Whee! Guess what?
I shall now share with you very vital information....on what's to be tested for bio tomorrow! haha... for those who hate bio, then you don't have to read. But really, give it a shot.It's all about us and the human body! :)

Presenting...the dummie's guide to the human body!
Written by: a fellow dummy :P

Topic #1: Excretion

First to clear up a few misconceptions...Excretion is the process by which metabolic(involves reactions in body) waste products and toxic materials are removed from the body. Therefore, the removal of faeces is NOT, and I repeat, is not part of excretion, like many people believe.

If you're bored already, forget about going on...go back to sleep :)

Let's have a few examples on the metabolic activities in our bodies then.
Activity #1: Tissue respiration in our body cells. After we've eaten our delicious food, our body breaks down those large chunks of carbohydrates into simpler substances like glucose. This is then absorbed into our bloodstream. So why do we need glucose? Well, we need energy right? Energy comes easiest from glucose, compared to protein and fat. So what actually happens? Well, the glucose goes through oxidation and when that happens, energy is produced! Along with carbon dioxide and heat, of course. With this energy, we can carry on with many wonderful activities :)

Activity #2: Amino acid deamination: What's that? Actually, it's a chemical process whereby amino acids from proteins are broken down (when there's excess in our body. It's acidic, so we don't want too much). Anyway, they get broken down into urea and pyruvic acid. The urea is got rid of at the kidney, and comes out of our body as part of urine. For the pyruvic acid, however, again, this is where energy can come from. Yay!

Activity #3: Formation of new protoplasm from amino acids. From the amino acids formed after translation and transcription of DNA, a chain of proteins is formed, and all these connected actually form our wonderful protoplasm of our cells :)

Activity #4: Conversion of glucose to glycogen. For what? Well, it's not good for our body to contain too high a concentration of glucose, so the pancreas actually secretes more of a hormone called insulin, which in turn stimulates the cells in the liver and muscles to convert the excess into glycogen, which can be stored until needed. When there is a shartage of glucose, the reverse happens with the aid of the hormone, glucagon.

All these are either catabolic(breaking down of complex substances into simpler ones) or anabolic(forming complex substances from simpler ones) activities. Catabolic activities+anabolic activities= METABOLISM!


In humans, these are the main unwanted excretory products:

1. Carbon dioxide: got rid of by the lungs as a gas in expired air(breate out...)
2.Nitrogenous wastes like urea(from the deamination of amino acids), creatinine and uric acid(from breakdown of protein). The main organ that helps remove these substances is the kidney. These substances come out of our body as part of urine.
3. Excess water: Don't we need water? Isn't it the more the better? Well, of course we need water, but if there is too high a concentration, our cells will really burst! Yes, so we must get rid of excess water. How? 3 ways. a)by our kidneys as a main constituent of urine b)by our skin in the form of perspiration c)through our lungs as water vapour in expired air

To get rid of all these harmful metabolic wastes, we need an excretory system!
main members: kidneys, ureters, urinary bladder, urethra, lungs, skin
function: to transport and remove excretory products

For this chapter, however, we would like to focus on the urinary system. The hero of this system, is of course, the kidneys. Don't look down on these beany babies, I mean, bean-shaped things. They help remove unwanted substances from the blood, retain the essential elements(maintain pH and blood composition), and can control the amount of water in the body. This results in the production of urine consisting of wastes like excess water and urea, and urine can also contain variable amounts of water, depending on how much excess there is in the body. Too much water: cells burst. Too little: cells become crenated and shrunken. Eventually die. As an osmoregulator, the kidneys regulate salt and water balance of the body fluid by a negative feedback system.

Let's get to know the kidneys a little better. They're embedded in a mass of fat(prevent shock and damage to kidneys which could result in kidney failure) in the abdominal cavity and are attached to the dorsal wall. There is a special concave surface on each kidney, to which the renal artery, vein and nerves are attached to. If I'm not wrong, the ureter's conected there too, to transport the waste products via the ureter to the urinary bladder.The kidney has a capsule(the outermost layer), followed by a cortex and theh medulla. Beneath that lies 12-16 conical structures that we call pyramids, and last of all, the enlarged portion of the ureter, the renal pelvis. Within each kidney, there are about a million nephrons (kidney tubules). These are where the substances move in and out of the bloodstream. FYO, each tubule is 5 cm long when stretched out to full length.

This is how the magic works:
The blood travels into the kidney, carrying all sorts of rubbish along with the good stuff (water, red blood cells, whilte blood cells, dissolved mineral salts, protein molecules like globulin and albumin). "That's it!" it declares. "It's time to unload my weight". The blood travels along the renal artery and then enters a renal capsule vis the afferent arteriole. Ultrafiltraion takes place in the glomerulus, where blood rushes through at very high pressure(125 cubic cm per min). Everything that is soluble flows through as glomerulate filtrate, leaving only rbc, wbc, big proteins and some fluid platelets. As we can see, things pass through the glomerulus according to their size, not their usefulness. The filtrate probably contains precious substances like glucose that we cannot afford to lose in our urine.

Reabsorption to the rescue! As the filtrate steadily flows downwards, other mechanisms kick into action. The super egoistical proximal convoluted tubule tries to steal as much greenlight, i mean, limelight, as possible. At this place, glucose, amino acids, vitamins, hormones, sodium ions, soluble minerals, and essential water are absorbed back into the veins. Glucose is reabsorbed by active transport, while the rest get absorbed mainly through diffusion. As the smart guy, the proximal convoluted tubule can control the amounts of each substance absorbed, and therefore can control the amount of water that goes back into the blood. In us humans, about 85% of water is reabsorbed in the proximal convoluted tubule. However, this is not enough. There are 2 other very important mechanisms that fine-tune the water absorption: The loop of Henle and distal convoluted tubule. The loop of Henle(obviously discovered by this guy called Henle) provides extra length for all glucose, some salts and water to be reabsorbed. But that's not over. Depending on the state of the person, the distal convoluted tubule will control the permeability of the tube to allow more or less water to be absorbed. Pretty cool, I must say :) However, tragicly, some of the unwanted substances will also go back into the bloodstream because of passive diffusion. When the concentration of these substances is lower in the bloodstream than in the tubules, they will also diffuse across into the renal vein. Therefore, it's impossible to have 0% of urea in our body, and that's why, blood has to constantly be pumped into the kidneys to be "cleaned".
"Phew! Finally a load off my mind."

Besides all these reabsorption stuff, kidneys are also great osmoregulators (control amount of water in our body). Our water potential in the blood has to be kept constant, and this water potential depends on the amount of water and salts in the blood. the water content can be controlled by a hormone called anti-diuretic hormone, or ADH. ADH is secreted by the pituitary gland, which is the "master" gland found in our brain. When there is too little body in our body, the osmotic pressure increases, and this is detected by the hypothalamus(regulates thirst, temp, etc) in our brain. When this happens, the pituitary gland secretes more ADH, which in turn stimulates the kidney to absorb more water. The distal convoluted tubule will then increase its permeability and allow more water to be reabsorbed. The water potential in the body goes back to normal again.

What is urine?

Presenting...the culprits who could have harmed us, but were got rid of in time... 1) excess water 2) mineral salts 3) nitrogenous waste products (urea, uric acid, creatinine). They are allowed to pass through the nephron and out through the collecting tubule into the renal pelvis as urine. The urine then flows through the ureter and into the urinary bladder and is stored there. When the bladder is about half full, a nerve signal is sent to the brain to say, "please be smart and go to the toilet now. You are ready for discharge :)". Anyway, when you reach the toilet bowl, the brain sends a message to the shincter muscle just below the urinary bladder, and the muscle relaxes, allowing the hole to become bigger and the urine to flow through. The urine flows through the short urethra before finally facing daylight. Yay! It's out!

Attention all health conscious people! Some interesting facts about our urine:

1) Protein-ruch diet results in a higher concentration of urea, because there will be more excess proteins digested, and the amino acids will be deaminated to form more urea

2) Foods rich in water or have a larger liquid intake will result in a greater volume of urine. Duh! haha... because there will be more excess water in the body, and we don't want our cells to burst...not me, anyway.

3) Salty food can result in excess salts being excreted. See lah, eat too much. Waste salt ah!

(Clear throat) The next section is very serious, and probably quite a sensitive issue too. Please be nice.

KIDNEY FAILURE. It could either be genetic, or could also be caused when the kidney is damaged through an infection or accident (that's why the layer of fat is there to help as much as possible). Because of the damage, the kidney is unable to effectively remove harmful substances. Its job as an osmoregulator is also compromised. Because of this, kidney patients may look very yellow(a lot of toxins not removed), swollen(too mushc water in body), and be very weak. They need help.

This is where dialysis comes in. First, let's touch on haemodialysis. What happens is: blood is drawn fron an artery in the arm. This blood is allowed to flow through tubes in the dialysis machine. These tubes are highly coiled in order to increase the surface area to volume ratio to increase efficiency (patients endure the pain for a shorter time, hopefully). The partially-permeable tubes contain bathing fluid that contain essential salts for the body. This sets up a diffusion gradient for the salts to diffuse into the blood and for the waste products to diffuse into the bathing fluid. However, this is not all. The blood flowing through also moves in the opposite direction of the bathing fluid to maintain the diffusion gradient(constant) for effective diffusion. Osmoregulation also inevitably takes place. The filtered blood then returns into the vein.

Everything good comes with a price. Life can be restricted as dialysis takes up several hours each week. It is a tedious and painful routine :'( Patients going through dialysis may also suffer from medical problems such as anemia and infections. Only living kidneys secrete a hormone Erythropoietin that stimulates the formation of red blood cells in the bone marrow. The gland that produces this hormone is on top of the kidney, so when there's kidney failure, erythropoietin cannot be made. Hence, the patient has to endure even more pain by being injected with the hormone during dialysis too. We fortunate people enjoy such lovely food, but kidney patients really have to watch their diet, often against their wishes. They can onlt take 500ml of water each day (otherwise they'll be bloated or swollen), they can only have 30-40g of protein each day(otherwise too much urea produced) and they cannot take potassium rich foods like bananas, instant coffee and chocolate(excess salts in body, take a very long time to remove salts)

Is there a solution? Many look to getting a kidney transplant. Only one kidney is required, and this kidney(along with the donated renal artery, vein and ureter) is added near the bladder while the failed ones are left alone. Life donors are usually relatives, because this lowers the possibility of a tissue rejection from the host. If the kidney comes from a person who just died, kidneys must be removed within an hour of death, and used within 12 hours of removal.

Yes, yes, we know. This comes with a price too. Yup, there are problems. The patient may reject the transplanted kidney, and when that happens, it's just too bad :( Also, in order to decrease the possibility of rejection of the new kidneys, the patients are given drugs to supress rejection. However, because the activity of the immune system is compromised, the patient is more prone to infection and other illnesses because the body becomes weaker. Lastly, there are also very few donors. Many people are not willing to donate their kidneys, and so it is very difficult to find a kidney for transplant.

Whew! Finally finished Excretion! Decided not to torture anyone anymore (especially those who actually read to the bottom of this entry). I'll be off to study homeostasis, the endocrine system(hormones and stuff), as well as the nervous system (on neurons and all). Bye!

God bless you and have a nice day :)