Chapter 5: Cardiovascular System: Heart and Blood Vessels

Overview of the Cardiovascular System

  • Two components of the cardiovascular system:
    • The heart
    • Blood vessels
  • Functions of the cardiovascular system:
    • Pumps blood through blood vessels.
    • Brings nutrients to cells.
    • Helps remove wastes.
  • Exchange of substances occurs through interstitial fluid.
  • Lymphatic system:
    • Collects excess interstitial fluid.
    • Returns it to the blood.
    • Works with the immune system.
    • Lymph is interstitial fluid that enters the lymphatic vessels; is fluid connective tissue.

Circulation and Exchange of Materials

  • The cardiovascular system works with all other organ systems; blood vessels move blood and its contents to and from organs.
  • Gas exchange: Blood drops off carbon dioxide and picks up oxygen at the lungs, working with the respiratory system.
  • Nutrient exchange:
    • Nutrients enter the bloodstream at the intestines, transporting substances to the body’s cells, working with the digestive system.
    • The liver supports metabolism, detoxification, and homeostasis.
    • At the kidneys, the blood is purified, and water and salts are retained as needed.
  • The cardiovascular system maintains homeostasis by transporting:
    • Oxygen
    • Carbon dioxide
    • Waste products
    • Nutrients
    • Hormones
    • Immune system cells
    • Antibodies and chemical signals which help protect the body from infection.

Types of Blood Vessels

  • Three types of blood vessels:
    • Arteries
    • Veins
    • Capillaries

Arteries

  • Carry blood away from the heart under high pressure.
  • Walls have three layers:
    • Endothelium: thin, inner epithelium
    • Middle layer: smooth muscle and elastic tissue; allows arteries to expand and recoil
    • Outer layer: connective tissue

Arterioles

  • Small arteries with some elastic tissue, mostly smooth muscle.
    • Contraction constricts the vessel, reducing blood flow and raising blood pressure.
    • Relaxation dilates the vessel, increasing blood flow and reducing blood pressure.

Capillaries

  • Microscopic vessels between arterioles and venules.
  • Walls are made only of endothelium (simple squamous epithelium).
  • Form capillary beds where gas, nutrient, and waste exchange occurs.
  • Precapillary sphincters control blood flow through the capillary bed.
    • When closed, blood flows through an arteriovenous shunt.

Veins

  • Carry blood toward the heart; lower pressure than arteries.
  • Veins that carry blood against gravity have valves to keep blood flowing toward the heart.

Venules

  • Small veins that receive blood from the capillaries
  • Venule and vein walls have the same three layers as arteries but less smooth muscle in the middle layer.
  • Walls of veins are thinner than arteries so they can expand to hold more blood.
  • At any one time, they store 70% of the blood.
  • If blood is lost (hemorrhage), the nervous system causes the veins to constrict to increase blood volume.

The Heart as a Double Pump

  • Located between the lungs, behind the sternum; apex points toward the left.
  • Consists mostly of the myocardium, made of cardiac muscle tissue.
    • Muscle fibers are branched and connected by intercalated disks, which contain gap junctions (allow cells to contract in unison) and desmosomes (prevent overstretching by holding adjacent cells together).
  • The heart is surrounded by a sac called the pericardium, which secretes pericardial fluid for lubrication.
  • Internally, the heart’s septum divides the heart into right and left sides.
  • The heart consists of four chambers: two upper atria and two lower ventricles.
  • Two types of valves:
    • Pulmonary and aortic semilunar valves
    • Left (bicuspid or mitral) and right (tricuspid) atrioventricular (AV) valves, reinforced by chordae tendineae

Coronary Circulation

  • Coronary vessels supply and drain blood from the myocardium; found on the heart’s surface.
  • Coronary arteries supply blood to the myocardium. They are the first branches off the aorta.
  • Coronary veins drain blood from the myocardium and empty into the right atrium.
  • Coronary artery disease is a blockage in the coronary arteries that causes a myocardial infarction (heart attack).

Passage of Blood Through the Heart

  • The inferior and superior vena cava carry O<em>2O<em>2-poor, CO</em>2CO</em>2-rich blood from systemic veins to the right atrium.
  • The right atrium contracts, sending blood through the right AV (tricuspid) valve into the right ventricle.
  • The right ventricle pumps blood through the pulmonary valve into the pulmonary trunk, which branches into right and left pulmonary arteries, which lead to the lungs.
  • The pulmonary veins carry O<em>2O<em>2-rich, CO</em>2CO</em>2-poor blood from the lungs to the left atrium.
  • Blood flows through the left AV (bicuspid) valve into the left ventricle.
  • The left ventricle pumps blood through the aortic valve into the aorta.
  • The aorta branches into smaller arteries, which lead to arterioles, then capillaries, venules, veins, and back to the vena cavae.
  • The walls of the left ventricle are thicker than the right ventricle because it must pump blood to the entire body, not just to the lungs.
  • The walls of the atria are thinner than ventricles.
  • Pulmonary capillaries within the lungs allow gas exchange. Oxygen enters the blood; carbon dioxide waste is excreted from the blood.

Cardiac Cycle

  • First the atria contract together, then the ventricles, then the heart relaxes.
  • Systole: heart contraction
  • Diastole: heart relaxation
  • Heartbeat occurs 60–80 times per minute, creating two audible sounds: “lub-dub”
    • Lub: from the closure of the AV valves
    • Dub: from the closure of the semilunar valves
  • Murmur: a swishing sound between “lub” and “dub” from regurgitation of blood (leaky valves).

Internal Control of the Heartbeat

  • Internal (intrinsic) conduction system:
    • The SA node (pacemaker) in the right atrium initiates the heartbeat by sending out an electrical signal, causing the atria to contract.
    • This impulse reaches the AV node, also in the right atrium.
    • The AV node sends a signal down the AV bundle and Purkinje fibers, causing ventricular contraction.
    • These impulses travel through gap junctions in the intercalated disks.

External Control of the Heartbeat

  • External (extrinsic) control of heartbeat:
    • The cardiac control center in the brain (medulla oblongata) increases or decreases the heart rate depending on sympathetic and parasympathetic nervous system.
  • Hormones increase heart rate, such as epinephrine and norepinephrine.

Electrocardiogram (ECG)

  • Recording of the electrical changes in the heart muscle during a cardiac cycle.
    • P wave: electrical current produced by the atria when stimulated by the SA node
    • QRS complex: wave of electrical current traveling through the ventricles (contraction)
    • T wave: recovery of the ventricles
  • Ventricular fibrillation: caused by uncoordinated, irregular electrical signals in the ventricles due to injury, heart attack, or drug overdose.
    • The heart can’t pump blood; tissues become starved of oxygen.
    • Defibrillation: applying a strong electrical signal to reset the heart; hopefully, the SA node will start firing again.

Blood Pressure

  • The pressure that blood exerts against a blood vessel wall.
  • Highest in the aorta, right next to the heart.
  • Progressively decreases as blood moves through the body’s vessels—arteries, arterioles, capillaries, venules, and finally, the veins.
  • Lowest in the superior and inferior venae cavae, which enter the right atrium.
  • Pulse: surge of blood into an artery causes the walls to stretch and then recoil, measured in the radial artery at the wrist or carotid artery in the neck.
    • A measurement of the heart rate, averaging 60–80 beats per minute.
  • Blood pressure moves blood in arteries.
    • Contraction of ventricles creates blood pressure, which propels blood through the arteries.
    • Measured with a sphygmomanometer in the brachial artery of the arm.
  • Systolic pressure: the highest pressure when blood is ejected from the heart.
  • Diastolic pressure: the lowest pressure when the ventricles relax.
    • Average is 120/80 mm Hg (systolic/diastolic).
  • Hypertension: high blood pressure.
  • Hypotension: low blood pressure.
  • Blood pressure decreases as it flows away from the heart.

Blood Flow

  • Slowest in the capillaries to increase the exchange of gases, nutrients, and wastes, adjusted by the precapillary sphincters.
  • Blood pressure is very low in the veins, so doesn’t contribute much to the movement of blood.
  • Venous return depends on three additional factors:
    • Skeletal muscle pump (dependent on skeletal muscle contraction):
      • Compresses vein walls, forces blood past valves
    • Respiratory pump (dependent on breathing):
      • Inhalation reduces pressure in the thoracic cavity, allowing blood to flow into the lungs
    • Valves present in veins which prevents blood backflow

Two Cardiovascular Pathways

  • Blood flows in two circuits:
    • Pulmonary circuit circulates blood through the lungs.
    • Systemic circuit circulates blood through the body tissues.

Pulmonary Circuit

  • Right atrium pumps deoxygenated blood into the right ventricle, which pumps it into the pulmonary trunk (splits into right and left pulmonary arteries), then to the lungs.
  • In the lungs, the pulmonary arteries (deoxygenated blood) branch into arterioles, which lead to capillaries (gas exchange).
  • The pulmonary capillaries lead to venules, which merge into the four pulmonary veins that empty into the left atrium (oxygenated blood).

Systemic Circuit

  • The left ventricle pumps blood into the aorta, which gives off branches to all the tissues of the body.
  • Arteries eventually branch into arterioles, which lead to capillaries.
  • Capillaries lead to venules, which drain into veins, which lead to the superior and inferior vena cavae, that empty into the right atrium.

Hepatic Portal System

  • Usually, blood flows from the heart to an organ, then back to the heart.
  • The hepatic portal system is a specialized pathway.
  • Hepatic portal veins carry nutrient-rich blood from the digestive tract to the liver and then to the inferior vena cava.
  • The liver synthesizes blood proteins, stores glycogen, and removes toxins and pathogens that enter the blood through the digestive system.

Exchange at the Capillaries

  • Two forces drive fluid in and out of capillaries:
    • Blood pressure drives fluid out of capillaries, mainly at the arterial end of the capillary bed.
      • Interstitial fluid contains everything that blood contains except cells and plasma proteins.
    • Osmotic pressure draws water into the capillaries by osmosis, mostly at the venule end.
  • Some tissue fluid enters lymphatic capillaries and becomes lymph, which is eventually returned to the cardiovascular system.

Cardiovascular Disorders

  • Cardiovascular disease (CVD) is the leading cause of early death in Western countries, accounting for 20% of deaths in the United States.
  • Disorders of the blood vessels often lead to stroke, heart attack, or aneurysm, including hypertension and atherosclerosis.

Disorders of the Blood Vessels

  • Hypertension (high blood pressure):
    • A systolic pressure of 130 or greater or a diastolic pressure of 80 or greater.
    • A “silent killer” because there are few symptoms until it causes kidney failure, a heart attack, or stroke.
    • The best safeguard is regular blood pressure checks and a healthy lifestyle.
    • Treated with diuretics, which increase the production of urine, and other drugs (ACE inhibitors).
  • Atherosclerosis: buildup of atherosclerotic plaque in the walls of blood vessels.
    • Plaques narrow blood vessel diameter, decreasing blood supply to tissues and can cause clots to form in the roughened walls of arteries.
      • Thrombus: a clot that is stationary.
      • Embolus: a clot that detaches and moves to distant sites.
      • Thromboembolism: an embolus that has become lodged in a blood vessel.
    • Prevention: a diet low in saturated fats and cholesterol but high in omega-3 polyunsaturated fatty acids, and no smoking.
  • Stroke (cerebrovascular accident, or CVA):
    • Occurs when a cranial artery is blocked or bursts.
    • Part of the brain dies due to a lack of oxygen.
    • Symptoms may include numbness of hands or face, difficulty speaking, and inability to see in one eye.
  • Myocardial infarction (MI, or heart attack):
    • Caused by a blocked coronary artery.
    • Can begin with angina pectoris, pain in the chest from a partially blocked coronary artery.
    • Part of the heart dies due to a lack of oxygen.
    • Can be treated with drugs that dilate blood vessels.
  • Aneurysm:
    • A ballooning of a blood vessel, most often the abdominal aorta or blood vessels in the brain.
    • Atherosclerosis and hypertension can weaken a vessel and cause ballooning.
    • If a major artery ruptures, death can result.
    • Can replace a portion of damaged vessel with a plastic tube, or new research uses a patient’s stem cells to generate a new vessel.

Medical Treatments

  • Dissolving blood clots. Medical treatment for a thromboembolism—tissue plasminogen activator (t-PA), a biotechnology drug, converts plasminogen, a protein in the blood, into plasmin, an enzyme that dissolves blood clots.
  • New drugs prevent the plasma membrane of brain cells from releasing/receiving toxins caused by stroke.
  • Prevention of thromboembolism—aspirin lowers the probability of clot formation and first heart attacks, among other health benefits, when taken at low doses.
  • Treating clogged arteries:
    • Coronary bypass operation: a vein from the leg is taken and used to bypass a clogged artery.
    • Gene therapy: injection of the gene for vascular endothelial growth factor (VEGF) induces the growth of new vessels, eliminating the need for bypass surgery.
    • Angioplasty: a tube is inserted into the clogged artery to insert a stent—a mesh cylinder to hold it open. Stents are usually coated in drugs to dissolve blockages.
  • Heart failure
    • Heart no longer pumps properly.
    • Treatments:
      • Wrapping the heart in fabric to prevent enlargement.
      • Implantable cardioverter-defibrillator (ICD) corrects an irregular rhythm.
      • Heart transplant
      • Injection of stem cells to repair the damaged heart
      • Left ventricular assist device (LVAD)—battery-powered pump to assist the heart
      • Total artificial heart (TAH)—temporary solution