapter 2).