Homeostasis

All living organisms require complex set of interacting metabolic chemical reactions to maintain a constant environment. This include the simplest unicellular (prokaryote) organism to the most sophisticated organisms such as humans, animals and plants (eukaryote).

Homeostasis refers to any process that living things utilise to maintain stable internal environments, which are essential for survival. Homeostasis describes how living organisms maintain constant temperature, balance between acidity and basicity (pH) and maintain levels of water, minerals, vitamins, proteins, lipids, carbohydrates and blood – oxygen levels. Factors that affect homeostasis include strenuous exercise and survival in warm and cold climates. These factors affect temperature,

blood – oxygen levels, hydration and salt levels.
The hypothalamus, a section of the brain which is located at the base of the brain performs numerous functions that are essential for the survival of the human being. Even though the hypothalamus is small part of the brain, it is involved in behavioural, autonomic an endocrine functions. The hypothalamus acts as an integrator to regulate and coordinate basic functions, which include fluid and electrolyte balance; feeding and energy metabolism, wake – sleep cycles; thermoregulations; stress responses and sexual behaviour and reproduction. In short, the hypothalamus is the part of the brain which maintains equilibrium in the body.
Moreover, the hypothalamus is responsible for hormone production and secretion. This means that the hypothalamus is the link between the endocrine system and the nervous system. The hypothalamus produces releasing and inhibiting hormones, which stop and start the production of other hormones throughout the boy.
Digestion is an important bodily function, as it provides nutrients, which are broken down from the food that is consumed, to be used for energy, growth and cell repair. The digestive system is a collection of organs that collaborate to provide energy to power the body by processing the food that is consumed. The main nutrients that the digestive system provides include carbohydrates, protein, lipids, minerals and vitamins.
There are two main groups of carbohydrates, these include; simple and complex carbohydrates. Simple carbs include sugars, whereas complex carbs include starch and dietary fibre. It is the energy that is used to fuel our body. Protein that comes from food is broken down by our digestive system into amino acids. These amino acids are then utilised for repairing and constructing muscles, red blood cells, hair and other tissue. In addition protein aid in the construction of hormones. It can be used as energy by the body if there are not enough carbs available and that is why protein intake needs to be adequate. Lipids can be classified into two categories; saturated and unsaturated fats. Animal products such as meat and milk contain high levels of saturated fats whereas vegetable oils have high levels of unsaturated fats. Dietary fat plays a vital role in a healthy diet, as fat maintains skin and hair, cushions vital organs, provides insulation, and is pivotal for the absorption and production of certain hormones and vitamins. Vitamins aid in the regulation of chemical reactions in the human body. There are 13 vitamins, however, the common vitamins include A, B, C, D, E and K. Vitamins are not produced in our bodies, thus we obtain them from our diets. Minerals are the components of foods that are involved in numerous bodily functions. An example of this is Iron, it is needed for our red blood cells to transport oxygen around the body, and calcium and magnesium aid in the structural component of bones.
Hunger, the physical sensation is related to the contractions of the stomach muscles.

These contractions are believed to be caused or triggered by high concentrations of the hormone Ghrelin. Ghrelin is a peptide, which is produced by ghrelin cells in the gastrointestinal tract. It acts as a neuropeptide in the central nervous system, which is a small protein – like molecule, used by neurons to communicate with each other. Additionally, they influence the activity of the brain in specific ways, such as recognising rewards, metabolism, reproduction and social behaviours. Other than causing hunger, ghrelin plays a vital role in regulating the distribution and rate of use of energy. Ghrelin might also be secreted if the blood sugar levels are low, which can occur if food is not consumed for prolonged periods. The secretion of ghrelin acts on the hypothalamic brain cells, which leads to increased hunger, increased gastric acid secretion and gastrointestinal mobility to prepare the body for food intake. - lateral

hypothalamus
However, to counteract the hunger stimulation, the hormone leptin is secreted. Leptin is a hormone which consists of fat cells and the main purpose of this hormone is to regulate the amount of fat stored in the body. It does so by adjusting the sensation of hunger and adjusting energy expenditure. Hunger is inhibited when the amount of fat stored reaches a particular amount, thus activating the leptin receptors in the arcuate nucleus of the hypothalamus, which signals the body to stop consuming food. ventromedial hypothalamus
Cholecystokinin, which is a mixture of peptides, is secreted by cells in the duodenum and jejunum when they are exposed to food. This peptide acts on the gall bladder stimulating it to contract and force its contents of bile into the intestine. In addition, it also acts on the pancreas, where it stimulates the release of pancreatic digestive enzymes into pancreatic fluid. Moreover, this peptide also performs a satiety function, by sending signals to the medulla oblongata via the vagal neurons. A satiety signal is where the brain informs the body to stop consuming food to maintain equilibrium, as consuming too much food can cause a rapid change in the bodies intake of nutrients and a rapid change can have disastrous effects on the body.
Fibroblast Growth Factor 19 is a protein, which is secreted by cells in the lower portion of the small intestine. After its secretion it travels in the hepatic portal system to the liver, where it stimulates the uptake of glucose and its conversion into glycogen and the synthesis of bile acids.
Incretins are released when the consumed food travels through the gastrointestinal tract and arrives at the duodenum. These peptides enhance the ability to stimulate insulin secretion, enhances the ability of organs and muscles to take up glucose from the blood supply. In addition, it also acts on suppressing the appetite, thus reducing the food intake. In short, Incretins maintain homeostasis by preventing sharp rises in blood glucose levels while consuming high sugar foods.
The hormones and peptides that were aforementioned are released by glands that are located throughout the gastrointestinal tract and aid in the maintenance of equilibrium which is vital for the survival of humans. They have there specific functions that need to be performed, such as leptin, which is secreted when the amount of fat stored reaches a particular amount, thus informing the hypothalamus to stop the consumption of food. Hence, the production and secretion of these hormones and peptides enable equilibrium to be achieved since these substances prevent any rapid change in bodily functions.
Our bodies survive and die at cellular level. All the cells in our body will survive and perform its intended functions if the conditions are suitable. This includes the pH level and temperature to be constant and ideal.
Homeostasis also regulates acidity and basicity in the stomach. The human body is sensitive to changes in the pH level, therefore, to maintain equilibrium the pH levels have to between a certain range for the body to have optimal function. There are numerous definitions and theories on acid and bases. However, there are three main definitions on acids and bases, which can be categorised into Arrhenius Theory, Bronstead - Lowry Theory and the Lewis Theory. The Arrhenius theory states that acids are substances that ionise (break off) in an aqueous solution to produce hydrogen (H+) ions in solution, while bases produce hydroxide (OH-) ions in solution. However, the Bronstead - Lowry definition states that acids are substances that donate protons H+ ions whereas bases are substances which accept H+ ions. This is supported by the Lewis Theory which defines acids as electron pair acceptors while bases are electron pair donors.
Acidity and basicity levels are regulated in the body via buffering agents, the respiratory system and the renal system, which maintain the pH level between 7.38 and 7.42.

There are two types of buffering agents, which include extracellular and intracellular buffers. Bicarbonate and ammonia can be classified as extracellular buffers, while intracellular buffers consists of proteins and phosphate groups.

Acid has a significant effect in the digestion of proteins. Gastric acid, whose main component is HCL, is secreted by the cells, which line the stomach as a primary response when proteins and fats are ingested. The main purpose of the acid is to break down the long chains of amino acid, however, this process is time consuming, therefore, digestive enzymes are secreted, which act as catalysts. Pepsin, Gastrin and gastric lipase are some of the enzymes that are

secreted.
To maintain a constant environment for the enzymes to function, the brain signals for the secretion of hormones and peptides to maintain a constant acidity level. All enzymes have different pH levels at which they have optimal function. An enzyme is a protein, which is composed of amino acids. Amino acids are organic compounds, which are composed of amine (NH2) and carboxylic acid (COOH) functional groups and contain side chains, which give them their specific roles. These organic compounds have regions that are sensitive to pH, which means that these enzymes need to be in a suitable pH level to function. The human stomach has a pH of 2 and the enzymes that are secreted in the stomach have adapted to function at that pH level.
Somatostatin, which is a peptide hormone is secreted in several locations in the digestive system, however, in the stomach it acts on the acid producing parietal cells to reduce secretion of acid. Additionally, it prevents acid secretion by inhibiting production of other hormones such as gastrin, secretin and histamine which effectively slows down the digestive process.
Another hormone that is secreted to maintain constant environment is Secretin. Secretin is a hormone that is released by the duodenum when exposed to acidic contents. this occurs when the peristalsis moves the bolus from the stomach to the duodenum. It stimulates the exocrine portion of the pancreas, resulting in bicarbonate to be secreted into the pancreatic fluid. Bicarbonate is an important component of the extracellular buffering mechanism. It assists in the buffering of the acid, hence reducing the acidity to a more favourable level, resulting in homeostasis to be achieved.
In conclusion, in order to maintain homeostasis within the digestive system, the hypothalamus assists in the production and secretion of hormones and peptides, which in turn stabilises the internal conditions, thus resulting in the optimal functioning of our bodies.