The headgut is the external opening for food; it consists of organs that allow for feeding and swallowing. A human’s headgut has mouth parts, an oral cavity, a throat (pharynx), and more specifically; teeth, tongue, and salivary glands. Mechanical digestion occurs here using digestive enzymes, toxins, and anticoagulants. Saliva from the salivary glands gives lubrication that allows for ingestion (swallowing); the tongue helps with mechanical digestion and ingestion as well, but also contains chemoreceptor’s that allow for taste in humans (smell in other species). So, when a person begins to enjoy his steak and potatoes, his teeth chew up the food that is being eaten and along with mechanical digestion, saliva, and the tongue the food is broken down and swallowed where it enters the esophagus which is the beginning of the foregut.
The foregut consists of the esophagus and stomach; the esophagus forms the bolus which then is transported to the stomach. Human’s stomachs are monogastric; consisting of a single strong muscular sac. The bolus enters the cardiac sphincter and into the stomach where most of the digestion takes place. The lining of the stomach has gastric pits that are lined with goblet cells in order to produce mucus. The bottoms of these pits contain gastric glands which include parietal and chief cells which secrete HCl and pepsinogen respectively; aiding in the digestion process. Once the food is digested into their component molecules (chyme) it is ready to be passed on from the stomach (being very acidic at this time), it leaves through the pyloric sphincter and into the beginning of the small intestine or midgut, called the duodenum.
The midgut is the place of chemical digestion and absorption and is generally a very alkaline environment. This is the first place of lipid digestion and it starts with the duodenum. The duodenum contains Brunner’s glands, which produce an alkaline secretion; it also receives secretions from the liver (emulsify fats, neutralizes acid and pancreas through the pancreatic duct and common bile duct (this is merged into the Ampulla of Vater which is controlled by the sphincter of Oddi) which aids in continual breakdown of the food (chyme). Next the food travels through the ligament of Trietz to the next section of the small intestine called the Jejunum. The jejunum is suspended by mesentery to allow for free movement. There is some secretion of fluid and digestion, but mainly absorption takes place here. Plicae circulars are present in the jejunum to increase the surface area for nutrient absorption. The food passes through the final portion of the small intestine called the Ileum. The ileum is meant primarily absorption of digested food; substances such as Vit B12, bile salts, etc. that wasn’t absorbed prior are able to be taken up here. One secondary aspect of the ileum is that is possess Peyer’s patches which are used by the immune system as a surveillance and present sample antigens to the APC’s. After the small intestine has passed the remaining food particles, it goes through the ICV (ileocecal valve) and into the cecum of the large intestine which begins the hindgut phase.
The hindgut is meant for water and ion absorption and defecation. The cecum begins the large intestine, is followed by the colon, and finally the rectum. This is where any last remnants of the digested food are taken up and then the remaining insignificant parts are brought to the rectum as waste; defecation occurs through the anus and the cycle is finished.
Thursday, May 1, 2008
Thermoregulation
Humans use metabolic heat in order to maintain a constant body temperature; since human body temperature doesn’t change with the environment or have to be derived from the environment like poikilotherms or ectotherms, respectively, we are considered homeothermic as well as endothermic. Being endothermic, when the temperature drops, additional energy is released from where it is stored to generate larger amounts of heat. Besides the body releasing extra heat, there are other methods in order to produce heat (thermogensis); shivering and non-shivering are the two primary mechanisms which convert chemical energy into heat.
When a person shivers (other endothermic vertebrates and some insects also have this characteristic, but this essay is discussing humans), their muscles contract which produces heat, especially around the vital organs. In order to do this the nervous system triggers antagonistic skeletal muscles, which allows for the body to have the added shivering action in itself and not increasing the net muscle movement by too much. In order for this activated contraction, energy needs to be provided; hence ATP is hydrolyzed for this matter. The chemical energy released materializes into heat because there is no useful physical work for inefficiently timed and mutually opposed muscle contractions.
Non-shivering thermogenesis has to do with the activation of breaking down fats and then oxidizing these molecules in order to produce heat. Since most fats release very little energy in the form of synthesized ATP, a few mammals have were able to evolve an adipose type tissue into a fat-fueled deposit, know as brown fat. There are two types of adipose tissue, brown and white, and brown is primarily used to generate body heat. Brown fat is composed of mainly mitochondria and with such a wide spread vascularization, the utilization of fuel to produce energy in the form of ATP is high in this tissue. Humans have brown fat as infants, but as we grow up, the mitochondria disappears and this tissue becomes similar to white adipose tissue (there may still be functional brown tissue in certain areas of the adult body in many people). Despite whether or not brown tissue is present, white tissue can still be used in the non-shivering mechanism. The adipose tissue is activated by the sympathetic nervous system when norepinephrine attaches to the tissue’s receptors. This stimulates one of two different pathways; either an increase in heat production is caused by a cyclic ion pumping process that goes across the plasma membrane to stimulate hydrolysis of ATP or the ETC is maneuvered in a way that the energy that is usually oxidized and captured in ATP is degenerated as heat instead.
When a person shivers (other endothermic vertebrates and some insects also have this characteristic, but this essay is discussing humans), their muscles contract which produces heat, especially around the vital organs. In order to do this the nervous system triggers antagonistic skeletal muscles, which allows for the body to have the added shivering action in itself and not increasing the net muscle movement by too much. In order for this activated contraction, energy needs to be provided; hence ATP is hydrolyzed for this matter. The chemical energy released materializes into heat because there is no useful physical work for inefficiently timed and mutually opposed muscle contractions.
Non-shivering thermogenesis has to do with the activation of breaking down fats and then oxidizing these molecules in order to produce heat. Since most fats release very little energy in the form of synthesized ATP, a few mammals have were able to evolve an adipose type tissue into a fat-fueled deposit, know as brown fat. There are two types of adipose tissue, brown and white, and brown is primarily used to generate body heat. Brown fat is composed of mainly mitochondria and with such a wide spread vascularization, the utilization of fuel to produce energy in the form of ATP is high in this tissue. Humans have brown fat as infants, but as we grow up, the mitochondria disappears and this tissue becomes similar to white adipose tissue (there may still be functional brown tissue in certain areas of the adult body in many people). Despite whether or not brown tissue is present, white tissue can still be used in the non-shivering mechanism. The adipose tissue is activated by the sympathetic nervous system when norepinephrine attaches to the tissue’s receptors. This stimulates one of two different pathways; either an increase in heat production is caused by a cyclic ion pumping process that goes across the plasma membrane to stimulate hydrolysis of ATP or the ETC is maneuvered in a way that the energy that is usually oxidized and captured in ATP is degenerated as heat instead.
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