Excretion In Humans

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Notes

Excretion: Definition & Organs

**Excretion** is the removal of waste products of **metabolic reactions**, toxic materials, and substances in excess of requirements.
The main excretory organs are the **lungs** (excrete CO₂), **kidneys** (excrete excess water, salts, and urea), and the **liver** (produces urea via deamination).
**Egestion** (removal of undigested food via the anus) is **not** excretion.
CO₂ must be excreted because it dissolves in water to form an acidic solution, lowering pH and reducing enzyme activity.
Urea is toxic at high concentrations and must be removed.

The Urinary System

**Kidneys**: two bean-shaped organs that filter blood to form urine.
**Ureter**: tube connecting each kidney to the bladder.
**Bladder**: stores urine until it is released.
**Urethra**: tube that carries urine from the bladder to the exterior.
Note: **ureter** and **urethra** are different tubes — do not confuse them.

Kidney Function & the Nephron

Kidneys regulate **water content** of blood (maintains blood pressure) and excrete **urea** and excess **salts**.
Each kidney contains about **1 million nephrons** (kidney tubules).
Nephrons start in the **cortex**, loop into the **medulla**, and return to the cortex; urine drains into the renal pelvis then ureter.

Ultrafiltration

Blood enters the glomerulus via an **afferent arteriole** at high pressure.
Capillaries narrow, increasing pressure, forcing small molecules out into **Bowman's capsule** — this is **ultrafiltration**.
Filtrate contains: **glucose, water, urea, salts**. Large molecules like **proteins** remain in blood.
The filtrate then flows through the nephron for selective reabsorption.

Selective Reabsorption

**Glucose** is reabsorbed by **active transport** in the **proximal convoluted tubule** (PCT).
PCT cells have many **mitochondria** to supply energy for active transport.
In a healthy person, **all glucose** is reabsorbed; in diabetes, excess glucose remains in urine.
**Salts** are reabsorbed in the **Loop of Henle** by diffusion and active transport; **water** follows by osmosis.
Water reabsorption in the **collecting duct** varies according to the body's needs.

Role of the Liver: Deamination

Excess **amino acids** cannot be stored and are broken down in the liver by **deamination**.
The **amino group** (-NH₂) is removed; the carbon part is converted to **glycogen** and stored.
The nitrogen part is converted to **ammonia** (highly toxic), then immediately to **urea** (less toxic).
Urea dissolves in blood, is carried to the kidneys, and excreted in urine (a small amount in sweat).
High urea levels can cause **cell death**, reduced insulin response, and deposits in blood vessels.

Excretory Products Summary

**Lungs**: excrete CO₂ (from aerobic respiration).
**Kidneys**: excrete excess water, salts, and urea.
**Liver**: produces urea from deamination of excess amino acids; also breaks down alcohol and toxins.
**Skin**: excretes small amounts of urea and salts in sweat.

The human excretory system showing kidneys, ureters, bladder, and urethra.

A nephron showing Bowman's capsule, glomerulus, proximal tubule, Loop of Henle, distal tubule, and collecting duct.

Ultrafiltration at glomerulus and selective reabsorption in proximal tubule.

Deamination of excess amino acids in the liver produces urea, which is excreted by the kidneys.

Practice questions

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1.What is urea produced from?
Easy
- AAmino acids
- BFatty acids
- CGlucose
- DGlycerol
2.Which of the following is a correct example of excretion in mammals?
Easy
- AThe removal of excess salts from the blood.
- BThe production of carbon dioxide from aerobic respiration.
- CThe removal of undigested food via the anus.
- DThe release of hormones from glands.
3.Which of the following is not a main function of the kidneys in a healthy person?
Easy
- ATo breakdown toxins.
- BTo reabsorb all glucose.
- CTo regulate blood pressure.
- DTo eliminate excess sodium and potassium ions.
4.Which of the following substances would be reabsorbed by the kidneys?
Medium
- ASalts and glucose
- BSalts, urea and glycogen
- CSalts and glycogen
- DUrea and glucose
5.Which statement about urea is correct?
Medium
- AUrea is formed from excess amino acids in the liver and excreted by the kidneys.
- BUrea is formed from excess amino acids in the kidneys and excreted by the liver.
- CUrea is formed from excess glucose in the liver and egested by the kidneys.
- DUrea is formed from excess glucose in the kidneys and egested by the liver.
6.The table shows the composition of blood entering the kidneys. Which row shows the composition of urine in a healthy person? substance: glucose 0.20%, urea 0.01%, protein 7.00%
Hard
- Aglucose 0.00%, urea 2.50%, protein 0.00%
- Bglucose 0.00%, urea 2.50%, protein 3.50%
- Cglucose 2.00%, urea 0.00%, protein 7.00%
- Dglucose 0.20%, urea 0.01%, protein 0.00%
7.A person produced 1.75 dm³ of urine on a cool day. How will the volume and concentration of urine change on a hot day, if fluid intake is the same?
Medium
- Avolume decreases, concentration increases
- Bvolume increases, concentration decreases
- Cvolume decreases, concentration decreases
- Dvolume increases, concentration increases
8.Which of the following organs are responsible for removing excretory products from the blood?
Easy
- AKidneys and lungs
- BBladder and kidneys
- CLungs and liver
- DBladder and liver

Slides

What is Excretion?

Excretion is the removal of waste products of metabolic reactions, toxic materials, and substances in excess of requirements.
It is different from egestion, which is the removal of undigested food waste from the anus.
The main excretory organs are the lungs, kidneys, and liver.

Which of the following is a correct example of excretion in mammals?

The production of carbon dioxide from aerobic respiration.
The removal of excess salts from the blood.
The removal of undigested food via the anus.
The release of hormones from glands.

The Excretory System

The kidneys filter blood to remove urea, excess water, and salts, forming urine.
Urine travels from the kidney to the bladder via the ureter.
The bladder stores urine until it is released through the urethra.

The tube connecting the kidney to the bladder is the ____. The tube that carries urine out of the body is the ____.

The Kidney and Nephron

Each kidney contains about one million nephrons (kidney tubules).
Nephrons filter blood and produce urine.
The nephron has several parts: Bowman's capsule, proximal convoluted tubule, Loop of Henle, distal convoluted tubule, and collecting duct.

Which of these are true about the nephron?

Filtration occurs in the Bowman's capsule.
Glucose is reabsorbed in the Loop of Henle.
The collecting duct reabsorbs water.
Urea is reabsorbed into the blood.

Ultrafiltration

Blood enters the glomerulus under high pressure from the renal artery.
Small molecules (water, glucose, urea, salts) are forced out of the capillaries into Bowman's capsule, forming filtrate.
Large molecules like proteins and blood cells remain in the blood.

Which substances are forced out of the blood during ultrafiltration?

Glucose, water, urea, salts
Proteins, glucose, water
Urea, proteins, salts
Red blood cells, glucose, water

Selective Reabsorption

Glucose is reabsorbed by active transport in the proximal convoluted tubule.
All glucose is normally reabsorbed; none appears in urine.
Salts are reabsorbed in the Loop of Henle by diffusion and active transport; water follows by osmosis.
Water reabsorption in the collecting duct varies to maintain water balance.

Glucose is reabsorbed from the filtrate by ____ transport in the ____ convoluted tubule.

The Role of the Liver in Excretion

The liver breaks down excess amino acids in a process called deamination.
The amino group (-NH2) is removed and converted into ammonia, then into urea (less toxic).
Urea is released into the blood and excreted by the kidneys.

What is urea produced from?

Fatty acids
Glucose
Amino acids
Glycerol

Deamination Process

Excess amino acids cannot be stored, so they are broken down in the liver.
The carbon-containing part is converted to glycogen and stored.
The nitrogen-containing part becomes ammonia, then urea.

Which statements about deamination are correct?

Deamination occurs in the kidneys.
Ammonia is converted to urea in the liver.
Urea is less toxic than ammonia.
The carbon part of amino acids is excreted.

Regulation of Urine Concentration

On a hot day, if fluid intake is the same, urine volume decreases and concentration increases.
Exercise also reduces urine volume due to water loss through sweat.
The kidneys adjust water reabsorption to maintain blood water levels.

A person produced 1.75 dm³ of urine on a cool day. How will the volume and concentration change on a hot day, with the same fluid intake?

Volume increases, concentration decreases
Volume increases, concentration increases
Volume decreases, concentration decreases
Volume decreases, concentration increases

Excretion vs Egestion

Excretion: removal of metabolic waste (e.g., urea, CO₂).
Egestion: removal of undigested food from the anus (faeces).
Do not confuse these terms in exams!

The removal of metabolic waste products is called ____. The removal of undigested food is called ____.

Summary of Excretion in Humans

Lungs excrete carbon dioxide.
Liver deaminates amino acids to produce urea.
Kidneys excrete urea, excess water, and salts.
The urinary system includes kidneys, ureters, bladder, and urethra.

Which row correctly matches organs to their excretory products?

Lungs: carbon dioxide; Kidneys: urea; Liver: deamination
Lungs: urea; Kidneys: carbon dioxide; Liver: deamination
Lungs: carbon dioxide; Kidneys: deamination; Liver: urea
Lungs: deamination; Kidneys: urea; Liver: carbon dioxide

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