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Cardiac & Muscle Physiology — Simple Study Notes

Smooth Muscle

What is it? Smooth muscle is the type of muscle found in your organs — like your stomach, intestines, and blood vessels. Unlike the muscles you flex at the gym, you can't control it on purpose; it works automatically.

What does it look like? It's tiny — much smaller than other muscle types. Under a microscope it looks smooth (no stripes), hence the name.

How does it contract (squeeze)? Calcium enters the muscle cell from outside → calcium grabs onto a protein called calmodulin → that combo turns on an enzyme → the enzyme wakes up myosin (a motor protein) → myosin grabs actin (another protein) → the cell squeezes together.

Two types:

  • Multiunit — Each muscle cell works on its own, like independent light switches. Your brain tells each one what to do. Found in your eyes (iris) and large blood vessels.

  • Single unit — All the cells are connected and fire together as a team. Like a wave at a stadium. Found in your gut and small blood vessels. This teamwork is what creates peristalsis (the wave that pushes food through your intestines).

Cardiac Muscle (Heart Muscle)

What is it? The muscle that makes up your heart. It's in a category of its own — it borrows features from both skeletal and smooth muscle.

Borrowed from skeletal muscle: Striped appearance, uses calcium + troponin to contract, has an internal calcium storage system (SR) and t-tubules.

Borrowed from smooth muscle: Cells are connected through tiny pores (gap junctions) so electrical signals can pass cell to cell. The heart can beat on its own — no brain signal required.

Special feature — intercalated discs: Cardiac cells are joined end-to-end by special structures called intercalated discs. Think of them like puzzle piece connectors that hold cells together mechanically AND let electricity pass through.

Why does the heart never cramp (tetanus)? The heart's electrical signal lasts much longer than a normal muscle signal. This long signal creates a long "rest period" (refractory period) after each beat, which forces the heart to relax before it can beat again. This is why your heart can't get stuck in a permanent squeeze the way a leg cramp works.

The Heart's Electrical System

Think of the heart like a building with a fire alarm system. There's one main alarm (the SA node), and if it fails, backup alarms (AV node, Purkinje fibers) take over — just at a slower pace.

SA Node (the main pacemaker)

  • Located in the upper right chamber (right atrium)

  • Fires 70–80 times per minute

  • Sets the pace for the whole heart

  • Signal spreads immediately to both upper chambers (atria), making them squeeze at the same time

AV Node (the relay station)

  • Located at the bottom of the right atrium

  • Fires 40–60 times/min on its own (backup)

  • Intentionally slows the signal down by about 0.1 seconds

  • This pause lets the lower chambers (ventricles) finish filling with blood before they squeeze

AV Bundle → Purkinje Fibers (the wiring)

  • Carry the signal down through the wall between the ventricles, then fan out

  • Make the ventricles squeeze from the bottom up — like squeezing a tube of toothpaste from the bottom, pushing blood upward and out

The ECG (Heart Monitor Reading)

An ECG is just a way to "listen" to the heart's electricity from outside the body using sticky electrode pads. Three main blips appear:

  • P wave — The upper chambers (atria) are getting their electrical signal and squeezing

  • QRS complex — The lower chambers (ventricles) are getting their signal and squeezing (this is the big pump)

  • T wave — The ventricles are resetting/relaxing

Heart sounds ("lub-dub"):

  • Lub — valves between atria and ventricles snap shut at the start of the squeeze

  • Dub — valves leading to the arteries snap shut when the squeeze is done

The Cardiac Cycle (One Full Heartbeat)

  • Diastole = heart is relaxed and filling with blood (think: "di" = down time)

  • Systole = heart is squeezing and pumping blood out (think: "sys" = send it)

About 75% of the blood that fills the ventricles flows in passively during diastole — before the atria even squeeze. The atrial squeeze (P wave) just tops it off.

How the Brain Controls Heart Rate

Your brain can speed up or slow down your heart through two systems:

Parasympathetic (the brake — "rest and digest"):

  • Uses the vagus nerve (nerve from the brain to the heart)

  • Releases acetylcholine → opens potassium channels → the cell becomes harder to fire → heart slows down

Sympathetic (the gas pedal — "fight or flight"):

  • Releases norepinephrine (from nerves) and epinephrine/adrenaline (from adrenal glands)

  • Makes cells easier to fire, speeds up signals, and makes each squeeze stronger

  • Increases heart rate AND contraction force

Cardiac Output (How Much Blood the Heart Pumps)

Cardiac Output (CO) = Heart Rate × Stroke Volume

  • Heart rate = how many times your heart beats per minute (~72 at rest)

  • Stroke volume = how much blood squirts out with each beat (~70 mL)

  • At rest: 72 × 70 = about 5 liters per minute (basically all your blood, every minute!)

  • During hard exercise: can jump to 20–25 liters per minute

  • The difference between your resting and max output = your cardiac reserve

Endurance athletes have bigger, stronger hearts:

  • Lower resting heart rate (40–60 bpm) because each beat pumps more blood

  • Higher stroke volume (up to 160 mL per beat during exercise)

The Blood Vessel System

Arteries — thick, strong pipes that carry blood away from the heart under high pressure. The largest ones are springy (elastic) which helps keep blood flowing even when the heart is refilling.

Arterioles — tiny branches of arteries. They have a muscular wall that can squeeze shut or open wide, making them the main "traffic controllers" of blood flow. Little rings of muscle (precapillary sphincters) act like gatekeepers at the entrance to each capillary.

Capillaries — microscopic tubes where the actual exchange happens. Oxygen, nutrients, and waste pass through their paper-thin walls. There are 20–40 billion of them in your body, and most cells are within 20 micrometers of one. Red blood cells squeeze through single-file.

Veins — low-pressure return pipes. They hold about 60–65% of your total blood at rest, acting as a reservoir. They have one-way valves to stop blood from flowing backwards.

How Blood Gets Back to the Heart (Venous Return)

Blood has to fight gravity to get from your legs back up to your heart. Five things help:

  1. Pressure difference — blood pressure on the vein side (17 mmHg) is higher than in the relaxed heart (0 mmHg), so blood flows toward lower pressure

  2. Muscle pump — when your leg muscles squeeze during walking/exercise, they literally squeeze the veins and push blood upward

  3. Vein valves — one-way flaps inside veins stop blood from sliding back down

  4. Breathing — when you inhale, pressure in your chest drops, which sucks blood toward the heart

  5. Heart suction — after each squeeze, the heart chambers spring back open, creating a mild suction effect

Varicose veins happen when those one-way vein valves stop working properly, letting blood pool. This can cause dangerous clots.

The Capillary Fluid Exchange

Fluid is always leaking out of capillaries and being reclaimed. Two opposing forces control this:

  • Hydrostatic pressure (blood pressure) — pushes fluid OUT of the capillary

  • Colloid osmotic pressure (proteins in blood) — pulls fluid BACK IN (proteins are too big to leave, so they act like a sponge attracting water)

Fluid normally leaks out at the beginning of a capillary and gets pulled back in at the end. Small amounts that don't make it back are collected by the lymphatic system.

The Lymphatic System

Think of lymphatics as the body's drainage/sewage system. They collect the small amount of fluid that leaks out of capillaries and can't get back in, and return it to the bloodstream.

  • Returns about 120 mL per hour back to the blood (drains into veins near your collarbone)

  • Lymph fluid is rich in fats and proteins, and the lymphatics are actually the main highway for absorbing fats from your food

  • Lymph nodes are little filter stations along the way, packed with white blood cells that catch bacteria and foreign invaders

  • If lymph flow gets blocked → fluid builds up = edema (severe swelling)

  • Elephantiasis is an extreme example — a parasitic worm clogs the lymph vessels, causing massive limb swelling