Excretory System PDF

EXCRETORY SYSTEM The human excretory system functions to remove waste from the body through the skin as sweat, the lungs in the form of exhaled carbon dioxide, and through the urinary system in the form of urine. Here we focus on the urinary system. I. The urinary system I.1. Descripti...

EXCRETORY SYSTEM The human excretory system functions to remove waste from the body through the skin as sweat, the lungs in the form of exhaled carbon dioxide, and through the urinary system in the form of urine. Here we focus on the urinary system. I. The urinary system I.1. Description It includes the kidneys and urinary tract. Urinary systeme Kidneys are bean-shaped and richly vascularized organs just below the liver in the body cavity. A longitudinal section of a kidney shows three regions—an outer cortex, a medulla in the middle, and the renal pelvis, which is the expanded end of the ureter. The renal pelvis is extended by the ureter which opens into the bladder which is connected to the outside by the urethra. Diagram of kidney section  The renal cortex contains the nephrons—the functional unit of the kidney (around 1 million per kidney in humans). Each nephron is made up of a capsule covering a glomerulus and extended by a renal tube which is surrounded by a rich network of blood capillaries. The nephron opens into a collecting tube located in the pyramidal zone. The collecting tubes which form the pyramids open into the renal pelvis. The nephron diagram I.2. Urine production The kidneys make urine from blood supplied by the renal artery. The urine formed in the nephrons is channeled by the collecting ducts to the renal pelvis then carried to the bladder by the ureters where it is stored before being evacuated outside through the urethra. Substances Plasm Urine II. Kidney functions a Experiment Water 910 950 The composition of blood plasma is compared to that Phosphates (PO4---) 0.64 2 of urine. We obtain the following results: Chloride (Cl-) 3.65 5à7 Interpretation Sulfates (SO4 ) -- 0.02 2  Certain constituents are present in the blood and Sodium (Na )+ 3.25 3à6 absent in the urine: the kidneys function as selective Potassium (K ) 0.2 2à3 + Calcium (Ca ) ++ 0.1 0.1 à filters. Ammonium (NH4 ) + 0.001.03  Toxic waste is eliminated in concentrated form: Glucose 1 1à2 the kidneys play the role of purifier. Lipids 6 0  The elimination of useful substances depends on Protéins 70 0 their concentration in the plasma: the kidneys play Urea 0.30 0 the role of regulator. Uric acid 0.03 20  Certain substances are absent in the blood and Créatinine 0.01 0.5 present in the urine: the kidneys play the role of Hippuric acid 0 1 synthesis. Ammonium 0 0.5 hydroxide 0.2  Note: when the kidneys are no longer able to effectively purify the blood, we speak of kidney failure. III. The stages of urine formation III.1. Glomerular filtration Experiment The composition of the plasma is compared to that of the capsular fluid. We obtain the following results: Some Concentration Concentration in substances in Bowman’s capsule plasma (g/l) (g/l) Glucose 1 1 Proteins 70 0 Lipids 5 0 Na+ 3.30 3.30 Cl- 3.65 3.65 K+ 0.17 0.17 Urea 0.30 0.30 Interpretation The glomerulus filters the blood plasma, allowing all elements to pass except macromolecules. The glomerular filtrate is called primary urine. The rate of glomerular filtration depends on renal blood pressure. All the blood in the human body is filtered about 60 times a day by the kidneys. III.2. Tubular reabsorption Experiment The quantity of substances contained in the glomerular filtrate obtained in 24 hours is compared with that of the substances excreted in 24 hours. The results are recorded in the following table: Substance Amount in Excreted s glomerular amount filtrate Water 170 l 1.5 l Glucose 170 g 0g Na+ 560 g 5g Cl- 620 g 9g K+ 29 g 2.2 g Urea 51 g 30 g Interpretation Substances filtered by the glomerulus are reabsorbed during their transit through the renal tube. The glucose is completely reabsorbed; water and salts are largely reabsorbed; waste (urea, uric acid, etc.) is not reabsorbed. Tubular reabsorption allows the body to recover water and essential substances in order to preserve the stability of the internal environment. Certain substances (for example glucose and NaCl) are only eliminated by the kidneys if their plasma level reaches a certain threshold (1.8 for glucose and 5.6 for NaCl). These are threshold substances. Other substances are eliminated regardless of their plasma concentration: these are non- threshold substances (waste). III.3. The tubular secretion The cells of the renal tube secrete certain substances such as ammonia and hippuric acid which will constitute, with the substances not reabsorbed, the definitive urine. IV. Regulation of the indoor environment The Kidneys ensure water exits and entries. The same is true for many mineral elements. The amounts of water and ions reabsorbed into the circulatory system are carefully regulated and this is an important way the body regulates its water content and ion levels Therefore, urinary excretion, through its dual purifying and regulatory role, contributes to regulating the volume and composition of the blood or the internal environment. Conclusion Urinary Excretion allows us to reject certain waste resulting from the functioning of the organs. It is a mechanism which also participates in the regulation of the internal environment..