15.5 The Structure And Funtion Of The Mammalian Kidney

Question 1

What main two homeostatic roles does the kidney play in the body?

Answer 1

• excretion
• osmoregulation

Question 2

Describe the process of excretion of waste products

Answer 2

• Blood enters the kidney through the renal artery and then passes through the capillaries in the cortex of the kidney
• as the blood passes through the capillaries, substances are filtered out of the blood and into long tubules that surround capillaries
• this s ultrafiltration
• useful substances (e.g. glucose) are reabsorbed back into the blood from the tubules in the medulla and cortex - this is called selective reabsorption
• the remaining unwanted substances (e.g. urea) pass along the tubules, then along the ureter to the bladder, where they’re expelled as urine
• the filtered blood passes out of the kidneys through the renal vein

Question 3

Describe the kidney structure

Answer 3

Main three areas: cortex, medulla and pelvis

• the cortex is the dark outer layer.
  This is where filtering of the blood takes place and it has a very dense capillary network carrying the blood from the renal artery to the nephrons
• the medulla is lighter in colour
  It contains the tubules of the nephrons that form the pyramids of the kidney and the collecting ducts
• the pelvis of the kidney is the central chamber where the urine collects before passing out down the ureter

Question 4

What are nephrons?

Answer 4

The long tubules along with the bundle of capillaries where blood is filtered are called nephrons - there are around one million nephrons in each kidney

Question 5

State the main structures of the nephron

Answer 5

• Bowman’s capsule
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct

Question 6

Describe and explain the process of ultrafiltration

Answer 6

• blood from the renal artery enters smaller arterioles in the cortex
• each arteriole splits into a structure called a glomerulus - a bundle of capillaries looped inside the 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 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 capillaries and into the Bowman’s capsule
• the liquid and small molecules pass through three layers to get into the Bowman’s capsule and enter the nephron tubule — the capillary endothelium, a membrane (the basement membrane) and the epithelium of the Bowman’s capsule
• the wall of the Bowman’s capsule also involves special cells called podocytes that act as an additional filter
• larger molecules like proteins and blood cells can’t pass through and stay in the blood
• the liquid and small molecules, now called filtrate, pass along the rest of the nephron and useful substances are reabsorbed along the way

Question 7

Describe and explain the process of selective reabsorption

Answer 7

Selective reabsorption of useful substances takes place as the 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 of the nephrons 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 filtrate (in the tubules) into the blood (in the capillaries).
  The cells lining the PCT also have many mitochondria to provide the ATP needed in active transport systems
• useful substances like glucose, amino acids, vitamins, and some salts are reabsorbed along the PCT by active transport and facilitated diffusion. Some urea is also reabsorbed by 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 loop of Henle and the DCT, and the collecting duct
• the filtrate that remains is urine, which passes along the ureter to the bladder

Question 8

What is urine made up of?

Answer 8

Urine is usually made up of water and dissolved salts, urea and other substances such as hormones and excess vitamins.

Urine doesn’t usually contains proteins or blood cells as they’re too big to be filtered out of the blood.

Glucose, amino acids and vitamins are actively reabsorbed back into the blood, so aren’t usually found in urine either.