This video was made for Matric revision, covering the topic
Human homeostasis: Part 2, water, salts
Hopefully this is helpful in your revision.

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Hello, I am Dr Derek Keats, a former professor of biology, and I am continuing with you on the topic of homeostasis. In this video we are going to cover the notion of homeostasis – maintaining a constant, optimal internal environment. We will look at the role played by negative feedback in regulating the body’s water levels and salt concentrations.

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Remember that homeostasis is about maintaining constant – and optimal – internal conditions.

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The regulations of water in our bodies – >> also called osmoregulation is vitally important, since water makes up >> around 60% of our bodies by mass. It varies with age, being higher in babies and lower as we get older. >> If the water level is much higher, >> or much lower than that, then we have big trouble. >> So, now let us look at how this water balance is maintained.

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We meed to review a little about >> the kidneys from what >> you learned in grade 11, under the topic of excretion.

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Remember that the kidney is a three-dimensional structure, and when we look at images like this, we are looking at a stylized drawing very thin slice through a kidney. A typical kidney can be seen to have three obvious layers, >> a renal pelvis that collects the output of the kidney and sends it to the ureter, from where it moves to the bladder, >> a medulla (meaning middle) which is divided into rounded pyramid shaped regions, >> and an outer cortex. >> You can also see the renal artery, which brings in oxygenated blood – including the blood that is going to be cleansed by the kidney – and >> the renal vein which takes away the deoxygnaged and clensed blood. >> Each kidney connects to a ureter, which takes away waste to be stored in the bladder.

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We are going to introduce the most important structure of the kidney, the nephron. >> Remember the structures in the medulla called pyramids. You can see the shape from >> the pelvis through >> the medulla into >> the cortex. >> It is within these extended pyramids that you find the nephrons. >> A human kidney contains up to 1.5 million nephrons, so you can imagine that this one shown here is not drawn to the correct scale.

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A nephron is the basic structural and functional unit of the kidney. And you can see which part of the nephron here is found in the >> cortex, >> medulla, and >> pelvis.

The nephron is responsible for four key processes that happen in the kidney: >> filtration,
>> secretion, >> reabsorption and >> excretion.

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Let us take a superficial overview of what happens in the nephron, purely as review so we can understand how water is regulated. >> The front end to the nephron is the Bowman’s capsule, which in three dimensions is like a tennis ball with a hole in one end, and connected to a tube a tube at the other. Remember, what we are seeing in the drawing is just a slice through it, you have to imagine it in three dimensions. >> Entering through the hole at one end is a blood vessel, which branches and anastamoses into a capillary bed called the glomerulus. >> Here is a diagrammatic representation of the >> Bowman’s capsule and the >> glomerulus, and wends its way through the system and eventually back into the renal vein. While in the glomerulus, blood pressure forces >> blood plasma out into the Bowman’s capsule as the filtrate.

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The filtrate passes through the >> proximal convoluted tubule, which in reality is surrounded also by blood vessels. There is some excretion there, and also some reabsorption of some of the components. The filtrate passes trough the loop of Henle where reabsorption continues, as the filtrate passes through >> the distal convoluted tubule and >> into the collecting duct. >> Note that other nephrons also connect to the collecting duct. >> From the collecting duct, the filtrate passes into the ureter, >> from where it moves into the bladder as the substance we call urine.

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Now, we have reviewed this because we want to talk about homeostasis and the maintenance of the water level of the blood. >> Let’s imagine that the water level in the blood rises. >> This might happen because of cool weather (we are not sweating very much), or because we drink a lot of fluids, or maybe we are sitting around using the computer or watching TV. >> Osmoreceptors in the hypothalamus send a hormone signal to the pituitary gland, which >> causes a decrease in the secretion of antidiuretic hormone (also known as vasopressin). >> This causes the permeability of those areas of the nephron responsible for water reabsorption – the distal convoluted tubule and the collecting ducts – to decrease. >> This decrease in permeability means less water is resorbed and a diluted urine is produced, so >> more water is lost >> and normal water levels are restored.

>> Now let’s imagine that the water level in the blood decreases. >> That might happen because the weather is hot (we are sweating a lot), because we don’t drink enough fluids, or if we are engaged in heavy exercise activity (we sweat a lot). >> Osmoreceptors in the hypothalamus send a hormone signal to the pituitary gland, which >> causes an increase in the secretion of antidiuretic hormone (ADH). >> This causes the permeability of those areas of the nephron responsible for water reabsorption – the distal convoluted tubule and the collecting ducts – to increase, letting more water back into the duct. >> More water is resorbed, resulting in a concentrated urine, >> with less water lost. >> In this way, normal water levels are restored. >> All of this happens due to the way in which the negative feedback, through the secretion of hormones, influences how the nephron interacts with the surrounding tissues and blood vessels.

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Salt balance also follows a negative feedback loop, as we have seen before when we were studying the endocrine system.

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We have pretty much covered what you need to know for the South African grade 12 life science syllabus on this subject. If you want to take it further, and have a deeper understanding,

>> you could search Google or another search engine using terms such as kidney, urinary system, nephron, osmoregulation, salt balance or any of the other terms we have used in this video.

>> You can look for videos on this and related subjects on YouTube or other online sources of video material.

>> You can visit the local or school library, and read up on the topics there.

>> And remember, if you find something good, put it on the wiki so we can all share and learn from it..

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I am Derek Keats, and this resource is licensed under a Creative Commons, Attribution license.