Bio 233 Ch. 18: The Heart

Pacemaker cells

Keep your heart beating at the correct rhythm and ensure that each cardiac muscle cell contracts in coordination with the others, because you don't want your brain to have to send a series of action potentials every time you need your heart to beat. Therefore, pacemaker cells are, in a way, your heart's very own brain, generating the initial spark that sends a current through your heart's internal wiring system, known as the intrinsic cardiac conduction system.

Myocarditis

Virus, bacterial, parasites (causes), treatment: antibiotics, steroids, bed rest

Pericarditis

Viruses, bacteria, open heart surgery, friction rub may be heard, treatment: diuretics, pain meds, address cause

Your heart is divided laterally into two sides by a thin inner partition called the septum. This division creates the four chambers- two superior atria, which are the low pressure areas, and two inferior ventricles that produce the high pressures. Each chamber has a corresponding valve, which acts like a bouncer at a club at closing time- he'll let you out but not back in.

When a valve opens, blood flows in one direction, into the next chamber. And when it closes, that's it- no blood can just flow back into the chamber it just left. When you heart your heart thumping, you're really hearing your heart valves opening and closing.

Endocarditis

Bacterial infection, damages heart valves, treatment: address cause

Systemic Loop (transport of oxygen throughout the body)

Begins in the left ventricle, when it flexes to increase pressure. Now the blood would like to flow into the nice low pressure left atrium where it just came from, but the mitral valve slams shut, forcing it through the aortic semilunar valve into your body's largest artery- nearly as big around as a garden hose- the aorta, which sends it to the rest of your body.

Mitral (bicuspid) valve

Between the left atrium and the left ventricle

Aortic Semilunar valve

Between the left ventricle and the aorta, which carries blood from the heart to the rest of the body

Tricuspid valve

Between the right atrium and the right ventricle

Pulmonary semilunar valve

Between the right ventricle and the pulmonary artery, allows blood to flow from the heart to the lungs

Right ventricle

Blood through pulmonary circuit

Left ventricle

Blood through systemic circuit

Veins

Blue (your blood is never actually blue, it's simply a brighter red when there's more oxygen in it). Carry blood back toward the heart.

Coverings of heart

Double walled pericardium sac. Outer layer: Superficial fibrous pericardium (anchor), Serous pericardium: parietal (middle layer) and visceral (bottom layer aka epicardium)

Layers of heart

Epicardium (aka visceral)--> myocardium--> endocardium

The path the electrical impulse takes to the bottom of the heart is called the atrioventricular bundle, also known by the more rad name, the bundle of His, where it branches out to the left and right ventricles. Finally, the signal disperses out into Purkinje fibers, which trigger depolarization in all surrounding cells, causing the ventricles to contract from the bottom up like toothpaste tubes, at which point the whole cycle starts all over again

Everything described just now, from when the SA node fires to when the last of the ventricular cells contract- takes about 220 milliseconds. So that is how your heart beats.

The heart

Powers the entire circulatory system, transporting nutrients, oxygen, waste, heat, hormones, and immune cells throughout the body, over and over.

Left atrium

Receives blood from pulmonary circuit

Right atrium

Receives blood from systemic circuit

Arteries

Red. Carry blood away from the heart.

Pulmonary circulation loop

Right ventricle--> Pulmonary semilunar valve--> pulmonary trunk--> pulmonary arteries--> lungs--> gets oxygen and drops off carbon dioxide in capillaries--> travels back to heart through pulmonary arteries--> left atrium--> mitral valve--> left ventricle

Heart Anatomy--Size, shape, location in thorax

Size of fist, shaped like a slight triangle, located in mediastinum (like behind the sternum and in-between each of the lungs)

Parasympathetic NS

Slows the heart heat. Opposes sympathetic, acetylcholine (hormone) hyper polarizes pacemaker cells and slows HR. (Rest and digest)

Sympathetic NS

Speeds up heart beat. Norephinephrine causes pacemaker to increase contractility (fight or flight)

Difibulator

Stops the uncoordinated heart and them revives it, this time perfectly coordinated (like a restart button)

Skeletal muscle tissue

Striated and contracts using the actin-myosin sliding filament dance you're heard so much about. Have long multinucleate cells. Fibers are both structurally and functionally separate from one another, meaning some cells can work while others don't

Prolonged high blood pressure can damage arterial walls, mess with your circulation and ultimately endanger your heart, lungs, brain, kidneys, and nearly every part of you.

*CPR can help prolong heart function during cardiac arrest, but it usually can't save a life without help from a defibrillator.

*If your systolic blood pressure is too low, that could mean that the volume of your blood is too low- like maybe because you've lost a lot of blood, or you're dehydrated.

*If your diastolic is too high, that could mean that your blood pressure is high even when it's supposed to be lower.

Pericardium (coverings of the heart)

-Double walled sac -Superficial fibrous pericardium: anchor (so that heart doesn't fly all over chest when beating) outermost layer (helps protect the heart while anchoring it to some of the surrounding structures) -(Inner) Serous pericardium: consists of inner visceral layer (epicardium) and outer parietal layer. These 2 layers are separated by a thick fluid so that the layers don't rub against each other and cause friction and damage the heart -Pericardial cavity- fluid (pericarditis / cardiac tamponade)

Heart anatomy

-Size of fist (technically its the size of 2 fists clamped together and a child's heart is the size of 1 fist) -Beats 72/min -Located in Mediastinum (center of chest, right in between both lungs) -Above diaphragm (superior surface of diaphragm) -In front of vertebrae (anterior) and behind sternum (posterior) -Apex: bottom corner of heart on left side -Apical impulse: where beat of apex can be felt

Structure and functions of the 4 heart chambers

1) Right atrium:

Blood pressure measurement. 120/80

120 is systolic (high) blood pressure- essentially the peak pressure, produced by the contracting ventricles that push blood out to all of your tissues. 80 is diastolic (low) blood pressure which is the pressure in your arteries when they ventricles are relaxed.

There are 4 chambers of the heart

2 ventricles (high pressure) and 2 atriums (low pressure)

Blood Pressure

A measure of the amount of strain your arteries feel as your heart moves your blood around.

Wall of heart itself begins at the visceral layer aka the epicardium --> myocardium--> endocardium

All in all: Superficial fibrous pericardium--> Serous pericardium (parietal layer--> visceral layer)--> visceral layer aka epicardium--> myocardium--> endocardium

The paddles send so much electricity through the heart that they trigger action potentials in all of the cells at once. Then, the cells depolarize, and start leaking again, and then the most leaky cells in the pacemaker SA node, reach their threshold and fire first re-setting the rhythm that keeps everyone in harmony so your heart functions properly.

Cardio Pulmonary Resuscitation (CPR): It can't correct fibrillation like a defibrillator can. What CPR can do is force a fibrillating heart to keep circulating oxygenated blood until help arrives. But if a person is in cardiac arrest, just breathing into their mouth and compressing their chest won't deliver the electricity needed to give the pacemaking cells a chance to reset.

This system transmits electricity along a precisely-timed pathway that ends with atrial and ventricular contractions- also known as heart beats. And it begins with pacemaker cells generating their own action potentials.

In most cells, the action potential starts with the resting potential, which the cell maintains by pumping sodium ions out and potassium ions in. Then, when some stimulus causes the sodium channels open up, the sodium ions flood back in, which raises the membrane potential until it reaches its threshold.

Fibrillation

In the heart, we call this out-of-sync behavior fibrillation, and it can be caused by all sorts of problems, especially ones that affect the pacemaker cells in the SA node.

Endocardium

Innermost layer (smooth, continuous with blood vessels)

Systemic circuit

Left ventricle--> aortic semilunar valve--> aorta--> rest of the body--> unoxygenated blood travels back to heart through the superior and inferior vena cava veins--> right atrium--> tricuspid valve--> right ventricle

Myocardium

Middle layer (involuntary cardiac muscular- does most of the work when it comes to the heart beating)

Electrical marvel that is the action potential and how it triggers both neurons and muscle cells. That process started by depolarizing the cell- that is, pushing the cell's membrane potential from negative toward positive past a threshold triggered voltage-gated ion-channels to open.

Most cells in your body only depolarize after being triggered by an external stimulus, or by a neighboring cell, in a long chain reaction of action potentials that's set off by the nervous system. But that is not the case for a special group of cells found only in your heart- ones that can trigger their own depolarization. These are called pacemaker cells.

So the first lub you hear in that pub-DUB noise is made by the mitral and tricuspid valves closing. And they do that because your ventricles contract to build up pressure and pump blood out of the heart. This high pressure caused by ventricular contraction is called systole.

Now the DUB sound- that's the aortic and pulmonary semilunar valves closing at the start of diastole. That's when the ventricles relax to receive the next volume of blood from the atria. When those valves close, the high-pressure blood that's leaving the heart tries to rush back in, but runs into the valves

The general system of chambers, valves, veins, and arteries all work together to circulate blood around your body.

Of course, fluid likes to move from areas of high pressure to areas of low pressure, and the heart creates those pressures, form once again following function.

Epicardium

Outermost layer (visceral pericardium)

Right side of heart

Oxygen poor blood (pulmonary circulation loop begins in the right ventricle)

Left side of heart

Oxygenated blood from lungs (systemic circuit begins in the left ventricle)

Cardiac muscle tissue

Striated and uses sliding filaments to contract. Has squat, branched out, and interconnected with one or two central nuclei. The cells are separated by a loose matrix of connective tissue called the endomysium which is chock full of capillaries, to serve up a constant supply of oxygen. Cardiac cells are also loaded with energy-generating mitochondria. In fact, mitochondria take up as much as 25-35% of each cell, making it resistant to fatigue, which is partly why your heart can beat nearly 3 billion times in a lifetime. Both physically and electrically connected, all of the time. Cardiac cells need to be linked in order to have that perfect timing.

Ventricles

The discharging chambers that push the blood back out of the heart. The ventricles are beastly compared to the atriums. They're the true pumps of the heart and they need big strong walls to shoot blood back out of the heart with every contraction.

The average human heart is about the size of 2 fists clasped together.

The heart is hollow, vaguely cone-shaped, and only weighs about 250 to 350 grams- a

The heart is located in the center of your chest contrary to popular belief that its located more on the left side. It is snuggled in the mediastinum cavity between your lungs. It sits at an angle, though, with one end pointing inferiorly toward the left hip, and the other toward the right shoulder. So most of its mass rests just a little bit left of the midsternal line.

The heart is nestled in a double-walled sac called the pericardium. The tough outer layer, of fibrous pericardium, is made of dense connective tissue and helps protect the heart while anchoring it to some of the surrounding structures, so it doesn't bounce all over the place while beating.

Truth is, the heart is really just a pump- a big, wet, muscly brute of a pump. The heart has only one concern: maintaining pressure

The heart maintains pressure by generating high hydrostatic pressure to pump blood out of the heart, while also creating low pressure to bring it back in. This gradient of force is what we mean when we talk about blood pressure.

Pacemaker cells operate the same way, except for that initial stimulus. They don't need it. Their membranes are dotted with leaky sodium and potassium channels that don't require any external triggers. Instead, as their channels let sodium ions trickle in, they cause the membrane potential to slowly and inevitably drift toward its threshold. Since the leaking happens at a steady rate the cells fire off action potentials like clockwork. And the leakier the membrane gets, the faster it keeps triggering action potentials.

The pacemaker cells at the start of the conduction system have the leakiest membranes, and therefore the fastest inherent rhythms, so they control the rate of the entire heart. And those fast, leaky cells are found in the sinoatrial node, or the SA node, up in the right atria. They essentially turn the whole SA node into your natural pacemaker. After those pacemaker cells make themselves fire, they spread their electrical impulses. The impulses leap across synapse-liek connections between the cells called gap junctions and continue down the conduction system until they reach the atrioventricular node, or AV node, located just above the tricuspid valve.

All in all: a wave of blood was pumped from the right ventricle to the lungs and then followed the lowest pressure pack to the left atrium. This is known as the....Pulmonary Circulation Loop.

The pulmonary circulation loop is how your blood unloads its burden of carbon dioxide into the lungs, and trades it in for a batch of fresh oxygen.

Atria

The receiving chambers for the blood coming back to the heart after circulating through the body. Pretty thin-walled, because the blood flows back into the heart under low pressure, and all those atria have to do is push it down into the relaxed ventricles which doesn't take a whole lot of effort.

Overview of blood pathway

The right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk, which is just a big vessel that splits to form the left and right pulmonary arteries. From there, and this is the only time in your body where deoxygenated blood goes through an artery- the blood goes straight through the pulmonary artery into the lungs, where it can pick up oxygen. It finds its way into very small, thin-walled capillaries, which allow materials to move in and out of the blood stream. In the case of the lungs, oxygen moves in, and carbon dioxide moves out. The blood then circles back to the heart by way of 4 pulmonary veins (the pulmonary arteries and veins are the big exception to the artery/vein, oxygen/no oxygen rule), where it keeps moving to the area of lowest pressure- because that is what fluids do- and in this case, that's inside the relaxed left atrium. Then the atrium contracts, which increases the pressure so the blood passes down through the mitral valve into the left ventricle.

Meanwhile, the inner serous pericardium consists of an inner visceral layer, or epicardium- which is actually part of the heart wall- and an outer parietal layer. These two layers are separated by a thick film of fluid that acts like a natural lubricant providing a slippery environment for the heart to move around in so it doesn't create friction as it beats.

The wall of the heart itself is made of yet more layers, three of them: that epicardium on the outside, the myocardium in the middle, which is mainly composed of cardiac muscle tissue that does all the work of contracting, and the innermost endocardium, a thin white layer of squamous epithelial tissue.

Now, when the signal hits the AV node, it actually gets delayed for like a 10th of a second- so the atria can finish contracting before the ventricles contract. Without that delay, all the chambers would squeeze at once, and the blood would just splash around and not go anywhere. So instead the atria contract and blood drops down into the ventricles, and then a moment later, the signal moves on and triggers the ventricles to squeeze, making the blood flow out of the heart.

There are 2 tricks to a good ventricular contraction. One, the ventricles are so large that the signal has to be distributed evenly to ensure a coordinated contraction and 2, the ventricles need to squeeze like their squeezing a tube of toothpaste- from the bottom up- to accelerate the blood through the big arteries at the top of the heart. So from the AV node, the signal travels straight down to the inferior end of the heart and gets distributed to both sides.

Ventricles

Thicker walls than atrium, pumps of heart, right ventricle- blood to pulmonary trunk, left ventricle- blood into aorta, to body

Atria

Thin walled, three veins to right atrium, four pulmonary veins into left atrium

Once oxygen cycles the body, the now oxygen-poor blood loops back to the heart, entering through the big superior and inferior vena cava veins, straight into the right atrium. When the right atrium contracts, the blood passes through the tricuspid valve, into the relaxed right ventricle, and right back to where we started.

This whole double-loop cycle plays out like a giant figure 8, heart to lung to heart to body to heart again- and runs off that constant high pressure, low-presssure gradient exchange regulated but the heart valves.