vet systems unit 2

layers of heart wall

endocardium, myocardium, epicardium

endocardium

made up of endothelium, subendothelial layer, and subendocardial layer, provides non-thrombogenic non-turbulent surface for blood flow

myocardium

thickest layer with cardiac muscle cells, responsible for contractile force

cardiac muscle cells structure

branching fibers, central nuclei, intercalated discs

epicardium

mesothelium, connective tissue and fat, contains coronary vessels and nerves, has visceral pericardium that touches heart

subendothelial layer of endocardium

loose connective tissue, small vessels, nerves

endothelium of endocardium

simple squamous epithelium, continuous with circulatory endothelium

subendocardial layer of endocardium

contains purkinje fibers

purkinje fibers function

modified cardiac myocytes that have rapid impulse conduction

purkinje fiber structure

large diameter, abundant gap junctions, fewer myofibrils, more glycogen, and high Na+ channel density

why do purkinje fibers have less myofibrils and more glycogen

need space for conduction machinery

why does myocardium have rich capillary network?

supports high metabolic demand

myocardium generates _ to pump blood

contractile force

epicardium main function

protective outer covering of heart

mesothelium of epicardium makeup

simple squamous epithelium

cusps/leaflets

endocardium covering a fibrous connective tissue core that ensure unidirectional blood flow

Atrioventricular valves

anchored by chordae tendineae, send blood from atria to ventricles

semilunar valves

no chordae tendineae, send blood from ventricles to aorta/pulmonary artery

cardiac skeleton

dense fibrous connective tissue that forms hearts structural core, provides structural support for valves and myocardium, electrical insulation between atria and ventricles, only conducts through AV node

fibrous annuli

tough rings that make up cardiac skeleton, support AV and semilunar valves

vessel wall layers

tunica intima, tunica media, tunica adventitia

generally arteries have thicker _ for _

tunica media for pressure and flow regulation

generally veins have thicker _ for _

tunica adventitia for support and capacitance (dilation)

tunica intima layers

endothelium, subendothelial layer, internal elastic lamina

tunica intima function

maintains blood-tissue interface and regulates vascular tone, permeability, and clotting responses

internal elastic lamina

wavy elastic sheet more distinct in arteries and less prominent/absent in veins, creates smooth surface for blood flow

tunica media structure

concentric layers of smooth muscle cells, interspersed elastic fibers, reticular fibers, and proteoglycans, thickness varies with vessel type

tunica media function

regulates vascular diameter, maintains blood pressure and flow distribution

tunica adventitia structure

loose connective tissue, vasa vasorum, and nervi vasorum

vasa vasorum

small vessels that supply outer wall of large arteries and veins

nervi vasorum

autonomic nerves regulating smooth muscle tone

tunica adventitia function

anchors vessel to surrounding tissues and provides nutrient and neural support to vessel wall

elastic arteries function

maintain continuous blood flow by storing energy during systole and releasing it during diastole (Windkessel effect)

elastic arteries structure

lots of concentric elastic lamellae in tunica media, prominent endothelium and internal elastic lamina in tunica intima, and thin vasa vasorum and nerves in tunica adventitia

elastic arteries examples

aorta, pulmonary arteries, major branches like carotid artery

muscular arteries function

resistance vessels that control vascular resistance and organ perfusion

muscular arteries structure

prominent internal elastic lamina of tunica intima, thick layer of smooth muscle cells in tunica media, and collagen, elastin, and vasa vasorum of tunica adventitia

muscular arteries examples

radial, femoral, coronary, mesenteric

arterioles function

primary resistance vessels that regulate systemic vascular resistance, control blood pressure and capillary perfusion via vasoconstriction/dilation

arterioles structure

thin endothelial layer and IEL sometimes in tunica intima, 1-3 layers of smooth muscle cells in tunica media, minimal tunica adventitia

arterioles examples

smallest arteries leading into capillary beds

capillaries function

site of gas, nutrient, and waste exchange, regulate fluid balance and permeability

capillaries structure

5-10 um wide, just wide enough for RBC to pass through, only have endothelium, may have pericytes

pericytes

support, repair, regulation

continuous capillaries

tight junctions

fenestrated capillaries

pores allow higher exchange

sinusoidal/discontinuous capillaries

wide gaps, incomplete basement membrane

types of venules and small veins

postcapillary venules and collecting venules and small veins

postcapillary venules structure

endothelium and thin connective tissue, little to no smooth muscle

postcapillary venules function

major site of leukocyte migration (diapedesis) and inflammation

collecting venules and small veins structure

1-2 smooth layers of smooth muscle

collective venules and small veins function

drain blood from capillary beds, regulate exchange and immune cell trafficking, act as capacitance vessels that hold most of body’s blood volume

medium and large vein examples

femoral, jugular, portal, vena cava

medium and large vein makeup

folds of tunica intima that prevent backflow, much thinner tunica media than arteries, thickest layer is tunica adventitia

medium and large veins function

act as capacitance vessels, return blood to heart

lymphatic vessels structure

endothelium has incomplete basal lamina, thin to no tunica media, valves present for unidirectional flow

lymphatic vessels function

collect interstitial fluid, transport immune cells, maintain tissue fluid balance

right atrium

receives deoxygenated blood from body

right ventricle

pumps deoxygenated blood to the lungs

left atrium

receives oxygenated blood from lungs

left ventricle

pumps oxygenated blood to body

average cell resting membrane potential

-60 mV

cardiomyocyte excitation

gap junctions allow AP to travel through all cells, the cells depolarize and contract in unified waves

plateau region in cardiac muscle

causes cell to completely contract before relaxing

electrical conduction pathway

SA node → atria → AV node → bundle of His → bundle branches → purkinje fibers → ventricles

what do SA node, AV node, and purkinje fibers all have in common?

ability to spontaneously depolarize without any external input

how quickly SA node depolarizes depends on

how steep phase 4 current is

how does sympathetic system affect the heart?

increases heart rate and contractility

how does parasympathetic system affect the heart?

decrease heart rate and contractility

B1-adrenergic receptors

norepinephrine will bind to these to increase heart rate and contraction

muscarinic acetylcholine receptors

where acetylcholine will bind to decrease heart rate and contraction

norepinephrine leads to a _ funny current, acetylcholine leads to a _ funny current

steeper, shallower

B-blockers

will block binding of norepinephrine so heart does not speed up or have greater contraction

ECG

allows us to visualize electrical activity

voltage has _ and _

magnitude, direction

voltage magnitude

measured proportional to the mass of tissue depolarizing

voltage direction

angle of measurement is the angle of current propagation

lead 1, lead 2, lead 3

RA → LA, RA → LL, LA → LL

what do all three leads form?

einthoven’s triangle

P wave

atrial depolarization

QRS complex

ventricles depolarizing

T wave

ventricles repolarizing

lead aVR

LA and LL → RA

lead aVL

RA and LL → LA

lead aVF

RA and LA → LL

ECG interpretation components

rate, rhythm, complexes, direction, beats

when is a heartbeat considered arhythmic?

when there is more than 10% variation in the R-R interval

mean electrical axis

calculated angle of depolarization, angle should point towards left ventricle

left axis deviation

above lead II

right axis deviation

above lead III

ECG does not show us

if the heart is actually beating, just depolarization

respiratory sinus arrhythmia

slight difference between R-R interval, common in brachycephalic dogs

sinus bradycardia definition, ECG features, causes, and treatment

• definition: heart range below normal species range

• ECG features: regular rhythm but prolonged RR intervals

• causes: high vagal tone (athletic dogs, sleeping), pathologic (hypothermia, hyperthyroidism, hyperkalemia), drug-induced (beta-blockers, Ca2+ channel blockers, digoxin, opoids)

• treatment: none if asymptomatic, treat underlying cause or atropine if unstable

sick sinus syndrome definition, ECG features, causes, and treatment

• definition: dysfunction of SA node causes failure of impulse initiation/conduction

• ECG features: highly irregular rhythm, pauses/arrests, may alternate with atrial tachycardia

• causes: structural (fibrosis, degenerative SA node diseases), inherited (genetic in breeds), idiopathic

• treatment: often requires pacemaker placement if symptomatic or causing syncope

AV blocks definition, ECG features, causes, and treatment

• definition: impaired conduction of atrial impulses through AV node

• ECG features: 1st degree: prolonged PR interval, 2nd degree: intermittent dropped QRS, 3rd degree: no relationship between P waves and QRS, ventricular escape rhythm

• causes: physiologic (high vagal tone for mobitz II), pathologic (myocarditis, endocarditis, cardiomyopathy, degenerative conduction disease), drug-induced (digitalis toxicity, b-blockers, Ca2+ channel blockers)

• treatment: none if asymptomatic or mild, atropine may help in vagal cases, pacemaker for 2nd or 3rd degree

two types of 2nd degree AV blocks

mobitz I: progressive PR lengthening until a beat is dropped, mobitz II: constant PR interval with sudden dropped QRS

drop in third degree AV block

spontaneous dpeolarization of purkinje fibers

types of bradycardias

sinus bradycardia, sick sinus syndrome, av blocks

types of tachycardias

supraventricular/atrial premature contraction, atrial fibrillation, ventricular premature contraction, ventricular tachycardia

Supraventricular/atrial premature contraction definition, ECG features, causes, and treatment

• definition: early atrial depolarization making an extra premature contraction

• ECG features: premature p wave, shortened T-P interval before premature beat

• causes: structural (atrial disease, dilation, fibrosis), metabolic (electrolyte imbalances), other (sympathetic stimulation, hypoxia, systemic disease)

• treatment: none if isolated or asymptomatic, treat underlying cause if frequent or additional symptoms, may consider antiarrhythmics