The liver and the kidneys are both blood filtration organs. They work together to detoxify the body from its waste products floating in the blood and have an important role in nutrient metabolism through their regulating functions.
The liver is a large organ, part of the digestive system, that filters blood and produces bile (Bouchon & Marceau, 2022).
It is situated obliquely in the intrathoracic cranial abdomen, with a great proportion being to the right of the median plane. The liver is also situated caudally to the diaphragm (Raynor, 2008).
Figure 1: THORACIC, ABDOMINAL AND PELVIC VISCERS (MARE) - Deep lateral view (Raynor, 2008). The liver is coloured in green.
The horse liver is made of 3 lobes: the right lobe, the quadrate lobe, and the left lateral lobe.
The aorta leads to the liver, and the caudal vena cava, the portal vein and the hepatic veins leave the liver.
The liver also contains bile ducts that join into the common bile duct to bring bile to the duodenum (Bouchon & Marceau, 2022).
The liver is mainly composed of parenchymal cells (60% of its cells forming 80% of its volume) (Vekemans & Braet, 2005).
Figure 2: Horse liver structure - Diaphragmatic surface (adapted from Bouchon & Marceau, 2022).
The liver has vascular, secretory and excretory, and metabolic functions (Johns Hopkins University, 2019). (Metabolic functions will be detailed in the next paragraph on the liver’s role in nutrient metabolism).
Vascular functions include blood storage (10-15% of the blood volume is in the liver, which regulates the blood in general circulation), formation of lymph (approximately half of the lymph is formed in the liver), and phagocytic system host (Dixon et al., 2013).
Secretory functions include the production of bile salts that go in the duodenum and emulsifies lipids for digestion (Russell & Setchell, 1992), the production of cholesterol, and the synthesis of plasma proteins in blood.
Excretory functions are detoxification of the body waste products (such as ammonia) and pathogens (such as drugs), and the breakdown of old red blood cells of which product is excreted in the bile.
Body temperature regulation, thanks to its high metabolic rate and its large size.
According to the University of Nottingham, 2018, the role of the liver in nutrient metabolism can be summarized into 3 categories: protein, fat, and carbohydrate metabolism.
The liver has a role in protein metabolism by breaking down excess protein and amino acids in the body into ammonia, which it will synthetise into urea to be excreted by the kidneys into urine.
The role of the liver in fat metabolism is to break down blood lipids for excretion or alter their structure to be stored as body fats for later use.
Finally, the liver is a glycogen reserve and is able of glycogenesis when the animal hasn’t eaten for several hours (mainly for carnivore mammals because their diet doesn’t include starch, but it can also happen for herbivore mammals). Other carbohydrate metabolism example is the maintenance of blood glucose levels, thanks to glucagon and insulin, the liver stores the non-used glucose to regulate the amount of glucose in the general circulation.
The liver plays a major role in glycaemic homeostasis and is responsible for distributing glucose into the bloodstream for transport to the cells where it is metabolised by cellular respiration to provide energy, as described above. Excess glucose is converted into glycogen by a series of reactions (glycogenesis) in liver cells. When the concentration of glucose in the blood falls, the liver triggers glycogenolysis, i.e. the breakdown of stored glycogen into glucose-6-phosphate and its subsequent hydrolysis (by glucose-6-phosphatase) into free glucose, which is released into the bloodstream for transport to the cells. If the body's energy requirements exceed the amount of stored glycogen, the liver can produce glucose-6-phosphate from precursors such as lactate, glycerol and amino acids through gluconeogenesis. Again, this glucose is converted to free glucose (Szablewski, 2017).
3 major liver's functions (Johns Hopkins University, 2019):
- vascular
- secretory and excretory
- metabolism
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- protein
- fats (lipids)
- glucides
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Bibliography
Bouchon, F. and Marceau, R. (2022) “Biologie Animale,” Centre Européen de Formation.
Dixon, L.J. et al. (2013) “Kupffer cells in the liver,” Comprehensive Physiology, pp. 785–797. Available at: https://doi.org/10.1002/cphy.c120026.
Johns Hopkins University (2019) Liver: Anatomy and functions, Liver: Anatomy and Functions | Johns Hopkins Medicine. Available at: https://www.hopkinsmedicine.org/health/conditions-and-diseases/liver-anatomy-and-functions (Accessed: January 10, 2023).
Raynor, M. (2008) Anatomie du Cheval à colorier: Une aide à l’apprentissage de l’anatomie du cheval. Paris, France: Vigot.
Russell, D.W. and Setchell, K.D. (1992) “Bile acid biosynthesis,” Biochemistry, 31(20), pp. 4737–4749. Available at: https://doi.org/10.1021/bi00135a001.
University of Nottingham (2018) Nutrient metabolism, RLO: The Physiology of the Liver. Available at: https://www.nottingham.ac.uk/nmp/sonet/rlos/bioproc/liverphysiology/page_two.html (Accessed: January 10, 2023).
Vekemans, K. and Braet, F. (2005) “Structural and functional aspects of the liver and liver sinusoidal cells in relation to colon carcinoma metastasis,” World Journal of Gastroenterology, 11(33), p. 5095. Available at: https://doi.org/10.3748/wjg.v11.i33.5095.
Szablewski, L. (2017) « Glucose Homeostasis », Gluconeogenesis. Available at : http://dx.doi.org/10,5772/67222