> For the complete documentation index, see [llms.txt](https://rss-industries.gitbook.io/learn/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://rss-industries.gitbook.io/learn/sci/fundamental/biology/homeostasis.md).

# Homeostasis

As it is already known, homeostasis is one of the most necessary things a living thing must partake in in order to survive. Stability is important to ensure the internal balance in temperature, pH, humidity, and more.

## Nervous System

Previous notes to the nervous system are [here](/learn/sci/fundamental/biology/cellular-respiration.md#nervous-system).

In the nervous system, **neurons** send quickly send **neurotransmitters** to the **synapse**, where neurotransmitters can quickly diffuse into the **target cell** and trigger a response.

<div data-with-frame="true"><figure><img src="/files/JFeLSQg4jNE4VKyFKUW4" alt="" width="563"><figcaption><p><strong>Image 1</strong> — Path of neurotransmitters to their target cell.</p></figcaption></figure></div>

## Endocrine System

The **endocrine system** consists of **glands** which—when triggered—produce and secrete **hormones** into the bloodstream. Humans have tons of different glands that produce many different hormones.

{% hint style="success" icon="lightbulb" %}
Every gland in the endocrine system is *identical* in both genders except for the testes[^1] and ovaries[^2].
{% endhint %}

Hormones are chemical messengers that, upon binding to a target, cause something to happen. Hormones can be steroids[^3] or non-steroids[^4].

### Communication with Cells

Hormones travel from the cell where they were produced into the bloodstream by diffusion. They then travel through the bloodstream and diffuse out to enter the target cell, where they bind to **receptors** and trigger a response.

<div align="center" data-with-frame="true"><figure><img src="/files/S7X5HAtvdceZWPJgxoEt" alt="" width="563"><figcaption><p><strong>Image 2</strong> — Path of hormones to their target cell.</p></figcaption></figure></div>

As seen, in Image 1 and Image 2, there is a lot of space between the bloodstream and the target cells. This makes it harder for hormones to diffuse into the target cell.

To sum up, the endocrine system transfers signals much more slowly than the nervous system, but they have a wider, more versatile range of places to send signals to.

### The Hypothalamus

The **hypothalamus** is a region in the brain that tells endocrine glands to secrete hormones and links the endocrine and nervous systems together. Funnily enough, the hypothalamus does this by producing hormones itself.

### The Pituitary Gland

The **pituitary gland** is a small gland attached to the hypothalamus by a thin stalk. It consists of two bulb-like lobes.

The [**posterior**](#user-content-fn-5)[^5] **lobe** stores hormones from the hypothalamus. The [**anterior**](#user-content-fn-6)[^6] **lobe** secretes pituitary hormones.

{% hint style="info" icon="circle-info" %}
Most pituitary hormones control other endocrine glands, which is why the pituitary gland is often called the master gland of the endocrine system.
{% endhint %}

<div data-with-frame="true"><figure><img src="/files/vFxunR5MiJ6GVF6v5Ncb" alt="" width="563"><figcaption><p><strong>Image 3</strong> — Diagram of the hypothalamus and the pituitary gland.</p></figcaption></figure></div>

## Feedback Loops

[**Feedback loops**](#user-content-fn-7)[^7] are mechanisms where the output or response of a loop impacts or influences the input or stimulus. In most feedback loops, a **receptor** receives a **stimulus**, provoking it to signal the **control**, which tells the **effector** to do a **response**.

<div data-with-frame="true"><figure><img src="/files/hJcUTek9DxFKUCOoLwK3" alt="" width="497"><figcaption><p><strong>Image 4</strong> — A feedback loop.</p></figcaption></figure></div>

In a **negative feedback loop**, the response reverses the stimulus back the **set point**. In a **positive feedback loop**, the response amplifies the stimulus.

{% hint style="info" icon="vial-vertical" %}

### Blood Glucose Levels

This is one example of a negative feedback loop.

All food contains some form of sugar. When food is consumed, blood glucose levels rise. This tells beta cells in the pancreas to produce insulin. Insulin makes body cells take in glucose causing glucose levels in the blood to decrease back to the set point.

When blood glucose levels drop, alpha cells in the pancreas produce glucagon. Glucagon then goes to the liver, causing it to release glucose into the blood, reverting glucose levels back to the set point.
{% endhint %}

{% hint style="info" icon="vial-vertical" %}

### Sweating

This is another example of a feedback loop.

When body temperature rises, it is released from your skin cells in a process that is known as sweating. The skin cells are part of the integumentary system.
{% endhint %}

Breathing, being dehydrated, and even having a high temperature are all examples of feedback loops, positive or negative.

## Urinary System

The **excretory system** is the body system that removes wastes from the body.

{% hint style="info" icon="circle-question" %}
Only one subsystem of the excretory system is covered here: the urinary system.
{% endhint %}

The **urinary system** consists of many organs necessary for the excretion of wastes.

When proteins and nucleic acids break down, they are converted to ammonia. Since ammonia is toxic though, it is again converted into **urea** in the liver which is water soluble and can be excreted in **urine**.

<div data-with-frame="true"><figure><img src="/files/My9Tp9g1hn3R4Ttwwb31" alt="" width="563"><figcaption><p><strong>Image 5</strong> — The urinary system.</p></figcaption></figure></div>

### Nephrons

In each kidney, there are millions of **nephrons**, which are long tubules that filter blood to remove wastes and reabsorb essential materials. These processes are called **filtration** and **reabsorption** respectively.

{% stepper %}
{% step %}
**Glomerulus and Bowman's Capsule**

In the kidneys, capillaries bunch up into a twisted bunch called the **glomerulus**. Through diffusion, fluid, called **filtrate**, is diffused out of blood and into the **Bowman's capsule** where it travels down further into the **proximal tubule**.
{% endstep %}

{% step %}
**Proximal Convoluted Tubule**

In the **proximal convoluted tubule**...

|                                                    Filtered In | Reabsorbed Out                                                                            |
| -------------------------------------------------------------: | ----------------------------------------------------------------------------------------- |
| <p>Hydrogen Ions (H<sup>+</sup>)</p><p>Ammonium</p><p>Urea</p> | <p>Salt</p><p>Water</p><p>Glucose</p><p>Amino Acids</p><p>Potassium</p><p>Bicarbonate</p> |
|                                                  {% endstep %} |                                                                                           |

{% step %}
**Loop of Henle**

Filtrate then travels into the **loop of Henle**. While descending...

| Filtered In | Reabsorbed Out          |
| ----------: | ----------------------- |
|        Urea | <p>Salt</p><p>Water</p> |

While ascending...

| Filtered In | Reabsorbed Out                                  |
| ----------: | ----------------------------------------------- |
|        Urea | <p>Salt</p><p>(Water isn't reabsorbed here)</p> |

{% hint style="success" icon="lightbulb" %}
Organisms that live in the *dryest* environments have the ***longest*** loops of Henle, because they need to conserve as much water as they can to prevent dehydration.

Organisms that live in the *wettest* environments have the ***shortest*** loops of Henle, because they don't need to conserve as much water.
{% endhint %}
{% endstep %}

{% step %}
**Distal Convoluted Tubule**

Filtrate then travels up into the **distal proximal tubule** and there...

|                                                         Filtered In | Reabsorbed Out                            |
| ------------------------------------------------------------------: | ----------------------------------------- |
| <p>Hydrogen Ions (H<sup>+</sup>)</p><p>Potassium</p><p>Ammonium</p> | <p>Water</p><p>Salt</p><p>Bicarbonate</p> |
|                                                       {% endstep %} |                                           |

{% step %}
**Collecting Duct**

Filtrate finally enters the collecting duct, where they kidneys release it as urine.
{% endstep %}
{% endstepper %}

<div data-with-frame="true"><figure><img src="https://o.quizlet.com/lAa-v9jotDhJ8lA8WDWRRw.jpg" alt="" width="563"><figcaption><p><strong>Image 6</strong> — Diagram of a nephron.</p></figcaption></figure></div>

### P\*\*\*ing

After urea is released from the kidneys, it travels through the ureters and down into the bladder. It is then excreted out through the urethra.

### Osmoregulation

**Osmoregulation** is the process of maintaining salt and water balance—also known as **osmotic balance**—within the body's fluids. Osmoregulation is done by the **antidiuretic hormone** (ADH), which influences the nephrons to [reabsorb more water in](#user-content-fn-8)[^8].

[^1]: Only found in males.

[^2]: Only found in females.

[^3]: **Lipid-based** like testosterone or estrogen.

[^4]: **Protein-based** like insulin or calcitonin.

[^5]: Posterior means back.

[^6]: Anterior means front.

[^7]: Also known as feedback mechanisms.

[^8]: This is done by reabsorbing more salt in and letting osmosis diffuse water out by the concetration gradient.


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