Topic 6—C: Homeostasis- 4. The kidneys Flashcards
Where does blood enter the kidney?
- Through the real artery and then passes through capillaries in the cortex (outer layer) of the kidneys
What happens as blood passes through capillaries in the cortex?
- Substances are filtered out of the blood and into long tubules that surround the capillaries
Ultrafiltration
- The filtering of the blood that takes place under high pressure as blood passes from the glomerulus into the bowman’s capsule
What substances are reabsorbed into the blood?
- Glucose and the right amount of water
Selective reabsorption
The reabsorption of useful substances along the kidney nephron back into the blood
Where do the reaming unwanted substances pass along to?
The bladder and are excreted as urine
Nephrons
The long tubules along with the bundles of capillaries where the blood is filtered
How many nephrons are in each kidney?
1 million
Ultrafiltration
- Blood from the renal artery enters smaller arterioles in the cortex of the kidney
- Each arteriole splits into a structure called a glomerulus which is a bundle of capillaries looped inside a hollow ball called a bowman’s capsule (this is where ultrafiltration takes place)
- The arteriole that takes blood into each glomerulus is called the afferent arteriole and the arteriole that takes the filtered blood away from the glomerulus is called the efferent arteriole
- the efferent arteriole is smaller in diameter than the afferent arteriole so the blood in the glomerulus is under high pressure
- The high pressure forces liquid and small molecules in the blood out of the capillary and into the bowman’s capsule
- the liquid and small molecules pass through 3 layers to get into the bowman’s capsule and enter the nephron tubules
1. The capillary endothelium
2. Membrane (basement membrane)
3. Epithelium of bowman’s capsule - large molecules like proteins and blood cells can’t pass through so stay in the blood
- the substances that enter the bowman’s capsule are known as the glomerular filtrate
- the glomerular filtrate passes along the rest of the nephron and useful substances are reabsorbed along the way
- finally the filtrate flows through the collecting duct and passes out of the kidney along the ureter
What is a glomerulus?
- a bundle of capillaries looped inside a hollow ball called a bowman’s capsule
What arteriole takes blood into each glomerulus?
Afferent arteriole
Which arteriole takes blood away from the glomerulus?
Efferent arteriole
Why is the blood in the glomerulus under high pressure?
The efferent arteriole is smaller in diameter than the afferent arteriole
What does the high pressure force out of the capillary?
Liquid and small molecules in the blood out of the capillary and into the bowman’s capsule
What are the 3 layers the liquid and small molecules pass through to get into the bowman’s capsule and enter the nephron tubules?
- Capillary endothelium
- Membrane (basement membrane)
- Epithelium of the bowman capsule
What can’t pass through so stay in the blood?
Larger molecules like proteins and blood cells
Glomerular filtrate
- the fluid present in the nephrons of the kidney, following ultrafiltration of the blood at the Bowman capsule
Selective reabsorption
- selective reabsorption of useful substances takes place as the glomerular filtrate flows along the proximal convoluted tubule (PCT), through the loop of henle, and along the distal convoluted tubule (DCT)
- Useful substances leave the tubules and enter the capillary network that’s wrapped around them
- The epithelium of the wall of the PCT has microvilli to provide a large surface area for the reabsorption of useful materials from the glomerular filtrate (in the tubule) into the blood (in the capillaries)
- Useful solutes like glucose are reabsorbed along the PCT by active transport and facilitated diffusion
- water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate
- Water is reabsorbed from the PCT, loop of henle, DCT and the collecting duct
- The filtrate that remains is urine which passes along the ureter to the bladder
Urine
- Usually made up of water and dissolved salts, urea and other substances such as hormones and excess vitamins
- urine doesn’t usually contain proteins or blood cells as they’re too big to be filtered out of the blood
- glucose is actively reabsorbed back into the blood so its not usually found in the blood either
Why is the blood at high pressure?
- Renal arteries are short
- Renal arteries aren’t far from the heart
- Diameter of efferent arterioles are less than those of the afferent arterioles
Ultrafiltration- how the blood gets filtered
- afferent arteriole is wider than efferent so hydrostatic pressure of blood increases
- blood plasma forced out of the pores between endothelium cells and through basement membrane
- podocytes raised off the basement membrane by little feet
- filtrate passes around the podocytes into cavity of bowman’s capsule
What does blood plasma contain?
Dissolved substances
E.g.
- water
- amino acids
- glucose
- urea
- inorganic material
Endothelium of blood capillary
- very thin
- perforated with thousand of pores of about 10nm diameter
- provides a barrier to cells but not plasma proteins
Basement membrane
- mesh work of collagen and glycoprotein fibres
- water and small molecules can pass through
- proteins are too large and are repelled by negative charges on fibres
Epithelium of renal capsule
- made of cells which are modified for filtration - podocytes
What percentage of glomerular filtrate is reabsorbed at the PCT?
80%
What provides a large surface area for the reabsorption of useful materials from the glomerular filtrate into the blood?
Epithelium of the wall of the PCT has microvilli to provide a large surface area
How is glucose reabsorbed along the PCT?
By active transport and facilitated diffusion
How does water enter the blood and why?
- by osmosis
- because the water potential of the blood is lower than that of the filtrate
What happens in the proximal convoluted tubule?
- glucose and some inorganic ions pass from the filtrate to the capillaries surrounding the nephron by facilitated diffusion (down a concentration gradient)
- the solutes are quickly carried away in the blood maintaining the high concentration gradient
- The remaining glucose is moved to the blood by active transport
- it is all reabsorbed
- removal of solutes from the glomerular filtrate makes it even more hypotonic to the blood than it was when it entered the PCT
- So some water moves by osmosis from the filtrate into the blood and is then carried away
- this makes it more isotonic
