Circulatory System in Humans PDF

# Circulatory System in Humans ## Key Concepts - Internal structure of the heart in detail (including valves, septum, pacemaker) - Schematic diagram of the heart - Blood vessels - aorta, pulmonary trunk, coronary artery and vein and vena cavae - Circulation of blood as double circulation - Blood...

# Circulatory System in Humans ## Key Concepts - Internal structure of the heart in detail (including valves, septum, pacemaker) - Schematic diagram of the heart - Blood vessels - aorta, pulmonary trunk, coronary artery and vein and vena cavae - Circulation of blood as double circulation - Blood Groups (A, B, AB and O): universal donor and universal acceptor - Conditions related to the functioning of the heart: palpitations, cardiac arrest and hypertension - Introduction of lymphatic system as a parallel circulatory system ## You Have Learned You have learned in class 6 that human beings have a well-developed specialized system to transport nutrients, gases and hormones within the body. This system is known as the circulatory system. You have also learned about various parts of the circulatory system, namely blood, blood vessels and heart. ## Blood Blood is a fluid connective tissue. Blood circulates and the transports essential substances such as oxygen and nutrients to the cells all over the body. It is composed of plasma and blood cells. There are three types of blood cells or corpuscles: - Red blood cells (RBCs) or erythrocytes - White blood cells (WBCs) or leucocytes - Blood platelets or thrombocytes - RBCs are red in colour because of the presence of a red-coloured iron protein called haemoglobin. - WBCs are larger than RBCs and do not contain haemoglobin, so, they are colourless. - Blood platelets are colourless, disc-shaped cells without nuclei. Blood is carried throughout the body inside hollow tube-like structures called blood vessels. ## Blood Vessels Blood vessels are of three types: - arteries - veins - capillaries - **Arteries** are thick-walled, elastic, muscular vessels that carry blood away from the heart to different body parts. - **Veins** are thin-walled vessels with valves that carry blood towards the heart from different body parts. - **Capillaries** are the fine blood vessels connecting arteries with veins. ## The Human Heart ### Structure of the Heart The heart is a pear-shaped muscular organ located in the chest cavity. Your heart is roughly the size of your clenched fist. An adult heart is about 12 cm in length and 9 cm in breadth. It weighs about 300 g. The walls of the heart are made up of cardiac muscles, which work tirelessly to keep it working throughout the life (Fig. 6.1a). The heart is enclosed in a double-layered sac called pericardium, which is filled with pericardial fluid. The fluid protects the heart from shocks and jerks during contraction. ### Internal Structure Of The Heart The heart has four chambers - two auricles and two ventricles. All these chambers are separated from each other to prevent the mixing of oxygenated and deoxygenated blood (Fig. 6.1b). In addition, there are great blood vessels, septum and valves. **Diagram of the Heart** **a. External view** - superior vena cava - pulmonary artery - right auricle (RA) - inferior vena cava - right ventricle (RV) - aorta - pulmonary artery - left auricle (LA) - left ventricle (LV) **b. Section through the heart showing its internal structure** - Superior vena cava - Pulmonary veins - Left auricle (LA) - Bicuspid valve - Aortic valve - Right ventricle (RV) - Left ventricle (LV) - Pulmonary valve - Pulmonary artery - Inferior vena cava ### Chambers of the Heart #### Auricles - The Receiving Chambers - Auricles are the upper chambers of the heart. - They have thin walls. - They receive blood from different parts of the body. - These are also called atria (singular: atrium). #### Ventricles - The Discharging Chambers - Ventricles are the lower chambers of the heart. - They have thick walls. - They pump blood out of the heart. #### Septum Blood flows from the auricles to the ventricles. It does not flow from one auricle to another auricle or from one ventricle to another ventricle. This is because of a thick muscular wall present in between them. This wall is called the septum. The septum separates the heart into the left and right sides. The auricles are separated from each other by an inter-auricular septum. The ventricles are separated from each other by an inter-ventricular septum. #### Valves There are valves in the heart that regulate the blood flow from auricles to ventricles and various parts of the body. These valves work like a door, which opens only in one direction. The heart has four valves - **The bicuspid valve** is present between the left auricle and left ventricle. It controls blood flow from the left auricle to the left ventricle. - **The tricuspid valve** is present between the right auricle and right ventricle. It controls blood flow from the right auricle to the right ventricle. - **Pulmonary valve** is present at the base of the pulmonary artery. It allows flow of blood from the right ventricle to the pulmonary artery. - **Aortic valve** is present at the point of origin of aorta. It controls flow of blood from left ventricle to the aorta. ### Heart's Natural Pacemaker A cluster of cells situated in the upper part of the right auricle are called the Sinoatrial node or SA node. The SA node coordinates the heartbeats by generating electrical impulses. It is called the heart's natural pacemaker. When the SA node is defective, heart rhythms become abnormal – either too fast or too slow, or a combination of both. ### Blood Vessels in the Heart The blood vessels entering or leaving the heart are called the great blood vessels. These are as follows: - **Vena cavae:** The venae cavae are large veins which carry deoxygenated blood from the body to the heart. There are two vena cavae in the heart - superior vena cava and inferior vena cava. - **Coronary arteries and veins:** The coronary blood vessels include both coronary arteries and veins. There are two major coronary arteries. They supply oxygenated blood to the heart muscles. On the other hand, coronary veins return deoxygenated blood to the right auricle. **Diagram of the Major Blood Vessels of the Human Heart** - **Superior vena cava:** receives deoxygenated blood from the upper body - **Pulmonary veins:** supply oxygenated blood from the lungs to the heart - **Inferior vena cava:** receives deoxygenated blood from the lower body - **Systemic aorta:** carries oxygenated blood away from the heart to all body parts except lungs - **Pulmonary arteries:** supply deoxygenated blood from the heart to the lungs - **Coronary arteries:** supply blood to the cardiac muscle cells of the heart - **The pulmonary arteries** carry deoxygenated blood from the right ventricle to the lungs for purification. - **Pulmonary veins** carry oxygenated blood from the lungs to the left auricle. These are the only veins that do not have valves. ### Aorta (systematic aorta) - It is the main artery and it arises from the left ventricle. - It carries oxygenated blood from left ventricle to all parts of the body except the lungs. ### Pulmonary Arteries and Pulmonary Veins - **Pulmonary arteries** carry deoxygenated blood from the right ventricle to the lungs for purification. - **Pulmonary veins** carry oxygenated blood from the lungs to the left auricle. ### Circulation of Blood The heart pumps blood constantly. The right auricle receives deoxygenated blood (blood rich in carbon dioxide) through two major veins, that is, from the anterior parts of the body through the superior vena cava and from the posterior parts through the inferior vena cava. From the right auricle, the blood flows into the right ventricle and then to the lungs through the pulmonary arteries. In the lungs, exchange of gases takes place and the blood gets oxygenated. This oxygenated blood is brought to the left auricle by the pulmonary veins. From the left auricle, the oxygenated blood flows into the left ventricle and from there it is pumped into the aorta, the largest artery in the body. The aorta divides further to supply blood to all body tissues (Fig. 6.3). When the blood passes through capillaries, oxygen and nutrients are delivered to the cells while carbon dioxide and other wastes are collected by the blood. It thus gets deoxygenated and comes back to the right auricle, completing one round of circulation (Fig. 6.4). ### Double Circulation of Blood For completing one round of circulation in the body, the blood passes twice through the heart. This is called double circulation. Double circulation involves two processes: - pulmonary circulation - systemic circulation #### Pulmonary Circulation The circulation of blood between the heart and lungs (from the right ventricle to the lungs and from the lungs to the left auricle) is called pulmonary circulation. #### Systemic Circulation The circulation of blood between the heart and other body organs (from the left ventricle to the body parts through the aorta) is called systemic circulation. Both these circulations take place in a rhythmic manner, together constituting a cardiac cycle. **Diagram of the Circulation of Blood in the Body** - **Venae cavae carrying deoxygenated blood to the heart** -**Pulmonary veins carrying oxygenated blood to the heart** - **Aorta carrying oxygenated blood to the tissues** - **Pulmonary artery carrying deoxygenated blood to the lungs** **a. Auricles contract, valves open** **b. Ventricles filled, valves close** **c. Ventricles contract, blood passes into the aorta and pulmonary arteries** **Direction of Blood Flow** - **LA to LV** - **Lungs to Body Parts** - **RV to RA** - **Blood gets deoxygenated in tissues and is carried to the heart by the superior and inferior venae cavae.** - **Blood gets oxygenated in the lungs and is carried back to heart by the pulmonary veins** - **Oxygenated blood goes to all body parts through the aorta** - **Deoxygenated blood goes to the lungs through pulmonary arteries** ## Heartbeat and Cardiac Cycle All the heart muscles do not contract at the same time. The auricles contract first, forcing the blood into the ventricles. A fraction of a second later, the ventricles contract to push blood into the pulmonary arteries and the aorta. Then the auricles relax and blood flows into them from the pulmonary veins and venae cavae. By this time, the ventricles have also emptied and the whole heart is relaxed. This is referred to as a heartbeat. The sequence of events which take place during one complete heartbeat is known as cardiac cycle. It involves repeated contraction and relaxation of the heart muscles. A contraction is termed as systole and a relaxation is termed as diastole. One complete heartbeat or cardiac cycle consists of one systole and one diastole. It lasts for about 0.8 seconds. Each heartbeat is heard as two sounds. The contraction of auricles makes one weak sound. It is followed by a strong sound caused by the contraction of ventricles. The doctor uses a stethoscope to listen to this two-part heartbeat (lub-dub) (Fig 6.5). A normal heart on an average beats 72 times per minute. ### Pulse When the left ventricle contracts, blood moves into the arteries under high pressure. The walls of the arteries get stretched by this pressure. As the ventricles relax, the pressure comes down. The stretching and relaxing of arteries with each heartbeat is felt as a throbbing called pulse. Your pulse is the rate at which your heart beats. It is easy to feel the pulse at the side of the neck or at the wrist near the base of the thumb. In a healthy adult, it is about 70-75 times per minute. ### Blood Pressure **D.2** The heart must exert pressure to push blood through the blood vessels. The force of blood exerted against the arterial walls by the heart is called blood pressure. It varies from one part of the body to another and has two limits (upper and lower). The average pressure produced in the ventricles when they contract and pump blood into the aorta and pulmonary artery is called systolic pressure (upper limit). It is equal to the pressure exerted by a column of 120 mm Hg (millimetres of mercury). The average pressure produced when the ventricles get filled with blood from the auricles is called diastolic pressure (lower limit). It is about 80 mm Hg. Blood pressure is measured with a sphygmomanometer (Fig. 6.6). The normal value of blood pressure in an adult is considered to be 120/80 mm Hg. It varies from person to person and is affected by the age, sex and health of the person. ## Abnormal Functioning of the Heart Sometimes due to some defects in the structural components of the heart or blood vessels, the heart shows abnormal functioning. Some of these are as under: **Palpitation** - **D.3** Palpitations makes a person feel as though his heart is beating too fast, skipping a beat, or fluttering. The person may notice heart palpitations in the chest, throat, or neck. Palpitation is usually caused by anxiety or some coronary heart diseases. This may also caused due to the effect of some medicine. **Cardiac Arrest** - **D.5** Sometimes, the heart suddenly stops contracting. This happens because of the narrowing of the blood vessels. This occurs especially in the coronary arteries due to the deposition of cholesterol and calcium on their inner walls. As a result, there is no or less supply of blood to the heart and the heart stops beating. **Hypertension - High Blood Pressure** Sometimes there is constriction of arterioles. This creates difficulty in the flow of blood. It is called high blood pressure or hypertension.. Hypertension increases the risk of heart disease, kidney disease, diabetes and blindness. Nowadays, an increasing number of people have higher than normal values of blood pressure. ### Check Your Progress **Fill in the blanks** 1. The upper chambers of the heart have **thin** walls and are called **auricles**. 2. **Aorta** carries blood from the heart to various parts of the body. 3. **Veins** carry blood from different body parts towards the heart. 4. Circulation of blood between the heart and lungs is called **pulmonary circulation**. 5. The normal blood pressure in an adult is **120/80** mm Hg. ## Blood Groups and Blood Transfusion The giving of blood to one another person is called blood transfusion. **Antigens** are large protein molecules present on the surface of cells, bacteria and some non-living substances such as toxins and chemicals. Antigens stimulate the body to produce antibodies, which are chemical substances produced by the body in response to antigens. Each antibody binds to a specific antigen like a key does to a lock. **Giving blood - donor receiving blood - receiver** Antigen A and antibody A are incompatible and cause self-clumping. Similarly, antigen B and antibody B are incompatible. However, antigen A is compatible with antibody B and antigen B is compatible with antibody A. Sometimes, due to some injury or major surgery or certain illness, there can be a heavy loss of blood from the body. This blood loss is made up by transferring blood from a healthy person. The process by which blood is transferred from one person to another is called blood transfusion. However, blood cannot be transfused without matching the blood type of the donor and the recipient. Depending on the presence of proteins called antigens and antibodies, the human blood is classified into four groups and Rh(+) and Rh(-). ### ABO Blood Groups Antigens are proteins present on the surface of RBCs while antibodies are present in the plasma. There are two types of antigens - A and B and two types of antibodies - A and B. Based on the presence or absence of specific antigens, the human blood is classified into four groups, namely A, B, AB and O. - **Blood group A** has antigen A and antibody B. - **Blood group B** has antigen B and antibody A. - **Blood group AB** has both antigens but no antibodies. - **Blood group O** has neither antigen A nor B but both antibodies A and B (Table 6.1). The antigens and antibodies work against each other. When blood containing the opposite antigen is transfused into the body of a person, the antibodies of the recipient will attack the donated blood and cause clumping. This may cause blood infection and even death. Therefore, blood transfusion is done very carefully by matching the blood groups of the donor and the recipient. **Table 6.1 Composition and transfusion compatibility of different blood groups** | Blood Group | Antigen on RBC | Antibody in plasma | Can donate blood to | Can receive blood from | |:---|:---|:---|:---|:---| | A | A | B | A and AB | A and O | | B | B | A | B and AB | B and O | | AB | A and B | None | AB | A, B, AB and O | | O | none | A and B | A, B, O and AB | O | Table 6.2 shows the blood transfusion compatibilities of various blood groups. Blood group O is called the universal donor since it can be given to a person with any blood group. Blood group AB is called the universal recipient since it can receive blood from a person with any blood group. ### Rhesus Factor (Rh-factor) Another antigen called the Rh-factor is found on the surface of RBCs. It was first discovered in rhesus monkeys. About 85 per cent people have the Rh-factor; they are said to be Rh-positive (Rh+). The rest 15 per cent do not have it. They are said to be Rh-negative (Rh-). When Rh+ blood is transfused into an Rh- person, the recipient forms antibodies against the Rh+ factor. However, no complication occurs after the first transfusion as Rh antibodies are short-lived. In case of a second transfusion of Rh+ blood after a short period, the recipient's antibodies will attack the transfused blood and cause problems. Thus, before blood transfusion, the blood groups of donors and recipients as well as their Rh-factor have to be matched to prevent clumping of RBCs. An Rh- person can donate blood to an Rh+ person of compatible blood group, but not receive blood from an Rh+ person (Table 6.2) **Table 6.2 Transfusion compatibility of human blood based on blood groups and Rh factor** | Type | Can give blood to | Can get blood from | |:---|:---|:---| | A+ | A+ AB+ | A+ A- O+ O- | | O+ | O+ A+ B+ AB+ | O+ O- | | B+ | B+ AB+ | B+ B- O+ O- | | AB+ | AB+ | Everyone | | A- | A+ A- AB+ AB- | A- O- | | O- | Everyone | O- | | B- | B+ B- AB+ AB- | B- O- | | AB- | AB+ AB- | AB- A- B- O- | ## The Lymphatic System In addition to the circulatory system, your body also has a lymphatic system that transports lymph throughout the body. Lymph is a fluid that contains infection-fighting white blood cells. When blood reaches the capillaries, some of the water and dissolved solutes are filtered out from the plasma into tissue spaces to form a tissue fluid called lymph. Some of this lymph enters tiny channels called lymph vessels. Basically, lymph has all the elements of blood except RBCs and blood platelets. Therefore, it is slightly yellowish in colour and contains a special type of white blood cells called lymphocytes, which help in fighting infections. The lymphatic system consists of lymph vessels and lymph nodes (Fig. 6.7). The lymph vessels resemble veins due to the thin walls and presence of valves. The lymph nodes are small globular masses of lymphatic tissue. ### Functions of Lymph The main functions of lymph are transportation, defence and drainage. #### Transportation - The lymph helps in transporting nutrients and hormones to the body cells. - It also removes waste materials from the body cells to the tissues and finally pours them into the veins. - Lymph nodes produce lymphocytes. - Lymph carries lymphocytes and antibodies from the lymph nodes to the blood. - The lymph also absorbs and transports fats (fatty acids and glycerol) from the small intestine to the blood #### Defence As lymph carries lymphocytes and antibodies, it destroys the harmful microorganisms and foreign particles in the lymph nodes. Lymph picks up the bacteria entering the body and brings them to the lymph nodes, where they are destroyed. Lymph nodes also help in localizing the infection and preventing it from spreading to other tissues. Can you correlate this with the swelling of tonsils in case of a throat infection? #### Drainage The lymph drains excess tissue fluid from the intercellular spaces back into the blood. Tissue fluid is a fluid that surrounds the cells of most tissues, arriving through blood capillaries.