Cardiovascular System: The Heart

Cardiovascular System: The Heart

• The cardiovascular system's primary function is to transport blood, which carries oxygen, nutrients, hormones, and waste products throughout the body.
• The heart is a muscular organ that pumps blood through the blood vessels.
• The heart has four chambers: two atria (receiving chambers) and two ventricles (pumping chambers).
• Pulmonary circulation involves blood flow from the right ventricle to the lungs for oxygenation and back to the left atrium.
• Systemic circulation involves blood flow from the left ventricle to the rest of the body and back to the right atrium.
• Blood flow: right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta → body → vena cavae → right atrium.

Heart Location and Structure

• The heart is located in the thoracic cavity within the mediastinum.
• The pericardium is a double-layered sac surrounding the heart, protecting it and reducing friction.
• Serous fluid within the pericardial cavity lubricates the heart, preventing friction as it beats.
• The anterior aspect of the heart features the right ventricle and atrium prominently, while the posterior aspect shows the left ventricle and atrium.
• The three layers of the heart wall are the epicardium (outer), myocardium (middle), and endocardium (inner).
• The myocardium contains cardiac muscle tissue responsible for the heart's pumping action.
• The atria are smaller, thin-walled chambers that receive blood, while the ventricles are larger, thick-walled chambers that pump blood.
• Atrioventricular (AV) valves (tricuspid and mitral) prevent backflow from ventricles to atria, while semilunar valves (pulmonary and aortic) prevent backflow from arteries to ventricles.
• The left coronary artery branches into the anterior interventricular artery and the circumflex artery, supplying the left ventricle and atrium.
• The right coronary artery branches into the marginal artery and posterior interventricular artery, supplying the right ventricle and atrium.

Blood Flow and Cardiac Muscle

• Coronary arteries supply the heart muscle itself with oxygenated blood.
• During contraction, coronary arteries are compressed, and blood flow occurs primarily during relaxation (diastole).
• Intercalated discs are specialized junctions between cardiac muscle cells that contain gap junctions and desmosomes.
• Gap junctions allow for rapid electrical communication between cells, ensuring coordinated contraction.
• Desmosomes provide strong adhesion between cells, preventing separation during contraction.

Heart Conduction System and Innervation

• The heart's conduction system includes the sinoatrial (SA) node, atrioventricular (AV) node, AV bundle (bundle of His), right and left bundle branches, and Purkinje fibers.
• The SA node (pacemaker) is located in the right atrium and initiates the electrical impulse.
• The AV node delays the impulse, allowing the atria to contract before the ventricles.
• Parasympathetic innervation (vagus nerve) decreases heart rate.
• Sympathetic innervation (sympathetic cardiac nerves) increases heart rate and force of contraction.

SA Nodal Cell and Action Potential

• An SA nodal cell at rest has a resting membrane potential of about -60mV.
• The action potential spreads from the SA node through the atria to the AV node.
• From the AV node, it travels through the AV bundle, bundle branches, and Purkinje fibers, causing ventricular contraction.

Cardiac Muscle Contraction and ECG

• Cardiac muscle contraction involves the same basic steps as skeletal muscle contraction but with some differences in calcium regulation and action potential duration.
• An electrocardiogram (ECG) records the electrical activity of the heart.
• The main components of an ECG are the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization).

Cardiac Cycle

• Pressure changes during the cardiac cycle cause the AV valves to close when ventricular pressure exceeds atrial pressure.
• Pressure changes during the cardiac cycle cause the pulmonary/aortic semilunar valves to open when ventricular pressure exceeds the pressure in the pulmonary artery/aorta.
• The five phases of the cardiac cycle are: ventricular filling, isovolumetric contraction, ventricular ejection, isovolumetric relaxation, and atrial contraction.

Cardiac Output

• Cardiac output is the amount of blood pumped by the heart per minute.
• Cardiac output is calculated by multiplying stroke volume (blood pumped per beat) by heart rate (beats per minute).

Cardiovascular System: Vessels and Circulation

• The three tunics common to most vessels are: tunica intima (inner layer), tunica media (middle layer), and tunica adventitia (outer layer).
• Arteries have a thicker tunica media with more elastic fibers and smooth muscle than veins.
• Capillaries have only a tunica intima, allowing for efficient exchange of substances.
• Veins have thinner walls and larger lumens than arteries, and some contain valves to prevent backflow.
• Elastic arteries (e.g., aorta) have a large proportion of elastic fibers in the tunica media, allowing them to expand and recoil.
• Muscular arteries have a thicker tunica media with more smooth muscle, allowing them to regulate blood flow to specific areas.
• Arterioles are small arteries that regulate blood flow into capillaries.
• Capillaries are small vessels where gas exchange occurs within tissues.
• Continuous capillaries have complete endothelial lining with tight junctions; found in muscle, skin, lungs, and CNS.
• Fenestrated capillaries have pores (fenestrations) in the endothelial lining; found in kidneys, small intestine, and endocrine glands.
• Sinusoids have large gaps between endothelial cells and a discontinuous basement membrane; found in liver, bone marrow, and spleen.
• Blood flow through a capillary bed is regulated by precapillary sphincters, which control blood flow into the capillaries.
• Veins carry blood back to the heart and serve as a blood reservoir.
• Simple pathway: artery → capillary → vein.
• Alternative pathways include anastomoses (interconnections between vessels) and portal systems (blood passes through two capillary beds before returning to the heart).

Vasodilators and Vasoconstrictors

• Vasodilators are substances that widen blood vessels, decreasing blood pressure.
• Vasoconstrictors are substances that narrow blood vessels, increasing blood pressure.

Blood Pressure

• Blood pressure is the force exerted by blood against the walls of blood vessels.
• The blood pressure gradient is the difference in blood pressure between two points in the circulation.
• Increased resistance in the systemic circulation leads to increased blood pressure because the heart has to work harder to pump blood through the vessels.

Hormones Regulating Blood Pressure

• Hormones that regulate blood pressure include:
  • Angiotensin II (vasoconstrictor, increases BP).
  • Atrial natriuretic peptide (ANP) (vasodilator, decreases BP).
  • Antidiuretic hormone (ADH) (vasoconstrictor, increases BP).
  • Aldosterone (increases blood volume, increases BP).
  • Epinephrine/Norepinephrine (can increase heart rate and cause vasoconstriction, increases BP)

Pulmonary Circulation

• Pulmonary circulation: right ventricle → pulmonary trunk → pulmonary arteries → lungs → pulmonary veins → left atrium.

Systemic Circulation - Arteries

• Arteries transporting blood away from the left ventricle: aorta, which branches into numerous arteries supplying the body.
• Major arteries supplying the body: brachiocephalic trunk, left common carotid artery, left subclavian artery, etc.

Gastrointestinal Arteries

• The three major arteries branching from the descending aorta to supply the gastrointestinal tract:
  • Celiac trunk (supplies stomach, liver, spleen).
    • Major branches: left gastric artery, splenic artery, common hepatic artery.
  • Superior mesenteric artery (supplies small intestine and part of large intestine).
    • Major branches: intestinal arteries, ileocolic artery, right colic artery, middle colic artery.
  • Inferior mesenteric artery (supplies the rest of the large intestine).
    • Major branches: left colic artery, sigmoid arteries, superior rectal artery.

Lymphatic System

• Lymph is a fluid similar to plasma but contains less protein.
• Lymph contains water, dissolved solutes, and lymphocytes.
• Lymph movement through lymphatic vessels, trunks, and ducts is aided by skeletal muscle contraction, respiratory movements, and valves.
• Lymph flows from lymphatic capillaries → lymphatic vessels → lymphatic trunks → lymphatic ducts → subclavian veins.

Lymphoid Structures

• Two categories of lymphoid structures:
  • Primary lymphoid structures (where lymphocytes are produced): red bone marrow and thymus.
  • Secondary lymphoid structures (where lymphocytes are activated): lymph nodes, spleen, tonsils, and MALT (mucosa-associated lymphoid tissue).