The human excretory system and the kidney

The parts of the system

Think of a water filter in a house. Water comes in, the filter removes the dirt, and the clean water goes on. The kidney does the same for blood. The human excretory system has four parts.

• A pair of kidneys, which make the urine.
• A pair of ureters, which carry the urine away from the kidneys.
• A urinary bladder, which stores the urine.
• A urethra, which takes the urine out of the body.

Where the kidneys lie

The kidneys are reddish brown and bean shaped. They lie close to the dorsal, or back, wall of the abdominal cavity, one on each side of the backbone. They are between the level of the last thoracic vertebra and the third lumbar vertebra.

An adult kidney is about 10 to 12 cm long, 5 to 7 cm wide and 2 to 3 cm thick. It weighs about 120 to 170 g.

The shape of a kidney

On the inner, concave side of the kidney there is a notch, the hilum. The ureter, the blood vessels and the nerves pass through it. Inside the hilum there is a broad, funnel shaped space, the renal pelvis. It has projections called calyces (one is a calyx).

The outer layer of the kidney is a tough capsule. Inside, there are two zones.

• The cortex is the outer zone.
• The medulla is the inner zone. It is divided into a few cone shaped masses, the medullary pyramids, which project into the calyces.

The cortex also goes in between the medullary pyramids. These strips of cortex are called the renal columns, or the columns of Bertini.

What the kidney is made of

Each kidney holds nearly one million complex tubes called nephrons. The nephron is the structural and functional unit of the kidney. You will study it in the next lesson.

The nephrons make urine. The urine from many nephrons flows into the calyces, then into the renal pelvis, and then out through the ureter.

The way the urine leaves

The ureter is a thin tube that leaves each kidney at the hilum and runs down to the bladder. The urinary bladder stores the urine. A tube, the urethra, takes the urine from the bladder to the outside. Lesson 1 of the third course explains how the bladder empties.

The blood supply

A renal artery brings blood from the aorta to each kidney, and a renal vein takes it away to the inferior vena cava. The kidneys are small, but they receive about 1100 to 1200 mL of blood in a minute. This is roughly one fifth of the blood that each ventricle of the heart pumps out in a minute.

So the kidneys are well supplied with blood. This is what makes the filtration, which you study in the second course, possible.

You should find that the kidneys, which make up a small part of the body weight, receive about a fifth of the cardiac output. The part of the blood that the nephrons filter is the plasma.

Try it in Python

The program calculates the blood and the plasma that reach the kidneys in a minute, and the blood in a day.

Practise with new numbers

Common mistakes

• Mixing up the two zones. The outer zone is the cortex, and the inner zone is the medulla.
• Saying that the renal columns are in the medulla. They are strips of cortex that lie between the medullary pyramids.
• Saying that urine is made in the ureter or the bladder. It is made in the nephrons. The ureter only carries it and the bladder only stores it.
• Saying that the hilum is the funnel. The hilum is the notch. The funnel inside it is the renal pelvis.
• Giving the blood flow as one fifth of the blood volume. It is one fifth of the cardiac output, the blood pumped every minute.

Remember these

• The system: two kidneys, two ureters, one bladder, one urethra.
• Kidneys: reddish brown, bean shaped, between the last thoracic and the third lumbar vertebra. Length 10 to 12 cm, width 5 to 7 cm, thickness 2 to 3 cm, weight 120 to 170 g.
• Parts: capsule, cortex, medulla (pyramids), renal columns (of Bertini), hilum, pelvis, calyces.
• About one million nephrons in each kidney.
• The kidneys receive 1100 to 1200 mL of blood a minute, roughly a fifth of the cardiac output.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four parts of the human excretory system, and say what each one does.

2. What is the hilum, and what passes through it?

3. Name the two zones of the kidney, and say where the renal columns lie.

4. About how many nephrons does one kidney have?

5. The kidneys receive 1100 mL of blood in a minute. About what fraction of the blood pumped by a ventricle in a minute is that?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 16, Excretory Products and Their Elimination.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 25, The Urinary System.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapters on the urinary system.