Excretion and Homeostasis Notes

Excretion and Homeostasis

CSEC Objectives

• Distinguish between egestion and excretion.
• Discuss the importance of excretion in living organisms.
• State how metabolic wastes are excreted from plants and animals.
• Relate the kidney to its osmoregulatory and excretory functions.

Homeostasis

• Homeostasis is maintaining a constant internal environment.
• Even during sleep, the body uses energy to maintain a constant internal environment.
• Automatic control systems maintain constant temperature, water levels, ion concentrations, and blood sugar levels.
• Homeostasis allows the body’s cells to work at their optimum.
• Homeostasis is the process by which organisms maintain a constant internal environment despite external fluctuations.
• Body temperature should be $37$ °C in both cold and warm environments.

Examples of Homeostasis

• Oxygen and carbon dioxide concentrations are maintained in balance due to chemo detectors.
• Blood glucose is maintained due to hormones.
• Body temperature is maintained.
• Water balance is maintained.
• Blood pH is maintained within narrow limits around $7.4$.

Excretion

• Biological processes produce metabolic reactions that leave waste products.
• Excretory products are harmful by-products of metabolic or chemical reactions.
• Some byproducts become harmful if they accumulate beyond a specific concentration.
• Some excretory substances are toxic and must be removed.
• Living organisms must get rid of excretory products to ensure homeostatic balance.
• Excretion is the removal or elimination of metabolic waste products and excess substances produced by cells from the body.

Human Waste Products

• Humans produce a variety of wastes that need to be excreted, including:
  • Carbon dioxide
  • Lactic acid
  • Bile/Bilirubin
  • Urea
  • Cholesterol
  • Water

Carbon Dioxide

• Carbon dioxide is a metabolic waste product of respiration.
• $CO_2$ is toxic in large amounts.
• It lowers the pH because $CO_2$ forms a weak acid, carbonic acid, when dissolved in water.
• Enzymes are pH sensitive, so this gas has to be removed.
• In simple animals like amoeba, carbon dioxide is lost by diffusion through the cell membrane.
• In more complex organisms, it is removed via the tracheae in insects, gills in fish, and lungs in humans.

Lactic Acid

• During strenuous exercise, oxygen becomes limited, and muscle cells convert glucose to energy by anaerobic respiration.
• Muscle cells can continue anaerobic energy production at high rates for one to three minutes, during which lactic acid can accumulate to high levels.
• High lactic acid levels increase the acidity of muscle cells and disrupt other metabolites due to its effect on enzymes, causing muscle fatigue and post-exercise muscle soreness.
• The accumulated lactic acid is removed by oxidation during continued heavy breathing to obtain oxygen from the lung.

Bile, Urea, and Bilirubin

• The liver excretes bile, which contains bile pigments produced by the breakdown of old red blood cells (RBCs).
• Iron from haemoglobin is retained and stored to be reused.
• Excess haemoglobin is broken down by the body into excess amino acids.
• The amino acid groups ($NH<em>2$) of the excess amino acids are converted to ammonia ($NH</em>3$) and then to urea because ammonia is toxic.

Bilirubin

• Bilirubin is an orange-yellow pigment that occurs normally when part of your red blood cells break down.
• The liver takes bilirubin from your blood and changes its chemical make-up so that most of it is passed through your poop as bile.
• Higher-than-normal bilirubin levels indicate either red blood cells are breaking down at an unusual rate, the liver isn’t breaking down waste properly, or there’s a problem in the bilirubin pathway.

Water

• Water is produced as a by-product of respiration but is not usually considered a waste material.
• The concentration of water in cells must be kept within a narrow limit.
• Too much or too little water can have negative effects.
• Plant cells are protected from bursting or taking in excess water by their cell walls.
• Animal cells do not have a cell wall and will burst if they absorb too much water.
• In animals, some water is lost through the skin as sweat.
• Most of the regulation of water levels, however, takes place in the kidney.

The Kidney and Excretion

• The kidney is not the only organ of excretion, but it is the main organ.

Organ Specific Excretory Products

Excretion vs. Egestion

• Material in faeces is not metabolic waste; it was never digested or absorbed.
• Defecation is an example of egestion, not excretion.
• Egestion is the removal of undigested food.
• Organs that carry out excretion in higher animals are the skin, lungs, liver, and kidneys.
• Excretion is not the same as secretion, which is the release of substances, such as hormones, from cells.

Urea Production

• Proteins are digested into amino acids and absorbed into the blood.
• Amino acids travel to the liver through the hepatic portal vein.
• Excess amino acids get broken down into urea through deamination, which involves breaking down amino acids into two parts.
• One part is converted to carbohydrate and used in respiration for energy or stored as fat.
• The other part, ammonia that contains nitrogen, is too toxic and is converted to urea.
• Urea is a “nitrogenous excretory product.”

Urea Excretion

• Urea goes back into the blood and travels to the kidneys.
• Kidneys take urea out of the blood and mix it with water to make urine.
• Urine flows from the kidneys through 2 ureters to the bladder.
• Then it’s excreted through the urethra.

The Kidney

Two Main Functions

1. Osmoregulatory Function: maintenance of the water and salts balance in the blood
2. Excretory function: The removal of nitrogenous metabolic waste and other waste products via excretion.

• The kidneys are the main excretory and osmoregulatory organs.

Structure of the Kidney

• The two kidneys are situated inside the back of the abdominal cavity, one at each side of the vertebral column.
• The kidney is bean-shaped and has a depression on its inner side.
• On the inner side of each kidney, there are blood vessels that carry blood to and from it as well as a tube called the ureter.

Blood Supply

• The kidney receives blood (rich in waste) from the dorsal aorta via the renal artery.
• Blood (deoxygenated and purer) leaves the kidneys via the renal vein where it travels back to the heart via the posterior vena cava.

Internal Structure

• Internally, the kidney is differentiated into two regions:
  • An outer narrow dark layer called the cortex
  • An inner, lighter broad layer called the medulla.
• Each kidney contains about a million nephrons which span both the cortex and medulla.
• The medulla also contains a mass of tubules, connective tissue, and capillaries.
• The functional unit of the kidney is the nephron.
• Each kidney consists of about one million nephrons.
• Each nephron starts in the cortex with a cup-shaped, thin, double-walled Bowman's capsule.
• The nephron is where urea is filtered out of the blood.
• An intricate network of blood capillaries wraps around the nephron.

Structure of the Nephron

• The nephron starts with a cup-shaped structure called the Bowman’s Capsule.
• Then the first or proximal convoluted tubule in the cortex.
• Then to the loop of Henle in the medulla which is U-shaped.
• Then to the second or distal convoluted tubule in the cortex before joining the collecting duct.
• Many distal convoluted tubules are connected to one collecting duct.
• This duct opens in the cavity of the kidneys called the pelvis.

Urine Formation

• The pelvis drains the excretory products into the ureter, which passes the urine into a small muscular sac called the urinary bladder.
• A sphincter muscle closes the outlet of the bladder till urine accumulates, then the bladder contracts expelling the urine through a duct called the urethra.
• Urine production involves two basic processes:
  1. Ultrafiltration: Some components of the blood flowing through the kidney are pushed out of the blood vessels into kidney tubules. Only plasma and small particles can be filtered. Large proteins and blood cells stay in the blood.
  2. Selective Reabsorption: Substances useful for the body such as glucose and amino acids are reabsorbed and send back to the blood. The amount of water and salts reabsorbed is regulated to maintain homeostasis. Unwanted substances become urine and are excreted.

Ultrafiltration

• The blood supply from the kidney comes from the renal artery.
• This renal artery divides or branches into a great many arterioles and capillaries that extend into the glomerulus.
• The afferent arterioles receive blood rich in food substance and metabolic waste product from the renal artery and passes it into the glomerulus at the cup-shaped Bowman’s capsule.
• The efferent arterioles carry blood away from the glomerulus.
• The diameter of the afferent arterioles carrying blood to the glomerulus is larger in diameter than the efferent arterioles carrying blood away from it.
• There will be a high build up of pressure in the capillaries that make up the glomerulus due to the difference in diameter between the afferent and efferent arterioles.
• The glomerulus and Bowman’s capsule are permeable to small molecules and ions but impermeable to large protein molecules and red blood cells, this high pressure forces the small molecules out through the capillary walls and into the Bowman’s capsule. This is called Ultrafiltration or Pressure Filtration.
• If the blood pressure is too high, it can cause the capillaries of the blood vessels to burst and destroy the nephron resulting in kidney failure.
• The filtered fluid contains water, wastes, salts and glucose, but the blood cells and large protein molecules remains in the blood.
• This is because all but large protein molecules and blood cells can pass through the capillary wall and into the Bowman’s capsule. This is known as renal dialysis which is separation of smaller molecules by larger ones by a membrane.
• High pressure forces some of the plasma and substances dissolved in the blood into the proximal tubule. This process of filtration under pressure is called ultrafiltration.
• The filtered solution is called glomerular filtrate.

Selective Reabsorption

• The body reabsorbs all important nutrients.
• The glomerular filters go into the proximal convoluted tubule where most selective reabsorption takes place by active transport.
• Glucose, other nutrients, hormones, amino acids, salts and water (around $80$%) is reabsorbed into the plasma/blood of the capillaries wrapped around the tubules in the proximal convoluted tubule.
• Large molecules such as blood cells (erythrocytes and leucocytes) and plasma proteins are too large to pass into the Bowman’s capsule and so remain in the blood.
• Reabsorption continues in the loop of Henle where more water is removed by osmosis.
• The longer the loop of Henle, the more water is reabsorbed e.g. kangaroo rat with long loop of Henle.
• The filtrate then moves through the distal convoluted tubule then collecting duct where water balance is controlled.
• Water and salts may also be reabsorbed in the distal convoluted tubule and collecting ducts.
• This reabsorption is controlled by hormone, antidiuretic hormone (ADH) secreted by the pituitary gland in the brain.
• A diuretic is a substance that increases the making of urine.
• ADH affects the permeability of the collecting ducts and the distal convoluted tubule so that they can take back water into the blood and allow it to remain at a constant composition.
• When the blood has too little water (is highly concentrated), the hypothalamus in the brain stimulates the pituitary gland to secrete ADH.
• This ADH travels in the blood to the kidneys where it makes the distal convoluted tubule and the collecting duct more permeable to water.
• Thereby, more water can leave the urine/filtrate and go back into the blood.
• When the blood is diluted or contains too much water, little to no ADH is secreted.
• Thereby, the collecting ducts and distal convoluted does not become permeable.
• Urine then passes down the collecting duct to the pelvis of the kidney where it collects and continues down the ureter to the bladder to be stored.
• When the bladder contracts, urine is expelled through the urethra.
• Kidneys, ureters, urinary bladder and urethra are collectively known as "the urinary system".

Summary of Urine production

1. Filtration under pressure (ultrafiltration) occurs in the capsule.
2. Selective reabsorption of the useful substances from the tubules.
3. More waste can be added to the tubules by the capillaries around them (Steps 2 and 3 require energy for active transport and ATP provides the energy).
4. Water is removed or added to the collecting ducts from the loop of Henle.
5. Urine passes to the pelvis and ureter.
6. Blood with plasma at the correct concentration passes out in the renal vein.

• Excretion = Filtration - Reabsorption + Secretion

Section of Nephron/Excretion Processes Occurring

Kidney Damage

• An individual can live with one kidney, which grows and becomes slightly bigger to perform the function of two kidneys.
• No one can live without a kidney, nor can you live if the kidneys stop functioning. Accumulation of poisonous wastes in the blood leads to death.
• The human body contains about $5.6$ liters of blood. $1.2$ - $1.3$ liters of blood passes through the kidney per minute.

The Artificial Kidney

• When kidneys fail, people still need their blood “cleaned” of urea and other harmful substances and have to undergo dialysis treatment.
• They have to undergo dialysis treatment 2 – 3 times a week for up to 10 hours.
• A dialysis machine is a mechanical means of filtering blood particles based on size, using a selectively permeable membrane and concentration gradients to remove urea and other substances.
• Blood is filtered outside of the body and pumped back in.
• In treating cases of kidney failure, a tube is inserted into an artery and vein in the patient's arm and the blood is channeled through semi-permeable tube immersed in a bath containing all the normal blood chemicals except urea and other metabolic wastes.
• Since the concentration of harmful metabolic wastes is higher in the blood than in the bath, they pass through the wall of the tubes into the bath and purified blood is returned to the body.

Excretion in Plants

• Plants excrete a variety of substances which include:
  1. Oxygen (released from photosynthesis through the stomata during the day)
  2. Carbon dioxide (released from respiration, excreted during the night)
  3. Water (released from respiration, excreted during the night). The water and gases diffuse out the leaves through the stomata.
  4. Nitrogenous compounds such as tannins.
  5. Calcium oxalate

Excretory Products

1. Calcium oxalate is a waste product that is converted to insoluble calcium oxalate crystals. Calcium oxalate may be stored in living tissues such as the leaves of plants. These leaves may change into a brilliant, bright colour and eventually fall off.
2. Oils (Oils include the oils in citrus, pimento, eucalyptus, cinnamon and many other plants.)
3. Tannins Some waste substances are taken to plant structures which are dead, but are kept on the plant as they are useful. These substances include the tannins which are stored in the barks (dead) of many trees like the mangroves.
4. Dyes Dyes are also stored in the heartwood of logwood.

• These tannins, oils and dyes help to protect and preserve the trees. In the case of the tannins stored in bark, they serve to prevent the trees from being eaten.
• Many oils, tannins and dyes are also used commercially and some for medicine. For example; pimento and citrus oils.

Homeostasis

• The maintenance of a constant body environment.

Water

• Water is gained:
  • From drinking
  • From water contained in food
  • From chemical reactions in the body
• Water is lost:
  • As sweat
  • As water vapour when breathing out
  • In faeces
  • In urine

Osmoregulation (Importance)

• The regulation of the concentration of water and salt in the blood and tissue fluid.
• This is important because if the body fluids become too diluted then water will enter the cells by osmosis, causing them to swell or even burst.
• If the blood and tissue fluids become too concentrated, then water would be withdrawn from the cells causing shrinkage and dehydration.

Kidneys and Water Regulation

• What if you had too much water in your body?
• Kidneys excrete lots of watery urine when you have too much.
• What if you didn’t have enough?
• Kidneys excrete only small amounts when you’re short on water, so the urine is very concentrated.
• The kidneys can control the volume of urine produced depending on the volume of water taken in to the body by a mechanism called negative feed back mechanism.

Negative feedback Mechanism

• Homeostatic control is achieved using negative feedback mechanisms:
  • If the level of something rises, control systems reduce it again
  • If the level of something falls, control systems raise it again
• Negative feedback ensures that, in any control system, changes are reversed and returned back to the set level.
• Some factors controlled by negative feedback are:
  • Body temperature
  • Blood oxygen levels
  • Blood carbon dioxide levels
  • Blood glucose and salt levels
• A negative feedback control system responds when conditions change from the ideal or set point and returns conditions to this set point.
• There is a continuous cycle of events in negative feedback.

Osmoregulation

• An example of negative feedback can be seen in osmoregulation; the control of water concentration in blood and body fluids.
• If the amount of water in the body is low, special sense organ known as osmorecepters in the hypothalamus of the brain detect this.
• The hypothalamus send message to the pituitary gland. This gland produces the antidiuretic hormone (ADH).
• ADH is carried by the blood to the kidneys.
• ADH increases the permeability of the distal convoluted tubules and the collecting duct allowing water to be reabsorbed from them into the blood.
• As a result, small amount of concentrated urine are produced.
• Which urine is more concentrated?
• If blood water concentration rises, the hypothalamus detect it.
• Less ADH is released by the pituitary gland. The distal convoluted tubules and the collecting duct become less permeable so less water is reabsorbed so more water is lost as urine.
• In the case, large amount of dilute urine are produced.

The Skin

• The skin is considered as the largest excretion organ in man.
• It is the biggest organ in the body as it covers the whole body and the limbs from the outside.
• The skin consists of three layers:
  1. An outer epidermis
  2. An inner dermis.
  3. Under this is a layer of fat called the hypodermis or subcutaneous tissue.

Epidermis

• The skin is consist of several layers of epithelial cells.
• At the surface, the cells are dead, full of a tough, insoluble, structural protein, called keratin and always are subjected to friction, e.g. when wiping your face or body with a towel or rubbing your hands.
• This layer is worn away and is continually replaced from beneath.
• At the base of its inner layer, there are pigments cells which secrete melanin which is responsible for the color of the skin This melanin protect the lower layer of the skin from damage by ultraviolet rays in the sunlight

Dermis

• It is next to the epidermis, consisting of connective tissues. It contains blood capillaries (supply nutrients and remove waste), nerve endings (facilitate reception of stimuli/sense of touch), lymphatic, sweat glands, fat glands, fatty cells and hair follicles.

The Sweat Gland

• It is a coiled tube that reaches the skin surface through a pore.
• Functions of sweat glands: Regulate body temperature and removal of waste from the body.
• Note: when your sweet evaporates the remaining wastes ( salts and urea) causes the skin to be sticky. It is important to remove these wastes continually by washing; otherwise sweat pores become blocked resulting in a foul odor, blackheads and acne.

The Hair

• It is made of a hair follicle, surrounded by many blood capillaries.
• At its free end, there is a sebaceous (fat) glands, which produces an oily secretion that coats and protect the hair. This oil secretion facilitates the exit of the hair from the skin, keeping it soft and pliable. The hair follicle is supplied with an erector muscles, which contract and relax to move the hair.
• The sensory nerve endings respond to touch, pain and temperature.

Control of body temperature

• Human enzymes usually work best at $37$ºC, which is human body temperature. This can be measured in several places, including the ear, finger, mouth and anus..
• Extremes of body temperature are dangerous:
  1. high temperatures can cause dehydration, heat stroke and death if untreated
  2. low temperatures can cause hypothermia and death if untreated
• The maintenance of a constant internal body temperature is called thermoregulation.

Thermoregulation Control Mechanism:

• The body’s temperature is regulated by the hypothalamus of the brain.
• If you are too hot or too cold, the brain sends nerve impulses to the skin, which has three ways to either increase or decrease heat loss from the body’s surface:
  1. Hairs on the skin trap more warm air if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.
  2. If the body is too hot, sweat glands under the skin secrete sweat onto the surface of the skin, to increase heat loss by evaporation. Sweat secretion stops when body temperature returns to normal
  3. Blood vessels supplying blood to the skin can swell or dilate - vasodilation. This causes more heat to be carried by the blood to the skin, the skin becomes warmer which increases heat loss to the environment. Blood vessels can shrink down again - vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.
• When its too cold the muscles contract rapidly and we shiver. These contractions need energy from respiration, and some of this is released as heat.

Behavioral activities and physiological mechanisms for maintaining body temperature

• Removing or putting on thick clothes
• Having a hot or cold drink
• Moving to a warmer or cooler place
• Stop or increase activity

Control of blood glucose

• Special cells in the pancreas called islets of Langerhans detect the blood glucose level.
• The pancreas have chemoreceptors which are sensitive to levels of glucose
• Levels of glucose can go up after a meal and down after exercise
• If blood glucose levels are to high
• The pancreas secrete insulin
• Insulin is a protein hormone travel to all parts of the body. Its presence will make the muscle cells absorbs more glucose from the blood.
• The muscle cells and liver cells convert glucose into glycogen in the presence of insulin.
• In adipose tissue glucose is converted into fat in the presence of the hormone insulin.
• IF glucose levels are too low, special alpha cells in the islets of Langerhans secrete the hormone glucagon
• The cells of the liver will respond to the presence of glucagon by converting glycogen to glucose and releasing it to the blood.
• Amino acids and glycerol are also converted to glucose

Homeostasis of Glucose

• Glucose can cause multiple complications in the blood and can result in diabetes.

• There are two forms of diabetes:

  1. Diabetes type 2 (the target cells containing glucose do not respond to insulin)
  2. Diabetes type 1 (this occurs when the beta cells completely fail to make insulin). These patients normally obtain insulin through injections

• When the concentration of glucose in the blood is too high, this condition is referred to as hyperglycaemia.

• Note hypoglycaemia occurs when the blood sugar level drops too low.

Control of carbon dioxide level

• The carbon dioxide level increases in the blood (as a result of holding the breath or exercise) it is detected by the medulla oblongata. The body responds by directing the diaphragm and intercostal muscle to increase the rate and/or depth of breathing, which moves carbon dioxide from the blood and out into the air. As blood carbon dioxide levels return to normal, the rate of ventilation decreases.
• The carbon dioxide level in the blood has decreased far below normal (due to hyperventilation) it is detected by the medulla oblongata. The diaphragm and intercostal muscle are directed to slow the breathing rate, so that carbon dioxide again builds up in the blood and returns to the normal condition.