The Human Urinary System

Organs of the urinary system

• The two kidneys filter blood and form urine, removing urea and excess water and salts.
• Two ureters carry urine from the kidneys down to the bladder.
• The bladder is a muscular sac that stores urine until it is convenient to release it.
• The urethra carries urine from the bladder out of the body.

Structure of the nephron

Each kidney contains around a million tiny functional units called nephrons. At the start of each nephron is the Bowman's capsule, a cup-shaped structure that surrounds a knot of blood capillaries called the glomerulus.

The Bowman's capsule leads into a long renal tubule, which winds through several regions (including the loop of Henle) before joining a collecting duct that carries urine out of the kidney.

How urine is formed

1. Ultrafiltration: blood entering the glomerulus is under high pressure, forcing water, glucose, urea, mineral salts and other small molecules out of the capillaries and into the Bowman's capsule. Blood cells and large protein molecules are too big to be pushed through and remain in the blood.
2. Selective reabsorption: as the filtrate flows along the renal tubule, useful substances, all the glucose, most of the water, and some mineral salts, are reabsorbed back into the blood through the surrounding capillaries, using diffusion and active transport.
3. The fluid that remains, now containing urea, excess water and excess mineral salts, becomes urine and flows into the collecting duct, then down the ureter to the bladder for storage.

How it is examined

Questions on this standard often provide a labelled diagram of the kidney or nephron and ask you to name structures, or to explain what happens at the glomerulus and along the renal tubule. A frequent question asks why substances such as glucose and protein normally behave differently in the nephron, glucose is filtered out but then fully reabsorbed, while protein is never filtered out at all because its molecules are too large.

Worked exam-style question

Question. A doctor tests a patient's urine and finds glucose present, but no protein. (a) Name the structure at which both glucose and protein would normally be filtered from the blood if their molecules were small enough.

(b) Explain why protein is never found in the urine of a healthy person. (c) Suggest one explanation for why this patient's urine contains glucose.

(d) Name the process that normally removes all filtered glucose from the fluid before it leaves the kidney as urine.

Model answer. (a) Filtration occurs at the glomerulus and Bowman's capsule. (b) Protein molecules are too large to pass through the walls of the glomerulus capillaries during ultrafiltration, so they never enter the filtrate in the first place.

(c) The patient's blood glucose concentration may be too high (as in diabetes mellitus), so more glucose is filtered into the nephron than the renal tubule's reabsorption mechanism can reabsorb, leaving some in the urine. (d) Selective reabsorption along the renal tubule normally returns all the filtered glucose to the blood.

Practice question

Try this. A patient's kidney has a damaged glomerulus that allows large protein molecules to pass into the Bowman's capsule. Predict one change this would cause in the patient's urine, and explain why.

Exam tip

Key terms

These Chapter 13 terms name the structures and processes examined in this topic:

• Nephron, the tiny functional unit of the kidney where blood is filtered and urine is formed.
• Ultrafiltration, the filtering of blood at high pressure at the glomerulus, forcing small molecules into the Bowman's capsule.
• Osmoregulation, the control of the water and salt balance in the body, largely carried out by the kidneys.
• Homeostasis, the maintenance of a constant internal environment despite changes outside the body.

Source:SRC-DSKP-EN

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