Human Excretory system: Definition, Diagram, Function

What Is the Human Excretory System? (Definition & Importance)

The human excretory system is an important physiological system, responsible for carrying out an activity of the disposal of waste products and excess substances from the blood. This helps to keep the constancy of the internal environment of the body. This consists of four organs: the kidneys, the ureters, the urinary bladder, and the urethra, all functioning together in blood filtration and the resulting urine production and its elimination from the body.

Excretion is critical to homeostasis since it restores the conditions for the body's interior. Conditions for the volume and composition of body fluids, which in turn establishes blood pressure, electrolyte balance, and acid-base balance are thus maintained with the exact conditions needed to function at the cellular and consequently the health levels.

Major Organs of the Human Excretory System

The human excretory system contains important parts with varied excretory functions.

Kidneys (Primary Excretory Organ)

The kidneys are complex organs with several distinct structures, each serving a specific function in the filtration and excretion process:

Renal Capsule

• Tough, fibrous outer layer that serves to protect the kidney.

Cortex

• The outer region of the kidney contains glomeruli and convoluted tubules.

• Site of ultrafiltration and initial stages of urine formation.

Medulla

• The inner region consists of renal pyramids and renal columns.

• It contains the loops of Henle and collecting ducts.

Renal Pyramids

• Triangular-shaped structures within the medulla.

• It contains nephron loops and collecting ducts that transport urine to the calyces.

Renal Columns

• Bands of cortical tissue that separate the renal pyramids.

• Provide support for the passage of blood vessels through the kidney.

Nephrons

• The microscopic functional units of the kidney. Each nephron is made up of a renal corpuscle and renal tubule.

• There are approximately 1 million nephrons per kidney.

Renal Corpuscle

• It is made up of glomerulus and Bowman's capsule and is the first site of blood filtration.

• Glomerulus: A network of capillaries in which the blood filtration takes place. It is enclosed by Bowman's capsule.

• Bowman's Capsule: A cup-like structure that surrounds the glomerulus, collects the filtrate from the blood.

Renal Tubule

• Proximal Convoluted Tubule (PCT): The first segment is for the renal tubule, reabsorbs water, ions and nutrients back from the filtrate.

• Loop of Henle: The U-shaped part of the renal tubule extends into the medulla. It is responsible for concentrating urine by reabsorbing water and salt.

• Distal Convoluted Tubule (DCT): The last segment of the renal tubule is before the collecting duct which helps in further reabsorption and secretion of ions.

• Collecting Ducts: Ducts receive urine from several nephrons and transport urine to the renal pelvis.

Renal Pelvis

• A funnel-shaped cavity collects urine from collecting ducts.

• Channels urine into the ureter.

Renal Artery & Vein

• Supplies the kidney with blood.

• Branches further into smaller arterioles, which service the glomerulus

• Drains into the inferior vena cava while it conveys deoxygenated blood from the kidney.

Ureters

• Muscular tubes that connect kidneys to the urinary bladder.

• The conveyance of urine from the kidneys to the bladder.

Urinary Bladder

• A hollow, muscular organ located in the pelvis.

• Stores urine until it is eliminated.

Urethra

• Tube extending from the bladder to the outside of the body.

• Involved in the elimination of urine.

Mechanism of Urine Formation (Three Steps)

The net process of urine formation comprises filtration, reabsorption, and secretion. This occurs in the kidneys.

Glomerular Filtration

• Blood enters the glomerulus water and solutes get filtered into Bowman's capsule.

• Filtration is driven by blood pressure.

Tubular Reabsorption

• Essential substances like glucose, amino acids, and ions get reabsorbed into the blood.

• Basically in the proximal convoluted tubule.

• Plays a significant role in the reabsorption process of important nutrients and water balance.

Tubular Secretion

• The secretion of waste products hydrogen ions, potassium, and some drugs into the tubule.

• This contributes to excess unwanted compounds that are eliminated from the blood.

Regulation of Kidney Function

The regulation of kidney functions are mainly through hormones and specific structures to maintain the overall water balance of body fluids and homeostasis.

Hormonal Control

• ADH: increases the amount of water reabsorption by kidneys.

• Aldosterone: increases the amount of Na+ reabsorption and K+ secretion.

• ANP: decreases the amount of Na+ reabsorption and lowers blood pressure.

Mechanism of Osmoregulation

• Water and Electrolyte balance to sustain osmolarity of blood.

• Implores negative feedback loops and hormonal control.

Role of Juxtaglomerular Apparatus

• Specialised cells near the glomerulus

• Produces renin, thus regulating blood pressure and the filtration rate.

Disorders of the Excretory System

Many disorders of the excretory system, which inhibits the proper clearance of wastes and continued homeostasis.

Kidney Stones

• These are caused by low fluid intake, high salt consumption, and genetics.

• Symptoms include severe pain, blood in urine, and frequent urination.

• Treatment are increased fluid intake, medications, and surgery.

Urinary Tract Infections (UTIs)

• Caused by bacterial infection, poor hygiene, and use of a catheter.

• Symptoms include painful urination, an urge to urinate frequently, and cloudy urine.

• Treatment options are antibiotics and increased fluid intake.

Chronic Kidney Disease (CKD)

• Stages 1 to 5, based on GFR

• Caused by diabetes, blood pressure, glomerulonephritis.

• Symptoms include fatigue, swelling, nausea, and decreased urine output.

• Treatment options include behavioural changes, medications, dialysis, and kidney transplants.

Human Excretory System NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Major organs in excretory system

• Regulation of kidney function

Practice Questions for NEET

Q1. With its anatomical positioning and the surrounding peritoneum, what distinct characteristic defines the retroperitoneal kidney?

1. Kidney of a fish

2. Kidney covered by Peritoneum on ventral side

3. Kidney covered by Peritoneum on Dorsal side

4. Kidney uncovered by Peritoneum on ventral Side

Correct answer: 2) Kidney covered by Peritoneum on ventral side

Explanation:

In the retroperitoneal position, the kidney is located behind the peritoneum, specifically covered by it on the ventral side. This distinguishes it from other options where the kidney is either covered by the peritoneum on the dorsal side (option 3) or not covered by the peritoneum on the ventral side (option 4). The retroperitoneal positioning of the kidney is common in mammals, including humans.

Hence, the correct answer is option 2) The Kidney is covered by the Peritoneum on the ventral side.

Q2. What is the function of the renal pelvis in the human excretory system?

1. To filter waste from the blood

2. To store urine temporarily

3. To transport urine from the kidneys to the bladder

4. To collect urine from the nephrons and direct it to the ureters

Correct answer: 4) To collect urine from the nephrons and direct it to the ureters

Explanation:

The primary purpose of the renal pelvis, a funnel-shaped organ in the centre of the kidney, is to collect urine that has been produced by the nephrons. The tiny kidney structure known as a nephron filters waste and extra water from the blood. The nephrons also create urine, which is then transported through a network of tiny tubes known as collecting ducts and excreted into the renal pelvis. The ureters, which are muscular tubes that carry urine from the kidneys to the bladder for momentary storage before eventual elimination from the body, are then directed to receive the urine by the renal pelvis.

Hence, the correct answer is option 4) To collect urine from the nephrons and direct it to the ureters.

Q3. Select the incorrect pair and write the role of organs in excretion:

1. Lungs - a large amount of CO2

2. Skin - sweat, urea, lactic acid

3. Sweat gland - facilitates cooling effect on body surface

4. None of these

Correct answer: 4) None of these

Explanation:

The skin and lungs play significant roles in excretion. Sweat glands in the skin produce sweat, a watery fluid containing NaCl, urea, lactic acid, and other substances, facilitating the removal of metabolic wastes while primarily aiding in cooling the body's surface. Sebaceous glands also contribute by eliminating certain substances through their secretions. The lungs complement this process by expelling large amounts of carbon dioxide, approximately 200 mL per minute, along with significant quantities of water vapor daily, underscoring their critical role in maintaining homeostasis and excretory balance.

Hence, the correct answer is option 4) None of these.

Also Read:

• MCQs on Human Excretory System

• Ultrafiltration

• Hemodialysis

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