B9 Transport in animals
This is the lecture for the 'Transport in animals' chapter of IGCSE combined/coordinated science biology. In this chapter, we study basic structure of the circulatory system including the heart and the blood vessels, and the components and functions of the blood.
9.1 Circulatory system & Heart
Circulatory system, also known as the cardiovascular system is a system consisted of the heart, blood vessels and the blood. The heart pumps the blood, the blood circulates the entire body via the blood vessels. While circulating the body, blood supplies oxygen and nutrients to our cells and removes carbon dioxide and wastes from the cells.
The blood circulation begins at the heart. Heart is an organ made of muscles divided into four chambers: left atrium, right atrium, left ventricle, right ventricle. So there are two chambers on each side of the heart, and the left and right side of the heart are separated by the septum. Atria are located above the ventricles, allowing blood gathered in the atria to flow to the ventricles. Blood in the ventricles are then pump to the arteries by strong contraction of heart muscles. From arteries to capillaries and to veins, blood vessels are connected throughout the body. After circulating the body through blood vessels, blood return to the atria via veins. Thus, we now know that blood flows in the following direction: atria --> ventricles --> arteries --> capillaries --> veins --> atria. It is crucial for the blood to flow in this direction without flowing backwards. In order to prevent the back-flow, there are valves in the heart and blood vessels. In the heart, valves exist at the junction of two different structures: between atrium and ventricle, between ventricle and artery at both sides. Another place where the valves exist is at veins. At arteries, blood flows at a great pressure due to the strong contraction of heart muscles meaning that there is no danger of back-flow. However at the veins, blood needs to return to the heart from our limbs without heart to pump the blood. This means that blood in veins flows at a very low pressure, and needs valves to prevent it from going backwards.
.webp)
Image 1. Simplified drawing of our heart.
The above is a simplified drawing of the human heart. The pulmonary artery and aorta are both arteries. Pulmonary vein, superior vena cava, inferior vena cava are all veins. Blood pumped from the right ventricle goes through the pulmonary artery to the lung. Gas exchange, which is removing carbon dioxide from the blood and filling oxygen instead, occurs at the lung. The oxygenated blood then returns to the heart via pulmonary vein. This time, the blood enters the left side of the heart. Left atrium filled with blood pumps blood to the left ventricle. Left ventricle contracts very strongly with its thick muscles, and blood is pumped with high pressure through the aorta to our body. After circulating the entire body, the deoxygenated blood returns heart via the vena cava. Both superior vena cava and inferior vena cava puts blood back to the right atrium. When the right atrium is filled with blood, it contracts and sends bloods to the right ventricle. Right ventricle then contracts to send the deoxygenated blood to the lung. This is the double circulatory system of our body, meaning that blood passes through heart two times per circuit.
When we do physical activities, our heart beats faster and stronger. This is to pump more blood out to our body. Activity of the heart can be measure in multiple ways including electrocardiogram (ECG), pulse rate, and listening to sounds of valves closing. ECG records electrical activities of the heart and shows the heart rate (how fast heart beats) and the heart rhythm (how regularly heart beats). Pulse rate can be measure at the wrist or neck. Pulse is created by the expansion and contraction of the arteries as the heart pumps the blood out. Thus pulse rate is equal to the heart rate, and the normal range is about 50 to 100 beats per minute. Sound of the valves closing is also well known as the lub-dup sound. The first 'lub' sound is when the valves between atria and ventricles of both sides close, and the second sound 'dup' is when the valves between ventricles and arteries of both sides close. This is possible because two atria contract at the same time and two ventricles also contract simultaneously right after two atria contract. By listening to this sound, we can tell the heart rate and the rhythm.
9.2 Blood vessels
The blood vessels are the passage for blood to flow. There are three types of blood vessels: arteries, veins and capillaries. Arteries carry blood out from the heart while veins carry blood back to the heart. Capillaries exist all around our body connecting arteries and vein. As blood pumped from the heart flows through arteries, they have to endure high pressure created by the heart muscles. To endure this high pressure, arteries have thick outer wall, muscle layers and elastic fibres. Thus, the lumen of arteries become small. Veins, however, do not have to stand the high pressure. So veins have thin outer wall, muscle layers and elastic fibres, leaving more room for lumen. Additionally, as stated above, pressure is too low at veins that veins require valves to prevent blood backflow. Capillaries supply oxygen and nutrients to every cell and take carbon dioxide and wastes from it. To increase the exchange rate, capillaries have very thin wall and are very narrow, only about one red blood cell thick.
Image 2: Brief drawings of blood vessels
9.2.1 Coronary Arteries and Coronary Heart Disease
Heart also needs blood vessels to provide blood to its muscle cells. Coronary arteries are vessels that supply blood to the heart muscle cells. It is crucial for coronary arteries to supply nutrients and oxygen to heart cells. If coronary arteries become narrowed for any reason, heart cells are negatively affected, making the entire heart malfunction. When coronary arteries are partially or completely blocked, the condition is called coronary heart disease. Plaque made up of fat, cholesterol and other substances deposit on the artery wall, blocking the artery. When the coronary arteries are partially blocked, blood flow to the heart muscles are restricted causing angina. If the artery is completely blocked, cardiac muscle cannot get blood supply leading to myocardial infarction, one of the most common causes of heart attack.
Some factors can increase or decrease the risk of coronary heart disease. Poor diet, smoking, stress, being a male, family history and aging increases the risk while good diet and exercise can lower the risk of coronary heart disease. Eating excess saturated fat can increase cholesterol level which may lead to more plaques. Smoking and stress can increase blood pressure, increasing the risk of plaque build up and blockage of the artery. Males and people with coronary heart disease in their family line are more likely to develop coronary heart disease. Aging is the risk factor of almost all diseases including the coronary heart disease. In contrast, exercise can help reduce the risk of coronary heart disease by weight loss, lower stress level, fall in blood pressure, and decreased cholesterol level.
9.3 Blood
Blood is not just red water inside our body. There are a lot of essential components in our blood which can be classified into 4 categories: red blood cell, white blood cell, platelets and plasma. Red blood cells are cells that transport oxygen. Hemoglobin that constitute the red blood cells have heme groups in them. Oxygen binds to the iron ion of the heme group. More oxygen can be carried via the blood by binding to the hemoglobin compared to simply dissolving in the plasma.
White blood cells are cell that protect our body. There are different types of white blood cells and each type carries out different role in our body. Among them, two main roles are phagocytosis and antibody production. Phagocytes carry out phagocytosis, and B cells of lymphocytes produce antibodies. Phagocytosis is eliminating pathogens by ingesting and lysing them. After ingesting the pathogens, phagocytes produce digestive enzymes that lyse the pathogens. Phagocytes take crucial part in the innate immunity by providing a front line defense for our body.
Antibody is an Y-shaped protein that specifically targets and neutralizes foreign antigens of the pathogens. Once pathogens invade our body, B cells are informed about the invasion and become activated. B cells differentiate into plasma cells and memory B cells. Plasma cells produce antibodies that kill pathogens. Memory B cells keep memories of that specific antigen and remain dormant until that specific pathogen with the antigen invades again. When the memory B cells are re-exposed to the same antigen, the memory B cells rapidly proliferate and differentiate into either plasma cells or additional memory B cells. Plasma cells derived from memory B cells produce antibodies much more quickly and in larger quantities compared to the initial response during the primary infection. This is why we suffer less and heal faster when infected by the same pathogen. We now take advantage of this adaptive immunity by vaccination. B cells take crucial part in the adaptive immunity by providing long-lasting immunity after an initial infection or vaccination.
Image 3: Drawing of blood components observed by a microscope
Platelets are small cell fragments whose roles are blood clotting. They make blood clots at the wound to stop the bleeding and help healing. Clotting is essential to minimize blood loss and prevent the entry of pathogens. When observed under a microscope, platelets look like a small purple dot. Plasma are the watery part of the blood. Plasma transports the red blood cells, white blood cells, platelets, antibodies, ions, nutrients, urea, hormones, oxygen and carbon dioxide to all parts of our body.
Conclusion
Today we studied basics of human circulatory system. The contents include our heart, types and structure of our blood vessels, compartments and functions of our blood cells. These are the most fundamental concepts on our circulatory system that is crucial to know. This chapter can be one of the bases where we can build up more information when studying biology or medicine. Hopefully this can help students studying biology. I will comeback with another IGCSE or IB science studies.
.webp)