Heart Energy & Exercise

Controlling the heart

• Heart beats continually throughout life with an average of about 70bpm although in small children the heart rate is much higher. The heart can respond to need.
• During physical exercise, when the tissue needs more oxygen your heart beats faster to supply more oxygenated, glucose carrying blood to tissues and to remove the increased waste products.
• Stress can also raise the heart rate, while rest and relaxation can lower it
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How is the heart beat controlled

• The heart beat have intrinsic rhythmicity.
• The heart can still continue to contract outside of the body as long as it is bathed in a suitable oxygen-rich fluid.
• This rhythm is maintained by a wave of electrical excitation similar to a nerve impulse which spreads through special tissues in the heart muscle.
• The area of the heart with the fastest intrinsic rhythm is a group of cells in the right atrium known as the sinoatrival node (SAN)-it acts as the heart's own natural pacemaker
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The Steps in the Cardiac cycle in terms of SAN

• The sinoatrial node sets up a wave of electrical excitation (depolarisation) which causes the atria to start contracting.
• Excitation also spreads to another area of similar tissue called the atrioventricular node (AVN)
• The AVN is excited as a result of the SAN and from here the wave of depolarisation passes into the bundle of HiS
• The bundle of His carries the excitation from the AVN down to the Purkyne Tissue
• The Purkyne tissue penetrates down through the septum of the heart, spreading between and around the ventricles. As the depolarisation travels through the tissue it sets off ventricular contraction, starting at the bottom and so squeezing the blood out of the heart
• https://www.youtube.com/watch?v=FLBMwcvOaEo&index=26&list=PL8dPuuaLjXtOAKed_MxxWBNaPno5h3Zs8 (Embed)

ECG

• The speed at which excitation spreads through the heart makes sure that the atria have stopped contracting before the ventricles start. It's these changes in the electrical excitation of the heart which are measured in an electrocardiogram (ECG).
• Since the heart has its own basic rhythm, body resources aren't wasted on maintaining such a vital but continuous event. However, many people have a faster resting heart rate. This is because lots of other factors constantly influence heart rate.
• An ECG is used to investigate the heart's rhythms by producing a record of the electrical activity of the heart.
• The rhythm of the heart results from the spread of depolarisation wave (electrical activity) through specialised tissues within the heart muscle itself.
• This depolarisation in the heart causes tiny electrical changes on skin's surfaces. An ECG measures these changes at the surface of the skin.
• To take an ECG, 12 electrodes and leads are attached to the body. The skin is wiped with alcohol to remove any grease or sweat so the electrodes make good contact with the skin,
• Sometimes a special gel is applied to the electrodes to make sure they conduct electricity as effectively as possible. As the recording is made, info is fed back from each electrode, giving 12 views of the heart.
• An ECG can show you what's happening in a normal, healthy heart and it's often used to indicate different heart conditions & to monitor patients with heart disease.
• It's usually done with patient laying down & resting but can be done while patient is exercising as some heart diseases only appear during exercise.

Homeostasis & responding to exercise

• Human body's cell are very sensitive to change
• For aerobic respiration to take place inside cells, Oxygen & glucose levels needs to be maintained within very narrow limits
• For many of the enzymes to work efficiently, the core temperature has to be maintained around 37℃. So during vigorous physical exercise, the body must control the internal conditions within a narrow range.
• The maintenance of a steady internal state of the body almost regardless of changes in either the external or internal conditions is known as homeostasis.
• Homeostasis involves a high level of coordination and control. Changes in the body are detected by a sensor ( also called a receptor)
• This sends a message to an effector which either works to reverse the change or increase it.
• Homeostasis plays an important role during exercise, when conditions in the body are changing rapidly and demand on the systems are high.
• Communication in a feedback system may be hormones or by nerve impulses.

Negative feeback systems

• https://www.youtube.com/watch?v=zv8LHyH7_rI (Embed)
• Negative feedback systems are the most common & they provide a way of maintaining a condition such as the concentration of a substance within a narrow range. If concentration goes up, the effector brings it down again and vice versa.
• Negative feedback systems are vital in the coordinated response which enables you to exercise effectively as both your heart and your respiratory system responds.

Positive feedback system

• In a positive feedback system, effectors work to increase the effect which has triggered the response.
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Changing cardiac output

• The intrinsic rhythm of the heart isn't sufficient to supply the body's needs except at a most basic level.
• For ex: during exercise, more oxygen must be carried to the rapidly respiring muscle tissue & the waste carbon dioxide & lactate which accumulate is the muscle fibre need to be removed.
• Once the exercise stops, your body needs to return to normal-even cardiac muscle can't continue pumping at maximum output all the time! Heart response is the result of negative feedback system.
• When your body demands more glucose and oxygen, the heart can respond in 2 ways- look at other column
• The rate at which the heart beats can increase.
• In addition, the volume of blood pumped at each heartbeat called the cardiac volume, can be increased by a more efficient contraction of the ventricles.
• The combination of these 2 factors gives a measure of cardiac output:
• Cardiac output = Cardiac volume x heart rate
• (unit: dm^3min^-1) (unit: dm-3) (bpm/ min-1)

Adjusting the heart rate

• Different control systems enable the heart to respond to the varying demands of the body throughout the day.

Nervous control of the heart

• The cardiovascular control system, which controls changes to heart rate is found in the medulla of the brain.
• Chemicals & stretch receptors in the linings of the blood vessels & the chambers of the heart sends nerve impulses to the cardiovascular centre.
• 2 types of nerves then carry impulses out to the heart, controlling the rate at which it beats.
• Most of the nervous control of your heart is by the automatic (involuntary) nerves system.
• The autonomic system is divided into 2 parts:
• The sympathetic nervous system is usually excitatory-for example it speeds up heart rate.
• The parasympathetic nerve from the cardiovascular centre in the brain to heart stimulate the SAN.
• Most of the body organs are supplied by both types of nerve, giving a level of fine control.
• Nerve impulses travelling down the sympathetic nerve from the cardiovascular centre in the brain to heart stimulate the SAN
• This increase the frequency of the signals from the pacemaker region so that the heart beats more quickly. In contrast, nerve impulses in the corresponding parasympathetic nerve inhibits the SAN and slows down the heart.

Video of nervous control of heart

• https://www.youtube.com/watch?v=KiouveG278Y (Embed)

Hormone control of heart

• The heart beats faster whenever a person is nervous, frightened or excited and when a person is doing exercise.
• When a person gets stressed, the hormone adrenaline is produced.
• Adrenaline affects the SAN, speeding up the heart, supplying a person with extra oxygen and glucose to the muscle and brain in case you need to run away or stand and fight.
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Responding to exercise

• Some of the main control systems that regulate the heart rate involve a negative feedback system to the pressure of blood in the circulatory system.
• As the atria fills with blood at the start of the cardiac cycle, stretch receptors in the muscle walls of the heart respond to the stretching by sending nerve impulses to the cardiovascular centre.
• For ex: at the beginning of the period of exercise more blood than usual returns to heart however the big muscles blocks in the legs and arms squeeze more blood along veins as they work.
• As more blood flows into the atria, the receptors are stretched more than usual & sends more nerve impulses to the cardiovascular centre in the brain.
• This in turn sends more nerve impulses along the sympathetic nerve to the SAN, causing an increase in blood flow.
• The increased stretching of heart atrial muscles as blood returns from the body also makes the muscle contract harder, increasing volume of blood expelled at every stroke.
• This doesn't depend on the cardiovascular centre in the brain, this is a feature of cardiac muscle.

Baroreceptors

• Baroreceptors are sensitive to pressure.
• They are found in the sinus of the carotid arteries in the neck and are also important in feedback control of the heart rate.
• As blood pressure in the arteries increase, the baroreceptors are stretched. They send nerve impulse to the cardiovascular centre which then sends impulses through the parasympathetic system to slow down the heart rate & causes a widening of the blood vessels (vasodilation). These 2 actions decrease blood pressure.
• The reverse happens when exercise starts- the blood vessels dilate in response to the hormone adrenaline which is released with exercise & blood pressure falls a little. This reduces the stretch of the baroreceptors & they almost stop responding. When they don't stimulate the heart rate & increase blood pressure again.
• During exercise, impulse from the cardiovascular centre affects other effectors as well as the heart. When many impulses travel along the sympathetic nerve to heart to speed it up, fewer impulse are sent along the sympathetic nerves to many blood vessels, causing the smooth muscle lining in the vessel to contract narrowing or closing the vessel.
• In this way the blood flow is diverted from temporarily less important areas to provide more blood for the heart & muscle to use.

Baroreceptor video

• https://www.youtube.com/watch?v=ajLgwCygHsc (Embed)