Due to the multiplication of vessels tends to increase functional diagnosis pressure, the ever diminishing heart impetus, due to its distribution over an increasing area and its expenditure in overcoming resistance, furnishes a more than corresponding reduction of the other factor in pressure, with the net result that the pressure in the capillaries is lower than in the arteries, although the resistance is greater. In the veins, however, this coincidence between velocity and pressure is lost. From causes already explained, the velocity of the blood stream increases from the capillaries to the heart, and the pressure, on the contrary, steadily diminishes, owing to decreasing resistance (less and less vessels) until the pressure in the venae cavae is practically nil.
The pressure in the arteries, like the velocity, is increased and diminished during systole and diastole, the difference being the mean arterial pressure. Pressure is also increased by constriction and diminished by dilatation of the vessels through the vaso-motor mechanism, the former increasing and the latter diminishing peripheral resistance.
Adjustment of Pressure. — The well-being of the body demands a more or less constancy of blood pressure, and, more urgently still, demands the maintenance of a minimum pressure. This condition is preserved by means of a physiological adjustment between heart beat and resistance, sometimes effected by the heart, sometimes by the vaso-motors, often by both.
<Callout type="important" title="Critical Adjustment">Maintaining blood pressure involves balancing heart rate and vessel resistance to ensure adequate circulation.</Callout>
High Blood Pressure results from any pathological condition which (a) increases the volume of blood in the vessels; (b) increases the force of the heart beat; or (c) offers abnormal resistance to its passage. Conditions which increase the total volume of blood are rare, the increase usually occurring either on the venous or arterial side, at the expense of the other.
Simple Cardiac Hypertrophy throws an abnormally large quantity of blood into the aorta at each systole, at the expense of the veins, thus raising arterial and lowering venous pressure. The impetus with which the blood is propelled also contributes to the increase of pressure. Drugs and Toxins of certain kinds exaggerate the force of the heart and thus raise pressure.
Low Pressure. — Conversely, any influence which (a) weakens the heart beat, (b) lessens the volume of the blood, or (c) diminishes resistance to its passage, lowers pressure. Myocarditis, Dilatation and Fatty Degeneration of the heart are attended with low arterial pressure, both because of diminished impetus and because an abnormally small quantity of blood is thrown into the aorta at each systole.
PULSE. The Pulse.— If the vascular system consisted of a set of rigid tubes, each discharge of blood from the ventricle would push the whole mass of blood forward and simultaneously empty from the venae cavae into the auricle a similar quantity of venous blood. The elasticity of the aorta wall makes it easier to distend the artery than to move the whole mass of blood forward, and this is what happens primarily to make room for the blood discharged by the heart beat.
<Callout type="tip" title="Pulse Wave Dynamics">Understanding pulse wave dynamics helps in diagnosing cardiovascular issues.</Callout>
The normal propagation of the pulse wave may be interfered with by (a) conditions within the heart, or (b) conditions in the vessels. Such interference may make itself manifest in (a) the amplitude, (b) the regularity, (c) the velocity of the wave.
Hypertrophy. — In general, anything which increases the force of the heart beat increases both the amplitude and velocity of the pulse wave. Hypertrophy of the heart, especially of the ventricle, in its early effects, is one of those influences.
Key Takeaways
- Blood pressure regulation involves balancing heart rate and vessel resistance.
- High blood pressure can result from increased cardiac output, vasoconstriction, or arterial stiffness.
- Low blood pressure is caused by weakened heart function, reduced blood volume, or decreased peripheral resistance.
Practical Tips
- Monitor your pulse to assess cardiovascular health.
- Understand the relationship between heart rate and vessel dilation for better blood pressure management.
- Recognize that certain drugs can affect both heart strength and vascular constriction.
Warnings & Risks
- High blood pressure can lead to serious complications such as stroke or kidney failure.
- Low blood pressure may indicate severe health issues like hemorrhage or sepsis.
Modern Application
While the chapter's focus on physiological principles remains relevant, modern medical advancements have introduced precise instruments and medications for diagnosing and treating cardiovascular conditions. Understanding these historical concepts enhances comprehension of current practices and aids in emergency situations where advanced equipment may not be available.
Frequently Asked Questions
Q: What are the main factors that increase blood pressure?
High blood pressure can result from increased cardiac output, vasoconstriction, or arterial stiffness.
Q: How does heart hypertrophy affect pulse wave characteristics?
Hypertrophy of the heart increases both the amplitude and velocity of the pulse wave in its early stages. However, as compensation fails, these effects diminish due to weakened heart force.
Q: What causes low blood pressure according to this chapter?
Low blood pressure is caused by weakened heart function, reduced blood volume, or decreased peripheral resistance.