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Endocrine System

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Endocrine System

The endocrine system is one of the key regulatory systems within the body. It consists of a series of glands that interact with each other and external factors to control organs located far away from them by secreting hormones into the bloodstream. It works in concert with the nervous system, however, instead of short term rapid changes, it creates slower acting, longer-lasting changes in the body.

Types Of Cell Signalling

There are four main types of cell signalling used to convey messages within the body. These vary in the range and type of signalling molecule used. In increasing order of range, the four signalling strategies are:

  • autocrine signalling,
  • juxtacrine signalling,
  • paracrine signalling, and
  • endocrine signalling

The type of signalling used in the endocrine system is, as the name suggests, endocrine signalling. A brief overview of each of these is provided below.

Autocrine Signalling

Autocrine signalling is used by cells to effect changes within themselves. The cell releases a signalling molecule, such as a hormone or cytokine. This signalling molecule then binds to receptors on the same cell, triggering changes within that cell. This form of signalling is employed by cells of the immune system, amongst others.

en-biology-homeostasis-endocrine system- autocrine signalling

Cytokines are a type of protein that exert an effect on our immune system!

Juxtacrine Signalling

Juxtacrine signalling is used by cells to effect changes directly in neighbouring cells. In this mode of signalling, nothing is released from the signalling cell. Instead, proteins on the signalling cell interact with receptor proteins on or in the receiving cell.

There are three sub-types of juxtacrine signalling. In one type, surface proteins on the signaller cell interact with surface proteins on the receiver cell. The second features a protein in the extracellular matrix secreted by one cell binding to the receptor within another. The third has the signalling molecule move through channels connecting the cytoplasm of the two cells. A comically named example of this is the bride of sevenless proteins interaction with its corresponding receptor protein, sevenless.

Paracrine Signalling

Paracrine signalling is used by cells to effect changes within cells in the local area. The signalling cell releases paracrine factors into the surrounding area. These factors then spread into the surrounding area. Diffusion of the paracrine factors creates a gradient, with the concentration lowering the further from the signalling cell you get. The stronger the concentration of paracrine factors a cell is exposed to, the larger its response. This type of signalling is used for example within synapses.

Paracrine factors are small proteins produced by cells to signal other nearby cells. They trigger different changes when cells are exposed to different concentrations created by diffusion.

Endocrine Signalling

Endocrine signalling is the type of cell signalling used by the endocrine system. Endocrine signalling relies on the use of chemical signalling molecules called hormones.

Hormones are small signalling molecules that bind to receptor proteins expressed by different cells to trigger a response and help regulate physiological activities.

This form of signalling can act on cells throughout the body, outside of the local area, a significant increase in communication range when compared to the above signalling methods. This range is achieved by the gland secreting the hormone into the bloodstream, rather than the immediate extracellular area. The bloodstream then carries the hormone around the body, ready for it to interact with relevant receptors on its target.

Hormones within the human body belong to five classes, determined by the compounds they are formed from. Protein hormones are, as the name suggests, proteins. Amino acid hormones are modified amino acids, steroid hormones are formed from cholesterol, eicosanoids are formed from lipids and gas hormones are gases that exert a biological effect, such as nitric oxide.

The Function Of The Endocrine System

The endocrine system regulates virtually all factors within the body. Anything in which some degree of homeostasis is required is regulated by a part of the endocrine system such as blood glucose levels or metabolic rates. The endocrine system regulates factors within the body directly, through the action of hormones, or indirectly, through knock-on effects of hormone interactions.

Homeostasis is the maintenance of steady-state conditions in a biological system. Check out the article Blood glucose concentration to learn how the endocrine system helps regulate our blood glucose levels!

Because of this wide range of impacts, the endocrine system is involved in many diseases, ranging from metabolic disorders to cancer. The study of the role the endocrine system plays in disease is known as endocrinology. Diseases caused by the endocrine system generally stem from altered levels of hormone release, inappropriately timed hormone release, the gland being missing or damaged, or the gland being overgrown and therefore overactive.

The Nervous System vs the Endocrine System

While the nervous system and the endocrine system both allow signals to be carried throughout the body, this is where their similarity ends. Nerve cells conduct electrical signals around the body. They do use chemical messengers, but only over incredibly short distances at special junctions between cells known as synapses, rather than long-distance via the bloodstream like the endocrine system does. They also generally act over a very short time period, but the signals travel quickly. This is the opposite of the endocrine signal, where due to the requirement for hormones to circulate in the blood, the response is much slower but more long-lasting as it acts until the hormones are broken down.

Read our article The Human Nervous System to learn more about this important regulatory system!

Diagram Of The Endocrine System

The diagram below shows the key glands of the endocrine system including the ovaries, testes, pineal gland, adrenal gland, thyroid, parathyroid, pituitary gland, hypothalamus and pancreas, each of which is discussed in depth after.

Structure of the Endocrine System

The glands of the endocrine system differ in structure quite significantly from other glands, which make up the exocrine glands. Exocrine glands, such as those found in the digestive tract or the sweat glands, secrete their products outside of the body. In order to do this, they must have a duct to carry the product to the required area. As they don't secrete their product into the blood, they have few blood vessels. Endocrine glands in contrast generally do not have ducts as their product diffuses into the bloodstream. This need for access to the bloodstream also means they have a lot more blood vessels than exocrine glands.

The Endocrine System's Glands

There are nine main components that form the endocrine system, these are the ovaries, testes, pineal body, adrenal gland, thyroid, parathyroid, pituitary gland, pancreas, and hypothalamus. A brief overview of each of these is given below.

The Ovaries

The ovaries are the site of egg maturation and release, however, they also possess a significant role in hormonal control, both ovulation and many other systems. Hormones produced by the ovaries include oestrogen, inhibin and progesterone. Dysfunctions in the hormonal secretion of the ovaries can lead to several issues, one of the most common of which is Polycystic Ovary Syndrome.

PCOS or Polycystic Ovary Syndrome is a condition where the ovaries produce an excess of male sex hormones. Its name stems from the characteristic fluid-filled follicles which are overly enlarged in the ovaries.

Testes

The testes are the male equivalent to the ovaries, producing sperm cells instead of eggs. Like the ovaries, as well as producing gametes, they produce the hormone testosterone. Lowered hormone production by the testes, termed hypoandrogenism, leads to an array of symptoms such as genital shrinkage, low muscle mass, and frail bones amongst other symptoms.

Pineal Body

The pineal body, also known as the pineal gland, is a small gland located within the brain. It regulates the circadian rhythm via the production of melatonin, also known as the sleep hormone. Dysregulation of melatonin production can lead to a corresponding dysregulation of the circadian rhythm and reproductive cycles.

Circadian rhythm is a natural internal process which repeats a regular set of changes approximately over 24 hours, which is adjusted by external factors known as zeitgebers, meaning time givers in German. Some examples of these are temperature and daylight exposure.

Adrenal Gland

The adrenal glands are located at the top of each kidney. Like most glands of the endocrine system, they produce an array of hormones including steroid hormones which regulate blood pressure and salt levels and the metabolism of many compounds like androgens. These are usually converted to testosterone and DHT or estrogens, depending on the gender of their owner. The other hormones produced by the adrenal glands, for which they are named, are adrenaline and noradrenaline. These are responsible for the fight or flight response and act to increase blood pressure, heart and breathing rate, and blood sugar levels.

Thyroid

The thyroid is located in front of the windpipe within the neck. It produces several hormones, collectively known as thyroid hormones. Two variants of thyroid hormones are produced, numbered according to the number of iodine within their structure. T3 is the active form, and T4 its prohormone, with these being secreted in a 1:14 ratio. T4 is converted to T3 by deiodinases located throughout the body. Thyroid hormones act to increase metabolism throughout the body, increase respiratory and heart rate, and regulate brain development and function.

One of the most common disorders involving the thyroid is hypothyroidism, which causes decreased levels of thyroid hormone production. This is often caused by Hashimoto's thyroiditis, a condition where the immune system attacks the thyroid.

Parathyroid

The parathyroid glands are small glands that sit behind the thyroid. They are responsible for the regulation of blood calcium levels through the production of parathyroid hormone. This induces calcium release from the bones, along with making the kidneys increase vitamin D production and calcium ion retention. Vitamin D acts to raise calcium levels by increasing absorption within the gut. Over or underproduction of the parathyroid hormones can lead to disordered calcium levels within the blood.

Pancreas

The pancreas is an organ within the digestive tract. It is responsible for regulating blood sugar levels by producing the hormones glucagon and insulin. Glucagon is released from alpha cells in the pancreas and raises blood sugar. Insulin is produced by beta cells in the pancreas and lowers blood sugar levels. These work together in a pair of negative feedback loops, which maintain a narrow range of blood sugar levels. Disorders of the pancreas most commonly lead to diabetes mellitus.

Diabetes Mellitus is a disorder characterised by abnormally elevated blood glucose levels, triggered by improper production of or response to insulin, the hormone which normally lowers blood sugar levels by removing glucose from the blood and sequestering it in glycogen.

Check out our article Diabetes to learn more about this prevalent disease!

Hypothalamus

The hypothalamus is the control centre of the endocrine system. It takes input from around the body and triggers the release of hormones from the pituitary gland. It does this by producing a specialised kind of hormone known as a neurohormone. These then travel down the hypophyseal system to the pituitary where they control the release of hormones from the pituitary.

Pituitary Gland

The pituitary is a gland that sits at the base of the hypothalamus within the brain. It secretes a wide array of hormones, many of which regulate other glands around the body. Actions that are controlled by a series of glands and hormones are referred to as an axis, such as the hypothalamic-pituitary-adrenal axis, which regulates various systems throughout the body, including digestion, immunity, mood, libido and metabolism. Because of its role in regulating the activity of other glands, the pituitary is often referred to as the master gland. Hormones released by the pituitary include but are not limited to, human growth hormone, thyroid-stimulating hormone, luteinising hormone, follicle-stimulating hormone, anti-diuretic hormone and oxytocin. Due to the large number of systems impacted by the pituitary, many different diseases can arise from pituitary disorders.

Endocrine System - Key takeaways

  • The endocrine system uses hormones produced by glands to convey signals to locations far around the body.
  • Hormones may be steroids, proteins, lipids, amino acids or gases and circulate through the bloodstream.
  • The endocrine system is slower to respond than the nervous system, however, the response stays active much longer.
  • Endocrine glands don't have ducts to carry their product to another area, but they are heavily vascular so they can secrete their products into the blood.
  • The main glands of the endocrine system include the ovaries, testes, pineal gland, adrenal gland, thyroid, parathyroid, pituitary gland, hypothalamus and pancreas.

Frequently Asked Questions about Endocrine System

The endocrine system is a network of glands that use feedback loops to control organs located far away from them, by releasing hormones into the circulatory system. 

The endocrine system uses hormones secreted directly into the blood to control distant organs. This control is needed to maintain or adjust factors within the body.

The main glandular parts of the endocrine system are the ovaries, testes, pineal body, adrenal gland, thyroid, parathyroid, pituitary gland, pancreas and hypothalamus.  



Some of the main hormones of the endocrine system are: T3 and 4, the thyroid hormones; melatonin; progesterone; testosterone; cortisol; insulin; and oestrogen. 

The endocrine system regulates homeostasis within the body, maintains the rhythms of changes in the body and allows the body to respond to changes in the environment, like with adrenaline aiding in the fight or flight response. 

Final Endocrine System Quiz

Question

What type of hormone is insulin?

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Answer

Protein

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Question

What compounds concentration does insulin regulate?

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Answer

Glucose

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Question

What type of feedback system does insulin operate in?

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Answer

Negative

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Question

What cells are the main stores of glycogen?

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Answer

Muscle

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Is glycogen a short-term or long-term storage molecule?

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Answer

Short-term

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Question

What is the long-term storage molecule for glucose?

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Answer

Triglycerides

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Question

How many amine residues does insulin have before undergoing post-translational modification?

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Answer

110

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Question

What is the name of the small fragment excised when proinsulin is packaged into secretory vesicles?

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Answer

C-Peptide

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Where is insulin produced?

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Answer

Beta Cells

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How many insulin molecules form the stored form of insulin?

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Answer

6

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Question

What ion triggers the release of the secretory vesicles from the beta cells?

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Answer

Calcium

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Question

What organ contains the beta cells that produce insulin?

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Answer

Pancreas

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Question

How many insulin molecules make up the secreted form of insulin?

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Answer

1

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Question

How many main types of diabetes are there?

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Answer

1

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Question

Consistently elevated blood sugar levels results in insulin what?

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Answer

Resistance

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Question

Insulins counterpoint in the pancreas' two hormone system is?

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Answer

Glucagon

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The protein at the core of each glycogen molecule is?

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Answer

Glycogenin

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Diabetes is a catch-all term for disorders causing consistently elevated what?

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Answer

Blood glucose levels

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Insulin ______blood sugar levels?

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Answer

Raises

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The action of insulin contributes to maintaining what within the body?

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Homeostasis

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How many sub-types of diabetes insipidus are there? 

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Answer

4

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What differentiates diabetes mellitus and diabetes insipidus?

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Answer

Diabetes insipidus is characterised by extreme urine production without glucose being present in the urine. 

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Question

What does SIADH stand for?

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Syndrome Of Inappropriate Antidiuretic Hormone Secretion 

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What causes SIADH?

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The uncontrolled and excessive release of ADH into the body from either the pituitary or a non-pituitary source. 

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What does ADH regulate?

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Serum osmolarity and blood pressure. 

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What organ does ADH act on to increase water reabsorption? 

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Kidney

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What muscle type does ADH act on to increase blood pressure?

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Vascular

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How can SIADH cause death?

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Hypoosmolarity and hyponatremia causes cerebral oedema eventually leading to seizures, coma and death. 

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Hyponatremia is low levels of what ion?

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Sodium

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Hyperosmolarity is detected by the body using what receptors?

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Osmoreceptors

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Hypovolemia is detected with what receptors?

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Baroreceptors

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Which release trigger for ADH has priority? 

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Hypovolemia

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Does hyperosmolarity always mean hypertonicity?

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No as osmolarity accounts for all solvents, tonicity only for those that cannot cross the membrane. 

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Where is ADH produced?

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The hypothalamus.

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Where is ADH stored?

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The pituitary gland. 

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What proteins release is triggered by ADH and is used to absorb more water?

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Aquaporin-2

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How many amine residues make up the active form of ADH?

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Answer

9

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Is ADH translated into its active form?

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No it is translated into a preprohormone which is then repeated modified to eventually give the active form. 

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What is another name for ADH?

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Vasopressin

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What does ADH stand for?

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Antidiuretic Hormone.

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Question

What is dialysis?

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Dialysis attempts to perform the function of the kidneys and maintain the balance of the fluids and electrolytes in the blood when the kidneys are not working sufficiently. 

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Name some functions of the kidneys

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  • Extra water, waste, and other pollutants are prevented from collecting in your body by healthy kidneys. 
  • They aid in the regulation of blood pressure and the amounts of electrolytes in the blood.
  • Your kidneys also activate vitamin D which is important in promoting calcium absorption. 

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Question

What are the three types of dialysis?

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Haemodialysis, Peritoneal dialysis, and continuous renal replacement therapy (CRRT) 


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In haemodialysis, the blood is separate from the dialysis fluid via a __________. 

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Semi-permeable membrane

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The blood and the dialysis fluid flow in the same direction

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Answer

False

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What is the content of the dialysis fluid?

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The dialysis fluid contains the same concentration of glucose and electrolytes that are found in normal blood plasma while containing no urea. 

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What factors determine the duration of each dialysis session?

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Answer

The duration of the dialysis is determined by the amount of waste your body produces, your weight, and the present status of your health.


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Question

How does hemodialysis cause low blood pressure as a side effect?

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Answer

The fall in fluid levels during dialysis is the potential cause of this. Low blood pressure can in turn lead to lightheadedness, nausea and dizziness.  

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Question

Name three side effects of haemodialysis

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Answer

1. infection and sepsis

2. Muscle cramps 

3.  itchy skin 

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Question

What is peritoneal dialysis?

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Answer

Peritoneal dialysis involves inserting a catheter into the abdomen and injecting the dialysis fluid into the peritoneal cavity. The peritoneal membrane acts like a semi-permeable membrane filtering the waste products out of the blood. 

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