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Physiology, Arterial Pressure Regulation

 



In general, an individual’s “blood pressure,” or systemic arterial pressure, is the force of blood pushing against the walls of their arterial system. This pressure fluctuates depending on how much blood flows through one’s arteries at any given moment. Changes in blood pressure are regulated in order to maintain homeostasis and prevent injury-inducing falls in blood flow to essential organs and tissues.

Physiology, Pressure Regulation


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The human cardiovascular system is made up of three parts: the heart, which pumps oxygenated blood throughout our body; arteries, which carry this oxygenated blood; and veins, which return deoxygenated erythrocytes (red cells) and other substances back to the heart. The heart has 4 chambers, two atria and two ventricles. The atria contract rapidly, driving blood into the ventricles. The ventricles contract slowly and forcefully in order to drive blood through the tubes of our arterial system.

The four valves that separate the heart chambers also serve to keep blood flowing in the correct direction. These valves are named according to which side of their corresponding cavity they close towards. The tricuspid valve is the right atrioventricular valve, and it closes towards the right atrium. The pulmonary valve (or pulmonary semilunar valve) is the left atrioventricular valve, and it closes towards the left lung. The mitral valve is the septal/left ventricular outflow (from where blood flows out of both ventricles). It closes towards the left atrium. The aortic valve closes from top to bottom, preventing blood from flowing back into the left ventricle from above.

Arteries are tubular cavities within our body that branch out of our heart, supplying oxygenated blood to all organs and tissues except for our lungs. Each artery is made up of 3 layers: the innermost tunica intima (endothelium, loose connective tissue), the tunica media (muscle), and the tunica adventitia (loose connective tissue). Once blood leaves our heart, it flows towards our periphery where it branches off into smaller and smaller arteries known as arterioles. The innermost layer of these arterioles, the Tunica Intima, is lined with endothelial cells that form pores for red blood cells to travel through. These pores are called fenestrations.

Stroke volume is a measure of how much blood is ejected from a ventricle per beat. It is calculated by multiplying the systolic blood pressure times the end-diastolic blood pressure. Pumping efficiency can be calculated by dividing stroke volume by heart rate. The heart has a total of 3 chambers, 1 atrium and 2 ventricles. When the atrium contracts it will push blood into the ventricles. This is called preload. Once it gets to a maximum capacity of blood, it will contract and pump some amount of that blood out of our heart via the arterial system, this is called afterload. In order to maintain homeostasis, stroke volume and pumping efficiency must be balanced.

In general, if an individual’s body is under threat (i.e. not at rest), the heart will begin to contract more forcefully in order to pump more blood out of the body via the arterial system. This increases afterload, which means that more blood is being pumped through the vascular system than before. In this situation, the heart will contract harder and faster, increasing end-systolic pressure and ejecting more blood from the ventricles per beat. The pressure increase and ejection of blood will cause our arteries to dilate, expanding their diameter (eccentric contraction). Once the threat passes, our heart will relax and our arteries will return to their normal state (pulsation).

Changes in systemic arterial pressure are regulated by the baroreceptor reflex. Blood pressure is monitored by baroreceptors, highly sensitive stretch receptors located in our aortic arch, carotid sinus (below the tongue), and vena cava (right at the top of your abdomen under your breast bone). Baroreceptors respond to changes in blood pressure by sending signals to our heart to decrease or increase its rate and force of contraction, thus modulating systemic arterial pressure. In general, if blood pressure increases, signaling from our baroreceptors causes altered cardiac output. This causes an increased rate and force of ventricular contractions. Blood is ejected more forcefully from the heart through our arterial system. This increased blood flow is accompanied by an increase in systemic arterial pressure.

Systemic arterial pressure can be altered through pharmacological or surgical means. In the former case, diuretics or vasodilators will raise peripheral vascular resistance and lower capillary pressure, which decreases the osmotic gradient of plasma proteins into the capillary bed. This brings about a decrease in venous return, which decreases pulmonary hypertension. Other drugs that act upon vasodilation include nitrates, beta blockers, and calcium channel blockers. Vasodilators are commonly used to treat Raynaud’s syndrome, when peripheral arterial pressure is abnormally high due to spasm of smooth muscle cells in the arterioles. In the latter case, surgery can be used to sever major blood vessels in order to intentionally do harm. This can be done by severing the ascending aorta in order to cause death.

The two main types of shock are hypovolemic and cardiogenic shock. Cardiogenic shock is caused by a disorder of the heart muscle (myocardial infarction), whereas hypovolemic shock is caused by a decrease in total blood volume without pathology of the heart (hemorrhages, dehydration). Both types of shock affect systemic arterial pressure due to their effect on left ventricular preload/afterload.

When there is a drop in total blood volume, caused mainly by hemorrhage, peripheral vascular resistance increases and systemic arterial pressure decreases. Both of these factors contribute to low cardiac output, or reduced stroke volume. If we are in severe shock, the myocardium may also be damaged or destroyed by a lack of oxygen and nutrients.

In the first stage of shock, which can last from 30 seconds to an hour (a syndrome known as shock from any cause), the heart contracts at a faster rate than normal. The heart increases force generating capacity and ejects blood more forcefully into the arterial system with every beat. This causes our blood pressure to increase because the heart is working harder. When this happens, our arteries dilate and both venous return and cardiac output decrease. Therefore, our blood pressure decreases as these two factors contribute to low cardiac output.

If a patient is suffering from severe shock, their heart rate may decrease to a very irregular rate that is slower than 30 beats per minute (bradycardia). If the heart stops beating altogether (heart failure), recovery can take up to several days. In most cases, if the patient survives this first stage of shock, he or she will eventually stabilize and survive.

Rapid breathing will likely lead to a decrease in blood oxygen. If too much carbon dioxide is produced, acidosis will develop. If the blood volume is decreased and if the heart can’t pump enough blood to meet the body’s needs, hypovolemic shock occurs. This type of shock can come on very suddenly and may result in death if not treated appropriately within a few minutes of its onset. Hypovolemic shock has no known cause and usually follows an acute hemorrhage (blood loss).

The two main causes of hypovolemic shock are bleeding (internal or external) and dehydration such as long term diarrhea or other causes like vomiting in patients taking certain medications. Hypovolemic shock can also be caused by massive burns or the loss of blood due to intestinal perforation.

When the blood volume of a person is reduced, peripheral vascular resistance increases and systemic arterial pressure decreases. This causes low cardiac output, or reduced stroke volume. When an individual’s cardiac output is lowered, it can cause a heart attack as well as kidney failure and shock.

In the case of hypovolemic shock, the heart rate is usually high but irregular (sinus tachycardia). The most common sign of hypovolemic shock is a weak and irregular pulse that is extremely rapid in some cases (pulsus alternans). The patient’s pallor may also be noticeable in and around the lips and nail beds. The patient may have a weak, thready, or ultimately absent femoral pulse.

The most effective treatment for hypovolemic shock is to replace lost fluids with intravenous saline. If a person experiences this kind of shock, the best thing we can do is to get them to the hospital as quickly as possible. Once there, doctors can give these patients medications, such as vasopressors and fluids via an IV drip to increase pressure. A person with hypovolemic shock can receive more intravenous fluids if they are in shock.

In the case of cardiogenic shock, it is caused by a problem within the heart muscle (myocardial infarction). This kind of shock occurs when there is an inadequate supply of oxygen and nutrients to the heart. If a patient has this type of problem, the heart will not be able to create enough force to pump blood adequately.

If blood is not able to get into the myocardium after 2 or 3 minutes, there will be severe pain that begins in the abdomen and moves up through the chest. It may spread over both upper arms and jaw muscles as well as into one leg. The person’s skin may feel cold and the blood pressure may drop rapidly. If a patient is suffering from cardiogenic shock, their heart rate will often be irregular or slower than 80 beats per minute (bradycardia). The patient will have a low level of consciousness and may have difficulty breathing due to lung congestion.

Cardiogenic shock can be caused by myocardial infarction, ruptured aneurysms (especially if in the coronary artery), or cardiomyopathy (heart muscle disease). This condition may require surgery in an attempt to correct the problem. The first step is to restore blood flow through clot removal, angioplasty with stents, or coronary bypass surgery.

The two main goals of treatment for cardiogenic shock are to stabilize and then to control the patient’s blood pressure. Drugs may be used to raise the blood pressure and decrease the heart rate, but these medications can have many negative side effects. An important goal of management is to keep the person alive until he or she can be operated on either medically or surgically. The patient will also need immediate surgery because his or her quality of life and longevity depend on it.

Shock refers to a state in which oxygen demand exceeds oxygen supply (hypoxia). This condition is characterized by low blood pressure that causes poor tissue perfusion (ischemia). When this happens, hypoxia can lead to changes in the heart’s rhythm. The other systems of the body are also affected since they all depend on oxygen to survive. For example, if there is not enough oxygen available to the muscles and other tissues, cardiac output and blood pressure decreases.

When a patient experiences a lack of blood flow (ischemia), this can lead to dysfunction in many organs or tissues as well as death in extreme cases. With hypoxia, our cells require more energy because there is not enough oxygen for normal functioning. If there is not enough oxygen present within the tissues, this will lead to anaerobic metabolism, which produces lactic acid (lactic acidosis).

When a person suffers from shock, it’s essential to identify the underlying cause. In most cases, shock would be associated with some sort of severe trauma such as loss of blood due to internal hemorrhage or external bleeding. One common example of this type of trauma would be internal bleeding in patients who have experienced severe abdominal trauma. The other most common causes are sepsis and anaphylaxis (allergic reaction), although these types of shock are rarer than hypovolemic or cardiogenic shock.

In some cases, shock may be a complication of sepsis (infection) due to an underlying medical condition like chronic heart failure or kidney disease. If a patient does not respond to medical treatment for their underlying illness, sepsis may develop. Sepsis can also cause shock in patients who are already suffering from sinus bradycardia (heart rate less than 80 beats per minute or a very slow heart rate).

The most important thing that anyone can do when faced with the prospect of shock is to seek immediate medical attention. Shock is critical in the sense that it’s life-threatening if left untreated. The best way to respond to this problem is by contacting emergency services and receiving medical help quickly.



 

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