Sitem Endokrin

Hormon yang berhubungan dengan reproduksi wanita

Homon prolaktin
Hormon ini dilepaskan oleh kelenjar pituitari yang merangsang kelenjar susu dalam payudara perempuan sehingga menghasilkan susu. Produksi susu ini dikendalikan oleh hipotalamus.
Hormon Oksitosin
Hormon ini dihasilkan oleh hipotalamus dan disimpan dalam kelenjar pituitari belakang. Saat diperlukan, oksitosin dilepaskan oleh kelenjar pituitari ketika menerima rangsangan syaraf dari hipotalamus. Fungsinya termasuk mengerutkan saluran susu. Selain perannya dalam pembentukan susu, hormon oksitosin memiliki tugas lain. Hormon ini memastikan terjadinya kerutan otot rahim saat persalinan sehingga memperlancar proses persalinan. Saat persalinan, produksi oksitosin meningkat cepat. Pada saat yang sama, otot rahim mengembangkan kepekaan terhadap hormon oksitosin. Di waktu proses persalinan, sebagian perempuan diberi suntikan oksitosin untuk membantu mengatasi rasa sakit dan mempercepat proses itu.
Agar produksi oksitosin wajar, sel-sel yang membentuk hipotalamus harus mengetahui semua unsur yang terlibat dalam proses persalinan yang terjadi di tempat yang jauh darinya. Sel-sel ini harus mengetahui bahwa persalinan adalah proses sulit dan bahwa otot rahim harus dikerutkan agar menekan si bayi keluar. Selain itu, sel-sel harus mengetahui bahwa diperlukan produksi kimiawi untuk mendorong kerutan ketegangan otot rahim, dan harus mengetahui rumus kimia yang benar.

Ket. Gambar;
Hormon oksitosin dihasilkan oleh hipotalamus dan disimpan di kelenjar pituitari. Pada saat yang tepat, sebuah isyarat syaraf dikirimkan oleh hipotalamus ke kelenjar pituitari agar melepaskan hormon ini. Tujuannya adalah memastikan terjadinya pengerutan saluran-saluran susu dan otot-otot rahim ketika waktu kelahiran tiba. Dengan cara ini, hormon memudahkan proses persalinan.

Kerja Hormon Prolaktin dan Oksitosin
Hormon yang mengaktifkan kelenjar susu dalam payudara ibu, sebagaimana dikemukakan, adalah hormon prolaktin yang dilepaskan oleh kelenjar pituitari. Pada masa-masa awal kehamilan, faktor-faktor tertentu menekan pelepasan prolaktin. Faktor-faktor itu seperti kaki yang menekan rem sebuah mobil yang menuruni bukit. Menghentikan produksi prolaktin sangat penting, sebab saat bayi belum lahir, produksi susu ibu tak dibutuhkan. Bagaimanakah rem ini bekerja, dan bagaimana pelepasan dicegah di awal? Inilah keajaiban rancangan yang sesungguhnya. Hipotalamus dalam otak melepaskan sebuah hormon yang mencegah produksi prolaktin. Hormon ini, dikenal sebagai PIH (Prolactin Inhibiting Hormon — hormon penekan prolaktin), memperlambat produksi prolaktin.
Saat bayi mengisap susu, sel-sel syaraf di payudara ibu mengirim rangsangan syaraf ke hipotalamus. Rangsangan ini mempengaruhi dan memastikan hipotalamus agar membuang rem bagi prolaktin. Lewat cara ini, produksi prolaktin meningkat dan kelenjar susu dirangsang agar menghasilkan susu
Sejak persalinan, reseptor tertentu dirancang di payudara ibu untuk mengenali refleks isapan bayi. Indra reseptor ini terhubung lewat jalur-jalur neuron — mirip kabel-kabel listrik di sebuah bangunan — ke organ lain yang jauh, yaitu, daerah hipotalamus di dalam otak. Jadi, sebuah sistem khusus telah diciptakan untuk mengabari hipotalamus bahwa refleks isapan bayi telah bekerja. Namun, dari tak terhitung kemungkinan di dalam tubuh manusia yang terdiri dari daging dan tulang, isyarat-isyarat syaraf ini bergerak ke arah yang tepat. Isyarat-isyarat tak terhubung secara kebetulan ke pusat penglihatan di otak, lambung, atau usus; isyarat-isyarat ini terhubung ke tempat yang benar-benar tepat: hipotalamus.

Ket. Gambar;
1) Saat bayi mengisap, sejumlah sel syaraf di payudara ibu mengirimkan pesan ke hipotalamus.
2) Ketika menerima pesan itu, hipotalamus melepas ‘rem’ penahan prolaktin
3-4) Untuk mulai menghasilkan ASI, prolaktin yang dihasilkan kelenjar pituitari merangsang kelenjar-kelenjar susu di payudara ibu

.
Saat menerima isyarat-isyarat listrik itu, sel-sel hipotalamus memulai pekerjaan yang dibutuhkan demi menghasilkan ASI. Namun, sel-sel ini tak berkecerdasan atau kesadaran sendiri. Sel-sel ini tak mungkin mengetahui bahwa isyarat-isyarat itu datang dari payudara ibu atau bahwa sel-sel ini telah dikabari tentang refleks isapan bayi dan, oleh karena itu, bahwa ASI harus dilepaskan; sel-sel ini tak mungkin mengetahui bahwa sebuah fungsi penting dalam produksi ASI telah diserahkan kepada sel-sel ini, atau bahwa sel-sel ini harus meningkatkan produksi prolaktin untuk mengaktifkan kelenjar susu.Hormon oksitosin ini juga memastikan terjadinya kerutan otot di sekitar saluran susu, menggerakkan susu dari kelenjar susu ke puting yang mudah dicapai oleh bayi saat disusui.

Kerja hipotalamus dan hubungannya dengan kelenjar hormone
Hipotalamus adalah pemimpin umum sistem hormon; ia memiliki tugas penting memastikan kemantapan dalam tubuh manusia. Setiap saat, hipotalamus mengkaji pesan-pesan yang datang dari otak dan dari dalam tubuh. Setelah itu, hipotalamus menjalankan beberapa fungsi, seperti menjaga kemantapan suhu tubuh, mengendalikan tekanan darah, memastikan keseimbangan cairan, dan bahkan pola tidur yang tepat.
Hipotalamus terletak langsung di bawah otak dan ukurannya sebesar biji kenari. Sejumlah besar informasi sehubungan dengan keadaan tubuh dikirim ke hipotalamus. Informasi ini disampaikan ke sana dari setiap titik dalam tubuh, termasuk pusat indra dalam otak. Kemudian hipotalamus menguraikan informasi yang diterimanya, memutuskan tindakan yang mesti diambil dan perubahan yang harus dibuat dalam tubuh, serta membuat sel-sel tertentu menjalankan keputusannya.
Hal mendasar yang harus diperhatikan di sini adalah: hipotalamus itu sebuah organ yang terdiri dari sel-sel tak sadar. Suatu sel tak mengetahui berapa lama manusia harus tidur; ia tak dapat menghitung berapa seharusnya suhu tubuh. Sel tak dapat mengambil keputusan terbaik berdasarkan informasi yang ada, dan tak dapat membuat sel lain yang berjauhan letaknya dalam tubuh menjalankan keputusan itu. Namun, sel-sel dalam hipotalamus bertindak dalam cara yang luar biasa sadar demi menjamin bahwa keseimbangan yang dibutuhkan dalam tubuh terjaga. Pada halaman-halaman selanjutnya, kita akan menelaah secara rinci kegiatan luar biasa yang diperlihatkan oleh sel-sel tak sadar ini.

Sebagian besar informasi tentang tubuh manusia ada di hipotalamus. Hipotalamus menerjemahkan informasi ini, mengambil keputusan penting, dan memerintahkan sel-sel menjalankan keputusannya. Pada gambar, terlihat letak hipotalamus di otak.

Salah satu fungsi terpenting hipotalamus adalah menjembatani sistem hormon dan sistem lain yang mengatur dan memelihara tubuh—yaitu sistem syaraf. Hipotalamus bukan saja mengatur sistem hormon, namun juga sistem syaraf dengan tingkat keahlian yang tinggi.
Hipotalamus memiliki pembantu yang sangat penting dalam perannya mengatur tubuh; pembantu ini menyampaikan kepada bagian-bagian tubuh tertentu tentang keputusan yang telah diambil. Misalnya, ketika terjadi penurunan tiba-tiba tekanan darah, potongan-potongan informasi dikirimkan, dan mengabari hipotalamus tentang perubahan tekanan ini; lalu hipotalamus memutuskan tindakan-tindakan yang harus dilakukan untuk menaikkannya dan menyampaikan keputusannya kepada pembantu-pembantunya.
Untuk menjalankan keputusan, pembantunya mengetahui sel-sel yang mana yang harus menerima perintah itu. Ia menulis pesan-pesan dalam bahasa yang dimengerti sel-sel ini dan segera menyampaikan segenap pesan itu. Sel-sel tujuan mematuhi perintah yang diterima dan melakukan tindakan yang tepat untuk menaikkan tekanan darah.
Pembantu hipotalamus adalah kelenjar pituitari, yang juga berpengaruh amat penting dalam sistem hormonal.
Antara kelenjar hipotalamus dan pituitari terdapat sistem komunikasi yang mengagumkan. Kedua potong daging ini sebenarnya berkomunikasi bagai dua manusia yang sadar. Hipotalamus memiliki kendali menyeluruh atas kelenjar pituitari dan pelepasan penting beberapa hormon.
Sesuatu yang mirip terjadi saat sel-sel tubuh harus bekerja lebih cepat; di sini terdapat dua tingkat komando. Hipotalamus mengirimkan perintah ke kelenjar pituitari yang pada gilirannya meneruskan perintah itu ke kelenjar tiroid. Kelenjar pituitari melepaskan hormon tiroid dalam jumlah yang tepat dan sel-sel tubuh mulai bekerja lebih cepat.

Letak kelenjar-kelenjar hormon di dalam tubuh yang di bawah kendali hipotalamus.
Saat kelenjar adrenal (yang menghasilkan beberapa hormon yang sangat penting) harus diaktifkan atau organ reproduksi harus menghasilkan hormon-hormonnya, hipotalamus lagi-lagi mengirimkan pesan ke kelenjar pituitari, yang mengarahkan pesan itu ke daerah yang sesuai dan memastikan bahwa hormon-hormon yang dibutuhkan di bagian tubuh itu dilepaskan.
Hormon-hormon yang dihasilkan oleh hipotalamus untuk mengatur kelenjar pituitari antara lain:
Hormon pelepas hormon pertumbuhan
Hormon pelepas tirotropin
Hormon pelepas kortikotropin
Hormon pelepas gonadotropin
Dalam beberapa hal, untuk ikut serta dalam kegiatan sel, hipotalamus menggunakan dua hormon yang dihasilkannya sendiri. Untuk menyimpan hormon-hormon ini, hipotalamus lebih dulu mengirimkannya ke kelenjar pituitari, kemudian, saat dibutuhkan, memastikan bahwa hormon-hormon ini dilepaskan oleh kelenjar pituitari. Hormon-hormon tersebut adalah:
• Vasopresin (sebuah hormon antidiuretik, yaitu hormon penahan air)
• Oksitosin
Kedua molekul hormon yang dihasilkan oleh hipotalamus ini sangat kecil. Salah satunya hanya sebesar tiga asam amino. Hormon hipotalamus berbeda dari hormon-hormon lainnya bukan hanya karena kecil, namun juga karena jarak tempuhnya dalam tubuh. Hormon biasanya bergerak ke daerah yang jauh dari kelenjar tempat ia dihasilkan menuju organ-organ yang ditentukan. Namun, hormon hipotalamus mencapai kelenjar pituitari hanya dengan menembus pembuluh kapiler setebal beberapa milimeter. Hormon ini tak pernah memasuki sistem peredaran umum.
Hipotalamus menghasilkan hormon yang mengaktifkan kelenjar pituitari, dan saat dibutuhkan, menghasilkan juga hormon yang menghentikan kelenjar pituitari di saat yang tepat sehingga tak melepaskan hormon tertentu. Dengan cara ini, hipotalamus mengatur sepenuhnya kegiatan kelenjar pituitari.

Hipotalamus, yang terletak tepat di bawah otak dan seukuran biji kenari, mengatur berbagai fungsi penting, seperti pengaturan metabolisme tubuh, pengendalian kelenjar adrenal, produksi susu, dan pengaturan pertumbuhan tubuh. Saat menjalankan semua kegiatan ini, hipotalamus memerintahkan kelenjar-kelenjar hormon lain yang di bawah kendalinya. Pada gambar di atas, kita melihat hormon-hormon yang bekerja sama dengan hipotalamus.

Eye Anathomy and Physiology

WHAT IS RETINITIS PIGMENTOSA?

Retinitis Pigmentosa is the name of a group of retinal dystrophies that cause degeneration of the retina of the eye.  Retinitis pigmentosa is a disease of the eye that the affected individual is born with. The word "retinitis" derives from "retina" (a part of the eye) and "itis" (a disease). It is a disease of the retina, though not a contagious one. The word "pigmentosa" refers to an associated discoloration of the retina, which becomes visible to an eye physician on examination. For those people who find retinitis pigmentosa a difficult term to use, the shortened form "RP" serves as a simpler alternative.

 WHAT THE RETINA IS  

The retina is located at the back of the eye and is connected to the brain. It is made up of many millions of light-sensitive cells known as photoreceptor cells. These photoreceptor cells have the vital function of transmitting electrical impulses to the brain to enable seeing to take place.
Retinal dystrophies are caused by the gradual breakdown of these photoreceptors. Therefore it is important to understand the structure of the eye (as well as the ear in Usher Syndrome).

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Eye Anatomy and Physiology

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STRUCTURE AND FUNCTION OF THE EYE

• The eye consists of several parts that resemble a camera (see diagram)
• sclera - the eye's white outer protective coat, normally seen as the "white of the eye".
• cornea - the transparent, curved structure at the front of the eye.
• iris - the coloured part of the eye - blue, brown, green, grey etc - that can be seen through the cornea.
•  pupil - the black part of the eye in the middle of the iris. It constricts or dilates according to the amount of light passing through it.
•  lens - the transparent disc (with both sides being convex) immediately behind the iris and pupil.
• aqueous humour - the transparent fluid (with consistency similar to water) that circulates behind the cornea and in front of the lens.
• vitreous humour - the material (like transparent jelly) that fills the eyeball between the lens and the retina.
• retina - the light-sensitive layer of millions of nerve cells that line the back of the eyeball. The cells consist of two main groups, called rods and cones due to their appearance under the microscope.
• rods - more numerous, spread out over the entire retina with more toward outer edge, respond to low levels of light.
• cones - far fewer, concentrated around the retina's centre, respond to colour and to details.
• macula - the small centre of the retina, responsible for reading vision.
• retinal pigment epithelium - This is a dark coloured layer of cells at the back of the retina responsible for providing oxygen and other nutrients to the rods and cones.
• choroid - a large network of blood vessels (behind the retina) that transport oxygen and other nutrients to the retinal pigment cells.
• optic disc - a small yellow oval structure in the retina, to which nerve cell connections travel from all the rods and cones.
• optic nerve and beyond - the "cord" of nerve cell connections that passes from the eyeball to destinations throughout the brain.

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Function of the Eye

 When you see an object, the light travels from that object to the cornea, then passes through the aqueous humour, pupil, lens and vitreous humour to reach the retina. During this passage, the light becomes focused onto the macula.
At the macula, the light causes chemical reactions in the cones, that consequently send electrical messages from the eye to the brain. The brain recognises these messages and indicates to you that this particular object has been seen. The cones are therefore responsible for you being able to recognise colours and to read.
The rods are essential for you to see in the dark, and to detect objects to the sides, above and below the object on which you are directly focused. This function prevents you from bumping into obstacles when moving around.
All the retinal cells (rods and cones) are provided with oxygen and other nutrients from the retinal pigment cells (epithelium), which are kept supplied by the rich network of blood vessels in the choroid.

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 WHAT IS RESPONSIBLE FOR RP 

It is thought that one child is born with RP in approximately every 3,000 births in Australia. It is important to recognise that it is no one's fault and that RP can strike in a family with no known history of it. In fact, RP results from an imperfection in a tiny gene that causes an incorrect protein to be supplied to the retina. Over time this causes photoreceptor cells to die and progressive loss of vision results. 

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SYMPTOMS 

Typical symptoms of RP are what are commonly referred to as "night blindness" and "tunnel vision". Night blindness refers to a reduced ability to see at night time, but more than that it entails a diminishing ability to see in dimly lit conditions generally, as well as in judging changes in level such as steps and gutters. Adjustments to changes in light intensity are typically slower and sensitivity to glare greater than is the case with the normal, healthy eye. All this results from the eye having fewer and fewer photoreceptor cells with which to transmit visual images to the brain.
 The common pattern with RP is for photoreceptor cells to be lost progressively from the outer edges of the retina. This causes a progressive loss of peripheral vision, meaning side, upper and lower vision. It is as if the affected individual is viewing the world through a narrowing tunnel. Unlike a person with a normal field of vision, the person with tunnel vision cannot simultaneously look ahead and downwards or sideways without using a scanning technique by moving the eyes. However, the central vision of the person with RP may remain largely unaffected for a considerable number of years, enabling the individual to continue to read, watch television and perform other visually detailed tasks.
 It is important to understand that with RP there is no uniform age of onset of symptoms and no uniform rate and extent of vision loss. These can vary markedly from individual to individual and are not usually able to be predicted.
 

	When Richelle's family look at her they see her like this		When Richelle looks at herself in the mirror this is what she sees

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TREATMENT

At present, there is no treatment to halt the progression of any retinal dystrophies. No convincing scientific evidence demonstrates any benefits for individuals with RP from the many unproven treatments that have been attempted. However scientists investigate even the most remote possibilities.
There is no scientific evidence that normal light levels worsen the symptoms. However, individuals with retinal degenerations are encouraged to protect their eyes from long exposure to bright light as a precaution. In dim light, they may use their eyes to the degree possible without fear of worsening their condition.
When only one person in the family is affected, it is difficult to decide how the condition is inherited.
Many hereditary diseases may affect the retina, each with symptoms. Conditions include Retinitis Pigmentosa (RP), Stargardt disease, Usher Syndrome, Best disease, Cone Rod Dystrophy and Choroideremia. 
In order to determine the likelihood of your children and other members of your family being affected, you should consult your doctor or seek genetic counselling. Because retinal dystrophies usually run in families, all family members are encouraged to have a thorough eye examination.
Professional genetic counselling is generally advisable.

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 CATARACTS AND RP

 It is not unusual for people with RP to develop a cataract, a haziness of the lens of the eye. When cataracts significantly interfere with vision, it may be advisable to remove them surgically. Whether surgery improves vision often depends on how far the retinal changes have advanced. The usefulness and implications of surgery should be discussed with an ophthalmologist.

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GENETIC INHERITANCE

 Whereas other genetically transmitted diseases are the result of a single gene defect, RP results from a large and as yet unknown number of gene defects. So far around a hundred gene defects have been found to cause RP.

However, there are three main genetic pathways through which a child can come to be born with RP and the probability of this happening varies according to which of the genetic pathways applies.
Two of them are usually identifiable through a recurrent family history of RP but the third and by far the most prevalent one is not. 
In simple terms RP is inherited in different ways:-

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Autosomal Recessive Inheritance Pattern

This is the most common form of RP. This form means each parent is a carrier (have the gene present, but DNA testing is still unavailable) but is not affected by the disease themselves. Each of their children will have a 25% chance of being affected, with males and females having an equal chance of being affected. 
The diagram indicates how the recessive genes are passed from the two carrier parents to their children.
The mutant RP gene is represented by "r" and the normal gene by "R".

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Autosomal Dominant Inheritance Pattern

In this type of RP, one parent is affected and each pregnancy has a 50% chance that the child will be affected. Males and females are equally affected.
The diagram indicates the four possible combinations of genetic information that may be passed on by the parents.
The mutant dominant gene is represented by "D" and the normal gene by "d".

X-linked Recessive Inheritance Pattern

This is the least common form of RP. This form is carried by the female and passed onto the male, there is a 50% chance that each son will have RP. There is also a 50% probability that each daughter will be a carrier.  X-linked RP is the one form in which carriers can often be detected.

This diagram depicts the four possible combinations of genetic input from a couple, each contributing a copy of each of their genes. The woman is a 'carrier' of X-linked RP; the mutant gene on the X chromosome is represented by "r". The man can only convey normal genes.

D'Masiv - Apa Salahku (Vidio Clip & Karaoke)

STRUKTUR KELENJAR ENDOKRIN

Sistem endokrin terdiri dari kelenjar-kelenjar endokrin;

Kelenjar endokrin merupakan sekelompok susunan sel yang mempunyai susunan mikroskopis sangat sederhana. Kelompok ini terdiri dari deretan sel-sel, lempengan atau gumpalan sel disokong oleh jaringan ikat halus yang banyak mengandung pembuluh kapiler

Kelenjar endokrin mensekresi substansi kimia yang langsung dikeluarkan ke dalam pembuluh darah. Sekresinya disebut : hormon. Hormon yaitu penghantar (transmitter) kimiawi yang dilepas dari sel-sel khusus ke dalam aliran darah. Selanjutnya hormon tersebut dibawa ke sel-sel target (responsive cells) tempat terjadinya efek hormon.

• Derivat asam amino – dikeluarkan oleh sel kelenjar buntu yang berasal dari jaringan nervus medulla supra renal dan neurohipofise, contoh epinefrin dan norepinefrin

• Petide /derivat peptide – dibuat oleh kelenjar buntu yang berasal dari jaringan alat pencernaan

• Steroid – dibuat oleh kelenjar buntu yang berasal dari mesotelium, contoh hormon testes, ovarium dan korteks suprarenal.

• Asam lemak – merupakan biosintesis dari dua FA, contoh hormon prostaglandin

KLASIFIKASI HORMON

• Hormon perkembangan/Growth hormone – hormon yang memegang peranan di dalam perkembangan dan pertumbuhan. Hormon ini dihasilkan oleh kelenjar gonad

• Hormon metabolisme – proses homeostasis glukosa dalam tubuh diatur oleh bermacammacam hormon, contoh glukokortikoid, glukagon, dan katekolamin

• Hormon tropik – dihasilkan oleh struktur khusus dalam pengaturan fungsi endokrin yakni kelenjar hipofise sebagai hormon perangsang pertumbuhan folikel (FSH) pada ovarium dan proses spermatogenesis (LH)

• Hormon pengatur metabolisme air dan mineral – kalsitonin dihasilkan oleh kelenjar tiroid untuk mengatur metabolisme kalsium dan fosfor.

SISTEM ENDOKRIN

• Sistem endokrin, dalam kaitannya dengan sistem saraf, mengontrol dan memadukan fungsi tubuh.

• Kedua sistem ini bersama-sama bekerja untuk mempertahankan homeostasis tubuh.

Struktur sistem endokrin;

• Kelenjar eksokrin melepaskan sekresinya ke dalam duktus pada permukaan tubuh, seperti kulit, atau organ internal, seperti lapisan traktus intestinal.

• Kelenjar endokrin termasuk hepar, pankreas (kelenjar eksokrin dan endokrin), payudara, dan kelenjar lakrimalis untuk air mata. Sebaliknya, kelenjar endokrin melepaskan sekresinya langsung ke dalam darah

Fungsi sitem endokrin;

1. Membedakan sistem saraf dan sistem reproduktif pd janin yang sedang berkbg
2. Menstimulasi urutan perkembangan
3. Mengkoordinasi sistem reproduktif
4. Memelihara lingkungan internal optimal

KLASIFIKASI SISTEM ENDOKRIN

• Hormon yang larut dalam air termasuk polipeptida (mis., insulin, glukagon, hormon adrenokortikotropik (ACTH), gastrin) dan katekolamin (mis., dopamin, norepinefrin, epinefrin)

• Hormon yang larut dalam lemak termasuk steroid (mis., estrogen, progesteron, testosteron, glukokortikoid, aldosteron) dan tironin (mis., tiroksin).

Karakteristik sistem endokrin;

Sekresi diurnal adalah pola yang naik dan turun dalam periode 24 jam. Kortisol adalah contoh hormon diurnal. Kadar kortisol meningkat pada pagi hari dan turun pada malam hari.

Pola sekresi hormonal pulsatif dan siklik naik turun sepanjang waktu tertentu, seperti bulanan. Estrogen adalah non siklik dengan puncak dan lembahnya menyebabkan siklus menstruasi. Tipe sekresi hormonal yang ketiga adalah variabel dan tergantung pada kadar subtrat lainnya.

Hormon paratiroid disekresi dalam berespons terhadap kadar kalsium serum.
Hormon bekerja dalam sistem umpan balik, yang memungkinkan tubuh untuk dipertahankan dalam situasi lingkungan optimal.

Hormon mengontrol laju aktivitas selular dan hormon tidak mengawali perubahan biokimia, hormon hanya mempengaruhi sel-sel yang mengandung reseptor yang sesuai, yang melakukan fungsi spesifik.
Hormon mempunyai fungsi dependen dan interdependen. Pelepasan hormon dari satu kelenjar sering merangsang pelepasan hormon dari kelenjar lainnya.
Hormon secara konstan di reactivated oleh hepar atau mekanisme lain dan diekskresi oleh ginjal.

PERAN HIPOTALAMUS & HIPOFISE

Aktivitas endokrin dikontrol secara langsung dan tak langsung oleh hipotalamus, yang menghubungkan sistem persarafan dengan sistem endokrin. Dalam berespons terhadap input dari area lain dalam otak dan dari hormon dalam dalam darah, neuron dalam hipotalamus mensekresi beberapa hormon realising dan inhibiting.
Hipotalamus sebagai bagian dari sistem endokrin mengontrol sintesa dan sekresi hormon-hormon hipofise. Hipofise anterior dikontrol oleh kerja hormonal sedang bagian posterior dikontrol melalui kerja saraf. Hormon yang disekresi dari setiap kelenjar endokrin dan kerja dari masing-masing hormon. Setiap hormon yang mempengaruhi organ dan jaringan terletak jauh dari tempat kelenjar induknya. Misalnya oksitosin, yang dilepaskan dari lobus posterior kelenjar hipofise, menyebabkan kontraksi uterus.

Hormon hipofise yang mengatur sekresi hormon dari kelenjar lain disebut hormon tropik. Kelenjar yang dipengaruhi oleh hormon disebut kelenjar target.

SITEM UMPAN BALIK

Kadar hormon dalam darah juga dikontrol oleh umpan balik negatif manakala kadar hormon telah mencukupi untuk menghasilkan efek yang dimaksudkan, kenaikan kadar hormon lebih jauh dicegah oleh umpan balik negatif.

Peningkatan kadar hormon mengurangi perubahan awal yang memicu pelepasan hormon. Mis. pengsekresi ACTH dari kelenjar pituitari anterior merangsang pelepasan kortisol dari korteks adrenal, menyebabkan penurunan pelepasan ACTH lebih banyak.

AKTIVASI SEL-SEL TARGET

Ketika hormon melekat pada sel, kerja sel akan mengalami sedikit perubahan. Misalnya, ketika hormon pankreatik glukagon berikatan dengan sel-sel hepar, kenaikan kadar AMP meningkatkan pemecahan glikogen menjadi glukosa. Jika hormon mengaktifkan sel dengan berinteraksi dengan gen, gen akan mensitesa mesenger RNA (mRNA) dan pada akhirnya protein (mis., enzim, steroid). Substansi ini mempengaruhi reaksi dan proses selular.

STRUKTUR & FUNGSI HIPOFISE

• Hipofise terletak di sella tursika, lekukan os spenoidalis basis cranii.

• Berbentuk oval dengan diameter kira-kira 1 cm dan dibagi atas dua lobus, yaitu :

1. Lobus anterior, merupakan bagian terbesar dari hipofise kira-kira 2/3 bagian dari hipofise. Lobus anterior ini juga disebut adenohipofise•
2. Lobus posterior, merupakan 1/3 bagian hipofise dan terdiri dari jaringan saraf sehingga disebut juga neurohipofise.

Hipofise stalk adalah struktur yang menghubungkan lobus posterior hipofise dengan hipotalamus. Struktur ini merupakan jaringan saraf. Hipofise menghasilkan hormon tropik dan nontropik. Hormon tropik akan mengontrol sintesa dan sekresi hormon kelenjar sasaran sedangkan Hormon nontropik akan bekerja langsung pada organ sasaran.

Kemampuan hipofise dalam mempengaruhi atau mengontrol langsung aktivitas kelenjar endokrin lain menjadikan hipofise dijuluki master of gland.

THE RENAL SYSTEM

Introduction

The chemical composition of body fluids is important for the well-being of the cells of the body. The circulatory system is mainly responsible for the physical transport of fluids but not for the composition of those fluids. This function is largely the responsibility of the kidneys.

Although they help with various physiological functions, the kidneys' main roles are the removal of wastes and the maintenance of the body's water balance. The functions of the kidneys can be summarised as follows:

1. Control of the body's water balance. The amount of water in the body must be balanced against the amount of water which we drink and the amount we lose in urine and sweat etc.

2. Regulation of blood pressure via the renin-angiotensin-aldosterone system

3. Regulation of blood electrolyte balance - Na+, Ca2+, K+ etc.

4. Excretion of metabolic wastes such as urea, creatinine and foreign substances such as drugs and the chemicals we ingest with our food

5. Help in the regulation of the body’s acid base balance

6. Regulation of red blood cell production via the hormone erythropoietin

7. Help in the production of vitamin D

As this list indicates, the renal system is very important to the normal functioning of the body.

THE STRUCTURE OF THE RENAL SYSTEM

Urine is produced in the kidneys from water and wastes extracted from the blood. The rest of the urinary system is concerned with the storage and ducting of the urine to the outside of the body - Figure 01.

Figure 01 - Structure of the renal system

The kidneys are large, bean shaped organs which lie on the dorsal side of the visceral cavity, roughly level with the waistline. Blood is supplied to the kidneys by the renal arteries which branch off the aorta. The kidneys and are drained by the the renal veins into the inferior vena cava. From the kidneys, urine passes to the urinary bladder via the ureters. Urine is passed to the outside environment via the urethra (this is routed differently in males and females).

MACROSTRUCTURE OF THE KIDNEY

The kidneys are protected by a tough fibrous coat called the renal capsule. Under the capsule, the arrangement of nephrons and capillaries in the kidney produce the appearance of distinct regions when viewed in longitudinal section. The outer cortex region surrounds darker triangular structures called pyramids which collectively form the medulla. The inner part of the kidney, the renal pelvis, collects the urine draining from the nephron tubules and channels it into the ureter - Figure 02.

Figure 02 - Sectioned view of the kidney

MICROSTRUCTURE OF THE KIDNEY

The basic functional unit of the kidney is the nephron. There are over one million nephrons in each human kidney and together they are responsible for the complex water regulation and waste elimination functions of the kidneys. The heads of the nephrons are in the cortical region and the tubular component then descends through the medulla and eventually drains into the renal pelvis - Figure 03.

Figure 03. Arrangement of nephrons in the kidney

The key area of interface between the circulatory system and the tubular part of the kidney is the knot of glomerular capillaries in the Bowman's capsule. Those liquid parts of the blood that are able to cross through the filtration membrane of the capillaries pass into the Bowman's capsule and then into the tubular section of the nephron - Figure 04. The filtration membrane only allows water to pass through it and small molecules that will dissolve in water such as waste (urea, creatinine etc.) glucose, amino acids and ions. Large proteins and blood cells are too large to be filtered and remain in the blood.

Figure 04. The Bowman's capsule and glomerulus

The filtered fluid or filtrate enters the proximal tubule and then into the loop of Henle which is the part of the nephron which dips in and out of the medulla. From the loop of Henle, the filtrate travels through the distal tubule and then into a common collecting duct which passes through the medulla and into the renal pelvis - Figure 05.

 

Figure 05. Structure of the nephron

PERITUBULAR CAPILLARIES

The nephrons are surrounded by a fine network of capillaries called the peritubular capillaries. These perform an important role in direct secretion, selective reabsorption and the regulation of water (see below).

DIRECT SECRETION

In addition to glomerular filtration, some substances are secreted directly from the adjacent peritubular capillaries into the proximal tubule. These substances include potassium ions and some hormones.

SELECTIVE REABSORPTION

Ultrafiltration is indiscriminate except for size of particle and useful substances are filtered from the blood as well as wastes. This situation is obviously unsatisfactory as the body would soon be depleted of amino acids, glucose and sodium etc. which would need to be replenished from external sources. To resolve this problem, useful substances in the filtrate are reabsorbed back into the peritubular capillaries as the filtrate passes along the tubule, leaving only the wastes which are eliminated in the urine. This process is shown in the animation in Figure 06.

Figure 06. Selective reabsorption of essential nutrients

WATER REGULATION BY THE KIDNEYS

The water content of the body can vary depending on various factors. Hot weather and physical activity such as exercise make us sweat and so lose body fluids. Drinking tends to be at irregular intervals when socially convenient. This means that sometimes the body has too little water and needs to conserve it and sometimes too much water and needs to get rid of it. Most of the control of water conservation takes place in the distal and collecting tubules of the nephrons under control of anti-diuretic hormone, (ADH), sometimes called vasopressin. This hormone is released by the posterior pituitary under control of the hypothalamus in the mid-brain area. The hypothalamus monitors the water content of the blood. If the blood contains too little water (indicating dehydration) then more ADH is released. If the blood contains too much water (indicating over-hydration) then less ADH is released into the blood stream - Figure 07.

Figure 07. Release of ADH from the posterior pituitary into the bloodstream

ADH released from the pituitary travels in the blood stream to the peritubular capillaries of the nephron. ADH binds to receptors on the distal and collecting tubules of the nephrons which causes water channels to open in the tubule walls. This allows water to diffuse through the tubule walls into the interstitial fluid where it is collected by the peritubular capillaries. The more ADH present, the more water channels are open and the more water is reabsorbed - Figure 08.

Figure 08 Reabsorption of water from the filtrate under the influence of ADH

Over 99% of the filtrate produced each day can be reabsorbed. The amount of water reabsorbed from the filtrate back into the blood depends on the water situation in the body. When the body is dehydrated, most of the filtrate is reabsorbed but note that even in cases of extreme of water shortage, the kidneys will continue to produce around 500 ml of urine each day in order to perform their excretory function.

THE MICTURION REFLEX

Micturition is another word for urination and in most animals it happens automatically. As the bladder fills with urine, stretch receptors in the wall of the bladder send signals to the parasympathetic nerves to relax the band of smooth muscle that forms the internal urethral sphincter. As the muscle relaxes, the urethra opens and urine is voided to the outside environment.

A second sphincter, the external urethral sphincter is skeletal muscle controlled by motor neurons - Figure 09. These neurons are under conscious control and this means we are able to exercise control over when and where we urinate. This control is a learned response that is absent in the new-born infant.

Figure 09. The urinary bladder and urethra

RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM

The long-term control of blood pressure is via the renin-angiotensin-aldosterone (RAA) system. This system is also one of the body's compensatory mechanisms to a fall in blood pressure. The kidneys release renin into the bloodstream and this converts angiotensinogen to angiotensin I which in turn is converted to angiotensin II by angiotensin converting enzyme in the capillaries of the lungs. Under the influence of Angiotensin II, aldosterone levels increase. This increases blood sodium levels by decreasing the amount of salt excreted by the kidneys. Retaining salt instead of excreting it into urine increases the osmolarity of the blood and so the blood volume. As the volume increases, so does the blood pressure. Angiotensin II is also a potent vasoconstrictor which raises blood pressure by increasing vascular resistance - Figure 10.

Figure 10. The Renin, angiotensin, aldosterone response to a fall in blood pressure

ACID BASE BALANCE

The body controls the acidity of the blood very carefully because any deviation from the normal pH of around 7.4 can cause problems - especially with the nervous system. Deviations in pH can occur due to trauma or diseases such as diabetes, pneumonia and acute asthma. The mechanisms that resist and redress pH change are...

1. Minor changes in pH are resisted by plasma proteins acting as buffers in the blood.

2. Adjustment to the rate and depth of breathing. An increase in acidity (decrease in pH) increases the rate and depth of breathing which gets rid of carbon dioxide from the blood and so reduces acidity.

3. The kidneys respond to changes in blood pH by altering the excretion of acidic or basic ions in the urine. If the body becomes more acidic, the kidneys excrete acidic hydrogen ions (H+) and conserve basic bicarbonate ions (HCO3-). If the body becomes more basic, the kidneys excrete basic bicarbonate ions and conserve acidic hydrogen ions.

Together, these three mechanisms maintain tight control over the pH of the body.

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