The kidney and urinary system: structure and organisation

Learning objectives

By the end of this lesson, you should be able to:

• identify the main organs of the mammalian urinary system
• describe the external and internal structure of a kidney
• explain how blood reaches and leaves the kidney
• relate the structure of the nephron to its role in urine formation
• distinguish excretion from osmoregulation

Why the urinary system is essential

The urinary system removes metabolic waste from the blood and helps maintain a stable internal environment. In mammals, the kidneys remove urea, excess mineral ions and excess water. The resulting urine passes through the ureters to the urinary bladder and leaves the body through the urethra.

The urinary system is also important in homeostasis. It helps regulate the volume and composition of body fluids, including water balance, salt concentration and blood pH. These functions are essential because cells can only work properly within a relatively narrow range of internal conditions.

Main organs and the route of urine

Route: kidney → ureter → urinary bladder → urethra → outside the body.

Do not confuse the ureter with the urethra. The ureter connects a kidney to the bladder; the urethra carries urine from the bladder out of the body.

External and internal structure of the kidney

Each kidney is a bean-shaped organ situated towards the back of the abdominal cavity, one on either side of the vertebral column. The renal artery brings blood containing dissolved substances to be regulated. The renal vein carries blood away after the kidney has removed or adjusted some substances. The ureter leaves the kidney at the hilum, a depression on its inner border.

In a longitudinal section, three important regions can be identified:

1. Renal cortex: the outer region. It contains the renal corpuscles (glomeruli enclosed by Bowman's capsules) and parts of the kidney tubules.
2. Renal medulla: the inner region, arranged into renal pyramids. It contains loops of Henle and collecting ducts.
3. Renal pelvis: a funnel-shaped collecting space that receives urine from the collecting ducts through the calyces and channels it into the ureter.

The arrangement of these regions supports the processes of filtration, selective reabsorption and the concentration of urine.

The nephron: the functional unit

A nephron is the microscopic functional unit of the kidney. Each nephron consists of a renal corpuscle and a long tubule. The renal corpuscle is made up of a knot of capillaries called the glomerulus, enclosed by Bowman's capsule.

The tubule has several regions:

• Proximal convoluted tubule (PCT): the first coiled section; it reabsorbs most of the filtered glucose, amino acids, salts and water.
• Loop of Henle: a U-shaped section that helps establish a concentration gradient in the medulla, enabling the kidney to conserve water.
• Distal convoluted tubule (DCT): a later coiled section where selective adjustment of ions and water continues.
• Collecting duct: receives fluid from nephrons and carries it towards the renal pelvis; its water permeability is regulated by antidiuretic hormone (ADH).

The nephron has a close blood supply. Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole. Capillaries around the tubule allow substances that are reabsorbed to return to the blood.

How structure supports function

The glomerulus contains many capillaries and is supplied at relatively high pressure, favouring filtration. The PCT has a large surface area, with microvilli, and cells containing many mitochondria. These features support selective reabsorption, much of which requires active transport. The loop of Henle extends into the medulla and helps create conditions that allow water to be conserved. The collecting duct passes through the medulla, where water can move out when the duct is made more permeable by ADH.

WASSCE examination focus

In a labelled kidney diagram, distinguish cortex, medulla, renal pelvis, renal artery, renal vein and ureter. In a nephron diagram, identify Bowman's capsule, glomerulus, PCT, loop of Henle, DCT and collecting duct. State that the nephron is the functional unit of the kidney. Do not describe the kidney as simply a sieve: it filters blood and then selectively returns useful substances to the blood.

Quick check

1. Name the four main parts of the urinary system.
2. What is the functional unit of the kidney?
3. Which region contains the renal corpuscles?
4. Distinguish the ureter from the urethra.
5. Give two structural features of the PCT that support reabsorption.

Answers: 1. Kidneys, ureters, urinary bladder and urethra. 2. The nephron. 3. The renal cortex. 4. The ureter carries urine from kidney to bladder; the urethra carries urine from bladder to outside. 5. Microvilli provide a large surface area, and numerous mitochondria supply ATP for active transport.