Kidneys & Osmoregulation
Applied Science: BTEC Level 3 National Extended Diploma, topic 4.7.2
Excretion and the Kidneys
The kidneys work alongside the liver in the excretion of waste products.
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.
Nephron structure
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.
Selective reabsorption
- Water, amino acids, glucose and ions are selectively reabsorbed first in the proximal convoluted tubule (PCT).
- This helps to maintain electrolyte balance.
- 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.
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 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
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
- 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)
- 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.
Test yourself on Kidneys & Osmoregulation
Which of these is NOT selectively reabsorbed into the bloodstream from the nephron?
What is the kidney made up from?
Excretion of Urine
What does antidiuretic hormone do?
Avoiding Dehydration