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Introduction to Lesson 3:

Blood Pressure and Fluid Exchange

Uncontrolled high blood pressure can lead to stroke, heart attack, heart failure or kidney failure. However, because there are no symptoms, nearly one-third of people with high blood pressure don't even know they have it. This is why high blood pressure is often called the "silent killer". The only way to tell if you have high blood pressure is to have your blood pressure checked. Do you know your blood pressure?

Lesson 3 Outcomes

By the end of this lesson, you should be able to:

5.3.1 Identify systolic and diastolic blood pressure using a sphygmomanometer.

5.3.2 List and describe extrinsic factors (e.g., exercise, caffeine, nicotine) which affect transient blood pressure.

5.3.3 Differentiate between vasodilation and vasoconstriction.

5.3.4 Describe the control of blood pressure by the autonomic nervous system.

5.3.5 List and describe factors which affect arteriolar resistance.

5.3.6 Explain how changes in blood pressure help to maintain homeostasis in the body.

5.3.7 Explain how blood pressure and osmotic pressure contribute to fluid exchange at the capillary level.

5.3.8 Describe the term hypertension and discuss its causes, effects and treatment.

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Lesson 3 Overview

Following is a list of topics covered in this lesson.

Blood pressure Measuring Blood pressure Hypertension Exchanges between Blood and Cells Regulation of Blood Pressure

Blood Pressure

Blood pressure is defined as ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ The highest pressure occurs in the aorta, the large vessel that carries oxygenated blood away from the heart. As the blood passes into smaller vessels and the distance from the heart becomes greater, the pressure becomes reduced.

The pressure in any artery varies as a result of two major factors.

1. Cardiac Output

o Volume of blood.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

o Heart rate.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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2. Arteriolar Resistance

o Size.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

o Elasticity.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

o Measuring Blood Pressure

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Two different pressures are measured and compared in a blood pressure reading.

Systolic pressure

____________________________________________________________________________________________________________________________________________

Diastolic pressure

____________________________________________________________________________________________________________________________________________

The pressure of the blood pressing against the arterial walls can be measured using a device called a sphygmomanometer. To measure blood pressure, an inflatable rubber cuff is wrapped around the upper arm. As air is pumped into the cuff, the cuff presses on the arteries of the arm. When the pressure in the cuff is high enough, the blood flow through the arteries ceases.

A stethoscope is placed over one of the arteries in the elbow, and the air in the cuff is gradually released. At first, the person listening through the stethoscope hears no sound. Then, a sharp tapping sound is heard. This sound is made by the blood spurting

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through a narrow opening in the compressed artery. The pressure reading just as this sound is heard is the systolic pressure. As more air is released from the cuff, the sound becomes muffled and then stops as the cuff ceases to press on the artery. The pressure reading just as the sound stops is the diastolic pressure.

Blood pressure is measured in millimeters of mercury (mm Hg). It is expressed as a ratio of systolic pressure to diastolic pressure. A reading of 120/70 means that the person's systolic pressure is 120 mm Hg and the diastolic pressure is 70 mm Hg. It is expressed verbally as "120 over 70."

Normal blood pressure is less than 130 mm Hg systolic and less than 85 mm Hg diastolic. Optimal blood pressure is less than 120 mm Hg systolic and less than 80 mm Hg diastolic. A typical reading for a healthy adult is 120/70. Readings for children and adolescents may be slightly higher.

A physician can infer much about a person's health by taking a blood pressure reading.

Hypertension

High blood pressure or hypertension is defined in an adult as a blood pressure greater than or equal to 140 mm Hg systolic pressure or greater than or equal to 90 mm Hg diastolic pressure. High blood pressure directly increases the risk of coronary heart disease (which can lead to heart attack) and stroke, especially along with other risk factors.

High blood pressure can occur in children or adults, but it's more common among people over age 35. It's particularly prevalent in middle-aged and elderly people, obese people, heavy drinkers and women who are taking birth control pills. It may run in families, but many people with a strong family history of high blood pressure never have it. People with diabetes mellitus, gout or kidney disease are more likely to have hypertension.

Recall:

Systolic Pressure:

_____________________________________________________________________

Diastolic Pressure:

_____________________________________________________________________

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Which is more dangerous to be higher than normal, systolic or diastolic pressure? ______________Why?_________________________________________________________________________________________________________________________

Blood pressure is normally controlled by nerves that have their center in the brain. If the blood pressure in certain vessels increases, the brain sends nerve impulses to the heart and to the blood vessels, causing the heart rate to slow and the blood vessels to widen. As a result the blood pressure decreases. If the blood pressure becomes too low, the brain sends impulses that cause the heart rate to increase and the blood vessels to narrow, increasing the blood pressure. This is another case of homeostasis—maintaining a constant internal environment. If this regulatory mechanism cannot bring the blood pressure to normal levels, a condition known as hypertension is evident and medical assistance is required.

The factors causing hypertension are not well understood. In 90% of the cases of hypertension, the cause is unknown.

What do you think can cause hypertension?

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Risk Factors You Can Control

Smoking Physical Inactivity Obesity Diet (Salt Intake) Diabetes Stress

Risk Factors You Can’t Control

Age Ethnicity (South Asians, First Nations/Aboriginal Peoples or Inuit and Blacks are

at increased risk) Family history

The goal of treatment is to reduce the diastolic blood pressure to less than 90 mm Hg. Treatment consists of a combination of no-added-salt diet, weight loss if the person is over-weight, and drug medication. The excess fluid that sodium (salt) holds in the body may also put an added burden on the heart and "waterlog" the blood vessels, causing them to contract or narrow more easily. The blood vessels then take less diluted blood to the organs of the body than the quantity of normal blood that is required, depriving

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the cells of some oxygen and nutrients that they need. For this reason, low-sodium diets are used in treating mild to moderately severe hypertension. However, in individuals with severe hypertension, salt restriction must be severe.

When the demand for blood in various parts of the body is high (e.g. during exercise), the heart must pump faster, increasing the blood pressure in the vessels. Fatty tissue requires a lot of blood to feed it. Therefore, another way to reduce blood pressure and the stress on the heart is to lose weight. In addition to possibly lowering blood pressure and reducing weight, a low-fat, low-cholesterol diet may also help delay the beginning of arteriosclerosis. Medications called diuretics, which help rid the body of excess salt and therefore, of excess water, are often prescribed by the doctor in the treatment of hypertension.

Blood Pressure in Capillaries and in Veins

Blood pressure in the Capillaries

The pressure of arterial blood is significantly reduced when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases.

Blood pressure in the veins

When blood leaves the capillaries and enters the venules and veins, little pressure remains to force it along. Blood in the veins below the heart is helped back up to the heart by the muscle pump. This is simply the squeezing effect of contracting muscles on the veins running through them. One-way flow to the heart is achieved by valves within the veins.

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Exchanges between Blood and

Cells

Our blood does not come into direct contact with the cells it nourishes. However, the distribution of capillaries is so extensive that cells are never farther away than 1 cell layer from a capillary.

When blood enters the arteriole end of a capillary, it is still under pressure produced by the contraction of the ventricle. As a result of this pressure, a substantial amount of water and some plasma proteins filter through the walls of the capillaries into the tissue space.

This fluid, called interstitial fluid, is blood plasma minus most of the proteins. Interstitial fluid bathes the cells in the tissue space. Substances in the fluid can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid.

Near the venous end of a capillary, the blood pressure is greatly reduced. Here, another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and a greater concentration of water. This difference sets up an osmotic pressure. Although the osmotic pressure is small, it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid re-enters the capillary at the venous end.

Regulation of Blood Pressure

An adult human has been estimated to have some 100 000 km of capillaries with a total surface area of some 800 -1000 m2(an area greater than three tennis courts). The total volume of this system is roughly 5 liters, the same as the total volume of blood. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. Therefore, a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. For example, during vigorous exercise, capillary beds in the skeletal muscles open at the expense of those in the abdomen. The reverse occurs after a heavy meal.

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The table below shows the distribution of blood in the human body at rest and during vigorous exercise. Note the increase in blood supply to the working organs (skeletal muscles and heart). The increased blood supply to the skin aids in the dissipation of the heat produced by the muscles. Note also that the blood supply to the brain remains constant. The total blood flow during exercise increases because of a more rapid heartbeat and also a greater volume of blood pumped at each beat.

Blood Flow mL/min

At Rest During

Strenuous Exercise

Heart 250 750

Kidneys 1,200 600

Skeletal Muscles

1,000 12,5000

Skin 400 1,900

Viscera 1,400 600

Brain 750 750

Other 600 400

Total 5,600 17,500

The walls of arterioles are encased in smooth muscle. Vasoconstriction (reduction in diameter of blood vessels) of arterioles decreases blood flow into the capillary beds they supply while vasodilation (increase in diameter of blood vessels) has the opposite effect. In time of danger or other stress, the arterioles supplying the skeletal muscles will be dilated while those supplying the digestive organs will decrease. These actions are controlled by the autonomic nervous system (ANS).

The autonomic nervous system consists of sensory neurons and motor neurons that run between the central nervous system (especially the hypothalamus and medulla oblongata) and various internal organs such as the heart, lungs, viscera, and glands (both exocrine and endocrine).

The ANS is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. For example, the contraction of both smooth muscle and cardiac muscle is controlled by motor neurons of the autonomic system. The ANS has two subsystems; the Sympathetic Nervous System which is involved in the fight or flight response and the Parasympathetic Nervous System which is involved in relaxation. Each of these subsystems operates in the

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reverse of the other (antagonism). Both systems affect the same organs and act in opposition to maintain homeostasis. For example: when you are scared, the sympathetic system causes your heart to beat faster; the parasympathetic system reverses this effect.

Baroreceptors

____________________________________________________________________________________________________________________________________________ When blood pressure exceeds acceptable levels, the receptors send nerve impulse messages to the medulla oblongata which causes the sympathetic (flight or fight) nerve impulses to decrease. This results in arteriole dilation and increased outflow of blood from the artery. The parasympathetic (relaxation) nerve impulses are increased, causing heart rate and stroke volume to decrease. The decreased cardiac output slows the movement of blood into the arteries, lowering blood pressure.

Low blood pressure is also adjusted by the sympathetic nerve. Without nerve information from the pressure receptors of the carotid artery and aorta, the sympathetic nerve will continue to be stimulated, causing cardiac output to increase and arterioles to constrict. The increased flow of blood into the artery, accompanied by a decreased outflow raises blood pressure to acceptable levels.

Control of Blood Pressure

Regulation of Blood Pressure by Hormones

The role of the kidney is __________________________________________________________________________________________________________________________________________________________________________________________________________________

Local Control of Blood Pressure in the Capillary Beds

Cells where infection or other damage is occurring release substances like histamine that dilate the arterioles and increase blood flow in the area.

Nitric oxide (NO) is also a potent dilator of blood vessels. When the endothelial cells that line blood vessels are stimulated, they synthesize nitric oxide. It quickly diffuses into the muscular walls of the vessels causing them to relax.

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Nitroglycerine is often prescribed to reduce the pain of angina (heart pain). It does so by generating nitric oxide, which relaxes the walls of the arteries and arterioles. The prescription drug sildenafil citrate ("Viagra ") does the same for vessels supplying blood to the penis. The effects of these two drugs are additive and using them together could precipitate a dangerous drop in blood pressure.

Shock

Trauma to the body or severe bleeding may cause shock which may result in capillary beds opening without others closing in compensation. Although the volume of blood remains unchanged, blood pressure declines abruptly as blood pools in the capillary beds. The heart can only pump as much blood as it receives. If insufficient blood gets back to the heart, its output - and hence blood pressure - drops. The tissues fail to receive enough oxygen. This is especially critical for the brain and the heart itself. If untreated, shock is usually fatal.

To cope with the problem, arterioles constrict and shut down the capillary beds - except those in the brain and heart. This reduces the volume of the system and helps maintain normal blood pressure. Air-breathing vertebrates that spend long periods under water (e.g., seals, penguins, turtles, and alligators) employ a similar mechanism to ensure that the oxygen supply of the heart and brain is not seriously diminished. When the animal dives, the blood supply to the rest of the body is sharply reduced so that what oxygen remains will be available for those organs needing it most: the brain and heart.

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Lesson 3 Exercise

1. a) What is blood pressure?

b) List and describe two major factors that affect blood pressure.

2. a) Differentiate between systolic and diastolic blood pressure.

b) How is blood pressure measured?

c) What are "normal" blood pressure measurements?

d) What effect does exercise have on blood pressure?

3. a) Define hypertension.

b) What are some of the possible side effects of hypertension?

c) Discuss the causes, risk factors and treatment of hypertension.

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4. a) Explain how blood pressure contributes to fluid exchange in the tissues

b) What other factor contributes to fluid exchange at the tissue level.

5. Differentiate between vasoconstriction and vasodilation.

6. How is blood flow to the muscles, kidneys and brain affected by vigorous exercise?

7. Describe how the Autonomic Nervous System contributes to homeostasis by controlling blood pressure. Include the role of blood pressure receptors in your answer.

8. Describe the role of the kidney in controlling blood pressure.

9. Why is nitroglycerin used as a treatment for angina?

10. Why is shock sometimes fatal if untreated?

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Lesson 3 Summary

In this lesson, you have learned about blood pressure, how it is measured, and some problems associated with high blood pressure. You have also studied how blood pressure helps to maintain homeostasis and fluid exchange at the cell level. In the next lesson, you will study the Lymphatic System.