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Nutrients keep us alive, and every cell in our body needs a constant supply of food and oxygen to function. In a simple single-celled organism, this can be easily achieved through diffusion. However, humans are complex organisms made up of trillions of cells. So how do we ensure that every cell, no matter how far from the surface, receives the substances it needs?
This is where the circulatory system comes in. Acting as the body’s transport network, it delivers oxygen and nutrients to cells while removing waste products such as carbon dioxide. Like a team of delivery couriers, the circulatory system ensures that materials are efficiently transported from one part of the body to another.
In this article, we will explore the key components of the circulatory system, the cardiac cycle, the exchange of materials between capillaries and tissue fluid, the components of blood, the ABO blood group system, and coronary heart disease.
The human circulatory system is a double circulatory system, meaning blood passes through the heart twice during one complete circuit around the body. The first passage sends blood to and from the lungs for gas exchange, while the second supplies the rest of the body with oxygen and nutrients and removes waste products.
The main components of the circulatory system are the heart, blood vessels and blood. In a double circulatory system, blood flow is divided into two circuits, the pulmonary circuit and the systemic circuit.
The pulmonary circuit carries blood between the heart and the lungs, allowing carbon dioxide to be exchanged for oxygen. The systemic circuit transports blood between the heart and the rest of the body, delivering oxygen and nutrients to tissues and carrying away waste materials.
You may have heard that the mitochondria is the powerhouse of the cell. Indeed, the mitochondria is a crucial organelle which generates energy from glucose in the form of ATP, which is then used to power our cells and ultimately, our whole body. Depending on whether oxygen is present, cells carry out either aerobic respiration (with oxygen) or anaerobic respiration (without oxygen).
Aerobic respiration takes place in the presence of oxygen. The food we consume is broken down into glucose, which is converted into carbon dioxide and water. This process also releases a lot of energy and heat which is used to keep our bodies warm.
Anaerobic respiration on the other hand, takes place when oxygen is lacking. Glucose can still be broken down to release energy, but much less energy would be generated compared to aerobic respiration.
In animal tissues, glucose would be converted to lactic acid and energy though the enzyme lactate dehydrogenase. A real-life example is during intense physical exercise, where oxygen is less than abundant and the body shifts towards anaerobic respiration. The build-up of lactic acid in the muscles results in the “sour” feeling and muscle aches after a tough workout.
In some microorganisms such as yeast, glucose would be broken down into ethanol, carbon dioxide and energy. This process is fundamental to making beverages (such as beer, wine and kombucha), breads and biofuels.
We’ve now seen how important oxygen is for respiration. A steady supply of oxygen is crucial so that cells can keep releasing energy.
Gas exchange is the exchange of gases between an organism and its environment. In a simple single-celled organism, oxygen from the surroundings can just diffuse through the cell membrane into the cell.
For multicellular organisms like animals, it is not so simple. How does the body make sure that every cell receives oxygen and has its carbon dioxide waste removed? This is where the respiratory system plays an important role, ensuring a constant supply of oxygen to the body and removal of carbon dioxide.
The respiratory system in humans is mainly responsible for gas exchange. It includes two lungs in the thorax (chest cavity) and the air passages that lead into them. The key parts of these air passages are the nose, trachea, bronchi (singular: bronchus) and bronchioles. At the ends of the bronchioles are tiny air sacs called alveoli (singular: alveolus), where oxygen and carbon dioxide are exchanged between the air and red blood cells.
Air enters your body through two nostrils. The walls of the nostrils are lined with hair, which filter our dirt and debris from inhaled air. The nostrils lead to the nasal passages, which are lined with a moist mucosal membrane. This membrane serves to trap foreign particles and moisten the air as it passes through the nasal passages.
From the nose to the trachea
From the nasal passages, air passes through the pharynx and larynx (voice box). After the pharynx, air enters the trachea (windpipe) which then splits into two bronchi.
The trachea is lined with tough C-shaped cartilage which keeps the trachea open. The trachea lumen is lined with epithelium. The epithelium comprises two types of cells, the gland cells which secret mucus to trap dust and bacteria, and ciliated cells which have hair-like structures (cilia). The cilia act to sweep away dust-trapped mucus up the trachea.
The trachea divides into two bronchi (singular: bronchus), where each bronchus carries air into each lung. Within the lung, the bronchi branch repeatedly, giving rise to bronchioles that end in a cluster of air sacs or alveoli. As the trachea progressively branches into bronchi and bronchioles, the amount of supporting cartilage in their walls decreases.
Gas exchange between the air and the blood takes place at the alveoli. Each lung contains numerous alveoli, providing a large surface area for efficient gas exchange. The alveoli are surrounded with numerous capillaries, ensuring a rich supply of blood carrying red blood cells ready for gas exchange. The walls of the alveoli are also only one cell thick, which provides a short diffusion distance for gases and allows a high rate of diffusion.
Gas exchange in the lungs occurs through diffusion, where gasses move from a region of higher concentration to a region of lower concentration.
The air that enters the alveoli is rich in oxygen, while the blood in nearby capillaries is poor in oxygen. Oxygen therefore diffuses from the alveoli into the blood, where it dissolves in the moist alveolar wall, crosses into the capillary, and is picked up by red blood cells for transport to the rest of the body.
At the same time, the blood in nearby capillaries is rich in carbon dioxide produced from cellular respiration, while the air in the alveoli is poor in carbon dioxide. Carbon dioxide thus diffuses from the blood into the alveoli, after which it is exhaled and removed from the body.
Breathing is the essential, nearly automatic process of taking in oxygen and giving out carbon dioxide. This process supplies the body with oxygen for cellular respiration, while expelling carbon dioxide that has been produced as waste from the body. The act of taking in air is known as inspiration or inhalation, while giving out air is known as expiration or exhalation.
Breathing is driven by Boyle’s law, which states that gas pressure is inversely proportional to volume at a constant temperature (P1V1 = P2V2). Simply put, when volume increases, pressure decreases and when volume decreases, pressure increases.
This law is the basis behind how air flows in and out of our lungs. The act of breathing is actually us changing the volume of our thoracic (chest) cavity, and varying the pressure as a result!
The muscles of the thoracic cavity, particularly the intercostal muscles and diaphragm, drive breathing by varying the volume and subsequently pressure within the thoracic cavity.
The intercostal muscles consist of the internal intercostal muscles which line the inside of the ribs, and the external intercostal muscles which line the outside of the ribs. The diaphragm is a dome-shaped sheet of muscle that separates the thorax from the abdomen.
During inspiration, the volume of the thoracic cavity increases, causing pressure to decrease. This results in the air pressure outside our bodies to be higher than within the thoracic cavity, causing air to flow into our lungs.
To increase the volume of the thoracic cavity, the diaphragm contracts and flattens, while the external intercostal muscles contract and pull the ribcage outwards.
This results in the lungs expanding and a decrease in air pressure within the lungs. The atmospheric air pressure is now higher than the pressure within the lungs, resulting in air flowing into the lungs.
Expiration is the process of “breathing out”, and can be either passive or active.
Passive expiration is an effortless, automatic process of breathing, where the lungs and chest wall naturally recoil due to its elasticity.
It involves the relaxation of the diaphragm and external intercostal muscles that were previously contracted during inhalation. As a result, the diaphragm moves upwards and the ribcage moves inwards, compressing the thoracic cavity and reducing its volume. This increases air pressure within the thoracic cavity above that of atmospheric air pressure, resulting in air flowing out of the lungs.
On the other hand, active expiration involves active contraction of the internal intercostal muscles, causing the ribcage to be pulled downwards towards the diaphragm. Other muscles such as abdominal muscles are also contracted to rapidly decrease the volume of the thoracic cavity. Active expiration occurs when rapid exhalation is required, such as laughing, huffing and sighing.
With rapid advances in science within the past few decades, we are now starting to understand the negative impacts on some activities that were historically widespread and assumed to be harmless. One of these activities is the smoking of tobacco.
Tobacco smoking has a long history, dating back to 5000BC where it was smoked on the American continent for ritualistic purposes. Up till several decades ago, smoking was widely accepted and tobacco companies have invested huge amounts of money into promoting cigarettes. However, as we are now starting to understand the harmful effects of smoking, numerous restrictions have been imposed to discourage smoking. So, what have we discovered in the past 50 years?
Tobacco contains many chemical compounds, many of which have been found to be harmful to the human body. Three that are are under constant scrutiny are:
Tobacco smoking has also been linked to various respiratory diseases, through frequent exposure to tobacco smoke. Hence, even non-smokers may be affected if they are in the presence of second-hand smoke.
Chronic bronchitis is the inflammation of the epithelium lining the air passages. Excessive mucus is secreted by the epithelium, which paralyses cilia and blocks air passages, making it difficult to breathe. The excess of mucus also increases the risk of lung infections.
Emphysema occurs when the partition walls between alveoli start to break down, due to persistent and violent coughing. This results in a loss of surface area for gas exchange. The lungs also lose their elasticity, which makes breathing difficult.
Lung cancer is the uncontrollable division of lung cells, resulting in a tumour. It has been shown that the risk of lung cancer increases with tobacco smoking. Due to the many harmful chemicals produced during tobacco smoking, the risk of developing cancers in other parts of the body also increase, such as in the mouth, throat, pancreas, kidneys and urinary bladder.
In this article, we saw how cells obtain energy from the food we eat through cellular respiration, how the respiratory system supplies oxygen and removes carbon dioxide, and how smoking damages these processes. The next time someone offers you a cigarette, remember that every puff harms your lungs and reduces the efficiency of gas exchange, ultimately affecting how well your cells can carry out respiration!
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Prepared by: Michelle
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