THE RESPIRATORY SYSTEM:The Heart and Blood Vessels, Blood Pressure

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Health Psychology PSY408

Lecture 10

THE RESPIRATORY SYSTEM

Breathing supplies the body with oxygen--but why do we need oxygen? The chemical reactions in

metabolism require oxygen, some of which joins with carbon atoms from food to form carbon dioxide

(C02) as a waste product. So breathing has another function-- it lets us get rid of this waste product. We

will begin our examination of the respiratory system by looking at its structures.

The Respiratory Tract

After air enters the body through the nose or mouth, it travels past the larynx, down the trachea and

bronchial tubes, and into the lungs. These organs are depicted on your TV screen. The bronchial tubes

divide into smaller and smaller branches called bronchioles inside the lungs. These branches finally end in

millions of tiny air sacs called alveoli. Each alveolus looks like a minute bubble made of a membrane that is

thin enough to allow oxygen, C02, and other gases to pass through. Alveoli are enmeshed in beds of

capillaries so that gases can be transferred to and from the bloodstream quickly and efficiently.

When we breathe, what makes the air go in and out? When we inhale, the rib muscles draw the ribs up and

outward and the diaphragm--a horizontal sheet of muscle below the lungs, contracts, pulling downward on

the bottom of the lungs. These actions pull air in and enlarge the lung chambers. When we exhale, these

muscles relax, and the elasticity of the lungs forces the air out, like a balloon.

Respiratory Function and Disorders

How do the muscles "know" when it's time to inhale and exhale? Our blood vessels contain sensors that

monitor blood gases and send this information to the medulla of the brain, which directs actions of the

muscles to cause us to inhale and exhale. When the CO2 level is high, the medulla increases the breathing

rate; when the level is low, breathing rate is decreased.

Foreign matter, such as airborne particles and microorganisms, can readily enter the respiratory tract. The

respiratory system therefore needs protective mechanisms to prevent foreign matter from reaching the

lungs and entering the bloodstream. Two protective mechanisms are reflexes: (1) sneezing in response to

irritation in nasal passages and (2) coughing in response to irritation in lower portions of the system.

Another protective mechanism is the mucociliary escalator. How does this mechanism work? Most of the

lining of the respiratory system is coated with sticky mucus that traps foreign matter. Furthermore, the air

passages leading from the mouth to the lungs are lined with tiny hair-like structures called cilia that move in

such a way as to force the mucus coating up toward the mouth, hence the name "mucociliary escalator."

When the mucus reaches the back of the mouth, it is usually swallowed. In this way, the respiratory system

cleanses itself and protects the body from harmful matter that we inhale.

The opening story of one of our earlier lectures was about a young man named Tom who was a victim of

cystic fibrosis, a fatal disease of the respiratory system. We will look at several of the many other disorders

that attack this system. Some of these disorders mainly affect the alveoli of the lungs, thereby impairing the

normal exchange of CO2 and oxygen.

For instance, there are several types of pneumonia, which can be caused by either bacterial or viral

infection (AMA, 1989). Although this disease often affects the bronchial tubes, the most serious types of

pneumonia cause the alveoli to become inflamed and filled with fluid.

In another respiratory disease called emphysema the walls between alveoli are destroyed. This decreases

the lungs' surface area for exchanging gases and their elasticity for exhaling CO2.

Pneumoconiosis is a disease that afflicts people who chronically inhale air containing high concentrations

of dust--generally at their workplaces. The black lung disease of coal miners provides an example. Dust

Health Psychology PSY408

that is not removed by protective mechanisms accumulates as thick sheets around the alveoli and

bronchioles, damaging these structures and blocking air exchange.

Other disorders of the respiratory system primarily affect the bronchial tubes, usually by narrowing the

tubes and reducing airflow.

Asthma is a disorder in which the bronchial airways narrow, because they become inflamed, develop

spasms, and secrete too much mucus. Attacks usually are temporary and occur in response to an irritant,

such as an infection or something to which the victim is allergic. Breathing becomes difficult and, in very

serious attacks, portions of the lungs may collapse temporarily.

In chronic bronchitis, inflammation and excess mucus occur in the bronchial tubes for an extended

period. This condition may be permanent or occur several times a year, lasting 2 weeks or more each

episode (Haas & Hass, 1990).

Lung cancer involves an unrestrained growth of cells that crowd out cells that aid respiration. This process

usually begins in the bronchial tubes and spreads to the lungs. In its final stages, the diseased cells enter the

bloodstream through the capillaries and spread throughout the body. At this point death is almost always

near. Many of the respiratory diseases we have discussed can be caused or worsened by smoking cigarettes.

This risk factor is also important in diseases of the cardiovascular system.

The Cardiovascular System

The physical design of every complex organism has to deal with a basic problem: How can the body service

its cells--supplying the substances they need to function properly and removing the wastes that metabolism

produces? In humans and many other animals, this problem is solved by having a cardiovascular system to

transport these materials. The blood circulates through blood vessels--capillaries, arteries, and veins--

within a closed system, one in which the blood does not directly contact the cells and tissues it services. All

transfers of oxygen, nutrients, waste products, and other substances occur through membranes that are

separated by fluid-filled spaces. The heart is the center of the cardiovascular system.

The Heart and Blood Vessels

The heart is a fist-sized pump made of muscle that circulates the blood throughout the body. It "beats," or

pumps, about 100,000 times a day (AHA, 1994). The muscular portion of the heart wall is called the

myocardium. The interior of the heart has four chambers, as the drawing on your TV screen illustrates.

The two upper chambers are called atriums, and the two lower ones are called ventricles; the left and right

sides are labeled from the body's perspective, not from ours. Looking at the drawing, we see several blood

vessels that connect to the heart. How are arteries and veins different? Arteries carry blood from the heart,

and veins carry blood to it. You will also notice in the drawing that the shading of some blood vessels is

light, and in others the shading is dark. The vessels with light shading carry blood that is laden with CO2

toward the lungs; the dark vessels carry blood away from the lungs after it has expelled CO2 and received

oxygen.

Now, let's follow the route of blood through the body. The blood that enters the right atrium of the heart is

laden with waste products, such as C02, from our cells and is deficient in oxygen, which makes the blood

bluish in color. After the atrium is filled, the blood passes through a valve to the right ventricle. The

ventricles provide the main pumping force for circulation as the heart muscle contracts, and their valves

prevent the blood from going back up to the atriums. From the right ventricle, the blood enters pulmonary

circulation to the lungs, where it becomes oxygenated and, consequently, red in color. The oxygenated

blood travels to the left atrium of the heart and is passed to the left ventricle, which pumps it out through

the aorta into systemic circulation. It then goes to various parts of the body before returning to the heart

and beginning the cycle again. The complete cycle takes about 1 minute in the resting person.

Health Psychology PSY408

Portions of each quantity of blood pumped by the heart travel through the liver and kidneys, where

important functions take place. The kidneys receive blood from the general circulatory system, cleanse it of

waste products, and pass these wastes on to be eliminated in the urine. The liver receives blood from two

sources: most of the blood comes from the intestinal tract, and the remainder comes from systemic

circulation. What does the liver do to the blood? First, it cleanses the blood of harmful debris, such as

bacteria. In fact, it is "so effective in removing bacteria that probably not one in a thousand escapes through

the liver into the general circulation" (Guyton, 1985, p. 467).

Second, the liver removes nutrients and stores them. The blood that comes from the intestinal tract after we

consume a meal is rich in nutrients, such as simple sugars and amino acids. Large portions of these nutrients

are retained in the liver until the body needs them. In this way, the ebbs and flows of nutrients in the blood

are kept relatively even over time.

Blood Pressure

Imagine you are holding a long balloon that is filled with air. Its end is tied off. If you squeeze it in the

middle, the rest of it expands. This is what happens when pressure is applied to a closed system. The

cardiovascular system is also closed, and the myocardium does the squeezing when it pumps blood from the

heart. Like the balloon, the cardiovascular system always has some pressure in it. The squeezing increases

the pressure.

Our arteries are elastic--they expand when pressure is applied. Blood pressure is the force exerted by blood

on the artery walls. The heart is at rest between myocardial contractions, while it fills with blood. The

resting force in the arteries that occurs at this time is called diastolic pressure. When the heart pumps, each

contraction produces a maximum force in the arteries, which is called systolic pressure. A person's blood

pressure is expressed with two numbers: a larger number, representing systolic pressure, followed by a

smaller number, representing diastolic pressure. Your physician might tell you that your blood pressure is

"120 over 80," for example.

Blood pressure varies. It changes from one moment to the next, it is higher in one part of the body than in

another, and different people have different blood pressures. What determines blood pressure? We can

answer this question in two ways--one involves the laws of fluid dynamics and the other involves factors in

people's lives that affect these dynamics. We will start with the first approach and examine five aspects of

fluid dynamics that affect blood pressure.

1. Cardiac output is the volume of fluid being pumped per minute. Blood pressure increases as cardiac

output rises.

2. Blood volume refers to the total amount of blood circulating in the system. The greater the volume, the

higher the blood pressure needed to move it.

3. Peripheral resistance refers to the difficulty fluid encounters in passing through narrow tubes or

openings. When you put a nozzle on a hose and turn on the water, the pressure is greater at the nozzle than

in the hose. Arteries vary in diameter. Arterioles are small arteries that connect larger arteries to capillaries.

Peripheral resistance is generally greater in arterioles than in larger arteries. Normally arterioles are highly

elastic and can expand or contract readily in response to messages from the nervous and endocrine systems.

After we eat a meal, extra blood is needed around the small intestine for the absorption of nutrients.

Messages to the arterioles in that region cause them to expand and accept more blood.

4. Elasticity, as we have seen, describes the ease in expanding and contracting. When blood vessels become

less elastic, blood pressure--especially systolic pressure--rises.

5. Viscosity refers to the thickness of the fluid. The viscosity of blood depends on its composition, such as

whether it contains high levels of red blood cells. Thicker blood flows less easily than thinner blood-and

requires more blood pressure for it to circulate through the cardiovascular system.

Health Psychology PSY408

What factors in people's lives affect these dynamics? In our everyday lives we experience a variety of states

that affect blood pressure. The temperature of our environment defines one of these states. When the

temperature is high, the blood vessels in our skin enlarge and our cardiac output and diastolic pressure fall,

which makes us feel drowsy. Low temperatures have the opposite effect. Another factor is activity. For

example, exercise increases blood pressure during and after the activity. Simply changing posture can also

affect blood pressure. When we go from a lying position to standing, blood flow in the veins that feed the

heart, lows down because of gravity. This causes a drop in cardiac output and blood pressure. As a result,

blood flow to the brain drops, sometimes making us feel dizzy. A third factor is emotional experience.

When we experience stress, anger, or anxiety, the sympathetic nervous system is activated. This causes a

variety of cardiovascular reactions, such as increased cardiac output. Both systolic and diastolic pressures

increase when people are emotionally aroused.

High blood pressure strains the heart and arteries. Some people have high blood pressure consistently over

a period of several weeks or more. This condition is called hypertension. How high is "high" blood

pressure? People whose pressure is at or above 140 (systolic) over 90 (diastolic) are classified as

hypertensive (AHA, 2000). When systolic pressure reaches 200, the danger is high that a rupture may occur

in a blood vessel, particularly in the brain. This is one way by which strokes occur. High diastolic pressure is

troubling because the arteries are constantly being strained, even between heartbeats, when they should

encounter little pressure.

As adults get older, they tend to get heavier, at least in industrialized countries. In a number of primitive

societies where adults do not show an increase in body weight as they get older, blood pressure does not

seem to increase with age.

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