Health - Biology
By: - at May 6, 2013

Cardiovascular & Energy Systems: Heart & blood vessels

Blood vessels:

cardiovascular systemArteries
Arteries always carry blood away from the heart, either to the lungs, or to the rest of the body. Arteries are responsible for delivering energy (nutrients) to the whole body. All arteries carry oxygenated blood, except from the pulmonary artery. Arteries have more smooth muscle, and thicker walls than other vessels as well as containing a lot of elastin as they need to withstand high pressures of blood being pumped out of the heart.

Arterioles connect arteries to the capillary network. Arterioles control the flow of blood with the large amounts of smooth muscle in their walls by using two processes known as vasodilation (dilation of the blood vessels) and vasoconstriction (constriction of the blood vessels). Due to these processes, arteries are able to control blood pressure and thermoregulation. They do this by constricting or dilating in certain places, to move blood closer to the surface of the body (when hot) or further from the surface (when cold).

anatomy of the human heart

Capillaries are the smallest vessels in the body with its walls at one epithelial cell thick. Capillaries receive their blood from the arterioles and feed to the Venules. Because of the thin capillary walls, the exchange of gases, fluids and nutrients happens in the capillaries as they can diffuse through the walls of the capillaries. Oxygen enters the capillaries from the arterioles and leaves though the capillary walls, into muscles or organs, whilst CO2 enters the capillaries through the capillary walls and leaves through the Venules.

Arteries, venule, capillaries, arteriole, metarteriole

Venules allow outflow of blood from the capillaries, and have many connective tissues in their walls, as the Venules move further away from the capillaries, they become larger and connect with other Venules and eventually merge into veins.

Veins have thinner walls, less smooth muscle and a thicker tunica externa than arteries or arterioles as they do not need to withstand high pressures as the blood in them can be forced back to the heart with the blood-muscle pump. Veins have valves all along their inside, this is stop the back flow of any blood and aid the blood-muscle pump and to help avoid blood pooling in the veins, as depicted in the picture below:

human body and veins

Blood pressure:

Blood pressure is the measurement of force applies to artery walls.
 Blood pressure is the measurement of force applies to artery walls.

Blood pressure is measured as the pressure acting on the walls of the artery as blood flows through it. Blood pressure results from two forces:

  1. The force of the blood as it is pumped out of the heart.
  2. And the other is the force created by the arteries to resist the blood flow.

During exercise, cardiac output and blood pressure both increase, however, there are biological mechanisms in place to prevent blood pressure from rising too high. Vasoconstriction and vasodilation both act upon the blood vessels, altering the blood pressure in different areas of the body to direct the majority of blood flow to the muscles that require it.

There are 2 main methods of measuring blood pressure, with auscultatoric and oscillometric instruments.

Below is using an Auscultatoric:
Below is using an Auscultatoric

Blood pressure is measured with two readings, these are systolic pressure (the pressure exerted when the heart contracts) and diastolic pressure (the pressure exerted when the heart relaxes). High blood pressure (Also called hypertension) is when blood pressure is measured at above 140/90 mmHg (Systolic/diastolic). Prehypertension means you do not have high blood pressure, but are likely to develop it in later life. A blood pressure between 120/80 mmHg and 139/89 mmHg is considered to be Prehypertension. Blood pressure can also be affected by the ANS (autonomic nervous system), more specifically the SNS (sympathetic nervous system) as this can be triggered by the stress of having your blood pressure measured and can cause an increase in blood pressure.  

Structures in the Heart

There are two atria in the heart, the left atrium, and the right atrium. These are both small chambers at the top of the heart that hold blood until the atrio-ventricular valves open, letting blood through to the ventricles. The atria receive blood through veins, the vena cava on the right, and the pulmonary vein on the left.

There are two atrio-ventricular valves. These are the Bicuspid valve, and the Tricuspid valve. The bicuspid valve is in the left side of the heart, and separates the left atrium from the left ventricle. The bicuspid valve is made up of two flaps, whilst the Tricuspid valve is made up of three flaps. The tricuspid valve is the other atrioventricular valve, and separates the right atrium from the right ventricle.  Both of these valves hold blood in the atria until the valves open due to atrial systole (contraction of the atria), forcing blood into the ventricles. Then valves then act to prevent blood flowing back from the ventricles into the atria during ventricular systole (contraction of the ventricles).

Heart Function Diagram

There are two ventricles in the heart, again, one on the left, and one on the right. The left ventricle needs to pump harder than the right ventricle as blood from the left ventricle needs to travel around the entire body, whilst blood from the right ventricle only needs to be pumped as far as the lungs. Both ventricles receive the blood that they pump, from the atria directly above them.

The septum is a thick muscular tissue separating both sides of the heart; the septum stretches from the bottom right to the top of the heart, changing from the interventricular septum (separating the ventricles) to the interatrial septum (separating the atria).

There are two semi lunar valves; these are called the aortic semi lunar valve, and the pulmonary semi lunar valve. The aortic semi lunar valve separates the left ventricle from the aorta, and much like the atrioventricular valves, they stop blood flowing back from the aorta into the left ventricle. The pulmonary semi lunar valve separates the right ventricle from the pulmonary artery and again, stops the back flow of blood, this time between the arteries to the ventricles.      

X-ray of heart:
heart xray

How the heart beats:
The heart is made up of a certain muscle type called cardiac muscle. This specific type of muscle has one major advantage. Due to the very high amount of mitochondria found, cardiac muscle never tires. The large amount of mitochondria is also why cardiac muscle appears to have a reddish colour.

The heart is naturally myogenic. This means that it does not require a stimulus from outside itself and can contract and relax without nervous stimulation. This is because of the Sino-Atrial Node.

The Sino Atrial Node (SAN) is the natural pacemaker for the heart. The SAN is located in the upper area of the right atrium, and creates an electrical impulse to start the beat of the heart. This impulse then travels through the walls of the atria, causing atrial systole (contraction in the walls of the atria) which forces the blood through the atrio-ventricular valves, and into the ventricles. Once the electrical impulse has passed through the walls of the atria, the atrial walls can begin to relax (atrial diastole) and the atria can begin to fill with blood again. This impulse then reaches the Atrio-Ventricular Node (AVN) and is redirected down the Bundle of His, towards the purkynje fibres. Once it reaches the purkynje fibres, the impulse is then conducted along the purkynje fibres, around the walls of the ventricles, starting from the bottom and moving upwards, causing ventricular systole (contraction of the ventricles) forcing blood upwards and into the arteries, either the Pulmonary Artery or the Aorta. Once the electrical impulse has passed through the ventricles, it stops and the ventricles begin to relax (ventricular diastole), the ventricles are then ready to receive the blood that has stored up in the atria, when the next impulse is sent out by the SAN. This process is repeated each time the heart beats.

Cardiac functions

vasodilation and vasoconstrictionThermoregulation is the control of temperature in the body and is achieved through the process of vasodilation and vasoconstriction. When the body becomes cold, it prioritises the important organs and areas of the body, and so vasoconstriction occurs in vessels closer to the surface of the skin, especially in areas like the fingers, or tip of your nose. When the vessels in these areas constrict, the blood flow directs more towards the core of the body, so the hot blood helps to maintain the core temperature of the body, which could be lifesaving.

When the body is hot, vasodilation occurs in the vessels closer to the surface of the skin, this allows the blood in these vessels to cool down, as the blood is closer to the cooler external environment. The cooler blood then continues around the body, cooling it down.

Arterioles are the main vessel involved with thermoregulation, this is because they have the highest percentage of smooth muscle of all the blood vessels and so can dilate or constrict more than other vessels can.

Transportation of blood and removal of metabolic waste:
Blood delivers the goodsThis function involves the removal of CO2, heat and H2O to the lungs and kidneys. Blood delivers oxygen, heat, hormones, nutrients and minerals and to areas of the body that require it, as well as maintaining body temperature and balance (homeostasis). During exercise, there is an increase in the muscle's demand for blood containing oxygen and different nutrients and minerals, there is also an increased demand for removal of CO2 from the body, as it is toxic to the body. The body reacts to this, by increasing the blood supply to this area of the body, which subsequently increase the levels of oxygen, nutrient and minerals as well as the increased amounts of CO2 being removed from the body.

Increased heart rate during exercise:
The heart rate is the amount of beats per minute an individualís heart makes. At rest, a normal adultís heart beats around 75 beats per minute, and this can increase to around 200 beats per minute during strenuous exercise. This increase during exercise is due to the higher demand for oxygen in the muscles, and to help remove increased levels of CO2 from the body at a faster rate.

The heart rate is controlled by the SAN (Sino atrial node). The heart rate changes when the SAN receives signals from the medulla, via the sympathetic or parasympathetic nervous systems.

Heart rate is controlled by the SAN (Sino atrial node)

The sympathetic nerve speeds up the heart rate by releasing a hormone called noradrenalin from the synapses at the end of the nerve, stimulating the heart to beat at a faster rate. When the sympathetic nervous system is active, most core bodily functions are slowed, e.g. digestion is slowed, which allows more of the bodyís blood to be redirected to the areas that require it. The Vagus nerve (parasympathetic nerve) slows the heart rate back to its resting rate by releasing a hormone called acetylcholine. When the parasympathetic nervous system is active, breathing rate is slowed and digestion returns to its resting rate. The sympathetic and parasympathetic nervous systems are both activated by the medulla in the brain.

Chemoreceptors in the carotid and aortic bodies detect changes in the PH level of blood. An increase in CO2 in the blood will increase the bloods acidity (decrease the PH) and this change will be picked up by the chemoreceptors. These receptors then send a signal to the medulla, causing the medulla to either activate the sympathetic nervous system, or the parasympathetic nervous system. Other receptors within the body that can also affect heart rate are mechanoreceptors. These receptors can be found in the joints and ligaments of an individual, and send feedback of the movement occurring at each joint, to the medulla, which can then cause an increase or decrease in heart rate via the sympathetic or parasympathetic nervous systems.

There are two main nervous systems in the body; these are the autonomic nervous system (ANS) and the voluntary nervous system (sometimes known as the somatic nervous system).

Autonomic nervous system (ANS) and the voluntary nervous system

The autonomic nervous system is split up into two sections; the first of these is the sympathetic nervous system (SNS). The SNS increases heart rate and breathing rate. The SNS is activated by emotional or physical stress on the body, this causes a release of Noradrenalin (NAd) which increases the work rate of the heart. Due to this increase of heart and breathing rates, the body slows down digestion to allow more blood and oxygen to get to the heart and lungs. The second part of the nervous system is the parasympathetic nervous system (PNS), this system reduces heart, and breathing rates. It also resumes normal digestion patterns and is also responsible for returning the body to a balanced state (homeostasis). The PNS is activated when the body needs to return to its resting state, after the SNS has been active for a period of time. PNS responses are controlled by Acetylcholine, which acts as a neurotransmitter in the PNS, causing an increase or decrease in the levels of activity in the PNS.

Increase in Stroke volume during exercise:
Stroke volume is the volume of blood pumped out of the left ventricle in one cardiac cycle. Around two thirds of the blood in one ventricle is pumped out with each beat, this volume progressively increases during exercise until it plateaus at a higher level, and will gradually return to normal after the exercise is finished. This increase in stroke volume allows the body to remove CO2 from the muscles faster, as well as delivering higher amounts of oxygen to the muscles that require it. The stronger an individualís heart is, the larger volume of blood is pumped out of the heart with each beat. So a trained athlete will most likely have a higher stroke volume than an untrained athlete.

Increase in Cardiac output during exercise
running uphillCardiac output is the amount of blood pumped from the heart in one minute. It is equal to stroke volume multiplied by heart rate (If a heart beats 65 times in one minute, and has a stroke volume of 80ml per beat, then the cardiac output would equal 65 X 80 = 5200ml per minute. The average adultís resting cardiac output is 4900ml per minute; however, cardiac output may reach up to 30 litres per minutes during extreme exercise. Increased sympathetic nervous system activity and decreased parasympathetic nervous system activity, resulting in an increased heart rate, would increase the cardiac output.

Red Blood Cells:
Red Blood Cells

Blood composition:                     
Blood makes up about 8% of our body weight, ranging from 4-6 liters of blood (4-5 for females, 5-6 for males). Our blood is regulated through the process of homeostasis, to keep the blood at a temperature of 38 degrees. Blood is around 5 times more viscous than water and contains plasma, erythrocytes, hemoglobin, leucocytes and thrombocytes.  

Plasma makes up 55% of the blood volume and is a mixture of proteins, enzymes, nutrients, waste toxins, hormones and gases, but is composed mostly of water (93%), and is a straw color in appearance. Plasma contains three main proteins; these are albumins, globulins and fibrinogen.  

Erythrocytes, also known as red blood cells, take and release oxygen into the capillary beds, as well as carrying Co2 back to the lungs to be removed from the body.  

Hemoglobin is an oxygen transport protein that gives blood its red color. Oxygen attaches onto the hemoglobin which is attached to the erythrocytes. Hemoglobin carries oxygen from the lungs to the rest of the body, and carries CO2 back to the lungs. One erythrocyte contains about 250,000,000 hemoglobin.

Leucocytes and Red Blood Cells:
Leucocytes and Red Blood Cells

The primary function of Leucocytes, also known as white blood cells, is to fight off diseases that enter the body, but they can also inhibit the growth of bacteria and help to prevent blood clotting. Leucocytes make up 1% of the volume of blood, but can be easily multiplied to fight infection.

Thrombocytes (platelets) are the main part of the blood clotting process, and are initiated following a trauma. The thrombocytes trap erythrocytes in the clot, to form a scab, protecting the wound and allowing time for the body to heal. The protein Thrombonin is a factor that helps form a blood clot.  

Energy systems

As exercise varies in intensity, different energy systems need to be used. There are three different energy systems within the body; these are the aerobic energy system, the lactic acid energy system and the phosphor-creatine energy system.

Aerobic energy system:
Aerobic energy systemThe aerobic energy system is active when the heart is working at 50% of its maximum output or below, and can keep working at this intensity for very long periods of time. The aerobic energy system requires oxygen to breakdown the glucose or fat. The energy is produced within the mitochondria inside the cells in the body. When one oxygen molecule aids the breakdown of 1 glucose molecule, 38 molecules of ATP are produced, but when one oxygen molecule is used to breakdown one fat molecule, 129 molecules of ATP are produced. The high amount of ATP being produced each day (120,000,000,000,000,000,000,000,000) is the reason behind the aerobic energy system being able to work for very long periods of time. If this energy system is used to its limits then the recovery time is the longest of all energy systems and can sometimes take up to three or four days.

Lactic acid energy system:
Glycolysis is the breakdown of glucose stored in the muscles and liver (known as glycogen).  The energy produced from this breakdown is used as energy for the muscles. When glucose is broken down, it produces 2ATP molecules, and when glycogen is broken down, it produces 3 ATP molecules. The lactic acid system lasts 60-90 seconds during high intensity exercise, and has a recovery time of around 1 hour, but this recovery time can be reduced with an appropriate cool down. Pyruvic acid is the by-product of this reaction, which is used in the kreb cycle.

Phospho-creatine energy system:
Phosphocreatine is made up of one phosphate and one creatine molecule. When the phosphocreatine energy system is used the phosphate from the phosphocreatine is donated to an ADP (Adenosine Diphosphate) molecule, causing it to instantly re-synthesise into an ATP (Adenosine Triphosphate) molecule, ready to be broken down again. Stored within the muscles there is enough phosphocreatine to last five to ten seconds of maximum output and so this energy system is only used for very short, high intensity (maximum) output. The phosphocreatine within the muscles is restored quickly after it has been used.

Thanks for reading, and I hope you are now a little bit more educated on the cardiovascular system and how the heart functions!





An Introduction to the Cells of Organisms
Cardiovascular System and Energy Systems

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