Excretion & Waste Control

Introduction to Excretion & Waste Control

• Every metabolic process in the human body uses energy and generates wastes.

• If these waste products are not removed, they would quickly accumulate in harmful proportions.

• In fact, some wastes are poisonous and pose a serious threat to health if they are not removed promptly.

• The process of getting rid of metabolic wastes is called excretion.

• There are many organs and systems responsible for excretion. For example, the skin removes salts and nitrogen-based wastes, the lungs remove carbon dioxide and the liver converts poisons and toxins to a form that can be eliminated by the kidneys.

• Cells produce water and carbon dioxide as by-products of metabolic breakdown of sugars, fats, and proteins.

• Chemical groups such as nitrogen, sulfur, and phosphorous must be stripped, from the large molecules to which they were formerly attached, as part of preparing them for energy conversion.

• The continuous production of metabolic wastes establishes a steep concentration gradient across the plasma membrane, causing wastes to diffuse out of cells and into the extracellular fluid.

• Multicellular organisms such as humans must have a specialized organ system to concentrate and remove wastes from the interstitial fluid and eventually from the body. The process of getting rid of metabolic wastes is called excretion.

• Excretion aids homeostasis by removing metabolic wastes and regulating the salt and water balance. The blood picks up carbon dioxide, excess salts, nitrogen compounds, and any excess water that may be present in the interstitial fluid and carries them to the excretory organs, which channel the wastes out of the body.

• Therefore, the excretory system regulates volume of internal body fluids as well as eliminates metabolic wastes from the internal environment.

• Fortunately, atoms and molecules do not wear out, but may be changed or rearranged and used over and over again. Many of the end products of various cell activities can be recycled and used in other processes. As a result, the amount of waste that actually needs to be discharged from the body is very small in relation to the amount of work done by all the cells of the human body.

• Several organs take part in human excretion. The skin, lungs, liver, and the transport system play very important roles. However, the main excretory organs are the kidneys.

Waste Product

How It’s Produced Excretory Organ

AmmoniaBreakdown of amino acids

in the liverLiver

UreaConversion of ammonia in

the liverKidneys, Skin

Carbon Dioxide

Cellular Respiration (breakdown of glucose in

cells)

Lungs, Intestines,

Skin

WaterCellular Respiration

(breakdown of glucose in cells)

Kidneys, Lungs,

Intestines, Skin

Minerals, Salts

Minerals, Salts Kidneys, Skin

Uric Acid Breakdown of DNA Large Intestine

Skin & Homeostasis• The skin has many functions in the body. These include protection, regulation of body temperature, sensory reception, water balance, excretion, synthesis of vitamins and hormones, and absorption of materials. The skin's primary functions are to serve as a barrier to the entry of microbes and viruses, and to prevent water and extracellular fluid loss.

• Your skin has approximately 2 million sweat glands.

• These tiny, coiled tubules are found in the dermis layer of skin, below the surface layer or epidermis.

• These glands secrete sweat continuously, even when you are not exercising.

• Sweat consists primarily of water, along with salts, and some urea (nitrogen-based waste).

• These materials pass from the blood vessels in the skin into the sweat glands.

• The wastes travel up the tubules and onto the surface of the skin through tiny openings called pores.

• The wastes form perspiration on the skin, which eventually evaporates.

• The solid wastes in the perspiration remain on the surface of the skin or on clothing.

• Perspiration itself has no odour. However, bacteria living on the skin decompose the urea in perspiration, producing new compounds that do have odours.

Heat & Cold Receptors• Heat and cold receptors are also located in the skin.

• When the body temperature rises, the hypothalamus, the region of the brain responsible for coordinating many nerve and hormone functions, sends a nerve signal to the sweat-producing skin glands, causing them to release about 1-2 liters of water per hour, cooling the body through evaporation.

Thermoregulation

• The hypothalamus also causes dilation of the blood vessels of the skin, allowing more blood to flow into those areas, causing heat to be dissipated from the skin surface.

• When body temperature falls, the sweat glands constrict and sweat production decreases.

• If the body temperature continues to fall, the hypothalamus will trigger the body to engage in thermiogenesis, or heat generation, by raising the body's metabolic rate and by shivering.

• This is an excellent example of negative feedback mechanisms in action.

Role of Lungs in Excretion• Carbon dioxide and water are waste products produced by all cells during cellular respiration. The blood carries these to the lungs. Carbon dioxide diffuses into the alveoli and is removed from the body when we exhale. Some water also leaves the lungs as water vapour.

• The lungs can also eliminate alcohol, which, since it is made up of small molecules, can be rapidly passed from the bloodstream into the alveoli of the lungs and exhaled. This is why the breathalyzer test is an effective and simple way of determining the amount of alcohol that people have taken into their systems.

Role of Large Intestine in Excretion• As you have learned, the process of digestion is pretty much complete when it reaches the large intestine.

• The food has been broken down into its smallest parts, and the nutrients have been absorbed.

• The final job of the large intestine is to absorb water and to eliminate waste.

The Role of the Liver in Excretion• Although, we obtain most of our energy from carbohydrates and fats, we sometimes use proteins for energy.

• Some proteins and other nitrogenous compounds are broken down in the liver by a process called deamination.

• The extraction of energy from proteins produces ammonia (NH3), a highly toxic substance.

• The liver converts ammonia to a non-toxic substance called urea by combining it with carbon dioxide. The blood then delivers urea to the kidneys, where it is excreted.

NH3 + CO2   UREA

• The liver also breaks down red blood cells. About 120 days after they are formed, erythrocytes are destroyed by special cells in the liver.

• These cells must be replaced if homeostasis is to be maintained.

• These cells engulf the red blood cells and break down their hemoglobin.

• Iron released from the hemoglobin is carried by the blood to various parts of the body.

• In the bone marrow the iron is used in the manufacture of new hemoglobin.

• Another part of the broken down hemoglobin is the yellow pigment bilirubin.

• This pigment enters the bile, passes into the small intestine, and is eventually removed from the body with the feces.

• The liver is also responsible for removing potentially hazardous chemicals from the blood. It "detoxifies" the blood.

• For this reason, alcoholics and other types of drug addicts have a higher incidence of liver disease.

• The liver breaks down harmful and foreign substances, such as many ingested poisons and drugs after they are absorbed by the digestive system and before they reach the rest of the body.

Since the liver has the enzymes to break down the natural materials, it is also able to destroy certain drugs.

• This function can be life-saving, because it makes destructive substances harmless, and prevents hormones, medications, and other chemicals from accumulating to harmful levels.

• Cortisone and sulfur-containing medicines are examples of drugs that the liver breaks down.

• The liver's ability to make substances harmless has limits. Materials can accumulate in the liver and destroy its tissues.

• For example, alcohol taken in excess over a period of time can deteriorate the liver. Pollutants, such as PCBs (polychlorinated biphenyls) and pesticides, such as DDT, can also harm the liver.

The Urinary System: The Kidneys• The kidneys are the major organs of excretion in the human body. They form part of the urinary system and function primarily in filtering wastes from the blood.

• The kidneys also play a key role in fluid balance in the body.

Kidneys

• The kidneys remove metabolic wastes from the body by filtering the blood.

• The entire blood volume is filtered through the kidneys about 65 times per day by way of the renal arteries.

• After the blood is cleaned, it flows back into the body through the renal veins.

• The kidneys work continuously, processing all of the blood in the body every 20 minutes.

• At this rate, every 24 hours the kidneys filter approximately 180 litres of blood. Of these 180 litres, only 1 to 2 litres of fluid are excreted as urine.

• The human urinary system consists of two kidneys, two ureters, the urinary bladder, and the urethra.

• Normally, there are two kidneys, one on either side of the spine under the lower ribs.

• The ureters lead from the kidneys to the urinary bladder, which lies low in the abdominal cavity.

• The urethra extends from the bladder to the exterior of the body, carrying urine through the urogenital organs, either the penis or vagina.

Importance of KidneysYour kidneys are important because they do these essential things:

1. Regulate water: For your body to work properly, it must contain just the right amount of  water. One of the important jobs of the kidneys is to remove excess water  from the body or to retain water when the body needs more.

2. Remove wastes: Many of the substances in the blood and body fluid must be kept at the  correct level for the body to function properly. For example, sodium and  potassium are minerals which come from food. These minerals are  needed by the body for good health, but they must be kept at specific  levels. When the kidneys are working properly, excess minerals, such as  sodium and potassium, are excreted from the body in the urine. The  kidneys also help to regulate the levels of other minerals, such as calcium  and phosphate, which are important for the formation of bone.

• Wastes, such as urea and creatinine, must also be removed from the body.  Urea and other wastes are made when the body breaks down protein, such as  meat. Creatinine is a waste product of the muscles. As kidney function  decreases, the levels of urea and creatinine in the blood increase. Many  waste products are toxic (poisonous) if they are not removed from the body  fluids. For example, when certain drugs are taken, chemical wastes are  produced which must be removed from the body by the kidneys.

• 3. Produce hormones: Normal kidneys also make important chemical messengers called  hormones. These hormones circulate in the bloodstream and regulate  some body functions such as blood pressure, the making of red blood  cells, and the uptake of calcium from the intestine.

Structure of the Kidney

Kidneys are reddish brown in colour and shaped like kidney beans. Each kidney is about the size of your clenched fist. If you were to cut a kidney in half, you would see the following parts:

Renal Capsule -  a thin, outer membrane that helps protect the kidney

Cortex -  a lightly colored outer region;  where blood is filtered

Medulla -  a darker, reddish-brown,  inner region; controls the   amount of salt and water in your urine.

Renal Pelvis -  a flat, funnel shaped cavity  that  collects the urine into  the ureters.

Renal Artery -  supplies blood to the   kidney from the left-side of  the heart

Renal Vein -  takes the blood back to the  right side of the heart

Nephron -  basic functional and   structural units of the kidney

Ureters -  funnels urine from the   kidney to the bladder.

Bladder - where the urine is held

Urethra - funnels urine from the   bladder to out of the body.

Urinary Sphincters  muscles to control the flow of urine.

The Urinary System: The Nephron• The working units of the kidney are microscopic structures called nephrons.

• Each kidney contains about one million nephrons.

• A nephron contains a network of capillaries called the glomerulus, which filters blood into a cup-shaped structure known as the Bowman's capsule.

• The filtrate then passes into a series of tubules.

Kidney & Nephron Overview

• Water and waste products are separated from the blood by the filtering process and the flow into and out of the tubules. Much of the water is reabsorbed by the tubules and the wastes are concentrated into urine.

• The main parts of the nephron are as follows:

Glomerulus - coiled capillaries inside the Bowman's capsule.

Bowman's (Glomerular) capsule - closed end at the beginning of the nephron. It is located in the cortex.

Proximal convoluted tubule - first twisted region after the Bowman's capsule. It is also in the cortex.

• Loop of Henle - long, loop after the proximal tubule. It extends from the cortex down into the medulla and back.

• Distal convoluted tubule - second twisted portion of the nephron after the loop of Henle. It is also in the cortex.

• Collecting duct - long straight portion after the distal tubule that is the open end of the nephron. It extends from the cortex down through the medulla.

• The nephron also has a unique and rich blood supply compared to other organs. They are listed below.

• Renal artery -  supplies blood to the kidney from the circulatory system

• Renal vein  -  returns blood from the kidney to the circulatory system

• Afferent arteriole -  connects the renal artery with the glomerular capillaries.

• Efferent arteriole -  connects the glomerular capillaries with the  peritubular capillaries.

• Peritubular capillaries - located after the glomerular capillaries and surrounding the proximal tubule, loop of Henle, and distal tubule.

• Many of the substances in the blood and body fluids must be kept at the correct level for the body to function properly. Regulating the composition of these fluids involves the following:

Keeping the concentrations of various ions and other important substances constant.

Keeping the volume of water in your body constant. Removing wastes from your body. Keeping the acid/base concentration of your blood constant.

The job of clearing the blood of wastes in the nephrons occurs in three stages:

• Filtration -  the passage of substances through the capillaries of the  glomerulus into Bowman's capsule.

• Reabsorption -  the transfer of essential solutes and water from the  nephron back into the blood

• Secretion -  the movement of materials from the blood back into the   nephron.

• Anything (fluid, ions, small molecules) that has not been reabsorbed from the nephron gets swept away to form the urine, which ultimately leaves the body.

• Through these processes, the blood is maintained with the proper composition, and excess or unwanted substances are removed from the blood into the urine.

Filtration

• The blood supply to the kidneys is carried by the large renal arteries (right and left).

• These large vessels divide into many smaller vessels as soon as they enter the kidneys.

• The result of the decrease in diameter of the artery is to produce an increase in the pressure of the blood.

• Think of the water pressure in a fire hose compared to a garden hose when using the same amount of water.

•This increased blood pressure helps to force the following components of the blood out of the glomerular capillaries:

Most of the waterMost/all of the salts Most/all of the glucose Most/all of the urea

• In addition, the efferent arteriole leaving the Bowman's capsule is considerably smaller than the afferent arteriole which delivers blood to Bowman's capsule.

• This increases the blood pressure within the glomerulus.

• In the kidney, blood pressure in the capillaries is 60 to 70 mm Hg, whereas elsewhere in the body capillary blood pressure is only 25 mm Hg or less.

• As a result of this high pressure in the glomerulus, about 20% of the blood plasma entering the kidneys is passed rapidly and easily into Bowman's capsule, creating a filtering effect.

• The substances that pass from the glomerulus into Bowman’s capsule comprise what is called the filtrate. Blood cells and large protein molecules are too large to pass through the tiny pores in the capsule capillaries.

• However, if blood is found in urine then there must have been some damage sustained to the glomerulus.

• Other substances such as, salts, sugars, water, and wastes are composed of smaller molecules and can pass easily through the pores to enter the capsule.

• About 80% of the plasma component of the blood entering the kidney is left behind and does not become part of the filtrate, but re-enters the bloodstream.

• Reabsorption

• The filtrate that enters Bowman's capsule contains metabolic waste products and other materials not needed by the body. It also however, contains salts, water, amino acids, and other useful substances which must be retained and recycled back into the blood.

• Sometimes it is necessary to move these molecules from an area of low concentration to an area of high concentration. Such movement is known as active transport.

• It requires energy and the presence of special carrier molecules, called transporters, which temporarily associate with the substance to help it pass through a membrane.

• Transporters are located on the membranes of the various cells of the nephron. These transporters grab the small molecules from the filtrate as it flows by them.

• As the filtrate passes through the proximal tubule of the nephron, glucose, amino acids, and some salts are reabsorbed back out of the tubules into the bloodstream by transporters.

• Each transporter grabs only one or two types of molecules. For example, glucose is reabsorbed by a transporter that also grabs sodium.

• Transporters are concentrated in different parts of the nephron. For example, most of the Na transporters are located in the proximal tubule, while fewer ones are spread out through other segments.

• Some transporters require energy, usually in the form of adenosine triphosphate (active transport), while others don't (passive transport).

• The efferent arteriole, which leaves the glomerulus, branches out into a network of capillaries called the peritubular capillaries which surround the tubules.

• It is this bed of capillaries that is constantly reabsorbing the needed products from the filtrate and enabling them to re-enter the bloodstream.

• The waste substances remain in the tubules, becoming gradually more concentrated until they are formed into urine.

•The cells of the proximal tubule are filled with mitochondria which produce energy for active transport to take place. The cells of the proximal tubules also contain microvilli which increase the efficiency of absorption by increasing the surface area of the tubule.

• The reabsorption process is similar to the "fish pond" game that you see in some amusement parks or state fairs. In these games, there is a stream that contains different colored plastic fish with magnets

STREAM = NEPHRON

FISH = FILTERED MOLECULES

CHILDREN = TRANSPORTERS

• Furthermore, each child is fishing for a specific colored fish. Most children start at the beginning of the stream and some spread out further downstream. By the end of the stream, most of the fish have been caught.

• This is what happens as the filtrate travels through the nephron.

• Two major factors affect the reabsorption process:

• 1) Concentration of small molecules in the filtrate - the higher the concentration, the more molecules can be reabsorbed. Like our children in the fish pond game, if you increase the number of fish in the stream, the children will have an easier time catching them.

• In the kidney this is only true to a degree because there is only a fixed number of transporters for a given molecule present in the nephron.

• There is also a limit to how many molecules the transporters can grab.

• 2) Rate of flow of the filtrate - flow rate affects the time available for the transporters to reabsorb molecules. As with our fish pond, if the stream moves by slowly, the children will have more time to catch fish than if the stream were moving faster.

• As salts (and ions) are forced out of the proximal tubule, the concentration of the filtrate changes.

• Eventually, there are more salts outside the tubule and more water inside the tubules.

• This establishes the condition under which normal diffusion takes place; water then flows out of the tubules by osmosis and is reabsorbed into the capillaries.

• Water continues to diffuse out of the top portion of the descending Loop of Henle. However, water does not pass out of the remainder of Henle's loop.

• Sodium ions (Na+) are moved out of the ascending portion of the loop by active transport, into the intercellular fluid.

• The walls of the ascending loop of Henle are impermeable to water, so no water leaves the loop as it would if the membrane did not block it.

• Sodium ions diffuse into the descending loop and increase the concentration of the filtrate there, so that a point is reached when the concentration of dissolved materials in the filtrate is equal to or greater than the concentration of the blood in the nearby capillaries and water ceases to diffuse out.

Kidneys & Water Balance• As mentioned earlier in the lesson, for your body to work properly, it must contain just the right amount of water.

• One of the important jobs of the kidneys is to remove excess water from the body or to retain water when the body needs more.

• Water conservation by the kidney is controlled by a hormone called antidiuretic hormone or ADH.

• ADH is produced in the pituitary gland in the brain. Low water levels in the blood signals the pituitary to release more ADH.

• The blood carries the ADH to the kidneys and increases the permeability of the tubule cell membranes to water causing more water to be reabsorbed from the tubule into the blood, producing more concentrated urine.

• If, on the other hand, a person has too much water, less ADH is released.

• Then the cell membranes of the tubules become impermeable to water.

• As a result, they do not allow water to return to the blood and the urine is very dilute.

Kidneys & Blood Pressure

• The kidneys play a role in the regulation of blood pressure by adjusting for blood volumes.

• A hormone called aldosterone acts on the nephrons to increase sodium ion reabsorption.

• This hormone is produced in the cortex region of the adrenal glands that lie on the upper surface of the kidneys.

• As Na+ reabsorption increases, the osmotic gradient increases and more water moves out of the nephron by osmosis.

• Low blood pressure through the body (resulting perhaps from loss of blood through a wound or loss of water and salts in diarrhea) is reflected in a lowering of the blood pressure in each glomerulus of the kidneys.

• This decreases the amount of plasma that leaves the glomerulus and enters the Bowman's capsule.

• Accordingly, this lowered blood pressure decreases the amount of water and salts filtered into the kidney tubules, helping to conserve them.

• High blood pressure results in an increase in the volume of filtrate in the glomeruli (plural form of glomerulus) and the excretion of more water and salts from the body.

• However, the activities of the tubule cells increase in an attempt to cause more water and salts to be reabsorbed into the bloodstream.

• While we think of the kidney as an organ of excretion, it is more than that. It does remove wastes, but it also removes normal components of the blood that are present in greater-than-normal concentrations.

• When excess water, sodium ions, calcium ions, etc. are present, the excess quickly passes out in the urine. On the other hand, the kidneys can step up their reclamation of these same substances when they are present in the blood in less-than-normal amounts.

• Thus the kidney continuously regulates the chemical composition of the blood within narrow limits. The kidney is one of the major homeostatic devices of the body.

Urinalysis•Why do we give urine samples when getting a physical examination at the doctors office?

•What kind of things can doctors find with a urine test?

•Why do athletes get their urine tested?

• How is it Used?

• The urinalysis is used as a screening and/or diagnostic tool because it can help detect substances or cellular material in the urine associated with different metabolic and kidney disorders.

• It is ordered widely and routinely to detect any abnormalities that require follow up.

• Often, substances such as protein or glucose will begin to appear in the urine before patients are aware that they may have a problem.

• It is used to detect urinary tract infections (UTI) and other disorders of the urinary tract. In patients with acute or chronic conditions, such as kidney disease, the urinalysis may be ordered at intervals as a rapid method to help monitor organ function, status, and response to treatment.

When is it ordered?

• A routine urinalysis may be done when you are admitted to the hospital. It may also be part of a wellness exam, a new pregnancy evaluation, or a work-up for a planned surgery. A urinalysis will most likely be performed when you see your health care provider complaining of symptoms of a UTI or other urinary system problem such as kidney disease. Some signs and symptoms may include: abdominal pain back pain painful or frequent urination blood in the urine

• This test can also be useful when monitoring certain conditions over time.

Kidney FailureWhat causes kidney failure?

• Kidney failure can occur from an acute situation or from chronic problems.

• In acute renal failure, kidney function is lost rapidly and can occur from a variety of insults to the body. The list of causes is often categorized based on where the injury has occurred.

• Prerenal causes (pre=before + renal=kidney) causes are due to decreased blood supply to the kidney.

• Renal causes (damage directly to the kidney itself) including sepsis, medications, Rhabdomyolysis

• Post renal causes (post=after + renal= kidney) are due to factors that affect outflow of the urine.

Obstruction of the bladder or the ureters can cause back pressure when there is no place for the urine to go as the kidneys continue to work. When the pressure increases enough, the kidneys shut down.

• Chronic renal failure develops over months and years. The most common causes of chronic renal failure are related to: Poorly controlled diabetes Poorly controlled high blood pressure Chronic glomerulonephritis

• Less common causes: Polycystic Kidney Disease Reflux nephropathy Kidney stones Prostate disease

Treatment

•Dialysis: The kidneys are responsible for filtering waste products from the blood. Dialysis is a procedure that is a substitute for many of the normal duties of the kidneys. Dialysis can allow individuals to live productive and useful lives, even though their kidneys no longer work adequately. Patients usually require dialysis when the waste products in their body become so high that they start to become sick from them.

• The level of the waste products usually builds up slowly. Doctors measure several blood chemical levels to help decide when dialysis is necessary. The two major blood chemical levels that are measured are the "creatinine level" and the "blood urea nitrogen"(BUN) level. As these two levels rise, they are indicators of the decreasing ability of the kidneys to cleanse the body of waste products.

•Organ Transplant: Kidney transplantation means replacement of the failed kidneys with a working kidney from another person, called a donor. Kidney transplantation is not a complete cure, although many people who receive a kidney transplant are able to live much as they did before their kidneys failed. People who receive a transplant must take medication and be monitored by a physician who specializes in kidney disease for the rest of their lives.

• The National Kidney Foundation estimates that about 350,000 people in the United States have end-stage renal disease and about 67,000 people die of kidney failure every year.

• In the year 2000, nearly 47,000 people in the United States were waiting for a kidney transplant.

• Because of a shortage of donor kidneys, each year only a small percentage of people who need a transplant actually receive a kidney. The wait for a donor kidney can take years.

• Among the 36,638 patients on renal replacement therapy in 2008: 14,884 (41%) had a functioning transplant 21,754 (59%) were on dialysis  

• In 2008, 215 Canadians died waiting for a transplant!

Donor Cards• The third card many drivers carry with their licence is the Donor Card. By signing a donor card, you can indicate your wish to donate organs or tissue for transplant, research or education.

• Donor cards are available wherever you renew your driver’s licence or Autopac policy. You can also download a donor card.