heart

Cardiovascular System Overview

  • The three principal components that make up the circulatory system are:

    • Heart: The pump

    • Blood Vessels: The pipes

    • Blood: The fluid to be moved

  • The function of the circulatory system is impacted by:

    • Endocrine System

    • Nervous System

    • Kidneys

  • Key Functions of the Circulatory System:

    • Supply oxygen and nutrients

    • Remove wastes

    • Regulate body temperature

    • Distribute hormones

    • Clotting of open wounds

    • Immuno-vigilance

Blood

  • Blood is composed of:

    • Cells

    • Cell Fragments

    • Plasma

  • Plasma Composition:

    • At least 90% water

    • Electrolytes and Nutrients:

    • Glucose

    • Amino Acids

    • Vitamins

    • Wastes:

    • Urea

    • Creatinine

    • Bilirubin

    • Gases:

    • O$2$, CO$2$

    • Hormones and Proteins:

    • Albumin

    • Fibrinogen

Erythrocytes (Red Blood Cells)

  • Function: Transport oxygen and carbon dioxide

  • Structure:

    • Biconcave disk shape with flexible membrane

    • Large surface area enhancing diffusion

  • Contents:

    • Hemoglobin: Binds oxygen and carbon dioxide

    • Enzymes:

    • Glycolytic enzymes

    • Carbonic anhydrase

    • Features:

    • No nucleus or organelles

    • No mitochondria

    • No DNA or RNA, thus no division of mature RBCs

  • Lifespan: Approximately 120 days

  • Production:

    • Triggered by Erythropoietin from kidneys

    • Synthesized in red bone marrow through erythropoiesis

    • Filtered by the spleen and liver

Circulatory System Loops

  • The circulatory system comprises two loops:

    • Systemic Loop:

    • Carries oxygenated blood from the heart to the rest of the body

    • Pulmonary Loop:

    • Carries deoxygenated blood to the lungs and back to the heart

  • The cardiovascular system is considered a closed system

    • Leaks are problematic

Heart Valves

  • Four Heart Valves:

    1. Right AV valve / Tricuspid (R.A. → R.V.)

    2. Pulmonary / Semilunar (R.V. → Pulmonary artery)

    3. Left AV valve / Bicuspid / Mitral (L.A. → L.V.)

    4. Aortic / Semilunar (L.V. → Aorta)

  • Function: Ensure a one-way flow of blood

    • Opens when pressure is greater behind the valve

    • Closes when pressure is greater in front of the valve

Heart Walls

Layers of the Heart Wall:

  1. Endocardium:

    • Thin layer of endothelial tissue lining the interior of each chamber; continuous with the lining of blood vessels

  2. Myocardium:

    • Middle layer, composed of cardiac muscle

    • Cells connected by intercalated disks

    • Desmosomes: Hold cells together

    • Gap-junctions: Allow electrical current to flow between cells, forming a functional syncytium

  3. Epicardium:

    • Thin external membrane covering the heart filled with pericardial fluid

Myocardium and Cardiac Cells

  • Myocardium contains intercalated discs comprising:

    1. Desmosomes: Mechanical connections between cells

    2. Gap Junctions: Low resistance pathways for electrical current

  • Cell Types in Heart:

    • Electrical Conduction System: ~1% of cells

    • Known as Pacemaker Cells, no contractile components

    • Force Producing Cells: ~99% of cells

    • Known as myocytes or contractile cells, containing striated muscle, allowing contraction via myosin-actin interaction

Electrical Activity of the Heart

  • Heart muscle has the capability to generate its own rhythmic electrical activity, known as autorhythmicity.

  • Accomplished by a subset of specialized cardiac muscle cells (pacemaker cells) that control the rate and coordination of cardiac contractions.

  • Pacemaker Activity:

    • Pacemaker cells initiate their own action potentials at regular frequencies.

    • Controlled through the generation of pacemaker potentials; uses multiple types of channels:

    • VG F-type Na Channel (F: funny)

    • VG-T type Calcium Channel (T: transient)

    • VG-L type Calcium Channel (DHP channel; L: long-lasting)

    • VG-Potassium Channels (various types)

Pacemaker Potential

  • Oscillation of the membrane potential leads to action potential generation at regular intervals, involving changes in K$^+$, Na$^+$, and Ca$^{2+}$:

    1. Hyperpolarization increases Na$^+$ permeability, causing membrane potential depolarization

    2. Increased Ca$^{2+}$ permeability leads to further depolarization, enabling threshold attainment

  • Action Potential Sequence:

    1. Membrane depolarization leads to increased K$^+$ permeability, causing repolarization

    2. A second increase in Ca$^{2+}$ permeability occurs, starting the cycle over

Electrical Conduction System of the Heart

Components:

  • Sinoatrial (SA) Node:

    • Located in the right atrium, near the superior vena cava; exhibits autorhythmicity of ~70 action potentials per minute

  • Atrioventricular (AV) Node:

    • Located at the base of the right atrium; exhibits autorhythmicity of ~50 action potentials per minute, follows SA node

  • Bundle of His:

    • Tract of specialized cardiac pacemaker cells from the AV node into the ventricles

  • Purkinje Fibers:

    • Small terminal fibers spreading throughout the ventricular myocardium; exhibit autorhythmicity of ~30 action potentials per minute, conduct rapidly

  • Interatrial Pathway:

    • Conducts pacemaker activity from right to left atrium

  • Internodal Pathway:

    • Conducts pacemaker activity from the SA node to the AV node

  • AV Nodal Delay:

    • Conduction through the AV node is relatively slow (approx. 100 ms) to ensure ventricular contraction follows atrial contraction

Cardiac Action Potentials

Action Potentials in Contractile Cells:

  1. Cardiac action potentials differ from those in pacemaker cells.

  2. Resting potential is very negative (-90 mV) until excited.

  3. Rising phase involves fast Na$^+$ influx (VG Na channel).

  4. Exhibits a plateau phase due to increased Ca$^{2+}$ permeability (L channel) and decreased K$^+$ permeability.

  5. Falling phase occurs with decreased Ca$^{2+}$ permeability and increase in K$^+$ permeability.

Cardiac Cycle

  • The cardiac cycle is defined as all events related to blood flow through the heart during one heartbeat:

    • Diastole: Ventricular relaxation phase

    • Systole: Ventricular contraction phase

  • Phases of Cardiac Cycle:

    • Late Diastole: Ventricular filling

    • Early Systole: Isovolumetric contraction

    • During Systole: Ejection of blood

    • Early Diastole: Isovolumetric relaxation

Heart Sounds

  • First Heart Sound:

    • Low-pitched, soft and long sound signifying closure of AV valves (dub)

  • Second Heart Sound:

    • High-pitched, sharp and short sound marking the closure of semilunar valves (dup)

  • Murmurs: Abnormal sounds often due to turbulent flow through malfunctioning valves:

    • Stenotic Valve: Narrow and stiff, creating a whistling sound

    • Insufficient Valve: Does not close properly, leading to swishing sound

Regulation of Cardiac Output

  • Cardiac Output (C.O.): Volume of blood pumped by each ventricle per minute; formula: C.O. = H.R. x S.V.

    • Average heart rate: ~70 bpm

    • Average stroke volume: ~70 mL

  • C.O. = 70 x 70 = 4,900 mL/min ~5 liters/min

  • Regulation of heart rate is controlled by:

    • Parasympathetic Nervous System

    • Sympathetic Nervous System

  • Regulation of stroke volume is influenced by:

    • Intrinsically (volume of returning venous blood)

    • Extrinsically (sympathetic nervous system)

Blood Vessels

Classification of Blood Vessels:

  • Arteries: Carry blood away from the heart

  • Arterioles: Smaller vessels branching from arteries

  • Capillaries: Smallest vessels enabling material exchange

  • Venules: Formed when capillaries join

  • Veins: Larger vessels formed from venules, returning blood to the heart

  • Exception: Pulmonary Arteries carry deoxygenated blood to lungs; Pulmonary Veins carry oxygenated blood from lungs to heart

Blood Pressure in Circulatory System

  • Systemic and pulmonary circulations are closed systems of vessels; C.O. ~5 liters/min

Key Equations:

  • Blood Flow (F):

    • F = ΔP/R

    • F = flow rate (mL/min)

    • ΔP = pressure gradient (difference in pressure between two points)

    • R = resistance (affected by blood viscosity, vessel length, and vessel radius)

  • Resistance (R):

    • Dependent on blood viscosity, length, and radius; R α 1/r⁴

    • Flow (F) α r⁴

Factors Influencing Vasoconstriction and Vasodilation

  • Caused by (Vasoconstriction):

    • Decrease in O$2$, increase in CO$2$

    • Sympathetic NE activity (α-adrenergic receptor activation)

  • Caused by (Vasodilation):

    • Increase in O$2$, decrease in CO$2$

    • Sympathetic NE activity (β₂-adrenergic receptor activation)

Capillary Exchange

  • Capillaries allow for exchange of materials via:

    • Diffusion: Based on concentration gradients

    • Bulk Flow: Mechanism maintaining fluid balance between blood and extracellular space

  • Ultrafiltration: Flow of plasma into tissues; Reabsorption: Flow back into capillaries

  • Properties of Capillaries:

    • Extremely small (average diameter ~7 μm)

    • Thin walls (single layer of endothelial cells)

    • Porous walls for small water-soluble substances and lipid-soluble substances for exchange

    • Dense network, high surface area, low velocity for effective exchange

Venous System

  • Venous Capacity: The volume of blood veins can accommodate; acts as capacitance vessels (blood reservoirs)

  • Venous Return: Blood volume entering each atrium per minute; facilitated by:

    • Valves: Prevent backflow of blood

    • Respiratory Activity: Pressure changes during respiration

    • Skeletal Muscle Pump: Squeezing by muscle contraction

Baroreceptor Reflex

  • Baroreceptors: Mechanoreceptors sensitive to changes in arterial blood pressure

    • Elevate or reduce their firing rate in response to arterial pressure changes

  • Signaling alters the balance of activity in parasympathetic and sympathetic systems for cardiac output and peripheral resistance regulation.

Summary of ANS Control on the Heart Rate and Cardiac Output

  • Parasympathetic Stimulation:

    • Decreases heart rate

  • Sympathetic Stimulation:

    • Increases heart rate and contractility

  • Total Peripheral Resistance: Increased by vasoconstriction leading to higher blood pressure

Practice quiz:

  1. true or false:- cardiac cells are electrically coupled to their neighbors, forming a functional syncytium

-true

  1. true or false: in cardiac muscle cells, Ca++ can enter cells via L-type Ca++ channels in the T-tubules

  • true

  1. the right half of the heart pumps blood through the___ circuit and the left half pumps blood through the ___ circuit

  • pulmonary; systemic

  1. the direction of impulses through the conduction system of the heart is normally

  • SA node- AV node- bundle of His- Purkinje fibers

  1. The AV nodal delay ensures that:

  • the atria contract and empty their contents into the ventricles prior to ventricular systole

  1. the refractory period of cardiac muscle

  • prevents tetanic contraction of the heart to occur to ensure smooth, coordinated ejection of blood from the ventricles

  1. the T wave in an electrocardiogram represents

-ventricular repolarization

  1. the f-type Na+ channel (funny) channel in cardiac pacemaker cells is important for the

-depolarixing pacemaker potential

  1. Ca++ enters cardiac contractile cells during the

  • plateau phase of the action potentilal

  1. true or false- with complete heart block, the frequency of the QRS events in the electrocardiogram decreases.

  • true