Exchange and Transport System
Imagine if animals took a couple of hours for the oxygen that they breathe to reach their cells. Well, if the giraffe relied on diffusion alone from oxygen to get to the tip of the head, by the time oxygen reaches cells would have died.
This is why large multicellular organisms have developed specialised exchange surfaces like lungs, gills, digestive systems, and specialised transport systems like circulatory systems. These systems help animals to exchange gases and nutrients with the surroundings and transport them where they are needed within the body. As we all know, animals have smaller surface areas compared to large volumes. It is slightly inefficient at exchanging substances. To cope with that, how animals have progressed exchange services and short diffusion distances are mentioned in the below sections.1.6.1 RespiratorySystem
The respiration system is a significant life-supporting system with many essential functions as gas exchange, oxygen supply to cells, maintaining the balance between gases, and pH in the body (Fig. 1.3). The respiratory system provides animals with oxygen and removes metabolic by-products such as carbon dioxide. Failure of the respiratory system is related to malfunction of various organ systems, with potentially fatal implications. For example, the COVID-19 pandemic during 2020 affected not only humans but animals too. Asiatic lions were found positive for COVID-19 in Hyderabad Zoo and Etawah Safari park, India, and mink farms in the Netherlands. A virus arrested the proper functioning of the lungs by compromising the immune status of the animals, which leads to further secondary complications, finally resulting in death.
Fig. 1.3 Description of the respiratory system and its components in cattle. (Courtesy: BioRender)
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Did You Know?
Lungfish have both gills and lungs. Many researchers even believe that lungfish might be the missing link between marine animals becoming landdwellers.
Now the question is why cells produce carbon dioxide following oxygen consumption? Animal depends on mitochondrial respiration to supply ATP to perform normal cellular activities. To summarise the whole process, animals breathe in oxygen, and lungs load up this inspired oxygen (O2) to red blood cells (RBC), which carries O2 to cells. Cells absorb oxygen through diffusion, where mitochondrial respiration comes into action. Mitochondria oxidise the available carbohydrates, amino acids (AA), and fatty acids to produce ATP, resulting in CO2 production. Produced CO2 is transported back to the lungs through RBC, then the expiration of CO2 from the lungs. On an outer look, it looks effortless. Still, if we dive deep, we can witness the impact of different factors like haemoglobin concentration, partial pressures of various gases, lungs pressure and movement of diaphragm concentration of alveoli, and many more. Unicellular organisms and aquatic animals may utilise diffusion gradients to drive gas exchange with the environment. In comparison, gaseous O2 requires an additional step to cross the cell membrane in terrestrial animals.
A common thought might be, what about the respiration in aquatic animals! Wouldn’t it catch your attention on mentioning, the exchange of gases in fish larvae is through the diffusion of gases across their body surfaces. Readers can appreciate complete comparative mechanisms in different species in detail in respective chapters.
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Octopuses have three hearts. A blue whale’s heart weighs about 400 pounds. A cheetah’s heart rate can speed up to 250 beats per minute within seconds.
We know that the respiratory system is more than just lungs; it includes the nose, followed by the nasal cavity, pharynx, larynx, trachea, primary bronchus, secondary bronchus, tertiary bronchus, respiratory bronchiole, alveolar duct, alveolus, and diaphragm.
But can you envision the design and fabrication of lungs outside the body? Yes, we are facing the future right in front of us. Bioengineered Lungs (BEL) is an exciting and rapidly progressing area in the veterinary biomedical field. It is an alternative for end-stage lung failures. Researchers are developing bronchial system circulation in non-immunosuppressed pigs to support BEL growth and animal survival after transplantation. Previously, researching and discovering medications for the treatment of respiratory disorders incurred very high variability and high costs. However, with the use of in silico and tissue- engineered lungs models, it is possible to understand various mechanical and biological variables that make in vivo research difficult. However, on the flip side, single-cell sequencing technologies, advancements in cellular and tissue imaging techniques, and improvements in tracking cell lineage systems have led to understanding the complex relationship between the respiratory system, cardiovascular system, and exchange of gases.1.6.2 CirculatorySystem
The circulatory system works to transport oxygen and nutrients throughout the body while removing waste products such as carbon dioxide, urea, and many metabolic wastes at the same time frame (Fig. 1.4). The heart, blood vessels, and blood are the major components of the circulatory system. The circulatory system is divided into two circuits: the pulmonary and systemic. In the pulmonary circuit, deoxygenated blood is pumped from the heart to the lungs to become oxygenated. In contrast, this oxygenated blood returned to the heart is pumped to the rest of the body in the systemic circuit. Unlike other muscles in the body, the heart, a muscular organ, never tires and works very hard to ensure that blood reaches all parts from head to hoof.
Is blood always red? Wrong, blood comes in a variety of colours. It may be red in humans and other mammals, but an octopus, for example, has blue blood and oscillated eyes.
Fish have completely clear blood, and in Papua New Guinea, they are green-blooded. The composition of blood is very simple yet extraordinary. Blood consists of red blood cells, white blood cells, platelets, and plasma. About 55% of the blood is the pale yellow sticky liquid found in animals called plasma. The plasma is mainly made up of water and proteins. Plasma carries nutrients, hormones, and proteins around the body. It contains about 92% water. About 45% of the blood is made up of RBCs. RBCs are tiny biconcave disc-shaped cells, also known as erythrocytes. Formation occurs in the bone marrow. It contains a special iron-containing protein called haemoglobin. Haemoglobin that makes the warm-blooded animal’s blood red, the octopus, has a unique protein called haemocyanin, making their blood blue.Fig. 1.4 Description of the circulatory system in animals. (Courtesy: BioRender)
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Blood Groups
Dogs—8 groups
Cats—3 groups
Horse—8 major groups
Cattle—11 groups
Goats—5 groups
Sheep—7 groups
Humans—4 groups
The icefish does not have either haemoglobin or haemocyanin, which makes it blood colourless. WBCs and platelets account for less than 1% of the blood. WBCs are savage fighters. Pathogenic microorganisms are prevented from entering the animal’s body by these organisms. Roughly about 70% of WBCs are phagocytes, which ingest and destroy invading pathogens in a process known as phagocytosis. Lymphocytes produce antibodies. Antibodies bind to the foreign pathogen and prevent it from spreading, whereas platelets form a minute blood fraction. Whenever an animal suffers a cut, they aid in clotting the blood, keeping the wound from becoming infected.
Transporting oxygen to all the cells in the body is the primary job of blood. The role is carried out by RBCs, which contain an essential protein called haemoglobin. So when animals breathe in, oxygen latches onto an active site in the haemoglobin with a single iron atom.
We can think of it as a seat on a shuttle bus. The oxygen molecules must first find their seat and put their seat belts on before the bus can move. Once the bus moves, the oxygen molecules are released when they reach their destination, anywhere in the body. The deoxygenated blood hops on while the empty shuttle bus returns to the heart. So the deoxygenated blood has arrived at the heart. Further functioning and structure of heart, arteries, veins, capillaries, and lymphatic system are described in respective chapters.In recent days, there have been many developments in veterinary cardiology like digital radiography, which helps diagnose congestive heart failures and monitor cardiac and noncardiac causes of cough (e.g. bronchial compression, tracheal collapse, inflammatory airway disease). Various cardiac biomarkers like cardiac troponin 1, N-terminal Pro-B- Type Natriuretic peptides released during contraction and stretch or damage to the heart have been identified. Technological advancements like cell phone heart monitor devices provide high-quality electrocardiograms (ECGs) and animal heart rate data. Moreover, a better understanding of blood transfusion is essential to interpret heart functionality. A technique called Xenotransfusion means transfusions between different species. It was practised before the identification of blood groups in humans. No allergy or agglutination events were observed in dogs receiving purified polymerised porcine haemoglobin. Oxyglobin is an ultrapurified polymerised bovine haemoglobin-based oxygen-carrying solution used to treat anaemia in cats and dogs. Using a cell separator system, a cell salvage technique collects blood intra- or post-operation in animals with severe haemorrhage. The blood is filtered and reintroduced to the patients within
6 h as packed red blood cells (pRBC) suspended in saline. Prior to transfusion, anticoagulants and systemic medicines such as plasma-activated clotting factors will be purified.
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