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Kidney
Excretion – the process of removing metabolic wastes from the body
The blood plays an important role in determining the chemical environment of the body. Wastes
must be removed so the blood can continue its job. The removal of wastes is a different
mechanism from the elimination of fecal material.
The average person eats more protein than is needed. Excess protein is converted into
carbohydrates by removing the nitrogen group. This process is called deamination and occurs in
the liver. Ammonia is formed which is a nitrogenous waste – NH3. Ammonia is too toxic
(too basic) to be left on its own to travel the blood so it is joined with CO2 to form urea.
The kidneys main function is eliminating urea and helping to regulate [water] and [salt] – so it is
excretion and osmoregulation.
Plants main excretory product is oxygen. The lungs and skin can also excrete – lungs excrete
CO2, skin excretes salts and nitrogenous wastes.
*Should be able to draw cortex, medulla, pelvis, renal arteries and veins – know which is which,
ureters and urethra, bladder.
Kidneys are bean-shaped on dorsal wall. Receive an extraordinary supply of blood. Enters
through artery and leaves through vein. (afferent means leads into and efferent means leads out).
Kidney composed of three distinct regions – cortex, medulla and pelvis. The structures that
actually filter the blood are called nephrons. Strung together, they would stretch 80 km. It is a
tube closed at on end composed of three regions – the proximal tubules, loop of Henle and distal
tubule. The whole nephron is surround by a rich network of capillaries.
should be able to annotate a drawing of glomerulus and nephron
The formation of urine – it is like cleaning out a drawer (blood). First you take out everything
(filtration) then you return all the articles you want to keep.(reabsorption). Then you add extra
things that you only need sometimes (secretion) and then you discard the rest. (excretion)
1. Ultrafiltration – Blood is brought to the nephron by the renal artery. The arteriole of the
glomerulus is very convoluted and narrow. There is also a difference in diameter between the
efferent and afferent arterioles. This causes high blood pressure, which forces the blood plasma
out of the arteriole into the Bowman’s capsule. There are several special features that facilitate
this action - a) basement membrane of the glomerulus is perforated – called fenestrated – and
only allows certain through due to size, b)special cells called podocyte cells in the basement
membrane have foot like extensions and c) the capillary walls of the glomerulus have pores
which allow passage of small substances. It all acts like a dialysis membrane (or dialysis
membrane acts like it). The movement is called ultrafiltration and is passive. Proteins, blood
cells and polypeptides remain in the blood due to their size. The rest of the blood enters the
Bowman’s capsule and is called the filtrate – is mainly plasma and what is dissolved in it which
is water, salts, glucose, amino acids, hormones, vitamins, ions and urea.
a tangle of yarn stuffed into a cupped hand = glomerulus and bowman’s, kidneys filter
abut 180 L of filtrate/day. The total volume of the plasma is only 3 L so the kidney does it
60 times a day. Most of the filtrate is reabsorbed back and only 1 – 1.5 L of urine is
produced/day.
2. Re-absorption – important to retain most …. As the filtrate passes through the proximal
tubule, the majority of the water and solutes are reabsorbed back into the surrounding capillaries.
Glucose is returned by facilitated diffusion and active transport. Amino acids and other nutrients
and ions return by active transport and pinocytosis. Water moves by osmosis. This process is
aided by microvilli lining the inside of proximal tubules – surface area. As the filtrate reaches
the loop of Henle, some more water maybe re-absorbed and more NaCl. The loop of Henle
drops down into the medulla of the kidney, which is hypertonic. This will allow water reabsorption. Also the descending loop is permeable to water and impermeable to salts. The
ascending loop is permeable to NaCl and impermeable to water , which allows salts to leave to
make the medulla hypertonic. The loop of Henle regulates [water] on a regular bases. It also
contains a lot of mitochondria and a lot of microvilli.
3. Secretion – is the movement of substances from the blood back into the nephron. It usually
includes things that couldn’t fit through the basement membrane or extra ions needed to maintain
pH balance. It is done by active transport and occurs in the distal tubule. Histamines, K+,
antibiotics, the by products of drugs ,and H+ and NH4+ ions for pH are some. (drug testing)
Urine leaves the distal tubule into the collecting ducts. It is high in urea and has a variable [ion]
based on the needs of the body. The collecting ducts of all nephrons drain into the pelvis of the
kidney. Urine flows to the bladder via the ureters.
The two ureters enter the bladder in such a way that their exists become forced shut when
pressure builds. If urination is delayed, urine begins to accumulate in the ureters and even
into the pelvis. Dangerous as toxic. When the bladder empties, the excess is allowed to
flow down and you need to urinate again. Urea + water = urine. Urination is under
voluntary control, sphincter muscles, intense emotion causes you to lose control, nerves
sense the stretch of the bladder.
*be able to compare blood in renal artery and renal vein and glomerulus filtrate with urine.
Renal artery
Renal vein
More oxygen
Les oxygen
Less CO2
More CO2
More urea
Less urea
More ammonia
Less ammonia
Don’t say “no” urea or CO2 as there
is always some
Osmoregulation *this is a homeostatic mechanism – if reabsorption is not able to maintain a
balance – when they ask for osmoregulation and/or homeostasis, this is what they want.*
Osmoregulation is maintaining water balance in blood, tissues and cytoplasm in living
organisms. The kidneys are ‘told’ the condition of the blood by hormones. Antidiuretic
hormone, ADH, is made by the hypothalamus and stored in the pituitary. Anything that tends to
dehydrate the body, like heavy perspiring, will sauce the [water] in the blood to decrease. This is
detected by osmoreceptors in the hypothalamus. It then stimulates the pituitary to release ADH,
which will be carried by the blood to the target organ – the collecting tubules. (receptors on cell
membrane) The collecting tubules are normally impermeable to water but ADH will cause it to
be permeable. More water will be reabsorbed and the urine will become concentrated or
hypertonic and the blood more dilute. Understand all this. Beer and coffee contain a
compound that inhibits the functioning of ADH so more urine will be made. Caffeine also
raises blood pressure which ….Cold weather will contract blood vessels…
If the blood is too dilute, it will inhibit the secretion of ADH – negative feedback and…
Diabetes mellitus (I) – lack of insulin, blood sugar rises, osmotic pressure and large volumes of
urine.
Kidney stones – precipitation of mineral solutes from the blood, may become lodged in
renal pelvis or ureters, sound waves can crush them. Gout – caused by the presence of uric
acid in urine. Man can’t break it down and accumulated in joints. Uric acid is found in
many rich foods.
Humans will die if they lose 12% of their body’s water.
There are different solutions to the nitrogenous waste problem – a trade off between
toxicity and water available. If you have access to lots of fresh water you convert your
nitrogen wastes to ammonia like fresh water fish – check the pH of your fish tank. Marine
animals and desert animals do not have access water so convert it to urea. Birds have even
less water to ‘play with’ as can’t transport a lot so they change it to uric acid paste which is
insoluble and less toxic. Urea and uric acid seem to be the methods used for terrestrial
animals as it is soluble in blood and relatively non-toxic. Dogs and cats have ammonia in
urine so it smells. Relative toxicity = ammonia > urea > uric acid