- The excretory system is vital for removing metabolic waste from the body. The kidneys, the primary organs, filter blood to produce urine through a three-stage process within microscopic units called nephrons.
- Components of the Excretory System
The human excretory system is a network of organs and tubes designed to eliminate waste.
Kidneys: These are the main filtering organs. They’re a pair of bean-shaped structures located on either side of the spine.
Ureters: These tubes carry urine from the kidneys down to the urinary bladder.
Urinary Bladder: A muscular sac that stores urine until it’s released from the body.
Urethra: This tube expels urine from the bladder to the outside.
- The Nephron: The Kidney’s Functional Unit
The nephron is the fundamental unit of the kidney responsible for filtering blood and creating urine. Each nephron has two primary parts:
Glomerulus: A cluster of tiny capillaries where initial blood filtration occurs.
Renal Tubule: A long, coiled tube that processes the filtered fluid.
Urine formation occurs in three key stages within the nephron:
Glomerular Filtration: Blood pressure forces water, salts, glucose, urea, and other small molecules from the glomerulus into a cup-like structure called the Bowman’s capsule. This fluid is known as the glomerular filtrate.
- Tubular Reabsorption: As the filtrate moves through the renal tubule, essential substances like glucose, water, and amino acids are reabsorbed back into the bloodstream, preventing the body from losing valuable nutrients.
- Tubular Secretion: Waste products and excess ions not removed during filtration are actively secreted from the blood into the renal tubule. This helps maintain the body’s pH and electrolyte balance. The remaining fluid is now urine.
- Urine Composition and Regulation
- Urine is a watery, yellowish fluid consisting of about 95% water. The remaining 5% is composed of waste products like urea (from protein metabolism), uric acid, and creatinine. The volume and concentration of urine are regulated by the hormone antidiuretic hormone (ADH), which signals the kidneys to reabsorb more water when the body is dehydrated.
- Role of Other Excretory Organs
- While the kidneys are the main players, other organs also contribute to waste removal:
- Lungs: Remove carbon dioxide and water vapor from the blood.
- Skin: Excretes salts and a small amount of urea through sweat.
- Liver: Breaks down and excretes bile pigments (from hemoglobin breakdown) into bile, which is then eliminated with feces.
- This collective effort ensures the body maintains a stable internal environment, a process known as homeostasis.
A. MULTIPLE CHOICE TYPE: (select the most appropriate option in each case)
1) Excretion primarily involves
(a) removal of all byproducts during catabolism
(b) removal by products during anabolism
(c) removal of nitrogenous wastes
(d) throwing out excess water
Ans: (c) Removal of nitrogenous wastes.
2) Maximum amount of water from the glomerular filtrate is reabsorbed in (a) proximal convoluted tubule
(b) descending limbs of loop of Henle
(c) ascending limb of loop of Henle
(d) distal convoluted tubule
Ans: (a) Proximal convoluted tubule
3) Which one of the following in real sense is NOT an excretory activity?
(a) giving out carbon dioxide
(b) passing out faecal matter
(c) sweating
(d) Removal of urea
Ans: (c) Sweating
4) In humans, urea is formed in
(a) ureter
(b) liver
(c) spleen
(d) Kidney
Ans: (b) liver
B. VERY SHORT ANSWER TYPE:
1) Name the following:
(a) The organ which produces urea
(b) The outer region of kidney containing the Bowman’s capsule
(c) The tuft of capillaries inside the Bowman’s capsule
(d) The part of kidney tubules where the term urine is first used for the fluid in it
(e) The vein in which urea concentration is maximum
Ans: (a) The liver is the organ that produces urea. It does so by converting ammonia, a toxic byproduct of protein metabolism, into the less harmful substance, urea.
(b) The outer region of the kidney containing the Bowman’s capsule is the renal cortex. This is where the initial filtration of blood takes place.
(c) This is a key component of the nephron, the functional unit of the kidney, and is where blood is filtered to form a fluid called glomerular filtrate.
(d) At this stage, reabsorption of water has largely been completed, and the fluid is now concentrated and referred to as urine.
(e) This vein carries blood rich in nutrients and toxins, including urea, from the digestive organs to the liver for processing.
2) Given below are two sets (a and b) of five terms each.
Rewrite the terms in their correct order so as to be in logical sequence.
(a) Afferent arteriole, renal vein, capillary network, glomerulus, efferent arteriole.
(b) Renal artery, urethra, ureter, kidney, urinary bladder.
Ans: (a) Afferent arteriole, glomerulus, efferent arteriole, capillary network, renal vein
(b) Renal artery, kidney, ureter, urinary bladder, urethra
3) In each one of the following sets of body parts or substances or processed, pick out the one item which overall includes the remaining four.
(a) Glomerular filtrate, bowman’s capsule, ultrafiltration, glomerulus, blood plasma. (b) skin, liver, lungs, kidney, excretion
(c) ADH, Water, pituitary, osmoregulation, urine
(d) CO2, bile pigments, water, excretion, urea.
Ans:(a) Ultrafiltration
(b) Excretion
(c) Osmoregulation
(d) Excretion
C. SHORT ANSWER TYPE:
1) Write down the functional activity of the following parts,
(a) Glomerulus …………………
(b) Henle’s loop ………………
(c) Ureter ………………………
(d) Renal artery ………………
(e) Urethra ……………………
Ans:
(a) Glomerulus: Acts as a fine filter. It filters blood under high pressure, separating water, waste products, and small molecules from blood cells and large proteins. This process is called ultrafiltration.
(b) Henle’s loop: This U-shaped tube is crucial for water conservation. It reabsorbs water and sodium ions from the filtered fluid, which helps in producing concentrated urine.
(c) Ureter: A muscular tube that transports urine from the kidney to the urinary bladder using wave-like contractions called peristalsis.
(d) Renal artery: This major artery brings impure, oxygenated blood (containing waste products like urea) to the kidneys to be filtered and cleaned.
(e) Urethra: The urethra is the tube responsible for carrying urine from the bladder to exit the body. In males, it uniquely serves a second role by also providing a path for semen during ejaculation
2) Why is excretion necessary? Name the common excretory substance in our body.
Ans: Excretion is essential because it’s the body’s system for getting rid of waste products and toxins that are produced during metabolism. Without this process, these harmful substances would build up to dangerous levels, potentially damaging cells and organs. It’s a critical function for maintaining homeostasis, which is the stable internal environment necessary for the body to function correctly.
The most common excretory substance is urea, a waste product from protein breakdown. Other common substances include carbon dioxide (exhaled from the lungs), excess water and salts (excreted in sweat and urine), and uric acid (from nucleic acid breakdown). Bile pigments, like bilirubin, are also eliminated in feces.
3) What is a uriniferous tubule? How does it function?
Ans: A uriniferous tubule, also known as a nephron, is the microscopic, functional unit of the kidney.
How It Functions
The tubule works in a three-step process:
- Filtration: Blood pressure forces water and small solutes, like salts and waste products, out of a capillary network called the glomerulus and into the tubule. Larger components, such as blood cells and proteins, stay in the blood.
- Reabsorption: As the filtered fluid, now called filtrate, moves through the tubule, the body reclaims essential substances. Most water, glucose, and salts are reabsorbed back into the bloodstream to prevent their loss.
- Secretion: The tubule actively secretes extra waste, excess ions, and toxins from the blood directly into the filtrate. This final step helps fine-tune the blood’s chemical balance
4) Why is it necessary to maintain a normal osmotic concentration of the blood?
Ans: Maintaining a stable osmotic concentration of the blood is critical for the body’s physiological balance. This is primarily because it protects cellular integrity and helps regulate blood pressure.
Cellular Integrity
Cells need to be in an environment with a stable osmotic concentration to function correctly. If the blood becomes too concentrated (hyperosmotic), water leaves the cells, causing them to shrink and become damaged. Conversely, if the blood becomes too dilute (hypoosmotic), water enters the cells, causing them to swell and potentially burst. Both scenarios are harmful to tissues and organs.
Blood Pressure Regulation
Blood osmolarity directly affects blood volume and pressure. When the blood’s solute concentration is high, water is drawn from surrounding tissues into the bloodstream, increasing blood volume and raising blood pressure. The opposite occurs when the concentration is low, leading to a decrease in blood volume and pressure. The kidneys and the hypothalamus work together to manage this balance through mechanisms that control thirst and the retention or excretion of water and solutes.
5) If you donate one kidney to a needy patient, would it cause any harm to you? Give a reason.
Ans: Donating one kidney is generally safe and does not cause significant harm to the donor. The reason is that the remaining single kidney is capable of performing the functions of both kidneys. Over time, the remaining kidney will increase in size (a process called compensatory hypertrophy) to handle the workload of filtering blood and producing urine for the entire body. While there are standard surgical risks and a recovery period, a person can live a long and healthy life with just one kidney.
6) In summer the urine is slightly thicker than in winter. Explain the reason.
Ans: In summer, the urine is slightly thicker due to the body’s need to conserve water. During hot weather, we sweat more to cool down. This process causes a loss of water and electrolytes from our bodies. To compensate for this fluid loss and prevent dehydration, the kidneys reabsorb more water from the urine.
This reabsorption makes the remaining urine more concentrated, or “thicker,” with waste products like urea. In contrast, during winter, we sweat less, and our bodies retain more water. This means the kidneys don’t need to reabsorb as much water, resulting in more dilute, or “thinner,” urine. The body’s ability to adjust urine concentration is a crucial part of maintaining homeostasis, the stable internal environment necessary for survival.
7) Differentiate between the following pairs of terms:
(a) Bowman’s capsule and Malpighian capsule.
(b) Renal cortex and renal medulla
(c) Renal pelvis and renal papilla
(d) Urea and urine
(e) Excretion and katabolism
Ans:
a) Bowman’s Capsule vs. Malpighian Capsule:
Bowman’s Capsule: A cup-like structure in the nephron that surrounds the glomerulus, involved in ultrafiltration of blood.
Malpighian Capsule: Refers to the Bowman’s capsule + Glomerulus together. It is the functional filtration unit of the kidney
b) Renal Cortex vs. Renal Medulla:
1)Location:
Renal Cortex: Outer, lighter region of the kidney.
Renal Medulla: Inner, darker region beneath the cortex.
2)Structure:
Cortex: Contains glomeruli, Bowman’s capsules, and convoluted tubules.
Medulla: Has renal pyramids, loops of Henle, and collecting ducts.
3)Function:
Cortex: Filters blood (ultrafiltration) and initiates urine formation.
Medulla: Concentrates urine and regulates water/electrolyte balance.
4)Appearance:
Cortex: Granular due to nephrons.
Medulla: Striated due to tubules and ducts.
Key Difference: The cortex filters blood, while the medulla fine-tunes urine concentration.
c)Renal Pelvis & Renal Papilla:
The renal pelvis is a funnel-shaped area in the kidney that collects urine and directs it to the ureter. The renal papilla is the tip of the renal pyramid where urine from collecting ducts enters the calyces.
d) Urea & Urine:
Urea is a waste product from protein breakdown in the liver and is the main nitrogenous compound in urine. It plays a crucial role in maintaining homeostasis by eliminating excess water, salts, toxins, and metabolic byproducts that could otherwise harm the body if allowed to accumulate. The primary components of urine include water, salts, urea, and various other dissolved substances that the body no longer requires.
e) Excretion & Catabolism:
Excretion removes metabolic waste from the body. Catabolism breaks down complex molecules into simpler ones, generating waste that is later excreted.
8) Name the main nitrogenous metabolic waste excreted out by mammals including humans.
Ans: Urea, creatinine, uric acid
9) Match the terms in Column I with those in Column II and write down the matching pairs.
| Column I | Column II |
| (a) Bowman’s Capsule | Renal artery |
| (b) Contains more CO2 and less urea | Regulates amount of water excreted |
| (c) Anti-diuretic hormone | Renal Vein |
| (d) Contains more urea | Glomerulus |
Ans:
| Column I | Column II |
| (a) Bowman’s Capsule | Glomerulus |
| (b) Contains more CO2 and less urea | Renal Vein |
| (c) Anti-diuretic hormone | Regulates amount of water excreted |
| (d) Contains more urea | Renal artery |
10) In a nephron, the ………….. flows through the ………….. under great pressure. The reason for this great pressure is that the ………….. (outgoing) ………….. is narrower than the ………….. (incoming). This high pressure causes the………….. part of the blood to filter out from the ………….. into the renal capsule.
Ans: In a nephron, the blood flows through the glomerulus under great pressure. The reason for this great pressure is that the efferent (outgoing) arteriole is narrower than the afferent arteriole (incoming). This high pressure causes the liquid part of the blood to filter out from the glomerulus into the renal capsule.
D. LONG ANSWER TYPE:
1) Define the following terms:
(a) Ultrafiltration
(b) Micturition
(c) Renal pelvis
(d) urea
(e) Osmoregulation
Ans: (a) Ultrafiltration is the initial step of urine formation where blood is filtered in the glomerulus under high pressure, separating water and small solutes from larger molecules like proteins and blood cells.
(b) It involves the contraction of the bladder muscles and the relaxation of the sphincter.
(c) The renal pelvis is the central, funnel-like structure in the kidney that receives urine from the collecting ducts and guides it into the ureter for transport to the bladder.
(d) Urea is a waste product of protein metabolism, synthesized in the liver and transported by the blood to the kidneys for excretion in urine.
(e) The kidneys are central to this process by regulating urine concentration.
2) Explain the terms ultrafiltration and selective absorption
Ans: Ultrafiltration
It happens in the glomerulus, a network of capillaries inside the Bowman’s capsule in the kidney. Here, blood pressure forces water, small solutes (like glucose, amino acids, and salts), and waste products (like urea) from the blood into the Bowman’s capsule. This process is called “ultrafiltration” because the glomerular capillaries act like a very fine filter, allowing small molecules to pass through while retaining larger components like blood cells and proteins. The fluid formed is called glomerular filtrate.
Selective Absorption
Selective absorption, or reabsorption, is the process that follows ultrafiltration. Here, the body actively and passively reclaims essential substances. The cells of the tubule walls transport useful molecules like glucose, amino acids, and most of the water and salts back into the bloodstream. This process is “selective” because the body only reabsorbs what it needs, while waste products like urea remain in the tubule to be excreted as urine.
3) What is dialysis? Under what condition is it carried out?
Ans: Dialysis is a medical procedure that removes waste products and excess fluids from the blood when the kidneys fail to function properly. It is carried out in conditions like kidney failure (end-stage renal disease – ESRD) or when the kidneys lose their ability to filter blood effectively. Dialysis helps maintain balance in the body by performing the kidney’s essential functions artificially
E. STRUCTURED / APPLICATION/ SKILL TYPE:
1) Look at the figure given below, it is a section of the human kidney as seen from the front.
(a) Is it the left kidney or the right one? Give reason in support of your answer.
(b) Is it a longitudinal section or a cross-section?
(c) Name the parts numbered 1-5
(d) which area/part (give its name and the number given on the diagram) contains the following respectively:
(i) malpighian capsule
(ii) The pyramids
(iii) Freshly collected urine
Ans: (a) In diagrams, if the kidney’s hilum (where vessels and ureters connect) faces left, it’s the left kidney, as it always points toward the midline.
(b) A longitudinal section cuts the kidney lengthwise, showing layers like the cortex, medulla, and pelvis.
(c) Parts:
Renal artery – Supplies blood.
Renal vein – Drains blood.
Ureter – Carries urine to the bladder.
Cortex – Outer layer.
Renal pelvis – Collects urine.
(d) Associations:
(i) Malpighian capsule (glomerulus + Bowman’s) is in the cortex (4).
(ii) Pyramids are in the medulla.
(iii) Urine flows from renal papillae → calyces → renal pelvis (5) → ureter.
2) Given alongside is the figure of certain organs and associated parts in the human body. Study the same and answer the questions that follow:
(a) Name all the organ systems shown completely or even partially
(b) name the parts numbered 1 to 5
(c) Name the structural and functional unit of the part marked ‘1’
(d) name the two main organic constituents of the fluid that flows down the part labelled ‘3’
(e) Name the two major steps involved in the formation of the fluid that passes down the part labelled ‘3’
Ans: (a) Left vs. Right Kidney: Your statement is correct. In anatomical diagrams, the hilum (the indented region where structures enter and exit) always faces the midline of the body. If the hilum on a single kidney diagram is on the left side, it represents the left kidney.
(b) Longitudinal Section: This is a correct description of a longitudinal section. It’s a vertical cut that allows you to view the internal structures of an organ, such as the cortex, medulla, and renal pelvis of the kidney.
(c) Part 1 is the Kidney: The kidneys filter blood and produce urine.
Part 2 is the Aorta/Inferior Vena Cava: The main blood vessels supplying and draining the kidneys. The renal artery branches from the aorta to supply the kidney, and the renal vein drains into the inferior vena cava.
Part 3 is the Ureter: This tube transports urine from the kidney to the bladder.
Part 4 is the Urinary Bladder: A muscular sac that stores urine.
Part 5 is the Urethra: The tube that allows urine to exit the body from the bladder.
(d) (i) Malpighian capsule: Found in the cortex of the kidney, not the bladder (part 4).
(ii) Pyramids: These are located in the medulla of the kidney.
(iii) Urine flow: Urine travels from the renal papillae, through the calyces, into the renal pelvis, and then down the ureter (part 3) to the bladder (part 4).
3) The following diagram represents a mammalian kidney tubule (nephron) and its blood supply. Parts indicated by the guidelines 1 to 8 are as follows:
1. Afferent arteriole from renal artery
2. efferent arteriole
3. Bowman’s capsule
4. Glomerulus
5. Proximal convoluted tubule with blood capillaries
6. Distal convoluted tubule with blood capillaries
7. collecting tubule
8. U-shaped loop of Henle
Study the diagram and answer the question that follow:
(a) where does ultrafiltration take place?
(b) Which structure contains the lowest concentration of urea?
(c) Which structure contains the highest concentration of urea?
(d) Which structure (normally) contains the lowest concentration of glucose?
(e) where is most water reabsorbed?
Ans: (a) Ultrafiltration
Occurs in the glomerulus and Bowman’s capsule. High-pressure filtration removes water and solutes from blood, forming glomerular filtrate.
(b) Lowest Urea Concentration
Renal vein—blood here has less urea after kidney filtration.
(c) Highest Urea Concentration
Collecting duct—urea becomes highly concentrated as water is reabsorbed.
(d) Lowest Glucose Concentration
This is because glucose is a valuable substance that the body reabsorbs almost entirely. The reabsorption happens primarily in the proximal convoluted tubule, which is the first part of the nephron after Bowman’s capsule. By the time the filtrate reaches the collecting duct, virtually all the glucose has been returned to the bloodstream, so its concentration in the remaining fluid is negligible.
(e) Water Reabsorption
Mainly in the proximal convoluted tubule (most water) and loop of Henle (further concentrates urine).
4) Given alongside is a highly simplified (but also somewhat wrong) diagram of the human kidney cut open longitudinally. Answer the questions that follow.
(a) Define excretion
(b) Name the functional units of the kidneys.
(c) Why does the cortex of the kidney show a dotted appearance?
(d) Mention two functions of the kidney.
(e) Write two differences in the composition of the blood flowing through the blood vessels, ‘A’ and ‘B’[There is an error in the diagram. Can you identify it?]
Ans: (a) Excretion removes waste like urea and salts from the body.
(b) Nephrons are the kidney’s functional units, filtering blood and making urine.
(c) The kidney cortex looks dotted due to many glomeruli and tubules.
(d) Kidney functions:
Removes waste (e.g., urea).
Balances water and salts (osmoregulation).
(e) If B = renal artery, A = renal vein:
Urea: High in B, low in A.
Oxygen: High in B, low in A.
Diagram error: Both A and B show blood leaving, but the renal artery should carry blood to the kidney.
5) Study the diagram given alongside and then answer the questions that follow:
(a) Name the region in the kidney where the above structure is present?
(b) Name the parts labelled 1, 2, 3 and 4.
(c) Name the stages involved in the formation of urine
(d) What is the technical term given to the process occurring in 2 and 3? Briefly describe the process.
Ans: (a) Nephron Location:
The nephron is found in the renal cortex (contains glomerulus, Bowman’s capsule, and convoluted tubules) and the renal medulla (holds loops of Henle and collects ducts).
(b) Nephron Components:
Malpighian capsule (glomerulus + Bowman’s capsule)
Proximal convoluted tubule (PCT)
Loop of Henle
Distal convoluted tubule (DCT) + collecting duct
(c) Urine Formation Stages:
Ultrafiltration: Blood filtered in glomerulus.
Selective reabsorption: Useful substances (glucose, water) reabsorbed.
Tubular secretion: Wastes actively secreted into tubules.
(d) Tubule Processes:
Reabsorption: Retrieves nutrients/water back into blood.
Secretion: Removes excess wastes into urine.
