(CONTINUED FROM 21/11/09)
THE CARDIOVASCULAR SYSTEM
HOW THE HEART AND BLOOD VESSELS WORK TOGETHER
Before we begin to examine how the heart and blood vessels work together in space, it is important to understand how the heart and the blood vessels normally function on Earth to carry blood and nutrients throughout the body. We will review some of the basic principles of blood pressure and blood flow regulation and control so that you can understand some of the expected differences in the operation of the human body in the environment of space. Let’s begin with a description of how the heart and blood vessels work together to deliver the fuel needed by all of the cells throughout the body.
Figure 1. Distribution of fluids in the body.
More than one-half of the human body is composed of fluid. Much of this fluid lies inside the body’s cells (intracellular fluid), but an important part of it lies in space outside the cells and in the blood stream (extracellular fluid). This extracellular fluid (which includes part of the blood) is in constant motion and provides what is known as the "internal environment" of the body. Approximately 80% of extracellular fluid surrounds the cells of the body. This is called interstitial fluid. The remaining 20% of the extracellular fluid is the plasma portion of the blood.
Humans have a well-defined circulatory system to transport the extracellular fluid to all parts of the body. The cardiovascular system includes the heart, the blood, and the blood vessels, and is that portion of the circulatory system that moves the blood around and about the body. (The complete circulatory system includes other elements, but we will limit our discussion to the cardiovascular portion.) As the blood moves around the body (all of the blood in the body circulates once each minute at rest), oxygen and other nutrients are carried to the cells of the body and carbon dioxide and other wastes are carried away from the cells.
The heart is the main pump that forces the blood through the blood vessels of the body. (Here is a movie of a dog heart in action.) The blood vessels, in turn, form a closed system of "highways" that transport blood and allow exchange of gases, nutrients, and wastes between the blood and the body cells. An estimated 62,000 miles of blood vessels throughout the body of an adult ensure that a continuous supply of nutrients and oxygen reaches each of the trillions of living cells and that the waste products are carried away.
BLOOD AND ITS CONSTITUENTS
Blood, the fluid that moves around throughout the blood vessels, is a complex fluid tissue consisting of cells (and cell fragments) and plasma, a straw-colored liquid that is part of the extracellular fluid of the body. Plasma, which makes up about 55 per cent of the blood, is mostly water with proteins, minerals and other substances dissolved in it. Some of the plasma proteins play important roles in keeping the body’s internal environment stable through such activities as blood clotting and maintenance of blood osmotic pressure. (Maintenance of a stable internal environment is calledhomeostasis.) Other plasma proteins are essential parts of the immune system which protects the body against invasion by organisms or foreign particles. The other substances in plasma include absorbed food molecules (such as simple sugars, amino acids, and fatty acids), as well as respiratory gases (oxygen and carbon dioxide), waste products, and regulatory substances (hormones and enzymes).
The remaining 45 per cent of the blood are the red blood cells (RBCs), the white blood cells (WBCs), and cell fragments called platelets (Table 1). More than 99 per cent of the cells of the blood are red blood cells. Red blood cells are the most abundant of all the cells of the body. They are formed (in the bone marrow) at a rate of 480,000,000 cells each minute. Each cell lives an average of 120 days before being destroyed. The major function of red blood cells is to carry oxygen from the lungs to the tissues of the body. As a red cell matures, it loses its nucleus and other cell structures and becomes filled with hemoglobin, an iron-containing pigment that combines readily with oxygen. In normal, healthy people, 100 ml. of whole blood contains 14-16 grams of hemoglobin, and each gram of hemoglobin can transport 1.39 ml. of oxygen. Thus, each 100 ml. of blood can carry 19-21 ml. of oxygen in combination with hemoglobin, which is more than 30 times the amount of oxygen that could be carried by the blood if hemoglobin were not present.
White blood cells play a major role in protecting the body against invading pathogens (bacteria, viruses, fungi, and parasites) which could produce disease. White blood cells are the mobile elements of the body’s defense system. Although there are several kinds of white cells, each different in size and function, all contain a nucleus. White blood cells can move like an amoeba, slipping through thin walls of capillaries and wandering among cells and tissues. Some white cells engulf bacteria or other pathogens, as an amoeba does, and then ingest them. Others synthesize antibodies, complex proteins that react with and destroy pathogens and other foreign substances.
Platelets are another constituent of blood. Platelets are small cell fragments which play an important part in blood clotting. Clotting begins when plasma and platelets come in contact with a rough surface, such as torn tissue. The body has its own internal "intelligence": it recognizes a wrong situation and goes into action. The platelets become sticky and attract more platelets, forming a plug that practically seals the wound. They also release substances that act with clotting factors in plasma to begin a chain of chemical reactions leading to the formation of the clot.
Element Diameter Number Scientific Main (in micrometers) (per mm cubed) Notation Function (per mm cubed) -------------------------------------------------------------------- red blood 7-8 4,500,000- 4.5 E6 oxygen cells 5,500,000 5.5 E6 transport white blood 9-12 7,000- 7 E3 defense cells 10,000 1 E4 against microorganisms platelets 2-4 300,000 3 E5 blood-clotting
THE HEART AND THE CIRCULATION
Figure 2. The structure of the heart and course of blood through the heart chambers.
The primary function of the heart is to pump blood through blood vessels to the body’s cells. Imagine a simple machine like a water pump working for perhaps 70 or more years without attention and without stopping. Impossible? Yet this is exactly what the heart can do in our bodies. The heart is really a muscular bag surrounding four compartments, with a thin wall of muscle separating the left- hand side from the right-hand side. The muscles in the heart are very strong because they have to work harder than any of the other muscles in our body, pushing the blood to our head and feet continuously.
Figure 3. The pulmonary circulation system.
The heart connects the two major portions of the circulation’s continuous circuit, the systemic circulation and the pulmonary circulation. The pulmonary circulation contains blood that flows through the lungs to pick up oxygen and get rid of carbon dioxide, while the systemic circulation contains all of the other blood flow pathways in the body. In each of these circulation subsystems, the arteries transport blood under high pressure to the tissues. The arterioles are the last small branch of the arterial system and they act as control valves through which blood is released into the capillaries. The capillaries are very small, thin-walled blood vessels that are capable of supporting exchange of gases, nutrients, and waste between cells and blood. Blood flows with almost no resistance in the larger blood vessels, but in the arterioles and capillaries considerable resistance to flow does occur. The venules collect blood from the capillaries and gradually coalesce into progressively larger veins. The veins transport the blood from the tissues back to the heart at low pressure. The walls of the veins are thin and pliable and can fold or expand to act as a reservoir for extra blood, if required by the needs of the body.
The heart actually has two separate sides, one designed to pump blood into the pulmonary circulation, and one designed to pump blood into the systemic circulation (Figure 3). Each side of the heart has two chambers or compartments. The first chamber on each side is called the atrium. The atrium forms the upper part of the heart and receives incoming blood from the tissues of the body. The thin-walled atrium bulges with this blood, and as the lower heart muscle relaxes, the blood flows into a second chamber, the thick muscular ventricle. The atrium and ventricle on each side are separated by tissue flaps called valves. The structure of these valves prevents blood from flowing backwards into the atrium when the ventricle contracts. The valve on the right side, between the atrium and the ventricle, is called the tricuspid valve. The valve on the left side, between the atrium and the ventricle, is called the bicuspid or mitral valve. There are two other important valves that help to keep the blood flowing in the proper direction. These two valves are located at the two points where blood exits the heart. The pulmonary valve is located between the right ventricle and the pulmonary artery that carries the deoxygenated blood from the heart to the lungs, and the aortic valve is located between the left ventricle and the aorta, the major artery that carries the oxygenated blood from the heart to the rest of the body.
Let us follow a single red blood cell (RBC) through one full cycle along the circulatory pathway. Remember that RBCs carry oxygen throughout the body.
- An RBC that has lost some of its oxygen in one of the body’s tissue beds (and is, therefore, said to be deoxygenated) returns to the heart. It enters either through the superior vena cava or the inferior vena cava. The superior vena cava returns deoxygenated blood from the upper part of the body to the heart. The inferior vena cava returns deoxygenated blood from the lower part of the body to the heart. These large veins lead into the right atrium.
- The RBC passes through the tricuspid valve into the right ventricle.
- The RBC is then pumped through the pulmonary valve into the pulmonary artery, and on to the lungs. There the RBC gives off carbon dioxide and picks up oxygen.
- The RBC returns to the heart through a pulmonary vein, enters the left atrium, passes through the mitral valve and flows into the left ventricle.
- The left ventricle pumps the fully oxygenated RBC through the aortic valve, into the aorta, and out to the body.
- From the aorta, the RBC flows into one of the many arteries of the body, through the arterioles and then to the capillaries, where the RBC will deliver oxygen and nutrients to the cells and remove wastes and carbon dioxide. Next it moves through the venules, veins, and on to the vena cava in a deoxygenated state, and returns to the heart, only to begin its repetitive journey once again.
Just as a bit of trivia, that single RBC will travel about 950 miles (1,528 kilometers) in its brief 4-month lifetime!
BLOOD PRESSURE, FLOW, AND RESISTANCE
The blood pressure (in a blood vessel) is defined to be the force exerted by the blood against the vessel wall. It is this pressure that keeps your blood circulating. Every blood vessel in the circulatory system has its own blood pressure, which changes continually. Note that in ordinary usage, the term blood pressure is most commonly used to refer to arterial pressure. Blood pressure is usually measured in millimeters of mercury (mm. Hg) because the mercury manometer has been used as the standard reference for pressure measurements. A blood pressure of 100 mm. Hg means that the force exerted by the blood is sufficient to push a column of mercury up to a height of 100 mm.
Arterial blood pressure rises and falls in a pattern corresponding to the phases of the cycles of the heart, the cardiac cycle. That is, when the ventricles contract (ventricular systole), their walls squeeze the blood inside their chambers and force it into the pulmonary artery and aorta. As a result, the pressures in these arteries rise sharply. The maximum pressure achieved during such ventricular contraction is the systolic pressure. When the ventricles relax (ventricular diastole), the arterial pressure drops, and the lowest pressure that remains in the arteries before the next ventricular contraction is termed the diastolic pressure.
The surge of blood entering the arterial system during a ventricular contraction causes the elastic walls of the arteries to swell, but the pressure drops almost immediately as the contraction is completed, and the arterial walls recoil. This alternate expanding and recoiling of an arterial wall can be felt as a pulse in an artery that runs close to the surface of the skin. The radial artery, for example, runs its course near the surface at the wrist and is commonly used to determine a person’s radial pulse.
The radial pulse is normally equal to the rate at which the left ventricle is contracting, which is why it is used to determine heart rate quickly and easily. A pulse also can reveal something about blood pressure, because an elevated pressure produces a pulse that feels full, while a low pressure is accompanied by a pulse that is easily compressed.
Flow through a blood vessel is determined by two factors: (1) the force that pushes the blood through the vessel, and (2) the resistance of the vessel to the blood flow. Ordinarily, blood flow is measured in milliliters or liters per minute. The blood flow in the entire human circulation is about 5000 ml. per minute at rest, but may be 5-6 times as great during heavy exercise. The amount of blood pumped by the heart in one minute is called the cardiac output.
It is important to note that the flow of blood in the body is directly influenced by gravity. When a person is standing, gravity helps pull the blood downward to the lower extremities. Without gravity, blood tends to remain closer to the heart. The force of gravity also makes it more difficult for the blood to flow upwards to return to the heart and lungs for more oxygen. Our bodies have evolved to deal with the ever-present downward force of gravity; our leg muscles function as pumps to help in the process of venous return (blood flow back to the heart) During walking or other leg movements, the muscles contract, forcing blood up through the veins of the calf towards the heart. The valves in the veins are arranged so that the direction of blood flow can only be in one direction. This mechanism effectively counteracts the force of gravity.
FACTORS THAT INFLUENCE ARTERIAL BLOOD PRESSURE
The arterial blood pressure depends on a variety of factors which are at work in the body at any given moment. These factors include the pumping action of the heart, blood volume, resistance to flow and the viscosity (thickness) of blood (Figure 4).
Figure 4. Some factors that influence arterial blood pressure.
The volume of blood discharged from the ventricle with each contraction is called the stroke volume and equals about 70 ml. for an adult at rest. As stated earlier, the volume discharge from the ventricle (or the heart) per minute is called the cardiac output. It is calculated by multiplying the stroke volume by the heart rate in beats per minute. (Cardiac output = stroke volume x heart rate.) Thus, if the stroke volume is 70 ml. and the heart rate is 72 beats per minute, the cardiac output is 5040 ml. per minute.
Blood pressure varies directly with the cardiac output. If either the stroke volume or the heart rate increases, so does the cardiac output and, as a result, the blood pressure rises. Conversely, if the stroke volume or the heart rate decreases, so do both the cardiac output and the blood pressure.
The blood volume is equal to the sum of the blood cell and plasma volumes in the vascular system. Although the blood volume varies somewhat with age, body size, and sex, for adults it usually remains about 5 liters.
Blood pressure is directly proportional to the blood volume. In other words, any changes in blood volume are accompanied by changes in blood pressure. For example, if blood volume drops drastically because of an injury which results in massive bleeding, the blood pressure also drops. If the normal blood volume is restored by a blood transfusion, with IV fluids, then blood pressure also returns to normal.
Friction between the blood and the walls of the blood vessels creates a force called resistance, which hinders blood flow and was discussed earlier. This force must be overcome by blood pressure if the blood is to continue flowing. Consequently, factors that alter resistance cause changes in blood pressure. Resistance in the systemic portion of the circulation is called peripheral resistance.
The viscosity of the fluid is related to the ease with which its molecules flow past one another. Measurement of viscosity could also be thought of as measuring how thick a fluid is. As viscosity increases, fluids flow less easily. Blood cells and plasma proteins increase the viscosity of the blood. This prevents the blood from flowing as easily as water (remember the saying "blood is thicker than water"). Therefore, greater force is needed to propel it through the vascular system. So, it is not surprising that blood pressure rises as blood viscosity increases and drops as viscosity decreases. Dehydration also increases blood viscosity.
CENTRAL VENOUS PRESSURE
All of the venous blood of the body drains into the right atrium. The pressure within this heart chamber is called the central venous pressure (CVP). This pressure is of special interest because it provides an indication of the volume of blood that is flowing through the veins and into the right atrium. If the heart is beating weakly, blood tends to back up in the veins. CVP, as a measure of this "back-up," will increase. Also, if the blood begins to back up (measured by increased CVP), the heart can respond in two ways: 1) it can beat faster, and 2) it can pump more blood with each beat. When the heart responds, then the back-up is relieved. If we were to measure CVP, it would decrease to normal.
The body has a number of automatic mechanisms for regulating blood pressure (and blood flow), so that a person doesn’t feel faint every time he/she stands. An important one of these mechanisms involves certain receptors, called baroreceptors (weight or pressure receptors), located in the carotid artery of the neck. These receptors sense changes in blood pressure in the neck, and react by sending signals to the heart to adjust the heart rate and to the blood vessels to constrict or relax, thus maintaining blood flow to the head. In space, since there is no "up" or "down," positional changes do not cause blood pressure changes, and the baroreceptors do not have to make automatic adjustments to postural movements. Thus, the baroreceptors may change their function during a space flight of several days duration, and not be able to function normally immediately following such a space flight because they have not been "exercised" while in space. Thus, when astronauts return to the gravity of Earth and stand up, the baroreceptors may not be able to react normally for a few hours or days of adjustment. This may contribute to the post-flight deconditioning experienced by the astronauts.
INTERDEPENDENCE OF ALL BODY SYSTEMS
The health of the body, indeed, its day-to-day functioning, depends on the close interaction of many different systems. It all boils down to keeping the body in a state of equilibrium in which the internal environment of the body remains relatively constant. As stated earlier, this is called homeostasis. For instance, the respiratory system is responsible for the exchange of oxygen and carbon dioxide between the environment and the blood. Respiration can be broken into three stages. The first is the process of breathing, which involves movement of air in and out of the lungs. The second stage is the exchange of gases between the lungs and the blood. The third is the exchange of gases between the blood and the cells of the body. The system that regulates this overall process is sometimes referred to as the cardiopulmonary system, because it involves both the heart ("cardio") and the lungs ("pulmonary").
Homeostasis also requires a proper balance between fluid volume regulation and the maintenance of a water and electrolyte balance. (Electrolytes are ions like sodium and potassium which the body needs to function properly.) This means that the quantity of liquids and electrolytes taken in by the body through food and drink should balance the quantity leaving the body. The urinary system, which includes the kidneys, helps to maintain the normal concentrations of water and electrolytes within body fluids. The kidneys play a large part in ‘the regulation of fluid volume in the body and aid in the control of such diverse things as red blood cell production and blood pressure.
The cardiovascular system interacts with every organ in the body. Therefore, small changes in any of these body systems can have "waterfall" effects and spread and create changes throughout the body. Every moment of our lives, hundreds of thousands of complex interactions take place in our bodies. Over millions of years, the human body has evolved in conjunction with the force of gravity. You have had a glimpse of the complexity of the body’s systems down to the microscopic level of the capillaries. What do you suppose happens when an environmental constant like the force of gravity is removed?
There are several countermeasures designed to alleviate the problems imposed on the cardiovascular system by the space environment. These include exercise, seating position, anti-G suits, and the Lower Body Negative Pressure (LBNP) device.
One way to prevent harmful effects of cardiovascular deconditioning is to start the space flight at a higher level of conditioning by athletic training before the flight. Many of the astronauts run, jog, or participate in aerobic exercise as part of their daily routine. (Do you?) This keeps their hearts strong and helps prevent ill effects caused by cardiovascular deconditioning.
Another way to prevent some of the cardiovascular deconditioning is to perform aerobic exercise during flight. Astronauts can use an exercycle or a rowing machine in the Space Shuttle. Exercising during flight keeps the heart strong.
During launch, when the largest acceleration affects the Space Shuttle, the astronauts are reclined on their backs with their legs higher than their heads. This prevents blood from pooling in the legs during ascent and assists the heart in pumping blood to the rest of the body.
Anti-G suits contain balloon-like pressure bladders in the pants which can be inflated with air by the astronaut. When the astronaut inflates the bladders in his pants, the bladder presses against his legs, forcing body fluid into the upper body. This helps the heart pump the blood more efficiently by pushing the blood out of the lower extremities.
(to be continued)
Raymond J. Noonan, Ph.D.
Health and Physical Education Department
Fashion Institute of Technology of the
State University of New York