Osmoregulation & The Kidney - The Kidneys and Excretion (GCSE Biology)
The Kidneys and Excretion
Excretion of Water
Importance of Water Excretion
Water leaves the body in excretion. Remember, excretion is the removal of waste products from the body and substances that are in excess.
You might find this odd – how is water a waste product? Well, we want to keep a good level of water, so its important to take in enough water (e.g. by drinking) and remove enough water (e.g. by urinating). If we have too much or too little water, there can be negative effects on the body, such as crenation or haemolysis of RBCs.
Examples of Water Excretion
There are a few examples of water excretion you need to know about:
Water leaves the body though the lungs, skin and urine.
- Lungs – When we breathe out, we give out water and also excrete carbon dioxide from the lungs which is important to prevent toxicity. This is uncontrollable. No matter how much breathe in or out, you cannot control how much water vapour or CO2 is lost.
- Sweat – Water is and ions are lost through sweat, as we have discussed before. Sweating can actually happen even when water levels are low – e.g. during exercise sweating is used as a cooling mechanism, but it can leave you low on water levels (hence the importance of drinking water during exercise). During sweating we also lose urea, which is another waste product. As with the lungs, you cannot control the loss of water and mineral ions in sweating.
- Urine – Urine is the main source of water excretion. Our kidneys filter the urine to control the amount of water lost. If we are very short on water, the kidneys will allow less water to leave in urine, and vice versa. Urea is also lost in urine to prevent toxicity. With urine, you can partly control the loss of water because urinating is a voluntary action. However, you cannot prevent it occurring altogether, you can just control the timing.
Excretion of Proteins
One of the hallmarks of proteins is that they contain nitrogen. This means that when they are digested, there is an excess of amino acids formed.
When proteins are digested, they are broken down to amino acids by protease enzymes. These amino acids can be used to build useful things like building muscle mass. The liver also has a role in the assimilation of amino acids by converting them to proteins, including plasma proteins, e.g. fibrinogen. However, excess amino acids need to be excreted:
- Excess amino acids cannot be stored. Amino acids which are left over need to be removed from the body because the body cannot store proteins or amino acids (like it can store glucose or fats).
- The liver deaminates the amino acids. The amino acids are deaminated in the liver. This means that the amine group (nitrogen and hydrogen atoms) are lost. This forms ammonia, a toxic compound which needs to be removed.
- The ammonia is converted to urea. The ammonia is toxic so can’t hang around. It is converted into urea, a compound that is safe for excretion.
- The urea is excreted. This urea is then transported to the kidneys. The kidneys are a site of excretion in the body. Here, the urea is passed into the urine and prepares to leave the body. Urine consists of water, urine and mineral ions. Some urea is also lost in sweat, as we learnt earlier.
Excretion in the Kidneys
Function of the Kidneys
The three main function of the kidneys include:
- Controlling ion levels. Kidneys decide the amount of ions (e.g. sodium) we excrete in urine.
- Excreting urea. Kidneys remove poisonous urea from the body in urine. This urea is a product of the breakdown of proteins by the liver.
- Water control. The kidneys control how much water is lost in urine and how much is retained in the blood.
Structure of the Urinary System
- The urinary system includes the kidneys, ureters, bladder and urethra.
- The blood supply to and from the kidneys is by the renal arteries and renal veins.
- The kidney has different parts. The outer part is the cortex and the inner part is the medulla.
Production of Urine: Step-by-Step Process
Blood that passes through the kidneys is filtered to produce urine. The kidney has many structures called nephrons. These nephrons are used in the filtration of blood and in the reabsorption process. Here is the step by step process:
- Blood is filtered in the kidneys. Blood enters the renal capillaries at
a high pressure. The high pressure means that the blood can be filtered by the glomerulus of the nephrons, which has small holes to allow molecules through.
Interpreting Data
Often exams can give you tables with the concentrations of glucose, ions and urea before and after filtration. You may have to make predictions:
- Water is always more concentrated in the urine than in the plasma. This is because lots of water is filtered through into the nephron, and only some is reabsorbed.
- Ions are usually more concentrated in the urine than in the plasma. Like water, lots of ions are filtered and only some reabsorbed.
- Glucose should not be present in the urine. If it is present, something is wrong and it could be a sign of diabetes mellitus.
- Urea should have a higher concentration in the urine than in the plasma. No urea is reabsorbed in the nephron.
- Proteins should have a concentration of 0 in the urine. Proteins are normally not present in the urine.
FAQs
Osmoregulation is the process by which organisms regulate the balance of water and salt in their bodies to maintain proper fluid balance and prevent dehydration or overhydration.
The kidney plays a critical role in osmoregulation by filtering waste products from the blood and regulating the balance of water and salt in the body. The kidney can retain water and salt when necessary, or excrete them when they are in excess.
The kidney filters waste products from the blood through a process called filtration. Blood enters the kidney through the renal artery and is filtered through tiny structures called nephrons. The waste products are then collected in the renal pelvis and excreted from the body as urine.
Osmoregulation is the process by which organisms regulate the balance of water and salt in their bodies to maintain proper fluid balance. Excretion is the process by which waste products are removed from the body. Although both processes are important for maintaining the health of an organism, they serve different functions.
The urinary system, also known as the renal system, consists of several organs that work together to produce, store, and excrete urine from the body. The main structures of the urinary system include:
Kidneys: The kidneys are two bean-shaped organs located on either side of the spine, in the upper abdomen. They are responsible for filtering waste products and excess water from the blood to form urine.
Ureters: The ureters are two narrow tubes that transport urine from the kidneys to the bladder.
Bladder: The bladder is a muscular sac that stores urine until it is ready to be excreted from the body.
Urethra: The urethra is a tube that carries urine from the bladder to the outside of the body during urination.
The urinary system is responsible for regulating the water balance and electrolyte levels in the body, as well as eliminating waste products and excess substances such as urea, creatinine, and uric acid. Dysfunction of the urinary system can lead to a variety of health problems, including infections, kidney stones, and kidney disease.
The kidneys have several important functions in the body, but here are the main three function of the kidneys:
Filtration: The kidneys filter waste products, excess fluids, and toxins from the blood and produce urine, which is transported to the bladder for excretion.
Regulation of electrolyte balance: The kidneys play a critical role in maintaining the balance of electrolytes such as sodium, potassium, and calcium in the body. They also regulate the acid-base balance of the blood to ensure that it remains within a narrow, healthy range.
Hormone production: The kidneys produce several hormones that are important for various bodily functions. For example, they produce erythropoietin, which stimulates the production of red blood cells, and renin, which helps to regulate blood pressure.
Overall, the kidneys are vital organs that play a crucial role in maintaining the overall health and wellbeing of the body. Dysfunction of the kidneys can lead to a variety of health problems, including kidney disease, electrolyte imbalances, and high blood pressure.
The kidneys are responsible for filtering waste products and excess water from the blood to form urine. The urine is then transported to the bladder for storage and subsequent excretion through the urethra. Along with urine, the kidneys secrete several important substances, including:
Hormones: The kidneys secrete several hormones, including erythropoietin (EPO), which stimulates the production of red blood cells in the bone marrow, and renin, which regulates blood pressure.
Enzymes: The kidneys secrete the enzyme renin, which helps to regulate blood pressure by controlling the production of the hormone angiotensin II.
Electrolytes: The kidneys regulate the balance of electrolytes in the body, including sodium, potassium, calcium, and magnesium. They can also secrete excess electrolytes into the urine if necessary.
Metabolites: The kidneys secrete metabolites such as urea, creatinine, and uric acid, which are waste products of metabolism that must be eliminated from the body.
Yes, water is excreted by the kidneys. The kidneys play a crucial role in regulating the water balance in the body by filtering excess water from the blood and excreting it in the form of urine. The amount of water excreted by the kidneys is regulated by the antidiuretic hormone (ADH), which is produced by the hypothalamus and released by the pituitary gland. ADH acts on the kidneys to increase the reabsorption of water, reducing the amount of water excreted in the urine. This helps to maintain the water balance in the body and prevent dehydration or overhydration.
Yes, the kidney secretes protein, but under normal circumstances, the amount is very small. The glomerulus, a network of tiny blood vessels in the kidney, filters waste products and excess water from the blood, but it also serves as a barrier to prevent proteins and other large molecules from being filtered out. This is because proteins are essential molecules that are needed in the blood to maintain various bodily functions, and their loss in the urine can be a sign of kidney damage or disease.
However, in some cases, the kidneys can secrete small amounts of protein into the urine due to damage or dysfunction of the glomerulus or the tubules of the kidney. This condition is known as proteinuria and can be a sign of kidney disease, infection, or other underlying health conditions. Proteinuria can be detected through a urine test and requires further evaluation and management by a healthcare professional.
An imbalance in water and salt levels in the body can lead to various health problems, such as dehydration or overhydration. The symptoms of dehydration include thirst, dry mouth, and fatigue, while the symptoms of overhydration include swelling, nausea, and headache.
The kidney regulates the balance of water and salt in the body by selectively retaining or excreting water and salt based on the body’s needs. It uses special structures called distal tubules and collecting ducts to control the reabsorption of water and salt.
Osmoregulation is important for the body because it helps maintain proper fluid balance, which is critical for the health and function of cells, tissues, and organs. An imbalance in water and salt levels can lead to various health problems, so it’s essential to maintain proper osmoregulation.
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