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Discover the vital role of excretion in removing waste products such as urea, ammonia, and uric acid. Learn about the urinary system, nephrons, and the process of peeing.
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Why Pee? • The body runs chemical reactions necessary for life. The products of these reactions tend to be useful to the body but some are not – like carbon dioxide in cellular respiration. • These byproducts that are not used must be removed from the cells/body through excretion.
Protein Problems • Proteins are the working molecules and they are an absolute necessity for the body. • When proteins are broken down the amine group (NH2) is removed in a process called deamination. These must be excreted as they will form ammonia which is toxic to the body. • The ammonia is combined with carbon dioxide by the liver to form urea. • Nucleic acidsbreak down to give a similar product – uric acid.
What Else is Excreted? • See Table 1 on page 342. This table shows what gets excreted and who gets rid of it. • Ammonia, Urea and Uric Acid Kidneys • Carbon dioxide Lungs • Lactic Acid Liver • Solid Waste (Feces) Large Intestine
How to Get Urine Out • Cells dump their waste into the blood – kind of like a sewer system. The blood travels through the renal arteries to the kidneys. (Renal = Kidneys) • The kidneys are the major filtration organ of the body. They filter the blood and remove waste products. • The kidneys have three parts: • The renal cortex – the outer layer of the kidney. • The renal medulla – the middle area of the kidney which houses the nephrons. • The renal pelvis – the collecting area for urine.
Nephrons • The nephrons are the actual filtration units of the kidneys. They are microscopic and each kidney has over a million of them. • The nephrons are housed in the renal medulla. • Nephrons are composed of several parts – glomerulus, Bowman’s capsule, proximal tubule, loop of Henle, distal tubule and the collecting duct.
What Happens in the Nephron? • Blood with waste in it is carried by the afferent arteriole to the glomerulus. • Dissolved matter – both useful and waste products are “pushed” into the glomerulus within Bowman’s capsule. Blood continues in circulation through the efferent arteriole. • The good and bad stuff transported into the glomerulus is separated as it travels through the proximal tubule and the loop of Henle – the good stuff is reabsorbed into the blood while the bad stuff stays in the nephron.
After the Nephron? • The bad stuff leaves the nephron through the distal tubule and enters the collecting duct. • The collecting duct leads to the renal pelvis where it collects and is ready to leave the kidney. • Ureters extend down from the kidney to the urinary bladder and the urine travels down these tubes. • Urine collects in the urinary bladder that has cells that are capable of stretching. When stretched far enough a nervous impulse is sent to the brain that says “ya gotta go!”. • Urine exits the urinary bladder and the body through a tube called the urethra.
The Process of Peeing • Urination is the result of three steps or processes…Filtration, Reabsorption & Secretion. • Filtration – Stuff is removed from blood (everything except large plasma proteins, blood cells, and platelets); this occurs at relatively high pressure in the glomerulus and Bowman’s capsule. • Reabsorption – The good stuff we need to keep is brought back into the blood from the nephron and into the capillaries. • Secretion – Taking the bad stuff from the nephron, mixing it with water and expelling it from the body.
Interesting Information • Once you get to 600ml – you go – like it or not! • Luckily, only 1ml of every 120ml/minute that is filtered by nephron is turned into urine. The other 119ml is reabsorbed into the blood. • The body will reabsorb all the good stuff until it hits a threshold level for that substance – after that, good or not, it is going to go out in the urine.
Interesting Information • A dialysis machine does the same job as the kidneys and nephrons except that it can take up to 4hours a day, 3 times a week to do so. This is a time consuming and emotionally draining activity for people with kidney disease/failure. • There are two types of dialysis: hemodialysis and peritoneal dialysis. • Peritoneal dialysis is appropriate if the patient is otherwise relatively healthy. It requires only a simple pumping device, and it is possible to do once a day, generally overnight.
