14.2.1
Kidneys & Osmoregulation
Excretion and the Kidneys
Excretion and the Kidneys
The kidneys work alongside the liver in the excretion of waste products. Urea is produced in the liver through deamination of amino acids. Urea is toxic and is excreted by the kidneys.


Kidney structure
Kidney structure
- The kidneys are surrounded by the capsule to protect the fragile structures underneath.
- Nephrons extend from the cortex into the medulla and are the site of ultrafiltration, selective reabsorption and urine formation.
- The renal tubule of each nephron is lined with microvilli to increase the surface area for absorption.


Ultrafiltration
Ultrafiltration
- Ultrafiltration is carried out in the glomerular capsule (often called the Bowman's capsule).
- Everything except larger proteins, red blood cells, and most white blood cells is forced out of the blood and into the glomerular capsule due to the high pressure caused by the difference in diameter between the afferent and efferent arterioles.
- The afferent arteriole is much wider than the efferent arteriole.
- Everything except larger proteins, red blood cells, and most white blood cells is forced out of the blood and into the glomerular capsule due to the high pressure caused by the difference in diameter between the afferent and efferent arterioles.


Selective reabsorption
Selective reabsorption
- Water, amino acids, glucose and ions are selectively reabsorbed first in the proximal convoluted tubule (PCT).
- The final composition of urine produced is dependent on:
- Selective reabsorption out of the nephron.
- Secretion of nitrogenous waste into the nephron by surrounding capillaries.
- The final composition of urine produced is dependent on:


Loop of Henle
Loop of Henle
- After the proximal convoluted tubule, water is also reabsorbed into the blood by osmosis in the descending limb of the loop of Henle.
- Ions are reabsorbed in the ascending limb of the loop of Henle.
- The ion concentration gradient set up in the area between the limbs drives osmosis in the descending limb.
- Approximately 95% of urine is water.


Urine excretion
Urine excretion
- Urine is formed from each nephron in collecting ducts.
- The collecting ducts converge in the renal pelvis to form the ureter of each kidney.
- The urine passes down the ureters and into the bladder.
- When the bladder is full, urine passes through the urethra and out of the body.
Osmoregulation and the Kidneys
Osmoregulation and the Kidneys
The kidneys are vital in osmoregulation and control water balance by reabsorbing more water when the body is dehydrated. This affects blood pressure and volume and the amount of water in the tissues.


Osmoreception
Osmoreception
- Osmoreceptors in the hypothalamus detect water composition in the blood.
- If the water level drops (due to increased sweating, decreased water intake or vomiting), the hypothalamus secretes antidiuretic hormone (ADH) and signals for the posterior pituitary gland to release it into the bloodstream.


Antidiuretic hormone (ADH)
Antidiuretic hormone (ADH)
- When the ADH reaches the cells in the collecting ducts, they become more permeable to water and so reabsorb more water.
- This increases the concentration of the urine and it appears darker.
- High concentrations of ADH constrict blood vessels, increasing blood pressure.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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