The Kidney

The kidney is a bean-shaped organ that removes waste products from the blood, balances body
fluids, controls electrolytes, and regulates blood pressure.

🔎 Functions of the Kidney

Function Explanation

Excretion Removes waste products like urea, creatinine, and uric acid from the blood.

Osmoregulation Regulates water balance to maintain blood osmotic pressure.

Electrolyte balance Controls levels of ions (e.g. sodium, potassium, calcium, chloride).

Acid-base balance Regulates blood pH.

Blood pressure regulation Produces renin, which helps control blood pressure.

Produces erythropoietin, stimulating red blood cell formation in bone

Red blood cell production
marrow.

Detoxification Helps remove toxins and drugs from the bloodstream.

🧬 Structure of the Kidney

Part Function

Cortex Outer layer; contains most nephrons.

Medulla Inner part; contains renal pyramids and collecting ducts.

Renal pelvis Collects urine before it passes to the ureter.

Ureter Carries urine to the bladder.

Renal artery Brings oxygenated, waste-rich blood into kidney.

Renal vein Carries clean, deoxygenated blood away from kidney.

🔬 The Nephron (Functional Unit of the Kidney)
Part Function

Bowman’s capsule Receives filtered substances from blood.

Glomerulus Filters small molecules from blood (ultrafiltration).

Proximal tubule Reabsorbs glucose, amino acids, and salts.

Loop of Henle Concentrates urine by reabsorbing water and salt.

Distal tubule Further adjusts salt and water balance.

Collecting duct Final site of water reabsorption, influenced by ADH.

🔄 Processes in the Nephron

Ultrafiltration (in glomerulus)

Selective Reabsorption (mainly in proximal tubule)
Secretion (removal of excess ions, toxins)
Concentration of urine (loop of Henle and collecting duct)

💧 Hormones Controlling Kidney Function

Hormone Effect

ADH (Antidiuretic Hormone) Increases water reabsorption in collecting duct.

Aldosterone Increases sodium reabsorption, affecting water retention.

Erythropoietin Stimulates red blood cell production.

🩺 Disorders of the Kidney

 Kidney failure → requires dialysis or transplant.

 Kidney stones
 Urinary tract infections (UTIs)
 Nephritis

🌍 Significance of the Kidney

 Essential for survival.

 Keeps internal environment (homeostasis) stable.
  Allows proper functioning of all body systems.

🚽 Process of Urine Formation

Urine formation occurs in the nephron (functional unit of the kidney) and involves 3 main
processes:

1. Ultrafiltration (Filtration)

 Where?
In the glomerulus and Bowman’s capsule.
 Process:
Blood enters the glomerulus under high pressure.
Small molecules like water, glucose, salts, urea, and amino acids are forced out of the
blood into Bowman’s capsule.
 What remains in blood?
Large molecules like proteins and blood cells (they are too big to be filtered).
 Result:
Formation of glomerular filtrate (filtrate containing waste and useful substances).

2.Selective Reabsorption

 Where?
Mainly in the proximal convoluted tubule, loop of Henle, distal tubule, and collecting
duct.
 Process:
The body reabsorbs useful substances from the filtrate back into the blood.
 Substances reabsorbed:
o Glucose (completely reabsorbed in proximal tubule).
o Amino acids.
o Most water (by osmosis).
o Ions (sodium, chloride, potassium, etc.).
 Hormones involved:
o ADH (Antidiuretic Hormone) → controls water reabsorption in collecting duct.
o Aldosterone → controls sodium reabsorption.
3. Tubular Secretion (Active Secretion)

 Where?
In distal convoluted tubule and collecting duct.
 Process:
Additional waste products (e.g. hydrogen ions, drugs, toxins) are secreted from the blood
into the filtrate.
 Purpose:
Helps regulate pH and remove unwanted substances not filtered initially.

4️. Excretion of Urine

 After filtration, reabsorption, and secretion, the remaining fluid is urine.

 Urine composition:
o Water
o Urea
o Salts
o Creatinine
o Excess ions
 Urine flows:
o Collecting duct → Renal pelvis → Ureter → Bladder → Urethra → Excreted.

📊 Summary Table:
Stage Location Main Activity

Ultrafiltration Glomerulus & Bowman’s capsule Filtration of blood

Selective Reabsorption Proximal tubule, Loop of Henle, Distal tubule Reabsorption of useful substances

Secretion Distal tubule & collecting duct Active addition of wastes

Excretion Collecting duct to urethra Removal of urine

🌀 The Loop of Henle

The Loop of Henle has two limbs:
  Descending limb
 Ascending limb

Its main role is to help concentrate urine and conserve water.

🔽 Descending Limb of Loop of Henle

What happens:

 Permeable to water.
 Impermeable to salts and ions.
 As filtrate moves down, water moves out into the surrounding salty medulla by osmosis.
 Filtrate becomes more concentrated as water leaves.

Adaptations:

 Thin walls which allow easy movement of water.

 Numerous aquaporin (water channels) which facilitate water reabsorption.
 Surrounded by salty medullary interstitial fluid which creates osmotic gradient.

🔼 Ascending Limb of Loop of Henle

What happens:

 Impermeable to water.
 Permeable to salts.
 Sodium and chloride ions are actively transported out into the medulla.
 This maintains the high salt concentration in the medulla needed for water reabsorption
from the collecting duct.
 Filtrate becomes more dilute as ions leave but water stays.

Adaptations:

 Thick segment (thick ascending limb) has many mitochondria which provides energy for
active transport of ions.
 Impermeability to water which ensures only salts leave.
 Sodium-potassium pumps which move salts out actively.

🔄 Overall Function of Loop of Henle

 Creates a counter current multiplier system.

 Establishes a high concentration of solutes in the medulla.
  Allows kidneys to produce concentrated urine, conserving water.

🏷 Adaptations of Other Parts of the Nephron

1️. Bowman's Capsule and Glomerulus (Ultrafiltration)

 Thin, one-cell thick capillary walls which easy filtration.

 Fenestrated capillaries which allow small molecules through.
 High blood pressure in glomerulus which forces filtrate out.
 Podocytes (special cells) in Bowman’s capsule which prevent large molecules from
filtering through.

2️. Proximal Convoluted Tubule (Selective Reabsorption)

 Long and coiled which provides large surface area.

 Lined with microvilli which further increase surface area.
 Many mitochondria which supply energy for active transport.
 Selective transport proteins which reabsorb glucose, amino acids, ions.
 Tight junctions which prevent leakage of substances back into the tubule.

3️. Distal Convoluted Tubule (Fine-tuning salt and pH balance)

 Responds to aldosterone which regulates sodium and potassium balance.

 Involved in pH regulation which secretes hydrogen ions.
 Contains mitochondria which energy for active transport.

4️. Collecting Duct (Water Reabsorption)

 Responsive to ADH which inserts aquaporins to increase water reabsorption.

 Surrounded by salty medulla which creates osmotic gradient That draws water out.
 Long length which allows maximum reabsorption.
🔬 Summary Table
Nephron Part Main Function Key Adaptations

Bowman’s capsule Ultrafiltration Thin membranes, high pressure, podocytes

Proximal tubule Reabsorption Microvilli, mitochondria

Descending limb Water reabsorption Permeable to water, thin wall

Ascending limb Salt reabsorption Active transport, mitochondria

Distal tubule Ion and pH regulation Aldosterone-sensitive, mitochondria

Collecting duct Final water reabsorption ADH-sensitive, osmotic gradient

🌿 Excretory Products in Plants and Their Excretion

🔎 Why Plants Excrete:

 Like animals, plants produce waste materials during metabolic activities such as
respiration, photosynthesis, and other biochemical reactions.
 However, plants produce less harmful wastes than animals and have simpler methods of
excretion.

🧪 Main Excretory Products in Plants

Excretory Product Source

Oxygen (O₂) Photosynthesis (light reaction)

Carbon dioxide (CO₂) Respiration

Water (H₂O) Respiration, transpiration

Tannins Secondary metabolism

Alkaloids (e.g. caffeine, nicotine) Secondary metabolism

Resins and gums Old or damaged cells

 Excretory Product Source

Latex Specialized cells (laticifers)

Anthocyanins By-products of metabolism

Organic acids (oxalic acid, citric acid) Metabolic activities

Essential oils Secondary metabolism

🌀 How Plants Excrete Wastes

Method Explanation Example

Gaseous wastes (O₂, CO₂) diffuse through O₂ released during

Diffusion
stomata, lenticels, or roots. photosynthesis.

Water vapor carrying some wastes is lost

Transpiration Water excreted in vapor form.
through stomata.

Toxic substances stored away from Alkaloids, tannins stored in

Storage in Vacuoles
metabolism. vacuoles.

Shedding of leaves, Waste products accumulate in old leaves, Deciduous trees shed leaves
bark, or fruit bark or fruit, which fall off. containing waste.

Wastes like resins, latex, gums are secreted

Exudation Rubber plant excretes latex.
to the plant surface.

🔬 Key Differences Between Plant and Animal Excretion

Feature Plants Animals

Metabolic rate Lower Higher

Waste toxicity Less toxic Often toxic

Excretion organs No specialized organs Kidneys, liver, lungs, skin

Main method Storage, diffusion, transpiration Specialized excretory systems

🌍 Significance of Excretion in Plants

 Prevents accumulation of harmful substances.

 Maintains internal chemical balance.
 Some waste products have economic value (e.g., resins, latex, essential oils).
🔬 Main Processes of Excretion in Humans

1.Excretion by the Kidneys (Urine Formation)

a) Ultrafiltration

 Occurs in glomerulus and Bowman’s capsule.

 Filters water, glucose, urea, and salts from blood.

b) Selective Reabsorption

 Occurs mainly in proximal tubule.

 Useful substances (glucose, amino acids, some water and ions) are reabsorbed into blood.

c) Tubular Secretion

 Occurs in distal tubule.

 Active removal of excess ions, hydrogen ions, and toxins.

d) Formation and Excretion of Urine

 The remaining fluid is urine → stored in bladder → excreted via urethr

2.Excretion by the Lungs

 Carbon dioxide (from cellular respiration) is transported to lungs via blood.

 Excreted by breathing out (exhalation).
 Water vapor also lost.

3️.Excretion by the Skin (Sweating)

 Sweat glands excrete:

o Water
o Salts
o Small amounts of urea
 Also helps regulate body temperature through cooling.

4️. Excretion by the Liver

 Converts toxic ammonia into urea (via urea cycle).

  Breaks down worn-out red blood cells.
 Excretes bile pigments (bilirubin, biliverdin) into bile.
 Detoxifies drugs and poisons.

5️.Excretion by the Intestines

 Small amounts of bile pigments and cholesterol are excreted with feces.

🔎 Summary Table
Process Main Product Organ Involved

Ultrafiltration & reabsorption Urea, water, salts Kidneys

Gas exchange CO₂, water vapor Lungs

Sweating Water, salts, urea Skin

Detoxification Urea, bile pigments Liver

Defecation (minor excretion) Bile pigments, cholesterol Intestines