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Blood pressure is the pressure exerted by the circulating volume of blood on the walls of arteries, the veins, and the chambers of the heart¹. To remain healthy the body must keep the blood pressure between certain levels.

The heart and the blood vessels are the main parts of the body’s cardiovascular system. Its job is to carry blood around the body so that it can feed the cells with oxygen and other nutrients needed to keep the body going. In order to simplify things we can think of the heart as a big pump, the blood vessels as a collection of pipes connected in a loop from the output of the heart back to the input, and the blood as red cordial made up of a mixture of water, sugar and a lot of other bits that give it its flavour and consistency.

To understand the factors effecting blood pressure we can break the cardiovascular system into four areas:

• things that can be changed (variables);
• things that detect what is going on in the system (input sensors);
• the brain (control centre); and
• things that actually make changes happen (control signals).

Variables

Using the analogy of the pump, pipes, and red cordial let’s look at all the things that can change.

The volume of the blood – you can imagine that the amount of fluid in the system is going to change the pressure on all the parts. If the volume changes so will the pressure. In technical terms this is called the blood volume.

The size of the pump – a bigger pump will pump out more blood than a smaller pump, but this is hard to change, and is usually proportional to the size of the person. Top athletes are an exception to this. They usually have larger hearts than normal people. For the sake of our discussion we will forget about the size of the heart.

The speed of the pump – you can imagine if the pump is working faster then more blood is going to be pumped around the pipes. This is an important variable in changing the blood pressure. In technical terms this is called the heart rate (HR).

The clearance of the pump – imagine a pump which is basically a hollow rubber ball with tubes coming from it (an inlet and an outlet). If you squeeze the ball slowly the air comes out gradually and without much force, but if you give it a good hard squeeze a lot of air comes out quickly. The same can be done with the heart, the body can increase the amount of blood it pumps out with each squeeze (contraction). The technical term for the amount of blood squeezed out with each contraction is stroke volume (SV). This variable is linked with the stretch of the heart prior to contraction (preload), the force of the contraction (contractility) and the pressure that must be obtained to actually get the blood out of the heart (afterload).

Preload - If the muscle walls of the heart are stretched prior to a stroke then they will squeeze harder on the stroke. To get a greater stretch more blood must be in the heart. This observation is called the ‘Frank-Starling law of the heart’.

Contractility - The contractility of the heart is influenced by inotropic agents that make it pump either stronger or weaker. Positive inotropic agents such as Calcium ions and adrenaline make it pump stronger, negative inotropic agents such as potassium ions make the stroke weaker.

Afterload - In order to get blood out the heart the back- pressure in the blood vessels must be overcome. The harder it is to get blood out, the less blood will actually leave the heart. You can imagine if the blood vessels are clogged or narrow that this will make things harder.

The amount of blood the heart pumps each minute is the Stroke Volume (SV) x Heart Rate (HR). The product is called the Cardiac Output (CO).

The diameter of the pipes – will make a difference to the pressure. The tighter the pipes are the harder it will be to get the blood through.

The stickiness of the cordial – when we make a strong glass of cordial we all know how sticky it gets, and you can imagine that it is easier to push a runnier, weaker, cordial through a pipe than thick, gooey cordial. The same happens with blood, it can be thick or thin, and when it is thicker it is harder to push through and thus the pressure increases. The stickiness of a fluid is called its viscosity.

The length of the pipes – if we have a short pipe to push the cordial around in you can imagine it does not take much effort on the part of the pump to move it. However, imagine if the pipes are really long, the pump is going to have to do more work to get the cordial to the other end. The same is true for the blood vessels, the length of the blood vessels influences the blood pressure. Like the size of the heart, the body cannot change this variable rapidly, it is mostly a problem for obese people.

The last three variables are often grouped together and referred to as factors contributing to peripheral resistance (PR).

Input Sensors

Since all these things can change at any time we really need something with a good deal of computing power to control them all. Control is carried out in certain parts of the brain, but the brain needs feedback on what is going on, and it gets this through the nervous system. There are two main types of sensors that provide information.

Pressure sensors – that measure the stretch in the walls of blood vessels. These sensors are called baroreceptors and they send messages to a part of the brain called the Cardiovascular (CV) centre.

Chemical sensors – that measure the levels of carbon dioxide, oxygen, and acidity in the blood. They are called chemoreceptors, and also send messages the CV centre.

Other sensors that send messages to the CV centre are the emotional and thinking parts of the brain, temperature sensors (thermoreceptors), and movement sensors (proprioceptors). These cause the brain to make adjustments that will affect the blood pressure, but are not as important as the two mentioned above.

The Control Centre

The part of the brain dedicated to controlling the cardiovascular system is called the Cardiovascular centre. It is in a part of the brain called the medulla oblongata. It controls the variables: heart rate (HR), stroke volume (SV), and blood vessel diameter.

There are a few different parts to this system and they include:

• The cardiostimulatory centre which makes the heart beat faster and stronger (increasing blood pressure);
• The cardioinhibitory centre which slows the heart down (decreasing blood pressure);
• The vasomotor centre consisting of the vasoconstrictor centre which decreases blood vessel diameter (increasing blood pressure), and the vasoconstrictor centre which increases blood vessel diameter (decreasing blood pressure).

Control Signals

There are two ways the body sends out signals to change the variables we talked about earlier: nerves and hormones. Nerves carry electrical signals to different parts of the body, whilst hormones are chemical signals carried in the blood that affect particular parts of the body. We shall now look at how these are used to maintain blood pressure.

The CV centre has two nerve links to the heart: a link to speed the heart up (sympathetic pathway) and a link to slow the heart down (parasympathetic pathway). When the baroreceptors detect a drop in pressure the CV centre sends signals via the sympathetic pathway and as a result the heart rate (HR) and the stroke volume (SV) increase. At the same time the diameter of the blood vessels is made smaller (vasoconstriction). When the blood pressure raises the opposite happens: the parasympathetic pathway receives signals and the heart slows and loses force, at the same time the blood vessels get wider (vasodilation), and the blood pressure decreases.

When the chemoreceptors detect an excess of carbon dioxide the CV centre decreases the diameter of the blood vessels increasing the blood pressure.

The same nerves that cause the heart to beat harder and stronger also cause the release of hormones called adrenaline and noradrenaline. These come from a gland called the adrenal gland, and cause the heart to beat faster (HR) and harder (SV), and causes blood vessels to get bigger (vasodilation).

Other hormones have different effects on the variables, these are summarised in table 1 below.

Table 1 Hormones involved in maintaining blood pressure

The body also has the ability to regulate blood pressure in a localised area. An example of this is heat and cold: heat will cause vasodilation, and cold vasoconstriction.

Problems

Having looked at all the factors that influence and control blood pressure we can now look at how all these can be overwhelmed. Table 2 lists some of the factors that could cause problems with blood pressure.

When the body problems with blood pressure all of the compensatory mechanisms will come into play. These are beneficial in the short term, but eventually they actually make the situation worse². Ultimately problems with blood pressure if left untreated can lead to heart-failure.

Table 2 Factors that can overwhelm the homeostasis of blood pressure.

References

1. Anderson, K.N., Mosby’s Medical, Nursing & Allied Health Dictionary 4^th Edition, Moseby, Sydney, 1994.

2. Porth, C.M., Pathophyisology: Concepts of Altered Health States 4^th Edition, JB Lippincott Company, Philadelphia, 1994.

Bibliography

Tortora, G.J., Grabowski, S.R., Principles of Anatomy and Physiology - 8^th Edition, Harper Collins, NY, 1996.

Willis Hurst, J (Ed.), Medicine for the Practicing Physician – 3^rd Edition, Butterworth-Heinemann, USA, 1992.