 Dear students, in this topic we shall discuss the mechanisms for thermogenesis in endotherms. The endothermic animals respond to temperatures below the lower critical temperature by generating additional heat. This additional heat is produced through three types of mechanisms, shivering thermogenesis, non-shivering thermogenesis and a specialized brown fat oxidation. The shivering and non-shivering thermogenesis both convert chemical energy into heat. These two processes use normal energy-converting metabolic processes, which body primarily uses to produce heat. But the difference is that all the released energy is utilized to produce heat. No energy is used for mechanical or chemical work. Usually, energy generated from the body metabolism process is used for some part-mechanical work and the rest is used to produce heat. But when heat is produced for purposefully thermogenesis, this mechanism works in the same way, but all that is 100% of energy is utilized to generate heat. Dear students, now we shall discuss shivering thermogenesis in detail. Shivering thermogenesis uses the involuntary muscle contractions to generate or liberate heat. This process is controlled by the nervous system, which controls the skeletal muscles. The nervous system at a time activates antagonistic pairs of skeletal muscles for contraction and relaxation. That is why there is no net movement in the shivering. Shivering is just like trembling or vibration. During this process, the muscle activation is hydrolyzed by ATP and energy is released as heat. Dear students, in non-shivering thermogenesis, enzyme systems are activated which metabolize fats. In non-shivering thermogenesis, conventional fats are broken down and oxidized and heat is produced. This process is activated by the sympathetic nervous system, which causes the release of two types of hormones. 1. Norepinephrine and 2. Thyroid. These two hormones activate the breakdown of fats and produce heat through non-shivering thermogenesis. Dear students, third important mechanism is the oxidation of brown fat. This is found in some mammals. A few mammals have specialized type of fat which is called brown fat. This fat contains large number of mitochondria in it. Its color is brown. A enzyme found in mitochondria is caused by cytochrome oxidase. All tissues in which the mitochondria is large appear in brown color. This brown fat, in addition to containing large number of mitochondria, it also has extensive vascularization that is supply of blood vascular system. Dear students, oxidation of brown fat is triggered by the thyroid hormones. This oxidation occurs within the cells which are rich with fat-medabilizing enzymes. The heat produced in this oxidation protein is rapidly dispersed to other body parts where this heat is needed. By the blood, because this brown fat is highly vascularized, there is a lot of blood flow in it, the heat generated by the blood flows to the tissues where the heat is needed. Dear students, brown fat is different from the ordinary body fat because ordinary fat deposits are not highly vascularized. Ordinary fat deposits are metabolically very inactive because they need to be reduced to fatty acids first and then those fatty acids cannot be oxidized by keeping them in the fat cells. In fact, these fatty acids enter circulation. They are taken up by the tissue cells which oxidize these fatty acids. In contrast, brown fat is oxidized within the containing tissue without delay with massive heat output. That's why brown fat is more efficient in heat generation and thermogenesis.