excretion in humans Flashcards
Excretory Products
Unlike plants, humans have organs which are specialised for the removal of certain excretory products
They include the lungs and kidneys
The Need for Excretion
Excretion is the removal of the waste substances of metabolic reactions (the chemical reactions that take place inside cells), toxic materials and substances in excess of requirements
Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can lower the pH of cells
This can reduce the activity of enzymes in the body which are essential for controlling the rate of metabolic reactions
For this reason, too much carbon dioxide in the body is toxic
Urea is also toxic to the body in higher concentrations and so must be excreted
The Role of the Liver
Many digested food molecules absorbed into the blood in the small intestine are carried to the liver for assimilation (when food molecules are converted to other molecules that the body needs)
These include amino acids, which are used to build proteins such as fibrinogen, a protein found in blood plasma that is important in blood clotting
Excess amino acids absorbed in the blood that are not needed to make proteins cannot be stored, so they are broken down in a process called deamination
Enzymes in the liver split up the amino acid molecules
The part of the molecule which contains carbon is turned into glycogen and stored
The other part, which contains nitrogen, is turned into ammonia, which is highly toxic, and so is immediately converted into urea, which is less toxic
The urea dissolves in the blood and is taken to the kidney to be excreted
A small amount is also excreted in sweat
Excretion and egestion are two terms that often get confused:
Excretion is the removal from the body of waste products of metabolic reactions, toxic substances and substances in excess of requirements
Egestion is the expulsion of undigested food waste from the anus
Structure
The urinary system in humans
The urinary system in humans
Main structures involved:
Changes in Urine
The colour and quantity of urine produced in the body can change quickly
Large quantities of urine are usually pale yellow in colour because it contains a lot of water and so the urea is less concentrated
Small quantities of urine are usually darker yellow / orange in colour because it contains little water and so the urea is more concentrated
There are various reasons why the concentration of urine will change, including:
Water intake – the more fluids drunk, the more water will be removed from the body and so a large quantity of pale yellow, dilute urine will be produced
Temperature – the higher the temperature the more water is lost in sweat and so less will appear in urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced
Exercise – the more exercise done, the more water is lost in sweat and so less will appear in urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced
The Kidney
The kidneys are located in the back of the abdomen and have two important functions in the body:
they regulate the water content in the blood
they excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)
Each kidney contains around a million tiny structures called nephrons, also known as kidney tubules
The nephrons start in the cortex of the kidney, loop down into the medulla and back up to the cortex
The contents of the nephrons drain into the innermost part of the kidney and the urine collects there before it flows into the ureter to be carried to the bladder for storage
The Nephron
Ultrafiltration
Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman’s capsule
The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them (which is already at high pressure because it is coming directly from the aorta)
This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate
This process is known as ultrafiltration
The substances forced out of the capillaries are: glucose, water, urea, salts
Some of these are useful and will be reabsorbed back into the blood further down the nephron
Selective Reabsorption
Reabsorption of Glucose
After the glomerular filtrate enters the Bowman’s Capsule, glucose is the first substance to be reabsorbed at the proximal (first) convoluted tubule
This takes place by active transport
The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules
Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that facilitate the active transport of glucose are only found in the proximal convoluted tubule
In a person with a normal blood glucose level, there are enough gates present to remove all of the glucose from the filtrate back into the blood
People with diabetes cannot control their blood glucose levels and they are often very high, meaning that not all of the glucose filtered out can be reabsorbed into the blood in the proximal convoluted tubule
As there is nowhere else for the glucose to be reabsorbed, it continues in the filtrate and ends up in urine
This is why one of the first tests a doctor may do to check if someone is diabetic is to test their urine for the presence of glucose
- Selective Reabsorption
Reabsorption of Water & Salts
As the filtrate drips through the Loop of Henle necessary salts are reabsorbed back into the blood by diffusion
As the salts are reabsorbed back into the blood, water follows by osmosis
Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time
Causes, Consequences, Treatments (kidneys might not work properly)
The kidneys might not work properly for several reasons, including accidents or disease
Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body which will be fatal if not removed
Kidney Dialysis
The usual treatment for someone with kidney failure is dialysis
This is an artificial method of filtering the blood to remove toxins and excess substances
Patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and salt balance of the blood
Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then returned to a vein in the arm
Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane – the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists
Dialysis fluid contains:
a glucose concentration similar to a normal level in blood
a concentration of salts similar to a normal level in blood
no urea
As the dialysis fluid has no urea in it, there is a large concentration gradient – meaning that urea diffuses across the partially permeable membrane, from the blood to the dialysis fluid
As the dialysis fluid contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient exists
As the dialysis fluid contains a salt concentration similar to the ideal blood concentration, movement of salts across the membrane only occurs where there is an imbalance (if the blood is too low in salts, they will diffuse into the blood; if the blood is too high in salts, they will diffuse out of the blood)
The fluid in the machine is continually refreshed so that concentration gradients are maintained between the dialysis fluids and the blood
Dialysis may take 3-4 hours to complete and needs to be done several times a week to prevent damage to the body from the buildup of toxic substances in the blood
An anticoagulant is added to blood before it runs through the machine to prevent the blood from clotting and slowing the flow
