Circulatory System PDF

**Circulatory System** The circulatory system (cardiovascular system) pumps blood from the heart to the lungs to get oxygen. The heart then sends oxygenated blood through arteries to the rest of the body. The veins carry oxygen-poor blood back to the heart to start the circulation process over. Ou...

**Circulatory System** The circulatory system (cardiovascular system) pumps blood from the heart to the lungs to get oxygen. The heart then sends oxygenated blood through arteries to the rest of the body. The veins carry oxygen-poor blood back to the heart to start the circulation process over. Our heart and blood vessels make up the circulatory system. The main function of the circulatory system is to provide oxygen, nutrients and hormones to muscles, tissues and organs throughout your body. Another part of the circulatory system is to remove waste from cells and organs so your body can dispose of it. Our heart pumps blood to the body through a network of arteries and veins (blood vessels). Your circulatory system can also be defined as your cardiovascular system. Cardio means heart, and vascular refers to blood vessels. ### Circulatory system work Your circulatory system functions with the help of blood vessels that include arteries, veins and capillaries. These [blood vessels](https://my.clevelandclinic.org/health/body/21640-blood-vessels) work with your heart and [lungs](https://my.clevelandclinic.org/health/articles/8960-lungs-how-they-work) to continuously circulate blood through your body. Here's how: 1. The heart's bottom right pumping chamber (right ventricle) sends blood that's low in oxygen (oxygen-poor blood) to the lungs. Blood travels through the pulmonary trunk (the main pulmonary artery). 2. Blood cells pick up oxygen in the lungs. 3. Pulmonary veins carry the oxygenated blood from the lungs to the heart's left atrium (upper heart chamber). 4. The left atrium sends the oxygenated blood into the left ventricle (lower chamber). This muscular part of the heart pumps blood out to the body through the arteries. 5. As it moves through your body and organs, blood collects and drops off nutrients, hormones and waste products. 6. The veins carry deoxygenated blood and carbon dioxide back to the heart, which sends the blood to the lungs. 7. Your lungs get rid of the carbon dioxide when you exhale. ### Circulatory system parts: The parts of your circulatory system are your: - **Heart,** a muscular organ that pumps blood throughout your body. - **Blood vessels,** which include your arteries, veins and capillaries. - **Blood,** made up of red and white blood cells, plasma and platelets. Structure of the Heart ====================== The human [[heart]](https://api.seer.cancer.gov/rest/glossary/latest/id/5502b492e4b0c48f31d64b2b) is a four-chambered muscular [[organ]](https://api.seer.cancer.gov/rest/glossary/latest/id/54da499ae4b07fe4ff776889), shaped and sized roughly like a man\'s closed fist with two-thirds of the [[mass]](https://api.seer.cancer.gov/rest/glossary/latest/id/5520eecfe4b0bc5c16bfb7b9) to the left of midline. The heart is enclosed in a [[pericardial sac]](https://api.seer.cancer.gov/rest/glossary/latest/id/5520547ce4b0bc5c16bf9707) that is lined with the [[parietal]](https://api.seer.cancer.gov/rest/glossary/latest/id/55022631e4b0c48f31d620fc) layers of a [[serous membrane]](https://api.seer.cancer.gov/rest/glossary/latest/id/550583c8e4b0c48f31d6f6e3). The [[visceral]](https://api.seer.cancer.gov/rest/glossary/latest/id/5502266ce4b0c48f31d6211d) layer of the serous membrane forms the [[epicardium]](https://api.seer.cancer.gov/rest/glossary/latest/id/5502b606e4b0c48f31d64bb5). Layers of the Heart Wall ------------------------ Three layers of [tissue](https://api.seer.cancer.gov/rest/glossary/latest/id/55097ed2e4b0c48f31d89a03) form the heart wall. The outer layer of the heart wall is the epicardium, the middle layer is the [myocardium](https://api.seer.cancer.gov/rest/glossary/latest/id/5502b4ebe4b0c48f31d64b4f), and the inner layer is the [endocardium](https://api.seer.cancer.gov/rest/glossary/latest/id/5502b634e4b0c48f31d64bf8). Chambers of the Heart --------------------- The [internal](https://api.seer.cancer.gov/rest/glossary/latest/id/55022573e4b0c48f31d62074) [cavity](https://api.seer.cancer.gov/rest/glossary/latest/id/5522c2dfe4b0bc5c16c04c7c) of the heart is divided into four chambers: - Right atrium - Right ventricle - Left atrium - Left ventricle  The two [atria](https://api.seer.cancer.gov/rest/glossary/latest/id/558d96c3e4b084b72edb2d11) are thin-walled chambers that receive [blood](https://api.seer.cancer.gov/rest/glossary/latest/id/54ac2835e4b0d965833ce0f4) from the [veins](https://api.seer.cancer.gov/rest/glossary/latest/id/5502b787e4b0c48f31d64ced). The two ventricles are thick-walled chambers that forcefully [pump](https://api.seer.cancer.gov/rest/glossary/latest/id/555388b1e4b0426fced97655) blood out of the heart. Differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which reflects the amount of force each chamber is required to generate. The right [atrium](https://api.seer.cancer.gov/rest/glossary/latest/id/558d96c3e4b084b72edb2d11) receives deoxygenated blood from systemic veins; the left atrium receives oxygenated blood from the [pulmonary](https://api.seer.cancer.gov/rest/glossary/latest/id/5553881de4b0426fced975a1) veins. Circulatory system has three circuits. Blood circulates through your heart and through these circuits in a continuous pattern: - **The pulmonary** **circuit: **This circuit carries blood without oxygen from the heart to the lungs. The pulmonary veins return oxygenated blood to the heart. - **The systemic circuit: **In this circuit, blood with oxygen, nutrients and hormones travels from the heart to the rest of the body. In the veins, the blood picks up waste products as the body uses up the oxygen, nutrients and hormones. - **The coronary circuit: **Coronary refers to your [heart's arteries](https://my.clevelandclinic.org/health/articles/17063-coronary-arteries). This circuit provides the heart muscle with oxygenated blood. The coronary circuit then returns oxygen-poor blood to the heart's right upper chamber (atrium) to send to the lungs for oxygen. There are three main types of blood vessels: - **Arteries:** Arteries are thin, muscular tubes that carry oxygenated blood away from the heart and to every part of your body. The aorta is the body's largest artery. It starts at the heart and travels up the chest (ascending aorta) and then down into the stomach (descending aorta). The coronary arteries branch off the aorta, which then branch into smaller arteries (arterioles) as they get farther from your heart. - **Veins:** These blood vessels return oxygen-depleted blood to the heart. Veins start small (venules) and get larger as they approach your heart. Two central veins deliver blood to your heart. The superior vena cava carries blood from the upper body (head and arms) to the heart. The inferior vena cava brings blood up from the lower body (stomach, pelvis and legs) to the heart. Veins in the legs have valves to keep blood from flowing backward. - **Capillaries: **These blood vessels connect very small arteries (arterioles) and veins (venules). Capillaries have thin walls that allow oxygen, carbon dioxide, nutrients and waste products to pass into and out of cells. Your heart is the only circulatory system organ. Blood goes from the heart to the lungs to get oxygen. The lungs are part of the [respiratory system](https://my.clevelandclinic.org/health/articles/21205-respiratory-system). Your heart then pumps oxygenated blood through arteries to the rest of the body. **Blood** Blood is a constantly circulating fluid providing the body with nutrition, oxygen, and waste removal. Blood is mostly liquid, with numerous cells and proteins suspended in it, making blood \"thicker\" than pure water. The average person has about 5 liters (more than a gallon) of blood. Liquid called plasma makes up about half of the content of blood. Plasma contains proteins that help blood to clot, transport substances through the blood, and perform other functions. Blood plasma also contains glucose and other dissolved nutrients. Blood is conducted through blood vessels (arteries and veins). Blood is prevented from clotting in the blood vessels by their smoothness, and the finely tuned balance of clotting factors. **blood**, [fluid](https://www.britannica.com/science/fluid-biology) that transports [oxygen](https://www.britannica.com/science/oxygen) and nutrients to the [cells](https://www.britannica.com/science/cell-biology) and carries away [carbon dioxide](https://www.britannica.com/science/carbon-dioxide) and other waste products. Technically, blood is a transport liquid pumped by the [heart](https://www.britannica.com/science/heart) (or an equivalent structure) to all parts of the body, after which it is returned to the heart to repeat the process. Blood is both a [tissue](https://www.britannica.com/science/tissue) and a fluid. It is a tissue because it is a collection of similar specialized cells that serve particular functions. These cells are suspended in a liquid matrix ([plasma](https://www.britannica.com/science/plasma-biology)), which makes the blood a fluid. If blood flow ceases, death will occur within minutes because of the effects of an unfavourable [environment](https://www.merriam-webster.com/dictionary/environment) on highly susceptible cells. About half of blood volume is composed of blood cells:\ \ Red blood cells, which carry oxygen to the tissues\ White blood cells, which fight infections\ Platelets, smaller cells that help blood to clot Types of Blood Cells -------------------- We have seen blood consist of cells known as formed elements of blood. These cells have their own functions and roles to play in the body. The blood cells which circulate all around the body are as follows: ### Red blood cells (Erythrocytes) RBCs are biconcave cells without nucleus in humans; also known as erythrocytes. RBCs contain the iron-rich protein called haemoglobin;  give blood its red colour. RBCs are the most copious blood cells produced in bone marrows. Their main function is to transport oxygen from and to various [tissues](https://byjus.com/biology/tissues/) and organs. ### White blood cells (Leucocytes) Leucocytes are colourless blood cells. They are colourless because it is devoid of haemoglobin. They are further classified as granulocytes and agranulocytes. WBCs mainly contribute to immunity and defence mechanism. Red Blood Cells Types of White Blood Cells ------------------------------------------ There are five different types of White blood cells and are classified mainly based on the presence and absence of granules. - Granulocytes - Agranulocytes ###  Granulocytes They are leukocytes, with the presence of granules in their cytoplasm. The granulated cells include- eosinophil, basophil, and neutrophil. #### Eosinophils - They are the cells of leukocytes, which are present in the immune system. - These cells are responsible for combating infections in parasites of vertebrates and for controlling mechanisms associated with allergy and [asthma](https://byjus.com/biology/asthma/). - Eosinophil cells are small granulocyte, which are produced in the bone marrow and makes 2 to 3 per cent of whole WBCs. These cells are present in high concentrations in the digestive tract. #### Basophils - They are the least common of the granulocytes, ranging from 0.5 to 1 per cent of WBCs. - They contain large cytoplasmic granules, which play a vital role in mounting a non-specific immune response to pathogens, and allergic reactions by releasing histamine and dilating the blood vessels. - These white blood cells have the ability to be stained when exposed to basic dyes, hence referred to as basophil. - These cells are best known for their role in asthma and their result in inflammation and bronchoconstriction in the airways. - They secrete serotonin, histamine and heparin. #### Neutrophils - They are normally found in the bloodstream. - They are predominant cells, which are present in pus. - Around 60 to 65 per cent of WBCs are neutrophils with a diameter of 10 to 12 micrometres. - The nucleus is 2 to 5 lobed and the cytoplasm has very fine granules. - Neutrophil helps in the destruction of bacteria with lysosomes, and it acts as a strong oxidant. - Neutrophils are stained only using neutral dyes. Hence, they are called so. - Neutrophils are also the first cells of the immune system to respond to an invader such as a bacteria or a virus. - The lifespan of these WBCs extends for up to eight hours and is produced every day in the bone marrow. ### Agranulocytes They are leukocytes, with the absence of granules in their cytoplasm. Agranulocytes are further classified into monocytes and lymphocytes. #### Monocytes - These cells usually have a large bilobed nucleus, with a diameter of 12 to 20 micrometres. - The nucleus is generally half-moon shaped or kidney-shaped and it occupies 6 to 8 per cent of WBCs. - They are the garbage trucks of the immune system. - The most important functions of monocytes are to migrate into tissues and clean up dead cells, protect against bloodborne pathogens and move very quickly to the sites of infections in the [tissues](https://byjus.com/biology/tissues/). - These white blood cells have a single bean-shaped nucleus, hence referred to as Monocytes. #### Lymphocytes - They play a vital role in producing antibodies. - Their size ranges from 8 to 10 micrometres. - They are commonly known as natural killer cells. - They play an important role in body defence. - These white blood cells are colourless cells formed in lymphoid tissue, hence referred to as lymphocytes. - There are two main types of lymphocytes -- B lymphocytes and T lymphocytes. - These cells are very important in the immune systems and are responsible for humoral and cell-mediated immunity. #### Platelets (Thrombocytes) - Thrombocytes are specialized blood cells produced from bone marrow. - Platelets come into play when there is bleeding or haemorrhage. - They help in clotting and coagulation of blood. Platelets help in coagulation during a cut or wound. Components Of Blood ------------------- There are many cellular structures in the composition of blood.  When a sample of blood is spun in a centrifuge machine, they separate into the following constituents: Plasma, buffy coat and erythrocytes. Thus blood contains RBC, WBC, platelets and plasma. ### Plasma The liquid state of blood can be contributed to plasma as it makes up  \~55% of blood. It is pale yellow in colour and when separated. Blood plasma consists of salts, nutrients, water and enzymes. Blood plasma also contains important proteins and other components necessary for overall health. Hence, blood plasma transfusions are given to patients with liver failure and life-threatening injuries. **Components of Blood Plasma ** Blood plasma has several protein components. Proteins in blood plasma are: - Serum globulin - Serum albumin - Fibrinogen The serum contains only globulin and albumin. Fibrinogen is absent in serum because it is converted into fibrin during blood clotting.  ### Red Blood Cells (RBC) Red blood cells consist of Haemoglobin, a protein. They are produced by the bone marrow to primarily carry oxygen to the body and carbon dioxide away from it. ### White Blood Cells (WBC) White blood cells are responsible for fighting foreign pathogens (such as bacteria, viruses, and fungi) that enter our body. They circulate throughout our body and originate from the bone marrow. ### Platelets Tiny disc-shaped cells that help regulate blood flow when any part of the body is damaged, thereby aiding in fast recovery through clotting of blood. The above-stated elements form the composition of blood in humans. The only vertebrate without haemoglobin is the crocodile icefish. It derives its oxygen requirement directly from the cold, oxygen-rich water where it lives. Functions of Blood ------------------ Blood is responsible for the following body functions: ### Fluid Connective Tissue Blood is a fluid connective tissue composed of 55% plasma and 45% formed elements including WBCs, RBCs, and platelets. Since these living cells are suspended in plasma, blood is known as a fluid connective tissue and not just fluid. ### Provides oxygen to the cells Blood absorbs oxygen from the lungs and transports it to different cells of the body. The waste carbon dioxide moves from the blood to the lungs and is exhaled. ### Transports Hormones and Nutrients The digested nutrients such as glucose, vitamins, minerals, and proteins are absorbed into the blood through the capillaries in the villi lining the small intestine. The hormones secreted by the endocrine glands are also transported by the blood to different organs and tissues. ### Homeostasis Blood helps to maintain the internal body temperature by absorbing or releasing heat. ### Blood Clotting at Site of Injury The platelets help in the clotting of blood at the site of injury. Platelets along with the fibrin form clot at the wound site ### Transport of waste to the Kidney and Liver Blood enters the kidney where it is filtered to remove nitrogenous waste out of the blood plasma. The toxins from the blood are also removed by the liver. ### Protection of the body against pathogens The White Blood Cells fight against infections. They multiply rapidly during infections. Blood Group System ------------------ The ABO blood group system consists of 4 types of blood group -- A, B, AB, and O and is mainly based on the antigens and antibodies on red blood cells and in the plasma. Both antigens and antibodies are protein molecules in which antigens are present on the surface of Red Blood Cells and antibodies are present in the plasma which is involved in defending mechanisms. On the other hand, the Rh blood group system consists of 50 defined blood group antigens. In the Rh system, the most important antigens are D, C, c, E, and e. The ABO and Rh blood systems  ### 1. ABO blood Group system The basis of ABO grouping is of two antigens- Antigen A and Antigen B. The ABO grouping system is classified into four types based on the presence or absence of antigens on the red blood cells surface and plasma antibodies. - **Group A **-- contains antigen A and antibody B. - **Group B **--contains antigen B and antibody A. - **Group AB **--contains both A and B antigen and no antibodies (neither A nor B). - **Group O **-- contains neither A nor B antigen and both antibodies A and B. The ABO group system is important during blood donation or blood transfusion as mismatching of blood group can lead to clumping of red blood cells with various disorders. It is important for the [blood cells](https://byjus.com/biology/blood/) to match while transfusing i.e. donor-recipient compatibility is necessary. For example, a person of blood group A can receive blood either from group A or O as there are no antibodies for A and O in blood group A. As shown in the above table, individuals of blood group O are called as ***universal donors***, whereas individuals of blood group AB are ***universal recipients***.  ### 2. Rh Blood Group System In addition to the ABO blood grouping system, the other prominent one is the Rh blood group system. About two-thirds of the population contains the third antigen on the surface of their red blood cells known as ***Rh factor*** or ***Rh antigen***; this decides whether the blood group is positive or negative. If the Rh factor is present, an individual is ***rhesus positive*** (Rh+ve); if an Rh factor is absent individual is ***rhesus negative ***(Rh-ve) as they produce Rh antibodies. Therefore, compatibility between donor and individual is crucial in this case as well. **Antibodies and** Antigens **Antibodies are proteins found in plasma**. They\'re part of your body\'s natural defences. They recognise foreign substances, such as germs, and alert your immune system, which destroys them. Antigens are protein molecules found on the surface of red blood cells. **PLASMA** **Plasma**, also known as blood plasma, appears light-yellowish or straw-colored. It serves as the liquid base for whole blood. Whole blood minus erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets) make up the plasma. Serum, sometimes mistakenly considered synonymous with plasma, consists of plasma without fibrinogen. Plasma contains 91% to 92% of water and 8% to 9% of solids. It mainly comprises of: 1. Coagulants, mainly fibrinogen, aid in blood clotting    2. Plasma proteins, such as albumin and globulin, that help maintain the colloidal osmotic pressure at about 25 mmHg 3. Electrolytes like sodium, potassium, bicarbonate, chloride, and calcium help maintain blood pH 4. Immunoglobulins help fight infection and various other small amounts of enzymes, hormones, and vitamins Function -------- As plasma forms the liquid base of blood, the functions carried out by plasma and blood overlap. The multitude of functions include:  - *Coagulation*: fibrinogen plays a major role in blood clotting along with other procoagulants like thrombin and factor X. - *Defense*: immunoglobulins and antibodies in plasma play an important role in the body's defense against bacteria, viruses, fungi, and parasites. - *Maintenance of Osmotic Pressure*: the colloidal osmotic pressure is maintained at around 25 mmHg by the plasma proteins like albumin synthesized by the liver. - *Nutrition*: transportation of nutrients like glucose, amino acids, lipids, and vitamins absorbed from the digestive tract to different parts of the body act as a source of fuel for growth and development. - *Respiration*: transportation of respiratory gases, i.e., carrying oxygen to the various organs and carrying carbon dioxide back to the lungs for excretion. - *Excretion*: the blood removes nitrogenous waste products produced after cellular metabolism and transports them to the kidney, lungs, and skin for excretion. - *Hormones*: hormones are released into the blood and transported to their target organs. - *Regulation of Acid-Base Balance*: plasma proteins contribute to acid-base balance through their buffering action. - *Regulation of Body Temperature*: this is maintained by balancing heat loss and heat gain in the body. - *Role in Erythrocyte Sedimentation Rate (ESR)*: fibrinogen, an acute phase reactant, increases during acute inflammatory conditions and contributes to the increase in ESR, which is used as a diagnostic and prognostic tool. Plasma Proteins: ================ Plasma proteins are the collection of intricate molecules found in blood plasma. Their roles are many and varied, and they are mostly synthesized by the liver. Albumin, globulins, and fibrinogen are the three most important plasma proteins. The three primary categories are albumin, globulins, and fibrinogen. ### **Albumin** - - - ### **Globulins** Alpha, beta, and gamma globulins are only a few of the many subtypes of globins, a class of proteins that are more complicated than albumin. - - - ### **Fibrinogen** Fibrinogen is a complex protein that is involved in the blood clotting process. It is synthesized by the liver and is converted to fibrin during the clotting process. Fibrin helps to form a clot, which is necessary to stop bleeding after an injury. In addition to these three main types of plasma proteins, there are also a number of other proteins that are found in smaller quantities in the blood plasma, including lipoproteins, which transport lipids throughout the body, and enzymes, which catalyze biochemical reactions. **BLOOD FORMATION**: Blood has been called the river of life, transporting various substances that must be carried to one part of the body or another. Red blood cells are an important element of blood. Their job is to transport oxygen to the body\'s tissues in exchange for carbon dioxide, which they carry to the lungs to be expelled. Red blood cells are formed in the red bone marrow of bones. Stem cells in the red bone marrow are called hemocytoblasts. They give rise to all of the formed elements in blood. Bone marrow is spongy tissue in the middle of certain bones. Most blood cells are made in your bone marrow. This process is called haemopoiesis. In children, haemopoiesis takes place in the long bones, like the thighbone (femur). In adults, it's mostly in the spine (vertebrae) and hips, ribs, skull and breastbone (sternum). You may have a bone marrow biopsy taken at the back of your hip (the iliac crest). ### How blood is produced Think of blood production like a family tree. At the top of the tree are the blood stem cells (or hematopoietic stem cells), which are the youngest (most immature) blood-forming cells. They can make copies of themselves. They also make new cells that are closer to being blood cells, called progenitor cells. There are two types of progenitor cells that split the family tree: lymphoid cells and myeloid cells. These cells then develop into various types of blood cells: **Myeloid stem cells** develop into red cells and some white cells (neutrophils, eosinophils, basophils and monocytes) and platelets. Immature myeloid stem cells are called myeloblasts (or just blast cells). **Lymphoid stem cells** develop into T-cells and B-cells. Immature lymphoid stem cells are called lymphoblasts (or just blast cells). Platelets are made from very large bone marrow cells called megakaryocytes. These are formed in the myeloid part of the tree. When megakaryocytes break apart, they form more than 1000 platelets each. Finally, at the bottom of the family tree are the mature red blood cells, white blood cells, and platelets. If a stem cell commits to becoming a cell called a proerythroblast, it will develop into a new red blood cell. The formation of a red blood cell takes about 2 days. The body makes about two million red blood cells every second! Blood is made up of both cellular and liquid components. If a sample of blood is spun in a centrifuge, the formed elements and fluid matrix of blood can be separated from each other. Blood consists of 45% red blood cells, less than 1% white blood cells and platelets, and 55% plasma. All normal blood cells live a short time: red blood cells 80--100 days, neutrophils 8--14 days, and platelets 4--5 days. They then die off and are replaced by new cells from the bone marrow. This means that your bone marrow remains very busy throughout your life. Chemicals in your blood called growth factors control blood cell formation. Different growth factors make the blood stem cells in the bone marrow become different types of blood cells. These days some growth factors can be made in the laboratory (synthesised) and are available for use in people with blood disorders. For example, granulocyte-colony stimulating factor (G-CSF) stimulates the production of white cells called neutrophils while erythropoietin (EPO) stimulates the production of red blood cells. Pathophysiology of anemia ========================= Anemia is the most common blood disorder, and according to the National Heart, Lung, and Blood Institute, it affects more than 3 million Americans. Anemia generally results from blood loss, decreased red blood cell (RBC) production, poor RBC maturation, or increased RBC destruction. [Red blood cells](https://www.hematology.org/education/patients/blood-basics) carry hemoglobin, an iron-rich protein that attaches to oxygen in the lungs and carries it to tissues throughout the body. Anemia occurs when you do not have enough red blood cells or when your red blood cells do not function properly. It is diagnosed when a [blood test](http://labtestsonline.org/understanding/analytes/hemoglobin/glance.html) shows a hemoglobin value of less than 13.5 gm/dl in a man or less than 12.0 gm/dl in a woman. Normal values for children vary with age. Cardiac Cycle The cardiac cycle attributes to a comprehensive heartbeat from its production to the commencement of the next beat. It comprises diastole, the systole, and the intervening pause. The occurrence of a cardiac cycle is illustrated by a heart rate, which is naturally indicated as beats per minute. A healthy human heart beats 72 times per minute which states that there are 72 cardiac cycles per minute. The cardiac cycle involves a complete contraction and relaxation of both the atria and ventricles and the cycle last approximately 0.8 seconds. Cardiac Cycle Physiology The [human heart](https://byjus.com/biology/human-heart/)** **consists of four chambers, comprising left and right halves. Two upper chambers include left and right atria; lower two chambers include right and left ventricles.  The key function of the right ventricle is to pump deoxygenated blood through the pulmonary arteries and pulmonary trunk to the lungs. While the left ventricle is responsible for pumping newly oxygenated blood to the body through the aorta. Cardiac Cycle Phases Following are the different phases that occur in a cardiac cycle: **Atrial Diastole**: In this stage, chambers of the heart are calmed. That is when the aortic valve and pulmonary artery closes and atrioventricular valves open, thus causing chambers of the heart to relax. **Atrial Systole**: At this phase, [blood cells](https://byjus.com/biology/blood/#types-of-blood-cells) flow from atrium to ventricle and at this period, atrium contracts. **Isovolumic Contraction**: At this stage, ventricles begin to contract. The atrioventricular valves, valve, and pulmonary artery valves close, but there won't be any transformation in volume. **Ventricular Ejection**: Here ventricles contract and emptying. Pulmonary artery and aortic valve close. **Isovolumic Relaxation**: In this phase, no blood enters the ventricles and consequently, pressure decreases, ventricles stop contracting and begin to relax. Now due to the pressure in the aorta -- pulmonary artery and aortic valve close. **Ventricular Filling Stage:** In this stage, blood flows from atria into the ventricles. It is altogether known as one stage (first and second stage). After that, they are three phases that involve the flow of blood to the pulmonary artery from ventricles. Duration of Cardiac Cycle ------------------------- In a normal person, a heartbeat is 72 beats/minute. So, the duration of one cardiac cycle can be calculated as: 1/72 beats/minute=.0139 minutes/beat At a heartbeat 72 beats/minute, duration of each cardiac cycle will be 0.8 seconds. Duration of different stages of the cardiac cycle is given below: **Atrial systole: **continues for about 0.1 seconds **Ventricular systole: **continues for about 0.3 seconds **Atrial diastole: **continues for about 0.7 seconds **Ventricular diastole: **continues for about 0.5 seconds **Cardiac Output** **"Cardiac output refers to the volume of blood pumped out per ventricle per minute."** Cardiac output is the function of heart rate and stroke volume. The amount of blood pumped by the left ventricle in one compression is called the stroke volume. Cardiac Output is an excellent example to exhibit the efficiency of the [human heart.](https://byjus.com/biology/human-heart/) It is related to the amount of blood pumped by the heart per minute. A normal cardiac output would be about five litres of blood per minute in a healthy individual. The cardiac output comprises 2 vital components: Heart rate: It refers to the number of times the heart beats per minute (bpm). Stroke volume: It refers to the quantity of blood pumped out of each ventricle with every heartbeat. Factors Determining Cardiac Output The following factors determine the cardiac output of a human heart: Venous Return This is the amount of blood that enters the heart through the veins per minute. After a certain time interval, the venous return becomes equal to the cardiac output. Force of Contraction The stroke volume and the cardiac output increases with the increase in the force of contraction. Heart Rate The cardiac output increases with the increase in heart rate. Cardiac Output Formula The equation for cardiac output is: **Cardiac Output (CO) = HR x SV** Cardiac Output Calculation For example: If the heart rate is 70 bpm and stroke volume is 70 ml. Using the formula: **HR X SV** = 70 X 70 = 4900 ml/min or 4.9 liters per minute. **Cardiac Output Measurement** There are different merits and demerits of measuring cardiac output using methods in an invasive and non-invasive manner. At present, there is no standard comparison system available for these methods using a reference measurement or standards. The invasive systems are well accepted, but there is progressing evidence that these systems are neither accurate nor effective in guiding therapy. Some of the methods to measure cardiac output are listed below: Doppler ultrasound Echocardiography Transcutaneous Transoesophageal Low output results in heart failure, a severe infection or [heart diseases](https://byjus.com/biology/heart-diseases/). The high output may be an indicator of blood infections. **Heart sounds** Heart sounds are created from blood flowing through the heart chambers as the cardiac valves open and close during the cardiac cycle. Vibrations of these structures from the blood flow create audible sounds --- the more turbulent the blood flow, the more vibrations that get created. The same variables determine the turbulence of blood flow as all fluids. These are fluid viscosity, density, velocity, and the diameter of the column through which the fluid is traveling. Auscultation of the heart sounds with a stethoscope is a cornerstone of physical medical exams and a valuable first-line tool to evaluate a patient. Some sounds are very characteristic of significant pathological lesions that have major pathophysiological consequences, and these first present on auscultation. These type of lesions can be heard in systole, diastole, or continuously through the cardiac cycle. **Electrocardiogram (ECG)** An ECG (electrocardiogram) is a test that records the electrical activity of your heart, including the rate and rhythm. It\'s usually quick and painless. Why an ECG (electrocardiogram) is done You\'ll usually have an ECG (electrocardiogram) if a doctor or healthcare professional thinks you\'re having symptoms of: - a heart attack - coronary heart disease - problems with how quickly or regularly your heart beats (arrhythmia) - You also may have an ECG: - if you\'ve been diagnosed with a heart condition or another condition that affects how well your heart works - before and while taking certain medicines