Chapter 4

Getting to the Heart of the Circulatory System

IN THIS CHAPTER

Breaking down the heart’s parts

Discovering arteries, capillaries, and veins

Uncovering blood’s main components

Following blood along its path through your body

The circulatory system’s main components are the heart and blood vessels. As a result, the functions of this system are all related to transportation. Nearly every substance made or used in the body is transported in the blood: hormones, gases of respiration, products of digestion, metabolic wastes, and immune system cells.

The blood also transports heat. Stimulated by the hormones of thermoregulation, blood flow can disperse heat to the environment at the body surface or conserve heat for essential functions in the body core.

In this chapter, we tell you what you need to know about the structures of your circulatory system and the blood it moves throughout your body. We also describe just how your circulatory system accomplishes its important task.

Analyzing the Cardiac Anatomy

The circulatory system — or cardiovascular system — consists of the heart and the blood vessels. The heart’s pumping action (or beat) squeezes blood out of the heart, and the pressure it generates forces the blood through the blood vessels. The autonomic nervous system controls the rate of the heartbeat.

In this section, we provide a brief overview of the heart, including its shape and collection of tissues. We also explain how the heart uses veins and arteries to pump blood.

Examining the shape of the heart

The heart is shaped like a cone and is only about the size of your fist (see Figure 4-1). It lies between your lungs, just behind your sternum, and the tip of the cone points to the left. In most individuals, the heart is situated slightly to the left of center in the chest.

FIGURE 4-1: A typical healthy heart.

The heart has four hollow spaces called chambers. The contraction of the heart muscle pumps blood into and out of all four chambers in a rhythmic pattern. (The arrows in Figure 4-2 show the direction of blood flow through the chambers.)

FIGURE 4-2: The valves of the heart and how blood flows through them.

The heart is also divided anatomically and functionally into left and right sides. Each side of the heart has one atrium and one ventricle, each with a separate function. A membrane called the interatrial septum separates the atria; a membrane called the interventricular septum separates the ventricles.

Between the chambers are several valves that allow measured quantities of blood to flow into the chambers and keep blood flowing in the right direction. The valves’ names tell you either their anatomical location or their characteristics. Consider the following:

• The two atrioventricular (AV) valves lie between the atrium and the ventricle on each side.
• The bicuspid valve (BV) has two flaps.
• The tricuspid valve (TV) has three flaps.
• The semilunar valves (SV) are shaped like half-moons.

Looking at the heart’s tissues

The tissues of the heart perform the functions required to keep the double-pump working strongly and steadily. Like other hollow organs, the heart is made up of layers of endothelial and connective tissue. Here’s the lowdown on those layers:

• Endocardium: The endocardium is a layer of endothelial tissue that lines the inside of the chambers. This layer is continuous with the vascular endothelium, which we talk about in detail in the later section “Beginning at the arteries.”
• Myocardium: The myocardium (literally, “heart muscle”) is the thick, muscular layer of your heart. It’s composed of cardiac muscle fibers that contract in a coordinated way to pump the blood out of the heart and into the aorta with enough force to carry it through the arterial system and out into the capillaries.
• Epicardium: The epicardium is the visceral layer of the serous pericardium — a double layered sac that covers the heart. The epicardium is a conical sac of fibrous tissue that closely envelops the myocardium and surrounds the roots of the major blood vessels. It secretes pericardial fluid into the pericardial cavity, which lubricates the tissues as the heart beats.
• Pericardial cavity: The pericardial cavity is a fluid-filled space between the epicardium and the parietal layer of the serous pericardium. The fluid reduces friction between the pericardial membranes.
• Parietal layer of the serous pericardium: This layer is a serous membrane that’s attached to the outermost layer of the heart, the fibrous pericardium. The fibrous pericardium is a thick, white sheet of fibrous connective tissue that anchors the heart and the major blood vessels, including the aorta, to the sternum and diaphragm. The parietal layer of the serous pericardium also secretes pericardial fluid into the pericardial cavity.

Observing how blood flows to (and from) the heart

Blood flows in and out of the heart every second of your life, and some of that blood needs to supply the cells of the heart itself with oxygen and nutrients. The coronary arteries supply oxygenated blood to the heart, while cardiac veins return deoxygenated blood to the pulmonary circulation loop. Here’s some more info on each of these:

• Coronary arteries: Two large coronary arteries and their many branches supply blood to the heart. Large arteries enter the heart on the left and right at the top. They’re called the left and right coronary arteries because they sit atop and encircle the heart, looking like a crown (see Figure 4-3).

  The right coronary artery and its two major branches, the marginal artery and the posterior interventricular artery, primarily supply the right atrium and ventricle with oxygenated blood and nutrients. The left coronary artery and its two branches, the anterior interventricular artery and the left circumflex coronary artery, primarily supply the left atrium and ventricle with oxygenated blood and nutrients.

• Cardiac veins: The cardiac venous system is similarly branched, often lying alongside the coronary arteries and their branches. Like everything about the heart, the cardiac venous system is composed of two parts, left and right. The left cardiac venous system, also called the coronary sinus system, receives deoxygenated blood from most of the superficial veins of the heart. The right cardiac venous system is composed of veins that originate on the anterior and lateral surfaces of the right ventricle and drain directly into the right atrium. The smallest vessels (the venae cordis minimae) drain the myocardium directly into the atria and ventricles.

FIGURE 4-3: The coronary arteries.

Meet Your Blood Vessels

Your blood vessels comprise a network of channels through which your blood flows. But the vessels aren’t passive tubes. Rather, they’re active organs that, when functioning properly, assist the heart in circulating the blood and influence the blood’s constitution. The innermost layer of the heart and of all vessels is continuous — it’s one convoluted sheet of epithelium.

The vessels that take blood away from the heart are arteries. The vessels that bring blood toward the heart are veins. (The smallest vessels are called arterioles and venules, respectively.) Generally, arteries have veins of the same size running right alongside or near them, and they often have similar names.

The arterial vessels (arteries and arterioles) decrease in diameter as they spread throughout the body. Eventually, they end in the capillaries, the tiny vessels that connect the arterial and venous systems. The venous vessels become increasingly larger as they converge on the heart. The smaller venules carry deoxygenated blood from a capillary to a vein, and the larger veins carry the deoxygenated blood from the venules back to the heart.

The following sections take a closer look at the three types of blood vessels.

Beginning at the arteries

Your arteries form a branching network of vessels, with the main trunk, called the aorta, arising from the left ventricle and splitting immediately into the brachiocephalic trunk, left common carotid artery, and left subclavian artery, which serve the head and upper limbs. The descending aorta — which serves the thoracic organs, abdominal organs, and lower limbs — gives off several branches:

• The mesenteric arteries, the main arteries of the digestive tract
• Two renal arteries, which supply the kidneys
• The common iliac arteries, which supply the pelvis and lower limbs

Although an artery looks like a simple tube, the anatomy is complex. As you can see in Figure 4-4, arteries are made of three concentric layers of tissue around a space, called the lumen, where the blood flows.

• Tunica externa: The outside layer, the tunica externa, is a thick layer of connective tissue that supports the vessel and protects the inner layers from harm. The larger the artery, the thicker the connective tissue supporting it. (However, this type of tissue can become stiff with age.)
• Tunica media: The next layer in, the tunica media, is a thick wall of smooth muscle and elastic tissue. This layer expands and contracts with every pulse wave (heartbeat), and sometimes it gives a little squeeze to increase the pressure and force the blood through.
• Tunica intima: The inner layer, the tunica intima, is a single-cell-thick endothelial layer that lines the lumen and is continuous through all the organs of the circulatory system. The vascular endothelium, as this tissue is also called, is active metabolically, releasing into the blood various substances that influence blood circulation and vascular health. It also specializes in transporting oxygen, nutrients, and other substances from the blood flowing in the lumen to the smooth muscle fibers of the tunica media.

FIGURE 4-4: The anatomy of an artery.

Moving on to the capillaries

After passing from the arteries and the arterioles, blood enters the capillaries, which lie between larger blood vessels in capillary beds. A capillary bed forms a bridge between the arterioles and the venules. Capillary beds are everywhere in your body, which is why you bleed anywhere that you even slightly cut your skin.

Your capillaries are your smallest vessels. Only the single-cell-thick epithelial layer surrounds the lumen. The precapillary sphincters of the metarterioles can tighten or relax to control blood flow into the capillary bed.

Lacking the structure and complexity of arteries and veins, capillaries rely on simple diffusion to do their job, which consists of moving oxygen and nutrients from the blood to the cells and waste materials from the cells to the blood. Capillary exchange is the term used to describe these processes (see Figure 4-5).

FIGURE 4-5: Capillary exchange.

The capillaries come into close contact with all the cells of your tissues. Consider the diffusion that occurs at both ends:

• At the end near the arteriole, oxygen diffuses out of the red blood cells and nutrient molecules diffuse out of the plasma, across the capillary membrane, and directly into the tissue fluid. The oxygen and nutrients dissolved in the tissue fluid diffuse across the membrane of the adjacent cells.
• At the venule end, the carbon dioxide and other waste materials diffuse out of the tissue fluid and across the capillary membrane into the blood. Then, the blood continues on through the venous system, and those waste materials are deposited in the proper locations on their way out of the body. The carbon dioxide diffuses out of the bloodstream in the lungs so it can be exhaled and thus removed from the body; other metabolic waste is filtered through the kidneys.

Besides aiding in the exchange of gases and nutrients throughout your body, capillaries serve two other important functions:

• Thermoregulation: Precapillary sphincters tighten when you’re in a cold environment to prevent heat loss from the blood at the skin surface. Blood is then shunted from an arteriole directly to a venule through a nearby arteriovenous shunt. When you’re in a warm environment or you’re producing heat through exertion, the precapillary sphincters relax, opening the capillary bed to blood flow and dispersing heat.
• Blood pressure regulation: When blood pressure (blood volume) is low, the hormones that regulate this pressure stimulate the precapillary sphincters to tighten, temporarily reducing the total volume of the blood vessel system and thus raising the pressure. When blood pressure is high, they stimulate the sphincters to relax, increasing overall system volume and reducing pressure.

Traveling through the veins

Small veins converge into larger veins, all merging in the inferior vena cava and superior vena cava, the largest vessels in the venous system. These major veins return blood from below and above the heart, respectively. The inferior vena cava lies to the right of, and more or less parallel to, the descending aorta. The superior vena cava lies to the right of, and more or less parallel to, the aorta.

Veins have a similar anatomy to arteries, but they tend to be wider and their walls thinner and less elastic. The tunica interna of a vein is also part of the continuous endothelial layer that lines the whole network. The tunica media has a layer of elastic tissue and smooth muscle, but this layer is much thinner in a vein than in an artery. The veins have virtually no blood pressure, so they don’t need a thick muscle layer to vary the vessel diameter or withstand fluid pressure. The outermost tunica externa is the thickest layer of a vein.

Because veins don’t have a thick muscle layer to push blood through them, they depend on contraction of skeletal muscles to move blood back to the heart. As you move your arms, legs, and torso, your muscles contract, and those movements massage the blood through your veins. The blood moves through the vein a little bit at a time. The larger veins have valves that keep blood from flowing backward. The valves open in the direction that the blood is moving and then shut after the blood passes through to keep the blood heading toward the heart.

Here are the details on how veins work in each of the different parts of the body:

• In the lower body: The internal iliac veins, which return blood from the pelvic organs, and the external iliac veins, which return blood from the lower extremities, converge into the common iliac veins. The renal veins return blood from the kidneys. Both of these major veins flow into the inferior vena cava.
• In the digestive tract: Blood from the digestive tract travels in the hepatic portal vein to the liver (see Figure 4-6). Specialized cells in the liver move glucose molecules from the blood into storage. Phagocytic cells in the liver destroy bacterial cells that make it through the digestive process and remove toxins and other foreign material from the blood. Blood exits the liver through the hepatic veins, which flow into the inferior vena cava. The inferior vena cava empties into the right atrium.

• In the head and upper extremities: Deoxygenated blood from the head and upper extremities drains into the brachiocephalic veins. The veins of the upper extremity — the ulnar veins, radial veins, and subclavian veins — also drain into the brachiocephalic veins. The jugular veins of the head and neck also drain into the brachiocephalic veins, which connect to the superior vena cava, which enters the right atrium.

  After the blood from the right atrium has been pumped into the right ventricle, it’s pumped into the lungs, where the blood is oxygenated, and then it flows back to the heart in the pulmonary veins, the only veins that carry oxygenated (red) blood.

FIGURE 4-6: Hepatic portal system (venous circulation).

Discovering What’s in Your Blood

Blood — that deep maroon, body-temperature-warm liquid that courses through your body — is a vitally important, life-supporting, life-giving, life-saving substance that everybody needs. And every adult-size body contains about 5 quarts of the precious stuff.

Blood consists of many different types of cells in a matrix called plasma, making it a connective tissue. The different types of cells — red blood cells, white blood cells, and platelets — are referred to as formed elements. We discuss plasma and these different types of cells in the following sections.

Plasma: Your protein carrier

Plasma is about 92 percent water. The remaining 8 percent is made of plasma proteins, including salt ions, oxygen and carbon dioxide gases, nutrients (glucose, fats, amino acids) from the foods you take in, urea (a waste product), and other substances carried in the bloodstream, such as hormones and enzymes.

The plasma proteins, which are produced in the liver or by immune cells in the blood, have some important functions. The following are some example of plasma proteins:

• Albumin: The smallest plasma protein and the most abundant, albumin maintains the osmotic pressure in the bloodstream within the homeostatic range.
• Fibrinogen: During the process of clot formation, fibrinogen is converted into threads of fibrin, which form a meshlike structure that traps blood cells to form a clot.
• Immunoglobulin (antibody): These proteins are created in response to an invading microbe by cells that make up the immune system (turn to Chapter 9 for more on the immune system).

Red blood cells: In charge of moving O₂ and CO₂

Red blood cells (RBCs), or erythrocytes (erythro is the Greek word for “red”), are the most numerous of the blood cells and one of the most numerous of all cell types in your body. About one-quarter of the body’s approximately 3 trillion cells are RBCs. They’re among the cell types that must be constantly regenerated and disposed of. In fact, you produce and destroy a few million RBCs every second!

The cytoplasm of RBCs is full to the brim with an iron-containing biomolecule called hemoglobin. The iron-containing heme group in hemoglobin binds oxygen at the respiratory membrane and then releases it in the capillaries. The ability of hemoglobin to bind and release oxygen is the sole mechanism by which all your cells and tissues get the oxygen they need to sustain their metabolism. RBCs containing heme-bound oxygen are bright red, the familiar color of the arterial blood that flows from wounds. RBCs in the venous system have less heme-bound oxygen and are a dark red.

As oxygen diffuses into a cell, carbon dioxide diffuses out, making its way in the interstitial fluid to the venous system. Some deoxygenated hemoglobin in the venous blood takes up carbon dioxide to form carboxyhemoglobin. At the respiratory membrane, carboxyhemoglobin releases the carbon dioxide and takes up oxygen again. Carbon dioxide is transported in several different ways in the blood to the respiratory membrane, where it enters the lung and is exhaled in the breath.

An RBC has about a four-month life span, at the end of which it’s destroyed by a phagocyte (a large cell with cleanup responsibilities) in the liver or spleen. The iron is removed from the heme group and is transferred to either the liver (for storage) or the bone marrow (for use in the production of new hemoglobin). The rest of the heme group is converted to bilirubin and released into the plasma (giving plasma its characteristic straw color). The liver uses the bilirubin to form bile to help with the digestion of fats.

Platelets: The clotting cells

Platelets are tiny pieces of cells. Large cells in the red bone marrow called megakaryocytes break into fragments, which are the platelets. Their job is to begin the clotting process and plug up injured blood vessels. Platelets, also called thrombocytes (thrombos means “clot”), have a short life span — they live only about ten days.

White blood cells: The body’s defenders

White blood cells (WBCs), also known as your immune cells, are derived from the same type of hematopoietic stem cells as RBCs. However, they take different paths early in the process of differentiation. The WBCs, also called leukocytes (except for the T-lymphocytes), leave the red bone marrow and enter into circulation in their mature form.

Focusing on the Physiology of Circulation

Your body makes it happen 100,000 times every day. Waking or sleeping, from a moment early in fetal development until the moment you die, the beat goes on. It may speed up slightly when you’re working hard physically or you’re excited or stressed, and some people can slow theirs down with meditation techniques. But for most people, it just plugs along, the same thing over and over.

What are we talking about? The heartbeat, of course. Your beating heart pushes blood around a double-circuit — out through your arteries, ultimately into your capillary beds, across your capillary beds and into your veins, and then back through your veins. The blood passes through the heart to the lungs and then back to the heart and out through the arteries again. Each complete double-circuit takes less than one minute.

You can feel the rhythmic pulsation of blood flow at certain spots around your body, most commonly on the inside of the wrist or on the carotid artery of the neck. What you feel as you touch these spots is your artery expanding as the blood rushes through it and then immediately returning to its normal size when the bulge of blood has passed.

The cardiac cycle: Conducting electricity

Five structures of the heart, together called the cardiac conduction system, specialize in initiating and conducting the electrical impulses that induce your heartbeat, keeping it regular and strong in every part of the organ (see Figure 4-7). We tell you about each structure and walk you through the cardiac cycle in the following sections.

FIGURE 4-7: The conduction system of the heart.

Anatomy of cardiac conduction

The cardiac conduction system is made up of the following five structures:

• Sinoatrial node (SA node; also called the sinus node): A small knot of cardiac musclelike tissue (the pacemaker cells look like cardiac muscle cells, but they’ve lost the ability to contract) located on the back wall of the right atrium, near where the superior vena cava enters the heart. The SA node’s cells initiate the electrical impulses that generate the heartbeat.
• Atrioventricular node (AV node): A small mass of tissue located in the right atrium near the septum that divides the right and left atria from the ventricles. Its function is to relay the impulses it receives from the SA node to the next part of the conduction system.
• Atrioventricular bundle (AV bundle): A bundle of fibers that extends from the AV node into the interventricular septum (which divides the heart into right and left sides). The AV bundle transmits cardiac impulses.
• Left and right bundle branches: Where the septum widens, the AV bundle splits into the right and left bundle branches, each extending down under the endocardium and up along the outside of the ventricles. These bundles carry cardiac impulses.
• Purkinje fibers: At the ends of the AV bundle branches are the Purkinje fibers, which deliver impulses to the myocardial cells, causing the ventricles to contract.

The sequence of events

The cardiac conduction system is responsible for keeping the cardiac cycle going. If the cardiac cycle stops for too long, you experience serious consequences. Here’s how the cycle goes:

1. The electrical impulse is initiated in the SA node. The right and left atria contract simultaneously, pumping blood into the right and left ventricles, respectively.
2. The impulse passes to the AV node, which sends it to the AV bundle. In the meantime, the right and left atria relax.
3. The impulse passes into the right and left bundle branches and ultimately into the Purkinje fibers, causing the right ventricle to contract and pump its deoxygenated blood into the pulmonary arteries and lungs. Simultaneously, the left ventricle contracts, pumping oxygenated blood into the aorta.
4. The ventricles relax while the atria begin contracting, and the cycle starts all over again.

Problems with signal conduction due to disease or abnormalities of the conducting system can occur anywhere along the heart’s conduction pathway. Abnormally conducted signals resulting in irregular heartbeat are called arrhythmias or dysrhythmias.

On the beating path: Blood flow through the heart and body

The heart is a double-pump, so it has two circuits:

• Pulmonary circulation: This path goes from the heart to the lungs and back to the heart.
• Systemic circulation: This path goes from the heart to the body and back to the heart.

Every drop of your blood travels around this double-circuit about once per minute. You can see both paths in Figure 4-8.

FIGURE 4-8: Pulmonary and systemic circulation working together through arterial and venous systems.

Pulmonary circulation

Deoxygenated blood enters the heart’s right atrium from the largest veins in the body, the superior vena cava and the inferior vena cava. When the SA node initiates the cardiac conduction cycle, the right atrium contracts and pumps the blood into the right ventricle.

When the impulse passes to the AV node and then on to the AV bundle, the right bundle branch, and the Purkinje fibers, the right ventricle contracts, pumping blood into the pulmonary arteries, which take it to the lungs for gas exchange. During the relaxation phase of the atria, the newly oxygenated blood flows into the left atrium.

The pulmonary arteries are the only arteries that carry deoxygenated blood.

Systemic circulation

When the SA node initiates the cardiac conduction cycle, the left atrium contracts, pumping the oxygenated blood into the left ventricle.

When the impulse passes to the AV node and on to the AV bundle, the left bundle branch, and the Purkinje fibers, the left ventricle contracts and pumps blood into the aorta. From the aorta, the blood travels through the arteries and arterioles to the capillary beds and then back to the heart through the veins. During the relaxation phase of the atria, the deoxygenated blood flows into the right atrium.

Measuring the importance of blood pressure

Blood pressure is a term used to describe the force of blood pushing against the wall of an artery. It’s measured in millimeters of mercury at both the highest point (systole, when the heart is contracted) and the lowest point (diastole, when the heart is relaxed) in the cardiac cycle. The systole pressure is always higher than the diastole. The higher the systolic and diastolic values, the more pressure present on the walls of the arteries. Two factors affect the blood pressure:

• Cardiac output: The amount of blood the heart pumps out per unit of time
• Peripheral resistance: A measure of the diameter and elasticity of the vessel walls

The cardiac output is determined by the heartbeat rate and the blood volume put out from a ventricle during one beat. When either of these rises, the blood pressure rises. Heart rate is increased by physical exertion, the release of epinephrine (a hormone), and other factors. The blood volume is influenced by the action of ADH (antidiuretic hormone) and other mechanisms in the kidneys to control the amount of water that’s removed from the urine and restored to the blood.

The arteries’ diameter changes locally and continuously. The pressure of the pulse wave increases pressure on the vascular endothelium, inducing it to release molecules, mainly nitrous oxide, that induce relaxation in the tunica media. The endothelium’s ability to respond to the pulse-wave pressure is extremely important to vascular health. Resistance in the arteries to expansion as the blood rushes through raises the blood pressure.

As part of homeostasis, receptors in the arteries called pressoreceptors measure blood pressure. If the blood pressure is above the normal range, the brain sends out impulses to cause responses that decrease the heart rate and dilate the arterioles, both of which decrease blood pressure.

Hemostasis: Stopping the flow

Not the least amazing thing about blood is its ability to stop flowing. The term for this is hemostasis (literally, “blood stopping”), which is not to be confused with homeostasis.

Hemostasis is the reason you didn’t bleed to death the first time you cut yourself. When vessels are cut, blood flows only long enough to clean the cut. As you watch, the blood stops flowing, and a plug, called a clot, forms. Within a day or so, the clot has dried and hardened into a scaly scab. Eventually, the scab falls off, revealing fresh new skin.

A blood clot consists of a plug of platelets enmeshed in a network of insoluble fibrin molecules. The clotting cascade is a physiological pathway that involves numerous components in the blood itself interacting to create a barrier to the flow. As soon as a blood vessel is injured, a signal is sent from the vascular endothelium to the platelets, summoning them to the injury site. On exposure to the air, their membranes become sticky, and they start to adhere together. Proteins in the blood plasma, called coagulation factors or clotting factors, respond in a complex cascade to form fibrin strands, which strengthen the platelet plug. Within minutes, your blood is safe in your body, where it belongs.

A blood clot in the right place is something to be grateful for. But blood tends to start clotting whenever it’s not flowing freely, and this tendency can cause problems in the peripheral vessels (the arteries and veins of the legs). Clots also form on the inner wall of blood vessels when the endothelium is injured by disturbances in blood flow (turbulence) or by free radicals in the blood. These tiny clots adhere to the wall, further disturbing flow and causing more turbulence and more injury. Atherosclerotic plaque may begin to form around the clot. Worst of all, perhaps, is when the clot, with the plaque attached, breaks off from the vessel wall and floats free (sort of) in the bloodstream. Sooner or later, this embolus lodges somewhere in a vessel, sometimes with sudden and fatal consequences.