Mammals

=__Mammals:__= =Reproduction, Development, Respiration, Transport, Nutrition, Excretion, and Regulation.= = =

The Class Mammalia is home to around 5,000 species of mammals, all of which are placed into 26 seperate orders. Orders are determined by reproduction, development, and evolutionary history. **__This tiny mouse and this enormous whale are both mammals.__**

But what defines a mammal? The answer, is very simple. Mammals are members of the Kingdom Animalia and the Phylum Chordata. Mammals are dictinct from other classes of chordates because of their reproduction. All mammals secrete milk from specialized sweat glands used to feed their young.
 * Chordates** are all animals that have a nerve cord sometime in their life.

__**REPRODUCTION**__

Most common of mammals, are the **Placental mammals,** which include nearly 4,000 species of known mammals. Placental mammals all bear live young, which are nourished before birth in the mother's uterus through a specialized embryonic organ attached to the uterus wall, called the **placenta.** The embryo develops over a long gestation period, and emerges from the womb fully formed. **Monotremes** and **Marsupials** have **placentae** as well, but they have little function in these different mammals.


 * Marsupials** are the group of mammals commonly thought of as pouched mammals, but the pouch is more complex than most think. This sub class is home to 260 species of known marsupials. Like most other mammals, marsupials give live birth, but have much shorter gestation periods than placental mammals. Instead, they give birth much earlier and the young animal, pretty much a helpless embryo, climbs from the mother's birth canal to the mother's pouch. Inside of the pouch are the mother's nipples, to which the embryo attaches to and remains until it is a mature juvenile.

The rarest of all mammals, are the **Monotremes.** Home to only three species, monotremes are the only known mammals to give birth to their young in eggs. Like reptiles, monotremes lay their young in eggs, but unlike reptiles the eggs are retained for some time within the mother, who actively provides the egg with nutrients.The **Duck-Billed Platypus** and two species of **Echidna** are found only in the continent of [|Australia].


 * __DEVELOPMENT__**

As like all **chordates**, mammals begin their lives as **embryos**. Mammals take this one step further, by developing their offspring in a placenta. When **sperm cells** from a male mammal enter the **ovary** of a female mammal of the same species, the sperm cell fertilizes the **egg cell** and forms a **zygote**. The zygote then begins to go into **mitosis,** rapidly growing until the zygote becomes an embryo. In humans, after eight weeks of gestation, the embryo is called a fetus. The unborn offspring is called a fetus until its birth. All mammals in embryo form have a set of **gills**, a **tail**, and **pharyngeal pouches**. However, all mammals lose their gills as they grow, and the pharyngeal pouches are replaced by lunds and other organs as the embryo matures. The fetus of most mammals develops similarly to the Human fetus. After the first stages of development, the human embryo reaches a stage very similar to all other chordates.


 * __RESPIRATION__**

The **lungs** of mammals have a spongy texture and are honeycombed with **cells**, having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung. Breathing is largely driven by the muscular **diaphragm** at the bottom of the **thorax.** **Contraction** of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward. Air enters through the **oral** and **nasal cavities**; it flows through the **larynx** and into the **trachea**, which branches out into **bronchi**. Relaxation of the diaphragm has the opposite effect, passively **recoiling** during normal breathing. During exercise, the diaphragm contracts, forcing the air out more quickly and forcefully. The **rib cage** itself also is able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its **pressure gradient**.
 * __The lungs of mammals are the most efficient lungs__** evolved on planet earth because the can extract much more oxygen from much less space than any other animal.


 * __TRANSPORT (Circulation)__**

Another advantage that mammals have over other animals is their highly advanced **circulatory systym**. A **mammalian heart** has four walled chambers, which efficiently pump **blood** to the **lungs**, to the **body**, and back to the **heart**. How does blood flow through the body of mammals?

Well, the heart pumps the blood through vessels called **veins** or **arteries**. The heart pumps blood through arteries, such as the **Aorta**, to the lungs. Other **veins** bring the newly **oxygenated blood** back to the heart. When the oxygenated blood gets to the heart, the heart pumps the blood through arteries flowing throughout the body. Veins then bring the unoxygenated blood back to the heart, and the cycle starts again. This has happened in just one heartbeat.


 * __NUTRITION__**

Mammals are one of only two classes to be considered **Endotherms.** Being an endotherm means that you do not need to waste precious time basking or collecting heat energy because your body heat is produced internally. This is not all of a blessing to mammals. Thanks to our internal "furnace," mammals need to consume much more "fuel" than other animals to keep their temperature regulated.

The mammalian diet is a vast variety of food, containing everything from grass to other animals.

Some mammals are **herbivores,** which means that their diet consists only of **plants** and plant **material**. The herbivorous diet is the most common diet found on Planet Earth.



Other mammals have evolved to a wider range of dietary needs. **Omnivores** also consume plant material, but also other animals as well.
 * __We humans are omnivoires__**, and our specialized teeth are examples of our needs.

Some mammals have evolved to the point that they no longer eat plants, but __only__ other animals. These mammals are called **Carnivores**. All cats and dogs are carnivores. Carnivores have a much faster **metabolic rate** than other mammals.

The size of mammals also has an affect of their diets. **__The larger the mammal, the more they need to eat to keep going.__** An example is the **Blue Whale**, who has to eat more than one ton of **Krill** each day to survive.

__**EXCRETION**__

When a mammal eats, it must **deficate** to release all of the undigested food, or it will die.
 * __Most mammals have similar digestive tracts to humans.__**

A mammal's diet also contributes to how the mammal digests.


 * __REGULATION__**

The mammal needs not only to remove wastes my means of excretion, bust also by means of its **Lymphatic System**. Over three liters of fluid leaks from the circulatory system each day, and it's the lymphatic system's job to collect and replace these fluids whil still filtering out bacteria and other waste products.

The Lymphatic sysetm is network of vessels, nodes, and organs that collect fluid that is lost by the blood and returns it back to the **circulatory system** (see above for circulatory system). The fluid found in the lymphatic system is called **Lymph**. Lymph collects into small **lymph capillaries**, which flow into much larger **lymph veins**. The collected lymph the travels through the veins and then goes to two openings in the **Superior Vena Cava**, thus returning the fluid back to the blood stream.

Along the lymph veins are small bean shaped objects called **Lymph Nodes.** When the lymph flows through the nodes, **bacteria** and **microorgansims** get trapped in tiny "filters" contained in the nodes. This cleans the lymph for re-entry into the blood stream.

Lymph vessels are also helpful in nutrient movement. **Fatty Acids** and **Glycerol** are pulled into the lymph vessels and transported throughout the body.

The **Spleen** is an important part of the lymphatic system. It stores **Phagocytes** that engulf and destroy harmful bacteria.

The **endocrine system** is an integrated system of small organs which involve the release of extracellular signaling molecules known as hormones. The endocrine system is instrumental in regulating metabolism, growth and development, tissue function, and plays a part also in mood. The **Pancreas** is a major part of the endocrine system because it releases hormones into the circulatory system. **Blood Plasma** also plays a large role in moving hormones throughout the body.

The **Nervous System** All mammalian brains possess a **neocortex**, a brain region that is unique to mammals. The Central command center of the mammalian nervous system is the **Brain**. The brain controls all of the functions of the body by sending **electrical impulses** throughout the nervous system.


 * The Brain**

The mammalian brain is the most evolved brain in the **Animal Kingdom**. There are many more lobes of the brain in mammals than in other animals. Mammalian brains also contain a region called the "**Reptilian Brain**." This small portion of brain is located near the back of the brain. It controls all of our instincts.

Hopefully this website will prepare you for the upcoming EOC in Biology. Good Luck!

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__MAMMALS__

 * __THE END__**