EXCRETION AND HOMEOSTASIS IN PLANTS AND ANIMALS.
At the end of this topic, you will be able to;
i) Distinguish between excretion and homeostasis.
ii) Explain various methods of excretion in plants.
iii)Describe excretory products in plants and their uses.
iv) Understand different excretory organs in mammals and how they work: the skin, lung, liver, and kidney.
v) Describe mechanisms of homeostasis; negative and positive feedback.
v)Describe the roles of the skin, liver, and kidney in homeostasis
vi) Describe disorders of various excretory organs.
Excretion on and homeostasis are characteristics of living things as it was introduced early in form one. Excretion is an essential characteristic of living things because it helps organisms to get rid of metabolic waste products. These two words are defined as follows; Excretion-Is the process by which living organisms remove their metabolic waste products.
Homeostasis-It is the maintenance of the internal environment of the body of the organism at a constant state for the cell’s normal functioning.
The plant does not have complex excretory organs because;
i)They can recycle their waste products; for example, oxygen, a waste product during photosynthesis, is used for respiration.
ii)They deposit their waste products in their various parts like the leaves and fruits which drop off or are utilized by man for food
iii). They do not move about hence have less toxic substances.
Method of Excretion in plants
i) Recycling; plants re-use some of their waste products in other processes. For instance, oxygen produced as a waste product is used for respiration during the day. ii) Exudation; is a process by which some plants release fluids at the back of their stems.
iii)Transpiration; plants remove excess salt and water through transpiration.
iv) Leaf fall, fruits fall, thereby releasing waste products deposited in them.
v) diffusion of gases from the leaves to the atmosphere through the stomata.
EXCRETION IN ANIMALS
a)Excretion in unicellular organisms
Unicellular organisms like amoeba use simple diffusion to remove their waste products because of their small size.
b)Excretion in multicellular organisms
Multicellular organisms possess complex excretory organs due to increased complexity in these organisms. We will describe the excretory organs in mammals like a man and their functions.
Excretory organ Excretory product(s)
i)Skin Excess salt, excess. Water Urea
ii)Kidney Excess water, excess salt Urea
iv)Lung Carbon(iv) Oxide, water
In this sub-topic, we will describe the skin, liver, and kidney structure and functions.
The kidney is an excretory organ that filters blood in the body. Man has two kidneys; the left kidney and the right kidney. Expand your knowledge!
” Did you know One can survive with one kidney?” Now you know
Anatomy of the Kidney Kidney
The kidney is bean-shaped. It has three regions;
-The basic functional unit of the kidney is the Kidney nephron.
Structure of the kidney nephron
The kidney receives blood through the renal artery. The renal vein takes away the blood from the kidney. How kidney nephron works Kidney nephron receives blood from the afferent arteriole. The blood entering the kidney is composed of molecules of both large and small molecular sizes.
Small molecular-sized(liquid part) molecules of blood
-Dissolve mineral salts -amino acids -Glucose
-Ion like sodium, chlorine, potassium
Large molecular-sized molecules in the blood
Point of thought-Why do we say kidney filters blood? Where are the sieves, the residue, and the filtrate?
As described above, the kidney separates the liquid and the large molecular-sized molecules of blood. As the blood enters the kidney through the afferent, it comprises both small and large molecular-sized molecules. Blood and its components are filtered at the glomerulus with network capillaries. At this point, the liquid part of blood is forced out of the blood into the Bowman’s capsule. This process is termed as ultrafiltration. The fluid portion of blood that is forced out into the Bowman’s capsule is called glomerulus filtrate. The large molecules remain in the blood and go back to circulation through the efferent. The process of ultrafiltration is achieved due to the following factor;
-The entering efferent is wider than the leaving efferent, which builds up pressure within the capillaries’ network at the glomerulus.
It is important to note that the glomerulus filtrate comprises both nutrients (amino acids, mineral salts, ions, glucose, some amount of water) and waste products (excess salt, excess water, urea). The body requires these nutrients, and the waste products need to be excreted. Therefore, at various parts of the nephron, different nutrients are reabsorbed, and the waste products left to flow until the collecting tubule where they form urine.
N/B. What kidney does is reabsorption and NOT absorption. Remember, the absorption took place at the ileum before these nutrients reach the kidney.
The summary of parts of the nephron and what they reabsorb.
i) Proximal convoluted tubule- Reabsorbs, amino acids, glucose some amount of water and vitamins
ii) Loop of Henle -Reabsorb ions mainly sodium ions by the help of aldosterone hormone secreted by the adrenal glands of the kidney.
iii)Distal convoluted tubule- Reabsorb water back to the body It is regulated by a hormone called antidiuretic hormone secreted by the pituitary glands.
After reabsorption of water at the distal convoluted tubule, the remaining waste products(excess salt, excess water, and urea) collects at the collecting tubule moves to the pelvis and then drained into the urinary bladder through the ureter and later released outside the body through the urethra. Reabsorption at the proximal convoluted tubule is similar to absorption at the ileum. Therefore proximal convoluted tubule and ileum have many common adaptations.
Adaptation of proximal convoluted tubule.
i)Numerous villi and micro-villi to increase the surface area for reabsorption.
ii)Numerous mitochondria to provide energy for active transport.
iii) long to give more time for efficient reabsorption.
iv)It is highly coiled to reduce the speed of the flow of glomerulus filtrate for efficient reabsorption.
The skin has both homeostatic and excretory functions. It is the largest organ in the body
General roles of the skin
I) Synthesis of Vitamin D
ii) Prevent dehydration by preventing loss of water. ii)Thermoregulation through vasodilation, vasoconstriction, and sweat, loses with the latent heat of vaporization and the skin’s hair.
iii) Perceive changes in the environment through nerve endings. iv)Protect the body’s inner delicate organs and tissues from physical damage.
v) Prevent of entry of disease-causing organisms and ultraviolet rays.
Skin Anatomy (Structure of the Skin)/ Adaptation of the skin to its functions.
The skin comprises two significant layers;
I) Epidermis layer-outer layer .
ii) Dermis layer-inner layer.
N/B- below the dermis is a layer of fats called the hypodermis. It is considered to be another layer of the skin by some authors.
Expand your knowledge!
Do you know what the meaning of the pre-fixes below is?
Therefore epidermis means above the dermis, and hypodermis means below the dermis.
- Epidermis layer
It is the outermost layer of the skin. It is made up of three sub-layers
i) Horny layer/Cornified layer- Is a layer of dead cells. The cells of this layer are made up of keratin. This layer protects the inner tissues of the skin.
ii) Granular layer- This layer is made up of actively dividing cells that give rise to a new cornified layer.
iii) Malpighian layer -Contain pigment called melanin, which;-prevents the entry of ultraviolet rays and provides the skin with its color.
2. Dermis layer
It is composed of various components as described below;
i)Nerve endings that perceive changes within the environment. ii)sweat glands for thermoregulation; when it is hot, excess salt and excess water are accumulated in these glands to form sweat. Sweat moves onto the skin surface through the sweat pore the sweat then evaporates with the latent heat of vaporization as it absorbs the body temperature.
iii)Blood vessels that;
-supply nutrients to the skin tissues
-Remove wastes of metabolism from the skin
-Supply oxygen to the body tissues.
iv)Hair and erector pili muscle; originates from the dermis and extends to the skin’s outer part. Hair is used for thermoregulation; when it is cold, erector pili muscles contract, making the hair stand straight/erect, the hair traps air, which acts as an insulator hence, reduced heat loss. When it is hot, erector pili muscles relax, hair lies flat on the skin, less air is trapped, reducing insulation effect hence reduced heat loss.
v)Sebaceous gland; secrets sebum which;
-Contain antiseptic substances that kill the disease-causing microorganisms
-Soften the skin and make it waterproof.
vi) Adipose tissue; is a layer of fats. Also called hypodermis, the layer of fats acts as an insulator hence reduces hearing loss when it is cold.
The liver is the second largest organ in the body. It is a vital organ that controls many body metabolic activities. The blood vessel that brings blood to the heart is the hepatic artery. The blood vessel that takes blood away from the kidney is the hepatic vein Blood vessel that brings blood and absorbing nutrients from the small intestines is the hepatic portal vein.
Primary functions of the liver.
i) Detoxification; the removal of toxic substances that may enter the body through drugs and other chemically.
ii)Deamination; the breakdown of amino acids, which involves removing an amino group from amino acids, combine it with carbon (iv) Oxide to form urea.
NH3 + CO2 -> HCOH + H2O
Deamination helps the body get rid of highly toxic ammonia by converting it into less toxic form urea, which is later excreted by the kidneys.
iii) Regulation of blood sugar level. The liver controls blood sugar level through the effect of two hormones; insulin and glucagon. When blood sugar level goes down, glucagon is secreted to the liver. Glucagon stimulates the liver to breaks down glycogen to glucose hence increasing blood sugar level to normal. When blood sugar level rises, insulin is secreted, making the liver increase intake of glucose from the blood and convert the excess glucose to glycogen hence reducing the level of glucose in the body.
iv)Elimination of the old red blood cells; Liver breaks down hemoglobin pigment in old red blood cells to heme and globin. Iron is recycled for the production of new red blood cells. vi)thermoregulation; when it is cold, the liver increases the rate of metabolic activities to generate heat while hot, the liver reduces the rate of heat-generating metabolic activities.
Other liver roles include -Elimination of excess cholesterol in the body -Production of red blood cells in the fetus when the bone marrow is not yet developed. -Storage of vitamins like vitamin K.
It is the maintenance of the internal environment of the body at a constant state. Excretion helps the body to achieve homeostasis. An organism is made up of cells that work at controlled conditions of their environment. However, the cells can not control their external environment.
The internal environment is made up of factors like temperature, PH, water, dissolved mineral salts, and ions, which need to be maintained in a constant state. Therefore, we will discuss the following under homeostasis;
i)Thermoregulation- regulation of body temperature. ii)Osmoregulation-Salt and water balance.
iii)Blood sugar balance. -regulation of blood glucose levels.
iv)Ionic balance-regulation of ions in the body like sodium.
Some excretory organs already discussed in earlier sub-topics are also essential in homeostasis, for example.
i)Skin takes part in thermoregulation and osmoregulation.
ii)The liver takes part in blood sugar balance and thermoregulation. iii)The kidney takes part in osmoregulation and ionic balance.
Mechanism of Homeostasis
There are two mechanisms through which the body corrects elevated levels or lower levels of internal body environment. These responses that rectify the changes are called feedback mechanisms. Feedback can be positive or negative. In the first case, the body responds to the change in the internal environment by reversing the change. This is called negative feedback. However, in some rare cases, the body responds to the change in the internal environment by elevating that change; that is, the change goes to the same direction in which it was initially. This is called positive feedback. Example of negative feedback.
i) Thermoregulation-When body temperature goes up; the body responds by reducing the increased temperature back to normal. When the temperature goes down, below the normal, the body responds by increasing the body temperature.
ii) Blood sugar level; The body reduces the blood sugar level when it is high and increasing it when it is low.
iii) Ion balance-the body increases the concentration of ions when its low in the body and reduces it when it is in high levels iii)Blood sugar level.
Example of positive feedback
i) Blood clotting process-more and more platelets are secreted in a damaged blood vessel until clotting is achieved.
ii) During birth, labor pain due to increased secretion of oxytocin hormone increases contraction and relaxation of uterine walls (leading to more and more pain) until the baby comes out.
It is the regulation of body temperature. It is achieved through negative feedback. The thermoreceptors detect changes in body temperature in the hypothalamus. The hypothalamus sends the information to respective organs like the skin and the liver, and these organs respond to correct the change in body temperature.
Role of homeostatic organs
a)Roles of the skin in thermoregulation
i) Sweat glands for thermoregulation; when it is hot, excess salt and excess water are accumulated in these glands to form sweat. Sweat moves onto the skin surface through the sweat pore the sweat then evaporates with the latent heat of vaporization as it absorbs the body temperature.
ii) Blood vessels which;
-supply nutrients to the tissues of the skin
-Remove wastes of metabolism from the skin
-Supply oxygen to the body tissues.
iii)Hair and erector pili muscle; originates from the dermis and extends to the skin’s outer part. Hair is used for thermoregulation; when it is cold, erector pili muscles contract, making the hair stand straight/erect, the hair traps air, which acts as an insulator, hence reducing heat loss. When it is hot, erector pili muscles relax, hair lies flat on the skin, less air is trapped, reducing insulation effect hence reduced heat loss.
iv) Adipose tissue; is a layer of fats. Also called hypodermis, the fatty layer acts as an insulator, thus reduces heat loss when it is cold.
b)Role of the liver in thermoregulation
When it is cold, the liver increases the rate of metabolic activities to generate heat. On the other hand, when it is hot, the liver reduces the rate of heat-generating metabolic activities. OSMOREGULATION
It is the salt and water balance. The role of the kidney achieves it. The hormone responsible is the Antidiuretic hormone (ADH).
Salt and Water Balance.
When the body has less amount of water, the osmotic pressure is increased. This is detected by the hypothalamus, which triggers the secretion of the antidiuretic hormone. The antidiuretic hormone is transported to the kidney tubules (distal convoluted tubule). ADH increases the permeability of kidney tubules, thereby allowing more water to be reabsorbed into the bloodstream, thereby reducing the blood’s osmotic pressure. This leads to the excretion of less and more concentrated urine. When the body has excess water, Osmotic pressure is low. In this condition, less ADH is secreted. This reduces the permeability of kidney tubules; hence; hence less water is reabsorbed. This leads to excretion of more and less concentrated.
Ionic balance is achieved through the role of the Loop of Henle of the kidney. When sodium ions level increase in the body, adrenal gland secrets less or no aldosterone hormone, and therefore, Loop of Henle reabsorbs less or no sodium ions. When the level goes up, more aldosterone hormone is secreted to increase reabsorption of the ions in the loop of Henle.
Blood sugar regulation in the liver.
When blood glucose goes up, insulin hormone is secreted in blood and transported to the liver. Insulin makes the liver to reduce blood sugar level by ;
-Conversion of excess glucose to glycogen
-converting excess glucose to fats
-Reducing the rate of respiration.
When blood sugar is low, glucagon hormone is secreted and transported to the liver, the liver responds by ;
-Increasing the breakdown of glycogen to glucose
-Increasing the conversion of fats to glucose
-Increasing the rate of respiration.
HEAT LOSS AND HEAT GAIN
Organisms whose body temperature remains constant despite the change in environmental temperature is called homoiotherms. Examples of homoiotherms are birds and mammals. Organisms whose body temperature fluctuates with that of .external environment are called poikilotherms. Examples of poikilotherms are fish, reptiles, and amphibians.
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