The Circulatory System

[Music] you [Music] more than 30 years of manned space travel has taught us that virtually every system in the human body is affected by the microgravity of space besides the issues you're probably already aware of such as decreased muscle and bone mass astronauts also experience circulatory problems such as arrhythmias or abnormal heartbeats they also suffer from anemia a reduced number of red blood cells that carry oxygen to the tissues and after just two weeks of spaceflight 20% of returning astronauts experience difficulty standing up without getting dizzy a condition called orthostatic intolerance the longer they stay in zero-gravity the more pronounced are the effects by studying the impact of virtual weightlessness on the circulatory system scientists have learned a lot about how it functions on earth in this program we'll take a look at the organs and processes involved in the human circulatory system and find out what scientists have learned why do we have a circulatory system basically we are a multicellular organism all of the cells in our bodies need oxygen and nutrients to survive and do their work in simple one-celled organisms like the amoeba or Paramecium this is accomplished through a process called diffusion diffusion is simply the random movement of molecules from a region of higher concentration to one of lower concentration using diffusion these one celled organisms can easily exchange respiratory gases food and waste materials with the environment through their cell walls the diffusion only works well over short distances large multicenter organisms like human beings need a rapid delivery system to bring the molecules of oxygen and nutrients close enough to each of our cells for the diffusion process to work and that's the role of the circulatory system the circulatory system consists of blood contained in a network of vessels called the vascular system that's connected to a pump the heart the left side of the heart receives oxygenated blood from the lungs and pumps it to cells and tissues throughout the body the blood then returns to the right side of the heart this is called the systemic circuit from the heart blood flows through large muscular arteries then into smaller and smaller arteries then arterioles and finally through capillaries it's in the capillaries where the diffusion process works to exchange molecules of oxygen and nutrients for carbon dioxide and other waste products of nearby cells after leaving the capillaries blood is collected into venules the smallest of the veins then intervenes of increasing size until it reaches the heart the right side of the heart receives blood returning from the tissues and pumps it toward the lungs in the lung capillaries carbon dioxide diffuses through vessel walls into tiny air sacs called alveoli and is exhaled oxygen diffuses in the opposite direction and is taken up by red blood cells this oxygenated blood then returns to the heart completing the pulmonary circuit in both the pulmonary and systemic circuits arteries carry blood away from the heart and veins bring it toward the heart blood is considered a fluid connective tissue that functions to distribute substances critical to life to cells throughout the body if we take a unit of blood and spin it in a centrifuge it will separate into its basic components at the top is a fluid called plasma that represents about 55% of our total blood volume next is a thin whitish layer called the Buffy coat this layer contains platelets and leukocytes which account for less than 1% of our blood volume the remaining nearly 45% consists of red blood cells called erythrocytes a mature red blood cell is a package of oxygen carrying hemoglobin molecules and respiratory proteins its biconcave shape provides a larger surface area for oxygen diffusion and allows it to fold and squeeze through small capillaries leukocytes are white blood cells that help fight disease platelets are cell fragments that circulate in the blood and promote clotting when the wall of a blood vessel is injured platelets adhere to exposed collagen fibers and release chemo attractants that bring more platelets to the site the injury is initially sealed by a plug of platelets than a clot of fibrin and red blood cells forms giving the vessel time to repair itself plasma the liquid portion of blood transports blood cells proteins and enzymes to sites where they are needed blood transfusions are often needed to treat patients whose red cells or volume of blood have dropped to low but it's critical that the blood to be transfused be matched to the recipients blood type there are 4 ABO blood types a B a B and O each is characterized by the presence or absence of different antigens found on the surface of red blood cells and the presence or absence of different antibody circulating in the blood Rh factor is another antigen found on the surface of red blood cells individuals with Rh positive blood have that antigen those with Rh negative blood do not any of the ABO blood types can be positive or negative for the Rh factor if the blood to be transfused isn't matched to an individual's Rh factor and ABO blood type antibodies can bind to and destroy the transfused red blood cells creating a potentially life-threatening situation red blood cells are packed with hemoglobin and iron containing protein molecule that transports oxygen each red blood cell is estimated to hold about 280 million hemoglobin molecules and each hemoglobin molecule can bind up to four oxygen molecules affinity is the term used to describe hemoglobins ability to bind oxygen once one molecule of oxygen is bound it's easier for hemoglobin to bind others this is called cooperative binding capillaries deliver oxygen and nutrients close enough to individual cells for the diffusion process to work outside the capillary cells are at rest and so they don't require much oxygen oxygen partial pressure there is at about 40 millimeters of mercury only slightly lower than it is inside the capillary hemoglobin molecules react accordingly releasing only one of their four molecules of oxygen when a person is active cellular respiration increases more oxygen is used to make energy as a result there is an oxygen deficiency in these tissues and the partial pressure of oxygen Falls a typical partial oxygen pressure in this situation is about 20 millimeters of mercury which would be substantially lower than within the capillary hemoglobin responds to this situation by losing its affinity for oxygen and releasing about three-quarters of its load these oxygen molecules diffuse out through the capillary walls and into the tissue cells also the concentration or partial pressure of carbon dioxide a waste product of cellular respiration has increased in the tissues outside the capillary this increases hemoglobins affinity for co2 molecules and it binds them in place of the oxygen it's given up the red cells leave the capillaries and travel through a series of venules and veins back to the heart completing the systemic circuit from the heart the deoxygenated blood is pumped to the lungs in the lung capillaries hemoglobin loses its affinity for carbon dioxide allowing the co2 molecules to diffuse into the lungs this happens because the partial pressure of carbon dioxide in the lung is lower than it is in the capillary at the same time partial oxygen pressure in the lungs is significantly higher than in the capillary this pushes oxygen molecules to diffuse from the lung to the capillary where hemoglobin binds them from the lungs the oxygenated blood returns to the heart where it will be pumped again throughout the body to supply tissues with the oxygen they need for respiration the heartbeats steadily from early embryonic development over a lifetime it will be more than three billion times the heart is located in the chest cavity with the lungs slightly to the left of center it's surrounded by the pericardium two layers of membranes separated by fluid the outer pericardium helps keep the heart in position in the chest cavity while the inner layer provides lubrication to reduce friction the heart has four chambers a left atrium and ventricle and a right atrium and ventricle the left and right sides are completely separated by a wall or septum the right atrium receives blood returning from the systemic circuit via two large veins the superior vena cava which drains all regions above the heart and the inferior vena cava which collects blood from all the lower regions when the right atrium contracts it pumps blood into the right ventricle through the tricuspid valve one of the atrioventricular or AV valves heart valves are designed to allow blood to flow in only one direction when the right ventricle is full it contracts pushing blood out through the pulmonary semilunar valve into the base of the pulmonary artery on its way to the lungs a similar cycle occurs on the left side of the heart here the left atrium accepts oxygenated blood returning from the lungs when it contracts blood passes through the bicuspid valve into the left ventricle the left ventricle then contracts pushing the blood out through the aortic semilunar valve into the ascending aorta from there it's distributed throughout the body cardiac muscle cells are smaller than skeletal muscle cells and are connected by structures known as intercalated discs desmosomes or adhesion molecules hold cardiac cells together during contractions gap junctions allow for synchronization of heart contractions the cells of the heart muscle need oxygen and fuel just like any other cells but they don't get them from the blood that the heart is pumping instead the needs of heart cells are met by the coronary arteries that branch off either side of the ascending aorta [Music] every time the heart beats the atria and ventricles contract in coordination this coordinated beating is accomplished by a number of tissues first the sinoatrial node contains pacemaker cells that generate an electrical pulse conducting cells relay the signal to the muscle tissue of the right and left atria to stimulate contraction they also send the signal to the atrioventricular or AV node which slows down the relay of the signal to the ventricles after the delay the signal is sent to the right and left atrial ventricular bundles which stimulate contraction of the ventricles this delay is critical to allow the atria adequate time to contract down on the blood they contain and force open the AV valves to fill the ventricles before the ventricles contract any damage to the conducting system of the heart leads to a loss of coordination and a diminished heart function we see arrhythmias such as tachycardia where the heart beats very rapidly in an uncoordinated way so it loses its ability to effectively pump blood when medical professionals take your blood pressure they can find out a great deal about how your circulatory system is working but what do those numbers mean the systolic pressure is measured when the ventricles are contracting forcing blood out of the heart and into the arteries this is when blood pressure is highest pressure is lowest when the heart is resting and refilling with blood this is the diastolic pressure blood pressure is measured using a blood pressure cuff the cuff is inflated to a pressure well above the higher systolic pressure blocking flow within the artery as the pressure in the cuff is slowly released blood begins to squirt through during the contraction of the ventricles at this point the pressure of the cuff equals the systolic pressure as cuff pressure is further reduced at some point it is no longer sufficient to disrupt blood flow the reading at this point is your diastolic pressure a typical blood pressure in a healthy individual is a systolic of 110 millimeters of mercury and a diastolic of 70 this is often stated as 110 over 70 systolic pressures of more than 120 and diastolic pressures of more than 80 are considered to be too high high blood pressure is an indication of hypertension and hypertension places people at higher risk for things like heart attacks strokes and kidney and heart failure all potentially fatal conditions there are several factors that affect blood pressure the first is cardiac output the volume of blood that is being pumped when cardiac output rises blood pressure rises a second factor is blood volume if an individual begins losing blood for example due to a major injury blood pressure will fall loss of pressure can mean insufficient blood flow and a lack of oxygen to the tissues which can cause permanent damage even death it's very important to restore blood volume and blood pressure as soon as possible to provide oxygen to these tissues the body automatically takes action to increase blood pressure vessels baso constrict in other words their diameters get smaller this increases the friction between the blood and the vessel walls that friction is called total peripheral resistance or TPR as TPR increases blood pressure increases blood is primarily water but it also contains cells and proteins as these cells and proteins increase in proportion to the liquid part of the blood viscosity increases the higher the viscosity or thickness of the blood the greater the resistance to flow and the more energy or pressure is required to push blood through the system one way to decrease blood pressure is to increase vessel diameter a process called vasodilation with vasodilation the friction between the blood and vessel walls or TPR decreases and blood pressure decreases blood pressure often increases with age one of the factors causing this is a thorough sclerosis atherosclerosis is chronic inflammatory response affecting the walls of the artery deposits of what we call plaque which are made up of low-density lipoproteins and triglycerides together with cells which migrate to the area collect within the walls of the arteries and make the artery more hardened this restricts the blood flow and limits the elasticity of the arterial vessels that supply various tissues of the body at times blood flow can be severely limited or completely blocked tissues that are served by these vessels don't receive enough oxygen then and may cease functioning or die and in the heart this is what we refer to as a heart attack in the brain a stroke blood flow is fastest in the larger arteries where resistance is lowest and slowest in the capillaries to allow time for capillary exchange to occur capillary walls are extremely thin they consist of only a single layer of cells water oxygen glucose molecules and amino acids can pass between the cells of the capillary wall into the interstitial compartment which is the fluid surrounding the tissue cells this type of diffusion is called filtration and it occurs when pressure in the capillary is greater than in the interstitial compartment reabsorption is the movement of carbon dioxide fluids and waste products from the interstitial back into the capillary reabsorption occurs when the concentration or pressure of a specific element is greater in the interstitial compartment than in the capillary filtration is higher on the arterial side and reabsorption is greater on the venous side there are several ways in which the circulatory system can adjust to the changing needs of body tissues some control occurs locally for example when a particular tissue is active carbon dioxide and other byproducts of cell respiration build up their presence causes the pre capillary sphincter a muscular ring around the vessel to dilate and allow blood to flow into the effect area this form of control is called Auto regulation there is also neural control of the circulatory function through the medulla oblongata in the brainstem the medulla can regulate blood flow by increasing or decreasing cardiac output and by stimulating vessels to dilate or constrict other regulation occurs when there is a change in blood pressure for example when an injury causes a hemorrhage a considerable blood loss that results in a significant drop in blood pressure baroreceptors located in vessel walls and even in the heart itself sense changes in blood pressure and signal the medulla the medulla sends out signals to the heart to increase its rate and stroke volume to the veins to vasoconstrict forcing more blood back to the heart and to the arterioles to vasoconstrict the net effect is an increase in blood pressure chemoreceptors in the aortic arch and carotid arteries serve much the same function as baroreceptors but in their case they signaled the medulla when they sense elevated levels of carbon dioxide in the blood indicating that tissues are in need of more oxygen exercise demands more oxygen and more blood flow to the coronary arteries that supply the heart to the exercising muscles into the skin the muscles and heart require more oxygen for their increased activity levels and the increased flow to the skin helps unload excess body heat to meet this demand blood flow output must increase from 5 liters per minute to as high as 35 or 40 liters per minute during vigorous activity this is partially accomplished through vasodilation of the vessels that supply these tissues at the same time arteries that supply blood to areas such as the digestive system and the kidneys are constricted because these systems are not needed during exercise venous return of blood to the heart must also increase this is aided by contraction of the skeletal muscles and respiratory pumps as well as by vasoconstriction the veins with years of physical exercise long-term changes in circulatory function can occur maximum cardiac volume is increased because the ventricular walls thicken and contract more forcefully to eject more blood with each beat the heart doesn't have to beat is fast for a given level of exercise venous return is enhanced through an overall increase in blood volume and muscles use oxygen and fuel more efficiently improving their endurance as you've seen the circulatory system is a complex and highly efficient delivery system that readily responds to your body's changing needs a healthy circulatory system is critical to your overall health and well-being and whether life keeps you here on earth or takes you out into space knowing more about your circulatory system can help keep it healthy and working for you [Music]

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