Cardiovascular System

System Overview

• The major components of the cardiovascular system include the heart, blood, and blood vessels.
• The cardiovascular system has four main functions: transporting blood, pumping substances with blood, facilitating delivery of vital materials/cellular waste exchange, and parallel circulation via the lymphatic system.

Heart

• The heart pumps blood throughout the body.

Blood

• Blood is connective tissue comprising plasma and cells.

Blood Vessels

• Blood vessels form a network of passageways for blood transport to and from cells.

Circulation

• Circulation refers to the movement of blood to and from the heart.
• Pulmonary circulation transports blood from the heart to the lungs and back.
• Systemic circulation moves blood from the heart to body tissues and back.

Heart Size, Location, and Orientation

• It is approximately the size of a fist that weighs less than 1 pound.
• It is located in the mediastinum between the second rib and the fifth intercostal space and is on the diaphragm's superior surface.
• Two-thirds of the heart lies to the left of midsternal line and is anterior to the vertebral column, and posterior to the sternum.
• The base (posterior surface) leans toward the right shoulder.
• The apex points toward the left hip.
• The apical impulse can be palpated between the fifth and sixth ribs, just below the left nipple.

Coverings of the Heart

• The pericardium is a double-walled sac surrounding the heart, made up of two layers.
• The superficial fibrous pericardium functions to protect, anchor the heart, and prevent overfilling.
• The deep two-layered serous pericardium is divided into:
  • Parietal layer: lines the internal surface of the fibrous pericardium.
  • Visceral layer (epicardium): located on the external surface of the heart.
  • Pericardial cavity: fluid-filled space between these two layers that decreases friction.

General Heart Structure and Function

• The heart can be understood as two pumps working together.
• The right side collects blood from the body and sends it to the lungs.
• The left side collects blood from the lungs and sends it to the rest of the body.

Heart Wall

• The heart is surrounded by the fibrous pericardium, a tough membrane.
• Inside the fibrous pericardium lies the serous pericardium, where the parietal layer of the serous pericardium lines fibrous pericardium.
• The visceral layer of the serous pericardium is fused to the heart surface, with a potential space between the layers called the pericardial cavity.
• The heart wall layers:
  • Epicardium: the outer layer, also known as the visceral pericardium.
  • Myocardium: middle layer, made of cardiac muscle.
  • Endocardium: epithelium lining the heart.

Internal Anatomy of the Heart

• The heart consists of four chambers; the small upper chambers are the atria.
• The large lower chambers are the ventricles.
• The chambers on the right and left sides are separated preventing blood mixing:
  • The interatrial septum is the wall that separates the two atria.
  • The interventricular septum is the wall between the ventricles.
• The atrial walls are thinner than the ventricular walls.
• Higher pressures are generated in the ventricles to move blood.
• The walls of the left ventricle are thicker than the walls of the right ventricle, because the right ventricle only pumps blood to the lungs.
  • The left ventricle pumps blood throughout the entire body.
• Blood enters the right atrium via two veins:
  • Superior vena cava: drains blood from the head, neck, chest, and upper extremities.
  • Inferior vena cava: drains blood from the trunk, organs, abdomen, pelvic region, and lower extremities.
• Pulmonary veins bring blood back to the left atrium.
• The pulmonary trunk carries blood from the right ventricle to the lungs.
• The aorta carries blood from the left ventricle to the body.
• To ensure correct blood flow through the heart, two sets of valves are present.
  • Atrioventricular (AV) valves are between each atrium and ventricle.
    • The tricuspid valve is located on the right side, having three cusps / folds.
    • The bicuspid, or mitral, valve is located on the left.
  • Semilunar valves are located between the ventricles and large arteries.
    • The pulmonary semilunar valve is on the right.
    • The aortic semilunar valve is on the left.
• In blood flow through the heart, blood returns to the right atrium via large veins.
• It drains to the right ventricle, then is pumped to the lungs.
• It then flows into the left atrium and left ventricle; it is then pumped into the body.

Electrical Pathway

• Cardiac muscle is autorhythmic, meaning it can contract without nerve impulses or hormones.
• Specialized cardiac cells create and distribute an electrical current for a controlled heart contraction.
• Nodal (pacemaker) cells create an electrical impulse at a regular interval.
• Nodal cells are divided into the sinoatrial node and the atrioventricular node.
• The sinoatrial (SA) node:
  • Located in the right atrium wall, near the superior vena cava entrance.
  • Generates an electrical impulse at approximately 70-80 impulses per minute.
• The atrioventricular (AV) node:
  • Located in the inferior wall of the right atrium.
  • Generates an electrical impulse at a rate of 40-60 beats per minute.
• The SA node sends its impulse to the AV node before the AV node can send a signal.
• Once the sinoatrial (SA) node generates an electric impulse, it transmits to the Atrioventricular (AV) node.
  • A slight signal delay allows for the atria to fill with blood before contraction.
• Once this charge reaches the AV node, it continues its journey through the AV bundle, or bundle of His.
• The AV bundle divides into right and left bundle branches in the interventricular septum and spread across the inner surfaces of both ventricles.
• Lastly, Purkinje fibers carry the impulse to the ventricles.

Microscopic Anatomy of the Heart

• Cardiac muscle cells have specific characteristics:
  • Striated
  • Short
  • Branched
  • Fat
  • Interconnected
• Cardiac muscle cells have one central nucleus, although some may have two. They contain large mitochondria volumes (25–35% of cell volume) which allows for resistance to fatigue. The muscle cells also include:
  • Z discs, A bands, and I bands all present
  • T tubules are wider and enter only once at the Z disc.
  • SR simpler than in skeletal muscle, with no triads.
• Intercalated discs are connecting junctions between cardiac cells:
  • Desmosomes: hold cells together; prevent cells from separating during contraction
  • Gap junctions: allow ions to pass from cell to cell; electrically couple adjacent cells
  • Allows heart to be a functional syncytium, a single coordinated unit
• Intercellular space between cells features a connective tissue matrix (endomysium) that:
  • Contains numerous capillaries.
  • Connects cardiac muscle to cardiac skeleton, providing something for cells to pull against.

Structure of Blood Vessel Wall

• Vessels consist of a lumen, which has central blood-containing space surrounded by a wall.
• Vessel walls, except for capillaries, made up of three layers, or tunics:
  1. Tunica intima.
  2. Tunica media.
  3. Tunica externa.
• Capillaries consist of endothelium with sparse basal lamina.
• The tunica intima contains:
  • Innermost layer in intimate contact with blood.
  • Endothelium with simple squamous epithelium lining the lumen.
    • It is continuous with endocardium.
    • Has a slick surface reduces friction
  • Subendothelial layer
    • Connective tissue basement membrane.
    • It is found only in vessels larger than 1 mm.
• Tunica media is:
  • Middle layer of smooth muscle and sheets of elastin.
  • Innervated by sympathetic vasomotor nerve fibers, controlling:
    • Vasoconstriction: decreased lumen diameter.
    • Vasodilation: increased lumen diameter.
  • Bulkiest and responsible for maintaining blood flow and blood pressure
• Tunica externa is the:
  • Outermost layer of the vessel wall
  • Also called tunica adventitia.
  • Primarily composed of collagen fibers that protect, reinforce, and anchor wall into surrounding structures.
  • It is infiltrated with nerve fibers and lymphatic vessels.
    • Large veins also contain elastic fibers.
    • Vasa vasorum is present to nourish outermost external layer.

Arteries

• Arteries are divided into three groups, by size and function:
  • Elastic, muscular, and arterioles.
• Elastic arteries are:
  • Thick-walled with large, low-resistance lumen.
  • The aorta and its major branches are called the conducting arteries, and conduct blood from the heart to medium sized vessels.
  • Elastin is found in all three tunics, mostly tunica media.
  • Contain substantial smooth muscle, but inactive in vasoconstriction.
  • Acts as pressure reservoirs, that expand and recoil as blood is ejected from heart.
    • Allows for continuous blood flow downstream between heartbeats.
• Muscular arteries:
  • Elastic arteries give rise to muscular arteries.
  • Also called distributing arteries because they deliver blood to the body organs.
  • Diameters range from pinky-finger size to pencil-lead size.
  • Accounts for most of named arteries.
  • Have thickest tunica media with more smooth muscle, but less elastic tissue.
  • Tunica media is sandwiched between elastic membranes.
  • Active in vasoconstriction
• Arterioles
  • They are the smallest of all arteries.
  • Larger arterioles contain all three tunics, while smaller ones mainly consist of endothelium, surrounded by a single layer of smooth muscle.
  • Control the flow into capillary beds via vasodilation, and vasoconstriction of smooth muscle.
  • They are also called resistance arteries.
  • They lead to capillary beds.

Capillaries

• Are microscopic with diameters so small that only a single RBC can pass through at a time.
• Walls are just thin tunica intima; in smallest vessels, one cell forms entire circumference.
• Pericytes help stabilize capillary walls, control permeability, playing a role in vessel repair.
• Supply all cells, except for cartilage, epithelia, cornea, and lens of eye.
• Their function to regulate the exchange of gases, nutrients, wastes, and hormones between blood, and interstitial fluid.

Types of Capillaries

• Capillary endothelial cells are joined by tight junctions and intercellular clefts.
  • Clefts allow passage of fluids and small solutes.
  • There are three types of capillaries:
  1. Continuous capillaries, abundant in skin, muscles, lungs, and CNS:
  • Continuous capillaries of brain:
  • Form the blood brain barrier, that is totally enclosed.
    • No intercellular clefts present in continuous capillaries of the brain.
  2. Fenestrated capillary
  • Found in areas involved in active filtration, absorption, and endocrine hormone secretion.
  • Contain Swiss cheese-like pores called fenestrations within the endothelial cells.
  • The fenestrations allow for increased permeability, and are usually covered with a thin glycoprotein diaphragm
  3. Sinusoidal capillaries
     • Few tight junctions; usually fenestrated with larger intercellular clefts; incomplete basement membranes.
     • Usually have larger lumens.
     • Found only in the liver, bone marrow, spleen, and adrenal medulla.
     • The blood flow is sluggish, allowing time for modification of large blood cells that pass between blood, and tissue.
     • Macrophages capture destroy foreign invaders.

Capillary Beds

• Capillary bed: interwoven network of capillaries between arterioles and venules
• Microcirculation: flow of blood through bed from arteriole to venule
• Terminal arteriole: branch off arteriole that further branches into vessels.
• 10-20 capillaries, which serve as exchange.
  • The capillaries then drain into postcapillary venule.
• Flow through bed is controlled by diameter of terminal arteriole.
• Upstream arterioles, local chemical conditions, and arteriolar vasomotor nerve fibers.
• In serous membranes of intestinal mesenteries there are two additional:
  • Vascular shunt: channel that directly connects arteriole with venule bypassing true capillaries. Consists of metarteriole and thoroughfare channel.
  • Precapillary sphincter: cuff of smooth muscle surrounding each true capillary. It branches off metarteriole, and acts as valve regulating blood flow into capillary bed. It is controlled by local chemical conditions and not innervated.

Veins

• Veins: carry blood toward the heart.
• Formation begins when capillary beds unite in postcapillary venules, and merge into larger veins. Capillaries unite to form postcapillary venules.
  • Consist of endothelium, and a little pericyte.
  • Very porous; allow fluids, and WBCs into the tissues.
  • Larger venules have one, or two layers of smooth muscle cells.
• Veins formed when venules converge.
• Have all tunics, but thinner walls with large lumens compared with corresponding arteries.
• Tunica media is thin, but tunica externa is thick.
• Contain collagen fibers, and elastic networks.
• Large lumen and thin walls make good storage vessels.
  • Called capacitance vessels, and blood reservoirs because hold up to 65% of total blood.
• Blood pressure is lower than in arteries, so adaptations ensure return of blood.
• Large diameter lumens offer little resistance. Other adaptations include: -Venous valves
  • Prevent backflow of blood
  • Most abundant in limbs -Venous sinuses
  • Flattened veins with a very thin wall
  • Composed of endothelium
  • Examples: coronary sinus of the heart; dural sinuses of the brain

Functions of Blood

• Functions include:
  • Transport
    • Transport functions include delivering O2 and nutrients to body cells; and transport hormones from endocrine organs to target organs.
  • Regulation
    • Regulation functions include maintaining body temperature by absorbing/distributing heat, and maintaining adequate fluid volume in circulatory system. -Protection
    • Protection functions include preventing blood loss and preventing infection.

Composition of Blood

• Blood is the only fluid tissue in the body and is a type of connective tissue.
  • The nonliving matrix is plasma, and living blood cells are the formed elements.
  • Cells are suspended in plasma and formed elements consist of:
    • Erythrocytes (red blood cells/RBCs)
    • Leukocytes (white blood cells/WBCs)
    • Platelets
• Spun tube of blood yields three layers:
  • Erythrocytes on bottom (~45%): known as hematocrit, the percentage of RBCs which is 47% ± 5% in males and 42% ± 5% in females
    • WBCs and platelets form a thin, whitish buffy coat (<1%) between RBCs and plasma layers.
  • Plasma then on top (~55%)
• Physical characteristics of blood:
  • Sticky, opaque fluid with metallic taste.
  • Color varies with O2 content. High O2 = scarlet red; Low O2 = dark red
  • pH 7.35–7.45
  • Makes up ~8% of body weight
  • Average volume: - Males: 5–6 L - Females: 4–5 L

Blood Plasma

• It is a straw-colored sticky fluid that is about 90% water.
• Includes over 100 dissolved solutes such as:
  • Nutrients, gases, hormones, wastes, inorganic ions. -Most plasma proteins are formed mostly by the liver.
  • Proteins remain in blood; and are not taken up by cells.
    • Albumin makes up 60% of plasma proteins.
      • It functions as a carrier of other molecules, as a blood buffer, and contributes to plasma osmotic pressure.