The Cardiovascular System

Chapter 11

Cardiovascular Overview

•  Major function: transportation

•  Blood transports oxygen, nutrients, cell wastes, hormones, and many other substances vital for homeostasis to and from the cells

The Heart: Anatomy

•  Enclosed within the mediastinum

•  Apex points toward the left at approximately the 5th intercostal space

•  Enclosed by double-walled pericardium (sac)

Pericardium

•  Outer Layer: fibrous pericardium

•  Protection & anchors heart to surrounding structures

•  Inner Layer: serous pericardium

•  slippery two-layer structure

•  parietal layer – lines fibrous pericardium

•  visceral layer – hugs heart and is part of heart wall (also called epicardium)

•  produces serous fluid to allow the heart to beat in a relatively frictionless environment

Heart Wall: three layers

•  Epicardium: visceral pericardium

•  Protects by producing serous fluid

•  Myocardium: thick bundles of cardiac muscle twisted and whorled into ringlike arrangements

•  Contractile layer of heart

•  Endocardium: thin, glistening sheet of endothelium that lines the heart chambers

•  Continuous with the linings of the blood vessels leaving and entering the heart

Heart Wall

•  Chambers of the Heart

•  Four chambers: two left & two right separated by septum

•  Each lined with endocardium

•  Upper: right & left atria - receiving chambers for blood from the body

•  Lower: right & left ventricles: thick-walled, discharging chambers, pumps blood from the heart circulation throughout the body

Circulation & Blood Flow

•  Heart acts as a double pump

•  Pulmonary circulation:

•  Superior & inferior venae cavae brings blood to the right atrium

•  Flows into right ventricle and is pumped through pulmonary trunk, which splits into pulmonary arteries (right & left) to take blood to the lungs

•  Oxygen & carbon dioxide are exchanged in the lungs, and the blood is returned to the left atrium through the four pulmonary veins

•  Systemic circulation:

•  Blood flows into the left ventricle from the left atrium, and is pumped out of the heart into the aorta

•  Systemic arteries branch to supply body tissues and oxygen-poor blood circulates via the systemic veins back to the superior & inferior venae cavae to the right side of the heart

Heart Valves

•  Four valves allow blood to flow in only one direction through heart chambers

•  Atrioventricular (AV) valves: located between atrial and ventricular chambers on each side (open during heart relaxation)

•  Bicuspid (mitral) valve – left AV valve (has two flaps of endocardium)

•  Tricuspid valve – right AV valve (has three flaps of endocardium)

•  Chordae tendinae – anchor flaps of valves to walls of the ventricles so that valves can prevent backflow of blood when ventricles contract

•  Semilunar valves: guard bases of two large arteries leaving the ventricular chambers (open during heart contraction)

•  Pulmonary semilunar valve: between right ventricle and pulmonary artery

•  Aortic semilunar valve: between left ventricle and aorta

Cardiac Circulation

•  Blood contained in the heart does not nourish the myocardium

•  Coronary arteries (right and left): branch from the base of aorta and encircle the heart as the functional blood supply

•  Oxygenates and nourishes the heart muscle

•  If blocked, causes ischemia (chest pain) and eventually myocardial infarction

•  Coronary veins: myocardium drains into coronary veins, which empty into an enlarged vessel (coronary sinus)

•  Coronary sinus empties into right atrium

Physiology of the Heart

•  In one day, pumps 6 liters of blood through the blood vessels over 1000 times

•  Made of cardiac muscle – contract spontaneously and independently, even if all nervous connections are severed

•  Fibers shorter than skeletal muscle & branching with striations

•  One central nucleus

•  Intercalated disc where cells join

•  Involuntary muscle

•  Muscle cells in different areas of the heart have different rhythms, so need to be coordinated with a control system

•  Atrial cells = 60 bpm

•  Ventricular cells = 20-40 bpm

•  Two systems regulate heart activity:

•  Autonomic nervous system: increase & decrease heart rate

•  Intrinsic conduction system (nodal system): built into the heart tissue & sets basic rhythm

Intrinsic Conduction System

•  Tissue is a cross between muscle and nervous tissue

•  Causes heart muscle depolarization in only one direction – from the atria to the ventricles

•  Contraction rate: 75 bpm as coordinated unit

•  Sinoatrial node: located in right atrium; increases rate of depolarization; starts heartbeat and sets pace (pacemaker)

•  Atrioventricular (AV) node: junction of atria & ventricles; in septum between two atria; only way impulse gets to ventricles (heart block if not conducting impulse)

•  Atrioventricular (AV) bundle (bundle of His) and right and left bundle branches located in the interventricular septum (wringing contraction of ventricle)

•  Purkinje fibers – spread within the muscle of the ventricle walls (causes contraction from apex to atria to eject blood superiorly)

Cardiac Cycle

•  Based on pressure and conduction of impulses to cause contractions

•  One complete heartbeat

•  Recorded on electrocardiogram (ECG)

•  P wave: atrial depolarization

•  QRS wave: rapic ventricle depolarization

•  T wave: ventricle repolarization

•  Cardiac Cycle: Fig 11.8 page 366

•  ECG: “A Closer Look” pg. 367

Cardiac Cycle & Heart Sounds

•  Atria contract simultaneously, then as they relax the ventricles contract

•  Systole: heart contraction (ventricular)

•  Diastole: heart relaxation (ventricular)

•  Cardiac cycle: events of one complete heartbeat, during which both atria and ventricles contract & relax

•  Mid-to-late diastole: semilunar valves closed and AV valves open

•  Ventricular systole: AV valves close, semilunar valves open

•  Early diastole: semilunar valves shut and AV valves forced open

•  Heart sounds – “lub-dup”

•  “lub” = closing of AV valves; “dup” = semilunar valves close at end of systole

Cardiac Output

•  Cardiac output (CO): amount of blood pumped out by each side of the heart in 1 minute

•  Stroke volume (SV): volume of blood pumped out by a ventricle with each heartbeat

•  CO = HR x SV

•  Average CO = 75 bpm x 70 ml/beat = 5250 ml/min

•  Entire blood supply passes through the body once each minute (adult blood volume is approximately 6000 ml)

•  Blood Vessels

Vascular system: closed transport system of blood vessels

•  Blood flows from arteries (0.1 mm-1 cm)  arterioles (10-100 µm)  capillary beds  venules (10-100 µm)  veins (0.1-1 mm)

Microscopic Anatomy of Blood Vessels

•  Blood vessels have three coats, or tunics (except capillaries)

•  Tunica intima: lines the interior of the vessels; thin layer of endothelium

•  Tunica media: bulky middle coat; smooth muscle and elastic fibers (constricts or dilates blood vessels)

•  Tunica externa (adventitia): outermost tunic; supports and protects vessels

Structural Differences in Blood Vessels

Arteries / Veins / Capillaries
•  Take blood away from heart in circulation
•  Thickest walls (thick tunica media) to withstand continuous changes in pressure
•  Have additional elastic membranes in the tunica media / •  Take blood towards the heart in circulation
•  Thinner tunica media, so larger lumen
•  Veins have valves to prevent backflow of blood and ensure that venous return equals cardiac output (amount of blood pumped out of heart)
•  Muscles surrounding veins aid in venous return as well / •  Exchange of materials between blood and tissues happens through the walls of capillaries
•  Wall is one cell layer thick (only tunica intima)
•  Form capillary beds in tissues (flow from arteriole to venule through capillary bed is called microcirculation)

Capillary Exchange

•  Number of capillaries in a tissue is related to metabolic activity

•  Types of capillary exchange:

•  Diffusion (O2, CO2, glucose, amino acids, hormones) & osmosis (water)

•  high to low (slow process)

•  Proteins & blood cells are too large to leave the vessel

•  Transcytosis – active transport, material enclosed in pinocytic vessels (large lipid insoluble molecules, i.e. insulin)

•  Bulk flow – pressure driven flow of particles that regulates relative volume of blood and interstitial fluid (faster process)

•  Blood pressure pushes fluids out of the vessels; osmotic pressure draws fluid back in (filtration and reabsorption)

•  Lymphatic system returns fluid left in the interstitial fluid back to the blood

Vital Signs: Physiology of Circulation

•  arterial pulse, blood pressure (along with respiratory rate & body temperature) are vital signs taken in clinical setting

•  Arterial pulse: arteries expand and recoil with each beat of the left ventricle and creates a pulse (pressure wave)

•  normally equals heart rate: 70-76 bpm in normal resting person

•  influenced by activity, postural changes, emotions

•  Blood pressure: pressure blood exerts against the inner walls of blood vessels that keeps blood circulating continuously even between heartbeats

•  Highest in arteries

•  Systolic – peak of ventricular contraction; diastolic – ventricles relaxing

•  Normal is 120/80 mm Hg

•  Figure 11.22 on page 386

Factors Affecting Blood Flow

•  Blood flow: volume of blood that flows through any tissue/time (mL/min); moves down pressure gradient

•  Affected by:

•  Blood pressure: decreases as move away from aorta

•  Resistance: friction increases as size of lumen decreases; differences in blood viscosity; friction increases as blood vessel length increases

•  Venous return: valves keep blood from backing up

•  Velocity: flows faster in larger vessels