Excretion in Humans
Senior 2 · Biology
Excretion in Humans
Lesson 2 of 4
22 min
Let's begin
Your two kidneys are each about the size of a clenched fist, yet all the blood in your body passes through them many times every day. They use nearly a quarter of the oxygen you breathe in. In this lesson you will follow the route of urine through the body and look inside a kidney at the tiny tubes that make it.
Worked example
Trace the path of a urea molecule from where it is made to where it leaves the body.
Answer: liver (where it is made) → hepatic vein → vena cava → heart → aorta → renal artery → kidney (filtered in a nephron) → collecting duct → renal pelvis → ureter → bladder (stored) → urethra → out of the body.
A kidney cut lengthways shows three regions:
Each kidney contains over a million microscopic tubes called nephrons. Each nephron has these parts, in the order the fluid flows through them:
Blood supply: a small branch of the renal artery, the afferent arteriole, brings blood into the glomerulus. A narrower vessel, the efferent arteriole, takes it out. Because blood leaves through a narrower vessel than it enters, the pressure in the glomerulus is high. The efferent arteriole then branches into capillaries around the tubules before the blood drains into the renal vein.
Tip
Nephron order: Blood Passes Long Distances Carefully: Bowman's capsule → Proximal tubule → Loop of Henle → Distal tubule → Collecting duct.
Don't mix up ureter (kidney to bladder, two of them) and urethra (bladder to outside, one). Glomeruli and convoluted tubules are in the cortex; loops of Henle and collecting ducts run through the medulla.
Name the blood vessel that brings blood to the kidney, and state how its blood differs from the blood in the renal vein.
Solution: The renal artery. Its blood contains more urea (and more oxygen) than the blood leaving in the renal vein.
What is the job of the bladder?
Solution: To store urine until it is convenient to release it through the urethra.
Which parts of the nephron are found in the medulla?
Solution: The loop of Henle and the collecting duct.
Explain why the blood pressure in the glomerulus is higher than in other capillaries.
Solution: Blood enters through the wider afferent arteriole and leaves through the narrower efferent arteriole, so blood is squeezed into the glomerulus faster than it can leave, raising the pressure.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
