Proses Pengeluaran
Berdasarkan zat yang dibuang, proses
pengeluaran pada manusia dibedakan
menjadi:
Defekasi: pengeluaran zat sisa hasil pencernaan
(feses)
Ekskresi: pengeluaran zat sisa hasil
metabolisme (CO
2
, keringat dan urine)
Sekresi: pengeluaran getah yang masih berguna
Sistem Ekskresi
Adalah sistem pengeluaran zat-zat sisa
metabolisme yang tidak berguna bagi tubuh
dari dalam tubuh, seperti:
Menghembuskan gas CO
2
ketika kita bernafas
Berkeringat
Alat Ekskresi manusia terdiri atas:
1.
Ginjal
2.
Kulit
3.
Hati
GINJAL
Functions of the Urinary System
The urinary system produces urine and conducts it to outside the body. As the kidneys produce urine, they carry out four functions: 1. excretion of metabolic wastes,
2. maintenance of water-salt balance, 3. maintenance of acid-base balance, 4. and secretion of hormones.
Excretion of Metabolic Wastes
The kidneys excrete metabolic wastes, notably nitrogenous wastes. Urea is the primary nitrogenous end product of metabolism in human beings, but humans also excrete some ammonium, creatinine, and uric acid. Urea is a
by-product of amino acid metabolism. The breakdown of amino acids in
the liver releases ammonia, which the liver combines with carbon dioxide to produce urea. Ammonia is very toxic to cells, but urea is much less toxic. Because it is less toxic, less water is required to excrete urea.
Creatine phosphate is a high-energy phosphate reserve molecule in muscles. The metabolic breakdown of creatine phosphate results in creatinine. The breakdown of nucleotides, such as those containing adenine and thymine, produces uric acid. Uric acid is rather insoluble. If too much uric acid is present in blood, crystals form and precipitate out. Crystals of uric acid sometimes collect in the joints, producing a painful ailment called gout.
Maintenance of Water-Salt Balance
A principal function of the kidneys is to maintain the appropriate water-salt balance of the blood. As we shall see, blood volume is intimately associated with the salt balance of the body. Salts, such as NaCl, have the ability to
cause osmosis, the diffusion of water—in this case, into the blood. The more
salts there are in the blood, the greater the blood volume and the greater the blood pressure. In this way, the kidneys are involved in regulating blood
pressure. The kidneys also maintain the appropriate level of other ions (electrolytes), such as potassium ions (K), bicarbonate ions (HCO3), and calcium ions (Ca2), in the blood.
Maintenance of Acid-Base Balance
The kidneys regulate the acid-base balance of the blood. In order for a person to remain healthy, the blood pH should be just about 7.4. The
kidneys monitor and control blood pH, mainly by excreting hydrogen ions (H) and reabsorbing the bicarbonate ions (HCO3) as needed to keep blood pH at about 7.4. Urine usually has a pH of 6 or lower because our diet often contains acidic foods.
Secretion of Hormones
The kidneys assist the endocrine system in hormone secretion. The kidneys release renin, a substance that leads to the secretion of the hormone
aldosterone from the adrenal cortex, the outer portion of the adrenal glands, which lie atop the kidneys. Aldosterone promotes the reabsorption of sodium ions (Na) by the kidneys. Whenever the oxygen-carrying capacity of the blood is reduced, the kidneys secrete the hormone erythropoietin, which stimulates red blood cell production. The kidneys also help activate vitamin D from the skin. Vitamin D is the precursor of the hormone calcitriol, which promotes calcium (Ca2) absorption from the digestive tract.
Juxtaglomerular apparatus. This drawing shows that the afferent arteriole and the distal
convoluted tubule usually lie next to each other. The juxtaglomerular apparatus occurs where they touch. The juxtaglomerular apparatus secretes renin, a substance that leads to the
release of aldosterone by the adrenal cortex. Reabsorption of sodium ions followed by water then occurs. Therefore, blood volume and blood pressure increase.
Aldosterone Hormones control the reabsorption of sodium at the distal convoluted tubule. Aldosterone, a hormone secreted by the adrenal cortex, promotes the excretion of potassium
ions (K) and the reabsorption of sodium ions (Na). The release of aldosterone is set in motion by the kidneys themselves. The juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule. When blood volume, and therefore blood
pressure, is not sufficient to promote glomerular filtration, the juxtaglomerular apparatus secretes renin. Renin is an enzyme that changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin I. Later, angiotensin I is converted to angiotensin II, a powerful
vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. The reabsorption of sodium ions is followed by the reabsorption of water. Therefore, blood volume and blood pressure increase.
Atrial natriuretic hormone (ANH)
ANH is a hormone secreted by the atria of the heart when cardiac cells are stretched due to increased blood volume. ANH inhibits the secretion of renin by the juxtaglomerular apparatus and the secretion of aldosterone by the adrenal cortex. Its effect, therefore, is to promote the excretion of Na, called
natriuresis. When Na is excreted, so is water, and therefore blood volume and
STRUKTUR GINJAL
BENTUK GINJAL SEPERTI KACANG MERAH, JUMLAHNYA SEPASANG DAN
TERLETAK DI DORSAL KIRI DAN KANAN TULANG BELAKANG DI
DAERAH PINGGANG. BERAT GINJAL DIPERKIRAKAN 0,5% DARI BERAT
BADAN, DAN PANJANGNYA ± 10 CM. SETIAP MENIT 20-25% DARAH
DIPOMPA OLEH JANTUNG YANG MENGALIR MENUJU GINJAL
GINJAL TERDIRI DARI TIGA BAGIAN UTAMA YAITU:
A.
KORTEKS (BAGIAN LUAR)
B.
MEDULLA (SUMSUM GINJAL)
BAGIAN KORTEKS GINJAL MENGANDUNG BANYAK SEKALI
NEFRON ± 100 JUTA SEHINGGA PERMUKAAN KAPILER GINJAL
MENJADI LUAS, AKIBATNYA PEREMBESAN ZAT BUANGAN
MENJADI BANYAK. SETIAP NEFRON TERDIRI ATAS BADAN
MALPHIGI DAN TUBULUS (SALURAN) YANG PANJANG.
PADA BADAN MALPHIGI TERDAPAT KAPSUL BOWMAN YANG
BENTUKNYA SEPERTI MANGKUK ATAU PIALA YANG BERUPA
SELAPUT SEL PIPIH. KAPSUL BOWMAN MEMBUNGKUS
GLOMERULUS.
GLOMERULUS BERBENTUK JALINAN KAPILER ARTERIAL. TUBULUS
PADA BADAN MALPHIGI ADALAH TUBULUS PROKSIMAL YANG
BERGULUNG DEKAT KAPSUL BOWMAN YANG PADA DINDING SEL
TERDAPAT BANYAK SEKALI MITOKONDRIA.
PADA RONGGA GINJAL BERMUARA PEMBULUH PENGUMPUL.
RONGGA GINJAL DIHUBUNGKAN OLEH URETER (BERUPA
SALURAN) KE KANDUNG KENCING (VESIKA URINARIA) YANG
BERFUNGSI SEBAGAI TEMPAT PENAMPUNGAN SEMENTARA URIN
SEBELUM KELUAR TUBUH.
DARI KANDUNG KENCING MENUJU LUAR TUBUH URIN
Glomerular filtration occurs when whole blood enters the afferent arteriole and the glomerulus. Due to glomerular blood pressure, water and small molecules move from the glomerulus to the inside of the glomerular capsule. This is a filtration process because large molecules and formed elements are unable to pass through the capillary wall. In effect, then, blood in the glomerulus has two portions: the filterable components and the nonfilterable components:
Filterable Nonfilterable
Blood Components Blood Components
Water Formed elements (blood cells and platelets) Nitrogenous wastes Plasma proteins
Nutrients Salts (ions)
The glomerular filtrate contains small dissolved molecules in approximately the same concentration as plasma. Small molecules that escape being filtered and the nonfilterable components leave the
glomerulus by way of the efferent arteriole. Nephrons in the kidneys filter 180 liters of water per day, along with a considerable amount of small molecules (such as glucose) and ions (such as sodium). If the
composition of urine were the same as that of the glomerular filtrate, the body would continually lose water, salts, and nutrients. Therefore, we can conclude that the composition of the filtrate must be altered as this fluid passes through the remainder of the tubule.
Tubular reabsorption occurs as molecules and ions are both passively and actively reabsorbed from the nephron into the blood of the peritubular capillary network. The
osmolarity of the blood is maintained by the presence of both plasma proteins and salt. When sodium ions (Na) are actively reabsorbed, chloride ions (Cl) follow passively. The reabsorption of salt (NaCl) increases the osmolarity of the blood compared to the filtrate, and therefore water moves passively from the tubule into the blood. About 67% of Na is reabsorbed at the proximal convoluted tubule. Nutrients such as glucose and amino acids also return to the
blood at the proximal convoluted tubule. This is a selective process because only
molecules recognized by carrier molecules are actively reabsorbed. Glucose is an example of a molecule that ordinarily is completely reabsorbed because there is a plentiful supply of carrier molecules for it. However, every substance has a maximum rate of transport, and after all its carriers are in use, any excess in the filtrate will appear in the urine. For example, as reabsorbed levels of glucose approach 1.8–2 mg/ml plasma, the rest appears in the urine. In diabetes mellitus, excess glucose occurs in the blood, and then in the filtrate, and then in the urine, because the liver and muscles have failed to store glucose as glycogen, and the
kidneys cannot reabsorb all of it. The presence of glucose in the filtrate increases its osmolarity compared to that of the blood, and therefore less water is reabsorbed into the peritubular capillary network. The frequent urination and increased thirst experienced by untreated diabetics are due to the fact that water is not being reabsorbed.
We have seen that the filtrate that enters the proximal convoluted tubule is divided into two portions: components that are reabsorbed from the tubule into the blood, and components that are not reabsorbed and continue to pass through the nephron to be further processed into urine:
Reabsorbed Filtrate Nonreabsorbed Filtrate
Components Components
Most water Some water
Nutrients Much nitrogenous waste
Required salts (ions) Excess salts (ions)
The substances that are not reabsorbed become the tubular fluid, which enters the loop of the nephron.
Tubular secretion is a second way by which substances are removed from blood and added to the tubular fluid. Hydrogen ions, potassium ions,
creatinine, and drugs such as penicillin are some of the substances that are moved by active transport from the blood into the distal convoluted tubule. In the end, urine contains (1) substances that have undergone glomerular
filtration but have not been reabsorbed, and (2) substances that have undergone tubular secretion.
B. PROSES-PROSES DI DALAM GINJAL
DI DALAM GINJAL TERJADI RANGKAIAN PROS FILTRASI, REABSORBSI,
DAN AUGMENTASI.
1. PENYARINGAN (FILTRASI)
FILTRASI TERJADI PADA KAPILER GLOMERULUS PADA KAPSUL
BOWMAN.
PADA GLOMERULUS TERDAPAT SEL-SEL ENDOTELIUM KAPILER YANG
BERPORI (PODOSIT) SEHINGGA MEMPERMUDAH PROSES
PENYARINGAN.
BEBERAPA FAKTOR YANG MEMPERMUDAH PROSES PENYARINGAN
ADALAH TEKANAN HIDROLIK DAN PERMEABILITIAS YANG TINGGI
PADA GLOMERULUS.
SELAIN PENYARINGAN, DI GLOMELURUS TERJADI PULA PENGIKATAN
KEMBALI SEL-SEL DARAH, KEPING DARAH, DAN SEBAGIAN BESAR
PROTEIN PLASMA.
BAHAN-BAHAN KECIL TERLARUT DALAM PLASMA, SEPERTI GLUKOSA,
ASAM AMINO, NATRIUM, KALIUM, KLORIDA, BIKARBONAT, GARAM
LAIN, DAN UREA MELEWATI SARINGAN DAN MENJADI BAGIAN DARI
ENDAPAN.
HASIL PENYARINGAN DI GLOMERULUS BERUPA FILTRAT
GLOMERULUS (URIN PRIMER) YANG KOMPOSISINYA SERUPA
DENGAN DARAH TETAPI TIDAK MENGANDUNG PROTEIN.
PADA FILTRAT GLOMERULUS MASIH DAPAT DITEMUKAN ASAM
AMINO, GLUKOSA, NATRIUM, KALIUM, DAN GARAMGARAM
LAINNYA.
2. PENYERAPAN KEMBALI (REABSORBSI)
VOLUME URIN MANUSIA HANYA 1% DARI FILTRAT
GLOMERULUS.
OLEH KARENA ITU, 99% FILTRAT GLOMERULUS AKAN
DIREABSORBSI SECARA AKTIF PADA TUBULUS KONTORTUS
PROKSIMAL DAN TERJADI PENAMBAHAN ZAT-ZAT SISA SERTA
UREA PADA TUBULUS KONTORTUS DISTAL.
SUBSTANSI YANG MASIH BERGUNA SEPERTI GLUKOSA DAN ASAM AMINO
DIKEMBALIKAN KE DARAH.
SISA SAMPAH KELEBIHAN GARAM, DAN BAHAN LAIN PADA FILTRAT DIKELUARKAN
DALAM URIN.
TIAP HARI TABUNG GINJAL MEREABSORBSI LEBIH DARI 178 LITER AIR, 1200 G GARAM,
DAN 150 G GLUKOSA.
SEBAGIAN BESAR DARI ZAT-ZAT INI DIREABSORBSI BEBERAPA KALI.
SETELAH TERJADI REABSORBSI MAKA TUBULUS AKAN MENGHASILKAN URIN SEKU
NDER YANG KOMPOSISINYA SANGAT BERBEDA DENGAN URIN PRIMER.
PADA URIN SEKUNDER, ZAT-ZAT YANG MASIH DIPERLUKAN TIDAK AKAN DITEMUKAN
LAGI.
SEBALIKNYA, KONSENTRASI ZAT-ZAT SISA METABOLISME YANG BERSIFAT RACUN
BERTAMBAH, MISALNYA UREUM DARI 0,03, DALAM URIN PRIMER DAPAT MENCAPAI 2%
DALAM URIN SEKUNDER.
MERESAPNYA ZAT PADA TUBULUS INI MELALUI DUA CARA. GULA DAN ASAM MINO
MERESAP MELALUI PERISTIWA DIFUSI, SEDANGKAN AIR MELALUI PERISTIWA OSMOSIS.
REABSORBSI AIR TERJADI PADA TUBULUS PROKSIMAL DAN TUBULUS DISTAL.
3. AUGMENTASI
AUGMENTASI ADALAH PROSES PENAMBAHAN ZAT SISA DAN UREA YANG
MULAI TERJADI DI TUBULUS KONTORTUS DISTAL.
KOMPOSISI URIN YANG DIKELUARKAN LEWAT URETER ADALAH 96% AIR,
1,5% GARAM, 2,5% UREA, DAN SISA SUBSTANSI LAIN, MISALNYA PIGMEN
EMPEDU YANG BERFUNGSI MEMBERI WARNA DAN BAU PADA URIN.
HAL-HAL YANG MEMPENGARUHI PRODUKSI URIN
HORMON ANTI DIURETIK (ADH) YANG DIHASILKAN OLEH
KELENJAR HIPOFISIS POSTERIOR AKAN MEMPENGARUHI
PENYERAPAN AIR PADA BAGIAN TUBULUS DISTAL KARENA
MENINGKATKAN PERMEABILITIAS SEL TERHADAP AIR.
JIKA HORMON ADH RENDAH MAKA PENYERAPAN AIR
BERKURANG SEHINGGA URIN MENJADI BANYAK DAN ENCER.
SEBALIKNYA, JIKA HORMON ADH BANYAK, PENYERAPAN AIR
BANYAK SEHINGGA URIN SEDIKIT DAN PEKAT.
KEHILANGAN KEMAMPUAN MENSEKRESI ADH MENYEBABKAN
PENYAKTI DIABETES INSIPIDUS. PENDERITANYA AKAN
MENGHASILKAN URIN YANG SANGAT ENCER.
Antidiuretic Hormone (ADH)
ADH released by the posterior lobe of the pituitary plays a role in water reabsorption at the collecting duct. In order to understand the action of this hormone, consider its name. Diuresis means flow of urine, and antidiuresis
means against a flow of urine. When ADH is present, more water is
reabsorbed (blood volume and pressure rise), and a decreased amount of urine results. In practical terms, if an individual does not drink much water on a certain day, the posterior lobe of the pituitary releases ADH, causing more water to be reabsorbed and less urine to form. On the other hand, if an individual drinks a large amount of water and does not perspire much, ADH is not released. In that case, more water is excreted, and more urine forms.
SELAIN ADH, BANYAK SEDIKITNYA URIN DIPENGARUHI PULA OLEH
FAKTOR-FAKTOR BERIKUT :
A. JUMLAH AIR YANG DIMINUM
AKIBAT BANYAKNYA AIR YANG DIMINUM, AKAN MENURUNKAN KONSENTRASI PROTEIN YANG DAPAT
MENYEBABKAN TEKANAN KOLOID PROTEIN MENURUN SEHINGGA TEKANAN FILTRASI KURANG
EFEKTIF. HASILNYA, URIN YANG DIPRODUKSI BANYAK.
B. SARAF
RANGSANGAN PADA SARAF GINJAL AKAN MENYEBABKAN PENYEMPITAN DUKTUS AFEREN SEHINGGA
ALIRAN DARAH KE GLOMERULUS BERKURANG. AKIBATNYA, FILTRASI KURANG EFEKTIF KARENA
TEKANAN DARAH MENURUN.
C. BANYAK SEDIKITNYA HORMON INSULIN
APABILA HORMON INSULIN KURANG (PENDERITA DIABETES MELITUS), KADAR GULA DALAM DARAH
AKAN DIKELUARKAN LEWAT TUBULUS DISTAL. KELEBIHAN KADAR GULA DALAM TUBULUS DISTAL
MENGGANGGU PROSES PENYERAPAN AIR, SEHINGGA ORANG AKAN SERING MENGELUARKAN
URIN.
2. PARU-PARU (PULMO)
FUNGSI UTAMA PARU-PARU ADALAH SEBAGAI ALAT PERNAPASAN.
AKAN TETAPI, KARMA MENGEKSKRESIKAN ZAT SISA METABOLISME
MAKA DIBAHAS PULA DALAM SISTEM EKSKRESI. KARBON DIOKSIDA
DAN AIR HASIH METABOLISME DI JARINGAN DIANGKUT OLEH
DARAH LEWAT VENA UNTUK DIBAWA KE JANTUNG, DAN DARI
JANTUNG AKAN DIPOMPAKAN KE PARU-PARU UNTUK BERDIFUSI DI
ALVEOLUS.
SELANJUTNYA, H2O DAN CO2 DAPAT BERDIFUSI ATAU DAPAT
DIEKSRESIKAN DI ALVEOLUS PARU-PARU KARENA PADA ALVEOLUS
BERMUARA BANYAK KAPILER YANG MEMPUNYAI SELAPUT TIPIS.
KARBON DIOKSIDA DARI JARINGAN SEBAGIAN BESAR (75%)
DIANGKUT OLEH PLASMA DARAH DALAM BENTUK SENYAWA HC03,
SEDANGKAN SEKITAR 25% LAGI DIIKAT OLEH HB YANG
3. HATI (HEPAR)
HATI DISEBUT JUGA SEBAGAI ALAT EKSKRESI DI SAMPING BERFUNGSI SEBAGAI
KELENJAR DALAM SISTEM PENCERNAAN. HATI MENJADI BAGIAN DARI SISTEM
EKSKRESI KARMA MENGHASILKAN EMPEDU.
HATI JUGA BERFUNGSI MEROMBAK HEMOGLOBIN MENJADI BILIRUBIN DAP
BILIVERDIN, DAN SETELAH MENGALAMI OKSIDASI AKAN BERUBAH JADI
UROBILIN YANG MEMBERI WARNA PADA FESES MENJADI KEKUNINGAN.
DEMIKIAN JUGA KREATININ HASIH PEMECAHAN PROTEIN, PEMBUANGANNYA
DIATUR OLEH HATI KEMUDIAN DIANGKUT OLEH DARAH KE GINJAL.
JIKA SALURAN EMPEDU TERSUMBAT KARENA ADANYA ENDAPAN KOLESTEROL
MAKA CAIRAN EMPEDU AKAN MASUK DALAM SISTEM PEREDARAN DARAH
SEHINGGA CAIRAN DARAH MENJADI LEBIH KUNING.
4. KULIT (CUTIS)
KULIT BERFUNGSI SEBAGAI ORGAN EKSKRESI KARNA MENGANDUNG KELENJAR
KERINGAT (GLANDULA SUDORIFERA) YANG MENGELUARKAN 5% SAMPAI 10%
DARI SELURUH SISA METABOLISME.
PUSAT PENGATUR SUHU PADA SUSUNAN SARAF PUSAT AKAN MENGATUR
AKTIFITAS KELENJAR KERINGAT DALAM MENGELUARKAN KERINGAT.
KERINGAT MENGANDUNG AIR, LARUTAN GARAM, DAN UREA. PENGELUARAN
KERINGAT YANG BERLEBIHAN BAGI PEKERJA BERAT MENIMBULKAN HILANG
MELANOSITNYA GARAM-GARAM MINERAL SEHINGGA DAPAT MENYEBABKAN
KEJANG OTOT DAN PINGSAN.
