Cardiac Output 1: Heart Rate

Introduction

  • Instructor: Dr. Erin Bruce

  • Topic: Cardiac Output (Focus on Heart Rate)

  • Learning Objectives:

    • Understand what cardiac output, stroke volume, and heart rate mean.

    • Learn about the autonomic nervous system and its two main parts (sympathetic and parasympathetic).

    • Explain how the autonomic nervous system helps control your heart rate.

Overview of Cardiac Output

  • Purpose of the Cardiovascular System:

    • Your cardiovascular system is like a delivery service, using blood to bring energy and nutrients to all parts of your body.

    • This is super important for everything your body does, like digesting food, moving your muscles, and thinking with your brain.

  • Normal Cardiac Output at Rest:

    • When you're just sitting still, your heart pumps about 5 liters of blood every minute. This amount is what your body needs to function normally.

  • Cardiac Output Formula:

    • To figure out how much blood your heart pumps per minute, you multiply your heart rate by how much blood it pumps with each beat:

    • Cardiac Output=Heart Rate×Stroke Volume\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}

  • Definitions:

    • Heart Rate (HR): This is simply the number of times your heart beats in one minute.

    • A normal heart rate for most adults when resting is usually between 60 and 100 beats per minute.

    • Stroke Volume (SV): This is the amount of blood your heart pushes out each time it beats.

    • For example, if an average adult at rest has a heart rate of 72 beats per minute and pumps 70 milliliters of blood with each beat, their cardiac output would be about 5 liters per minute (72 beats/min * 70 mL/beat = 5040 mL/min, which is about 5 L/min).

Factors Affecting Cardiac Output

  • We'll look at what can change your heart rate and how much blood your heart pumps with each beat.

  • All these things work together to adjust how much blood your heart pumps based on how much your body needs at any given moment.

Nervous System Overview

  • Central Nervous System (CNS):

    • This is like your body's main computer, made up of your brain and spinal cord.

    • It takes in all the signals from your body and decides how to react.

  • Peripheral Nervous System (PNS):

    • This includes all the nerves that branch out from your brain and spinal cord to the rest of your body.

    • There are different types of nerves:

    • Sensory Neurons: These are like sensors that detect things in your environment, such as changes in temperature or light.

    • Somatic Motor Neurons: These are the nerves that tell your skeletal muscles (muscles you control, like in your arms and legs) when to move.

    • Autonomic Neurons: These nerves control things you don't have to think about, like your heart beating or your digestion.

Autonomic Nervous System (ANS) Overview

  • Definition:

    • "Autonomic" basically means automatic or involuntary. You don't consciously control these functions.

  • Functions of ANS:

    • The ANS works to keep your body's internal conditions stable and balanced, a process called "homeostasis."

    • It has two main parts:

    • Parasympathetic Nervous System (PSNS):

      • This system helps your body calm down, "rest and digest," and recover energy.

      • It has long nerve fibers that allow for very precise control over specific organs.

    • Sympathetic Nervous System (SNS):

      • This system kicks in when you're stressed or need to react quickly – it's your body's 'fight or flight' response.

      • It has shorter nerve fibers that allow for a quick and widespread response throughout the body.

Sympathetic Nervous System (SNS) Details

  • Activation:

    • When the SNS is active, it leads to several changes:

    • Blood vessels in areas you don't need immediately (like your digestive system) get narrower (vasoconstriction), redirecting blood.

    • Blood vessels in your skeletal muscles (for running or fighting) get wider (vasodilation) to allow more blood flow.

    • Your heart beats faster and pumps blood more forcefully.

  • Key Neurotransmitters:

    • Norepinephrine (NE): This is a chemical messenger released by the SNS that tells your heart to speed up by binding to specific spots on heart cells called beta-1 adrenergic receptors.

    • Epinephrine (adrenaline): This is another chemical, often called adrenaline, released from your adrenal glands (small glands above your kidneys). It has similar effects to norepinephrine but can affect your whole body.

  • Chronotropic Action:

    • This term refers to anything that affects your heart rate. A "positive chronotropic action" means something that increases your heart rate.

  • Catecholamines:

    • This is a group of chemicals that act as both hormones and neurotransmitters. Epinephrine, norepinephrine, and dopamine are all examples of catecholamines.

Parasympathetic Nervous System (PSNS) Details

  • Activation:

    • The PSNS puts your body into a resting state, mostly by telling your heart to slow down. This is mainly controlled by:

    • Acetylcholine, a chemical messenger released by the PSNS that binds to specific spots on heart cells called muscarinic receptors (specifically type 2).

  • Mechanism:

    • When acetylcholine binds, it makes it easier for potassium to leave the heart cells and harder for sodium to enter. This makes the heart muscle cells less likely to create an electrical signal (known as hyperpolarizing the membrane).

    • This effectively slows down the natural pacemaker of your heart.

    • Result: Your heart rate decreases because it takes longer for the heart cells to get ready for the next beat.

Action Potential and Heart Rate Regulation

  • Action Potential: Think of an action potential as the electrical spark that makes your heart muscle contract. It's a rapid change in the electrical charge across the cell membrane.

  • SNS Influence:

    • The SNS makes it easier for sodium to enter heart cells (through something called the "sodium funny current"), which causes them to become electrically charged faster.

    • It also increases the flow of calcium into the cells during the initial rapid electrical change (phase 0).

    • Result:

      • Your heart cells generate these electrical sparks (action potentials) more quickly, leading to a faster heart rate.

      • The "refractory period" (the short time after a beat when the heart can't beat again) is also shortened, allowing for faster beating.

  • PSNS Influence:

    • The PSNS, on the other hand, makes it easier for potassium to leave the heart cells and harder for sodium to enter.

    • Result:

      • This slows down how quickly the heart cells become electrically charged.

      • It takes more time for the heart cells to reach the "threshold potential" (the specific electrical level needed to trigger an action potential), meaning fewer sparks per minute and a slower heart rate.

Summary of Key Points

  • Cardiac Output Equation:

    • The amount of blood your heart pumps per minute is found by:

    • Cardiac Output=Heart Rate×Stroke Volume\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}

  • Autonomic Nervous System Effects on Heart Rate:

    • Sympathetic Nervous System:

      • This system speeds up your heart rate by releasing norepinephrine, which acts on beta-1 receptors. This is called a positive chronotropic effect.

    • Parasympathetic Nervous System:

      • This system slows down your heart rate by releasing acetylcholine, which acts on muscarinic receptors. This is called a negative chronotropic effect.

  • SA Node and AV Node Control:

    • The main control points for your heart rate are two special groups of cells in your heart called the SA node (your heart's natural pacemaker) and the AV node. Both are affected by signals from both the sympathetic and parasympathetic nervous systems.