Biology – Care To Know How Your Excretory System Works? Read This!!!

Topic: Excretory Mechanism

Introduction – Excretion

Excretion is the process by which the body of human beings gets rid of or remove unwanted metabolic wastes e.g. urine, bile, carbon (iv) oxide, etc. Excretion, the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. Through excretion organisms control osmotic pressure—the balance between inorganic ions and water—and maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism’s internal environment. The various organs that help the body in removing these waste products are known as excretory organs. These organs include the kidney, lungs, skin and the liver. In humans, the various organs of excretion (kidney, lungs, liver and skin) make up the excretory system and they excrete the following: the kidney excretes urine, lungs excrete water, carbon (iv) oxide and the skin excretes sweat, salt and water, etc.

Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination, which may be considered to encompass all of the various mechanisms and processes by which life forms dispose of or throw off waste products, toxic substances, and dead portions of the organism. The nature of the process and of the specialized structures developed for waste disposal vary greatly with the size and complexity of the organism.

Excretory Mechanism

The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The dual function of excretory systems is the elimination of the waste products of metabolism and to drain the body of used up and broken down components in a liquid and gaseous state.

Excretory Organs and their Functions

The Kidney

• These are two beans shaped structured, reddish brown in colour and situated at the back of the abdominal cavity.
• A branch of interior vena cava and the aorta enter each kidney as the renal vein and renal artery.
• The left kidney sits slightly higher than the right one. The size of an adult kidney is approximately 4 inches (10 centimeters) long and 2 inches (5 centimeters) wide.
• To maintain human life, it is necessary for at least one of the kidneys to function properly.
• The kidneys remove from the blood the nitrogenous wastes such as urea, as well as salts and excess water, and excrete them in the form of urine.
• This is done with the help of millions of nephrons present in the kidney. The filtrated blood is carried away from the kidneys by the renal vein (or kidney vein).

The urine from the kidney is collected by the ureter (or excretory tubes), one from each kidney, and is passed to the urinary bladder. The urinary bladder collects and stores the urine until urination. The urine collected in the bladder is passed into the external environment from the body through an opening called the urethra.

These vessels further branch into capillaries in the kidney tissue. The kidney has millions of nephrones, which are the functional units where excretion takes place. Blood carries waste products to the kidneys via the renal artery. Inside each kidney, blood is transported to 1.2 million filtering units called nephrones (pronounced NEFF-rons). The cells in nephrones take in the liquid portion of the blood and filter out impurities (urea, mineral salts, and other toxins). Necessary substances such as certain salts, water, glucose (sugar), and other nutrients are returned to the blood stream via the renal vein.

The Nephrone

The bowman’s capsule and the glomerulus form the malphighian body. At the glomerulus, the substances that come with the blood are filtered under blood pressure through a process known as ultrafiltration.

At the Henle’s loop, re-absorption of useful substances such as sugar and water takes place to maintain the composition and volume of the blood by a process known as osmoregulation. Excess water enters the collecting tubules and is sent through the pelvis to the ureter which leads to the bladder. The fluid in the bladder which contains urea, uric acid, mineral salts and water, is known as urine. The urine is retained in the bladder until it is expelled. The problem associated with the kidney is diabetes in which the patient’s urine contains much sugar.

The job of clearing the blood of wastes in the nephrones occur in three stages. They are filtration, reabsorption, and tubular secretion:

1. The first stage in clearing the blood is filtration, the passage of a liquid through a filter to remove impurities. Filtration occurs in the glomeruli. Blood pressure forces plasma, the liquid portion of the blood, through the capillary walls in the glomerulus. The plasma contains water, glucose, amino acids, and urea. Blood cells and proteins are too large to pass through the wall, so they stay in the blood. The fluid, now called filtrate, collects in the capsule and enters the renal tubule.
2. During reabsorption, needed substances in the filtrate travel back into the bloodstream. Reabsorption occurs in the renal tubules. There, glucose and other nutrients, water, and essential ions materials pass out of the renal tubules and enter the surrounding capillaries. Normally 100% of glucose is reabsorbed. (Glucose detected in the urine is a sign of diabetes mellitus, which is characterized by too much sugar in the blood due to a lack of insulin.) Reabsorption involves both diffusion and active transport, which uses energy in the form of ATP. The waste-containing fluid that remains after reabsorption is urine.
3. Tubular secretion is the passage of certain substances out of the capillaries directly into the renal tubules. Tubular secretion is another way of getting waste materials into the urine. For example, drugs such as penicillin and phenobarbital are secreted into the renal tubules from the capillaries. Urea and uric acid that may have been reabsorbed are secreted. Excess potassium ions are also secreted into the urine. Tubular secretions also maintain the pH of the blood.

The kidney performs the following functions:

When it comes to components of the urinary system, the kidneys are multi-functional powerhouses of activity. Some of the core actions of the kidneys include:

• Waste excretion: There are many things your body doesn’t want inside of it. The kidneys filter out toxins, excess salts, and urea, a nitrogen-based waste created by cell metabolism. Urea is synthesized in the liver and transported through the blood to the kidneys for removal.
• Water level balancing: As the kidneys are key in the chemical breakdown of urine, they react to changes in the body’s water level throughout the day. As water intake decreases, the kidneys adjust accordingly and leave water in the body instead of helping excrete it.
• Blood pressure regulation: The kidneys need constant pressure to filter the blood. When it drops too low, the kidneys increase the pressure. One way is by producing a blood vessel-constricting protein (angiotensin) that also signals the body to retain sodium and water. Both the constriction and retention help restore normal blood pressure.
• Red blood cell regulation: When the kidneys don’t get enough oxygen, they send out a distress call in the form of erythropoietin, a hormone that stimulates the bone marrow to produce more oxygen-carrying red blood cells.
• Acid regulation: As cells metabolize, they produce acids. Foods we eat can either increase the acid in our body or neutralize it. If the body is to function properly, it needs to keep a healthy balance of these chemicals. The kidneys do that, too.

The Lung

The lung is an organ used for the excretion of gases such as carbon (iv) oxide. It is also an organ of respiration in which we breathe in fresh air (oxygen) from the atmosphere and give out carbon (iv) oxide as a waste product.

The lungs are found in the chest on the right and left side. At the front they extend from just above the collarbone (clavicle) at the top of the chest to about the sixth rib down. At the back of the chest the lungs finish around the tenth rib. The protective linings which cover the lungs (pleura) continue down to the twelfth rib. From front to back the lungs fill the rib cage but are separated by the heart, which lies in between them.

The lungs are divided into different parts which are called lobes. The right lung has three lobes called upper, middle and lower lobes. The left lung only has two lobes, the upper and lower.

Functions of the Lungs

The lungs’ main function is to help oxygen from the air we breathe enter the red cells in the blood. Red blood cells then carry oxygen around the body to be used in the cells found in our body. The lungs also help the body to get rid of CO₂ gas when we breathe out and also remove water vapour. There are a number of other jobs carried out by the lungs that include:

• Changing the pH of blood (whether the blood is more acid or alkali) by increasing or decreasing the amount of CO₂ in the body.
• Filtering out small gas bubbles that may occur in the bloodstream.
• Converting a chemical in the blood called angiotensin I to angiotensin II. These chemicals are important in the control of blood pressure.

    

The Skin

This is the largest organ of the body that protects the other internal organs of the body. The skin is one of the largest organs in the body in surface area and weight.  The skin consists of two layers: the epidermis and the dermis. Beneath the dermis lies the hypodermis or subcutaneous fatty tissue. The skin has three main functions: protection, regulation and sensation.

The epidermis is the outer layer of the skin. It is made up of three layers and they are:

        i. The cornified layer or stratum corneum

      ii. The granular layer or stratum granulosum

    iii.  The malphighian layer (the innermost layer)

The dermis is the inner layer of the skin. It contains blood vessels (arteries and veins), lymphatic vessels, muscles, fat cells, nerve endings, hair follicles and skin glands (sweat glands and sebaceous glands). All these are embedded in a matrix of connective tissues.

The sweat gland located on the dermis is a slender, coiled tube which continues as sweat pores. The dermis is surrounded by a network of blood capillaries.

As blood flows through these capillaries, the waste products such as water, urea and mineral salts are absorbed by the sweat glands. These are secreted into the sweat duct (tube) and they pass through the duct and the pores as sweat. By this, the skin is able to function as an excretory organ.

The skin is an organ of protection. The primary function of the skin is to act as a barrier. The skin provides protection from: mechanical impacts and pressure, variations in temperature, micro-organisms, radiation and chemicals.

The skin is an organ of regulation. The skin regulates several aspects of physiology, including: body temperature via sweat and hair, and changes in peripheral circulation and fluid balance via sweat. It also acts as a reservoir for the synthesis of Vitamin D.

The skin is an organ of sensation. The skin contains an extensive network of nerve cells that detect and relay changes in the environment. There are separate receptors for heat, cold, touch, and pain. Damage to these nerve cells is known as neuropathy, which results in a loss of sensation in the affected areas. Patients with neuropathy may not feel pain when they suffer injury, increasing the risk of severe wounding or the worsening of an existing wound.

Functions of the Skin

1. It protects the internal organs from damage
2. It prevents the entering of ultra-violet rays
3. Immunological function mediated by Langerhans cells.
4. It protects the body against bacteria and the invasion of other micro-organisms
5. Protection of the body from harmful effects of the sun and radiation.
6. The skin helps in the removal of waste products such as water, mineral salts and urea from the body
7. Vitamin D synthesis from its precursors under the effect of sunlight and introversion of steroids.
8. It also helps in regulating the body temperature
9. Regulates body temperature.
10. Prevents loss of essential body fluids, and penetration of toxic substances.
11. Mechanical support.
12. Sensory organ for touch, heat, cold, socio-sexual and emotional sensations.

Excretion and the Need for it

Waste products make one to be uncomfortable whenever they are present in the body. Waste products should be removed for the following reasons:

• Excess carbon (iv) oxide and urea are poisonous to the body
• Excess salt has to be removed to keep the blood salinity constant
• Excess water has to be removed to regulate the volume of fluid in the body
• Waste products make one feel uncomfortable and therefore, should be removed.

Excretory Products

Excretory products are waste products of metabolism. However, excretory wastes come from a number of sources such as:

• Products from metabolism of cells
• Products from natural breakdown of protoplasm
• Inability of the body to store some nutrients taken in excess of the needs of the body

Excretory products include water, mineral salts, uric acid, sweat, etc.