Life Processes (d) Excretion

Excretion refers to the removal of waste matter from the body. The unicellular organisms remove the waste products from their body by the process of simple diffusion which is defined as the movement of a substance from a region of higher concentration to a region of lower concentration through a membrane.

In unicellular organisms, excretion is performed by a single cell by the process of diffusion through the body surface. Whereas, in multi-cellular organisms, different life processes are performed by different cells and organ-systems (nervous system, excretory system, circulatory system, etc). Multi-cellular organisms use specialised organs such as lungs, kidneys, nephrons, skin, malphigian tubules, etc to eliminate waste products from their body.

Kidneys are bean-shaped organs meant for excretion. In human body, they are located in the upper abdominal area. The two kidneys are present opposite to each other, one on each side of the backbone.

Glomerulus is a tiny spherical structure present in nephrons (functional unit of a kidney). It is formed by the cluster of very thin-walled blood capillaries that are involved in filtration of blood to form urine. Each of this clustered group of capillaries is associated with a cup-shaped tube that collects the filtered urine.

The basic functional and filtration unit of a kidney is called a nephron. It is composed of renal corpuscle (Bowman’s capsule + glomerulus) and renal tubules. It is called so because it is actively involved in filtration of blood to produce urine. It removes waste products and excess substances from blood and forms urine.

In a nephron, ultra-filtration occurs in the renal corpuscle (glomerulus + Bowman’s capsule). Glomerular filtration of blood is also known as ultra filtration. It is a non-selective process and filtration is so fine that every component of plasma (water, glucose, amino acids, urea, etc) except proteins pass into the Bowman’s capsule. The epithelium of renal corpuscle has tiny filtration slits (also called slit pores) that help in ultra-filtration. This process ensures that only waste products and excess water are removed from the body and it also helps to concentrate the urine.

The second stage of urine formation, tubular re-absorption, is called selective re-absorption because some substances such as glucose, amino acids, urea, salts and excess water are selectively reabsorbed back into the blood as urine is being filtered along the tubes. All components of plasma are not re-absorbed but only a few of them, hence, selective re-absorption.

The urine is stored in urinary bladder, which is a muscular sac composed of smooth muscles that are able to relax and contract. The pressure built inside the expanded bladder due to storage of urine leads to the urge to urinate. As the activity of urinary bladder is under the control of nervous system, the urge to urinate can be controlled.

In case of kidney failure, an artificial kidney is needed. The kidneys lose the ability to remove the nitrogenous waste products from the body or their activity is highly reduced. They fail to perform their vital function. There are many reasons for kidney failure such as injury, infection and reduced blood flow that lead to accumulation of wastes in our body. Hence, an artificial kidney can be used.

Dialysis is the process of removing nitrogenous wastes and excess water from the body by using a clinical device, called as artificial kidney and a dialyzing fluid. It serves as a substitute for normal functioning of the kidneys. It is required when the kidneys stop functioning properly or are damaged.

In artificial kidneys, all tubes are semi-permeable and made of cellophane. These tubes are selectively permeable and allow only certain substances to pass through them. The semi-permeable nature allows only the nitrogenous waste products and water to pass through the tubes and does not allow the blood cells and proteins to pass through it.

Plants produce many waste products such as gases and water. They eliminate excess water through transpiration. Plants also produce gaseous waste products – oxygen during photosynthesis (yes, oxygen can be considered a waste product for plants) and carbon dioxide during respiration.

When water is lost in the form of vapours through the stomata present on leaves, it is called transpiration. Some plants lose excess water in the form of liquid droplets from the margins of leaves, through hydathodes. This process is called as guttation. Hydathodes are structurally modified pores present on leaf margin.

Stems of plants also possess stomata through which diffusion of gases occur. The oxygen from air diffuses into the stem for respiration through stomata and similarly, carbon dioxide diffuses out into the air. The stems of woody plants possess lenticels (large raised pores) to facilitate the exchange of gases, thereby help in excretion.

Plants lose excess water through stomata, lenticels and cuticle. The two processes involved are transpiration and guttation. When water is lost in the form of vapours through the stomata present on the aerial parts of plants, it is called transpiration. It occurs by pulling of water. It occurs only in daytime. When excess water is lost in the form of liquid droplets from the margins of leaves, through hydathodes, it is called guttation. It occurs by pushing of water towards leaf margins. It occurs only at night. Hydathodes are structurally modified pores present on leaf margin.

Apart from water, plants excrete out a number of liquid waste products into the soil around them. It includes essential oils, organic acids, aromatic oils, tannins, resins, latex and sap. Resins and gums are stored in old xylem of plants. Most of these wastes are useful to humans.

The latex is exuded as a waste product from plants. The tyres are made from the latex of rubber tree (Hevea brasiliensis).

The unicellular organisms (such as amoeba) have no specialized organs for excretion. They excrete out wastes by simple diffusion through their plasma membrane (general body surface) into their surroundings.

The urine is produced by the kidneys and is then passed to the urinary bladder, where it gets stored. The urinary bladder is a pear-shaped hollow muscular sac composed of smooth muscles that are able to expand.

The urine is made up of a number of constituents. It is an aqueous solution mainly composed of water (95%). It also consists of some organic and inorganic compounds. Other constituents include small amounts of urea, creatinine and some dissolved ions such as sodium, potassium and chloride.

The chemicals (toxic waste products) from the blood enter the dialyzing fluid through the process of diffusion when blood is passed along the tubes.

Normally, glucose is present in the blood, but not in urine of a healthy person. All of the filtered glucose is re-absorbed into the bloodstream by the kidneys through tubular re-absorption. The presence of glucose in urine (glycosuria) indicates that the person has diabetes.

Osmoregulation refers to the regulation of water and salt concentration across the membranes within the body fluid to maintain the required osmotic pressure. It serves to maintain the dissolved ions concentration of body fluid.

(i) The main osmoregulatory organs in humans are kidneys. They also function to filter blood.

(ii) Amoeba possesses contractile vacuoles as osmoregulatory organs. It serves to regulate the water content of the cell. These vacuoles help the amoeba to survive in hypertonic as well as hypotonic environment.

The circulatory system also serves to remove waste products from the body. Certain wastes such as excess water, urea, uric acid and carbon dioxide are transported from the liver to the kidneys, where the blood is filtered. The urea is produced in liver and released into the bloodstream. It is then transported to the kidneys by blood to be excreted out in urine. These waste products are exchanged through diffusion as blood is being filtered by kidneys.

Haemodialysis serves as a substitute for kidney. It works on the principle of diffusion. The nitrogenous wastes (urea), creatinine and potassium ions diffuse out into the dialyzing fluid across the semi-permeable membranes along the concentration gradient (from high concentration to low concentration). It corrects electrolyte imbalance and removes waste from the body.

Plants produce a number of waste products and use different mechanisms to get rid of them. Excess water is eliminated by transpiration and guttation. Oxygen (a by-product of photosynthesis) and carbon dioxide (produced during respiration) moves out through stomata. Some waste products are stored in plant parts like leaves that later fall off. Plants also excrete some waste products into the surrounding soil. Certain wastes are accumulated as resins or gums in old xylem tissue. All these methods help the plants to get rid of their excretory products.

Excretion is the removal of nitrogenous waste products of metabolism that are formed in the body, such as urine and carbon dioxide through kidneys and lungs. It is a part of excretory system.

Defecation (discharge of faeces) refers to the removal of undigested food and other waste material from the alimentary canal through the anus. It is a part of digestive system. It marks the end of digestion.

Human excretory system

● It consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.

● Urine is produced in the kidneys and passed to the urinary bladder through the ureters.

● It is then excreted out of the urinary bladder via urethra.

● Each kidney possesses about a million nephrons (basic functional unit) that are responsible to filter out blood and produce urine.

● The three processes involved are filtration, re-absorption and secretion.

● Certain substances such as glucose and amino acids are re-absorbed while other waste products are excreted out in the form of urine, which is carried by ureters from the kidney to the urinary bladder.

● Thus, help to concentrate the urine.

A. Renal artery – It supplies oxygenated blood from the heart to the kidney.

B. Renal vein – It carries deoxygenated blood from the kidney back to the heart.

C. Ureters – Paired tubes connecting kidneys to the urinary bladder. They transport urine from the kidneys to the bladder.

● The dialysis machine serves as artificial kidney.

● It is used for patients with damaged or dysfunctional kidneys.

● It filters blood and removes waste products and excess fluid from the body.

● It works on the principle of diffusion.

● It consists of semi-permeable tubes (made of cellophane) and a special dialyzing fluid.

● A small incision is made on the arm and a tube is connected. The blood is passed along the tubes placed in dialyzing fluid that filter waste products from the blood through diffusion.

● The filtered blood is transported back into the body.

(i) The multi-cellular organisms have a very complex body organization. They possess many different organ-systems to perform the basic life processes, such as respiration, excretion, etc. Also, the cells and tissues of these organisms are not in direct contact with the environment. Moreover, diffusion is quite a slow process. So, excretion cannot be performed through diffusion.

(ii) Ammonia is a toxic nitrogenous waste and is highly soluble in water. The removal of ammonia requires a large amount of water. Aquatic animals are ammonotelic as they need not conserve water. The conversion of ammonia into less toxic metabolite (urea, uric acid) is highly energy intensive. Therefore, aquatic animals prefer not to expend energy in conversion of ammonia.

(iii) Ritu possesses a number of values. She seems to be attentive, vigilant, observant and zealous. She has a curiosity for learning.

(i) Plants use different methods to perform excretion. They produce mainly two gaseous waste products, namely oxygen (produced during photosynthesis) and carbon dioxide (produced during respiration). The leaves of plants possess tiny pores called stomata and stems possess lenticels through which O₂ and CO₂ are diffused out of the plant body.

(ii) Apart from oxygen, plants produce a number of waste products such as excess water, essential oils, aromatic oils, tannins, resins, gums, organic acids, latex, etc. Almost all of these products are useful to human.

Gums and resins are used for varnishes and adhesives. Essential oils are used in cosmetics and soap making.

This is a negative feedback to reverse any change occurring in water concentration in blood. This system regulates and maintains required water amount in blood by excreting out excess water in the form of urine. If the amount of water in blood is reduced, more concentrated urine is formed to conserve water.

The labeled part is the urinary bladder, which is a pear-shaped muscular sac composed of smooth muscle. It is connected to the kidneys via ureters. It is meant for storing urine until it is released out through urethra.

The structures are labeled as:

X – Renal vein

Y – Renal artery

Z - Ureter

The renal artery (Y) carries oxygenated blood to the kidneys from the heart. This blood contains negligible amounts of waste products (such as urea). On the other hand, the ureters (Z) transport urine from the kidneys to the urinary bladder. The urine contains comparatively high amounts of urea to be excreted out. Thus, the relative concentration of urea in Y is always lower than that in Z.

The urea is formed by the urea-cycle in the liver, and not in the kidneys. The blood vessel coming out from the kidney will contain the lowest concentration of urea (although kidneys contain only a small amount of urea to maintain osmolarity). And the blood vessel coming out from the liver will contain the highest concentration of urea.

Consuming high amount of water leads to increased water concentration in blood. As a result, osmotic pressure of the plasma decreases because excess water must be eliminated to maintain normal functioning of the body. Decreased osmotic pressure of the plasma tends to push out excess water from the circulatory system.

Consequently, more volume of urine is produced and the person has frequent urination.

The process of glomerular filtration, also called ultra-filtration occurs in the renal corpuscle (glomerulus + Bowman’s capsule) through numerous tiny filtration slits present in the basement membrane of renal corpuscle. This filtration is so fine that every component of plasma, except proteins (as they are large in size) pass into the Bowman’s capsule. Therefore, the filtrate from the kidneys of a mammal contains glucose and salts, but not proteins.

The accumulation of Homogentisic acid (phenolic acid) causes alkaptonuria (a rare inherited disease that causes the urine to turn black when comes in contact with air). This accumulated acid is excreted out in urine giving it a dark colored appearance. Alkaptonuria is caused by the deficiency of an enzyme (homogentisic dioxygenase).

The kidney is dysfunctional in the patient; nitrogenous wastes will not be excreted out from his body. As a result, salts and urea will get accumulated in the blood. The dialysis fluid is meant for filtering the blood, so it contains no nitrogenous wastes. As compared to blood, urea and salts should be at a lower concentration in the dialysis fluid.