Human Excretion » Kidney Structure and Function

What you'll learn this session

Study time: 30 minutes

The structure of the human excretory system
The detailed structure of the kidney and nephron
How the kidney filters blood and forms urine
The processes of ultrafiltration, selective reabsorption and osmoregulation
Common kidney disorders and their impacts

Introduction to Human Excretion

Our bodies are constantly working to maintain a balanced internal environment. As cells carry out their functions, they produce waste products that need to be removed. This removal of metabolic waste is called excretion and it's vital for our survival. The kidneys are the stars of this show!

Key Definitions:

Excretion: The removal of toxic waste products of metabolism and substances in excess from the body.
Osmoregulation: The control of water balance in the body.
Homeostasis: The maintenance of a constant internal environment.

🔥 What We Excrete

The main substances our bodies need to excrete include:

Carbon dioxide - from cell respiration (via lungs)
Urea - from the breakdown of excess amino acids (via kidneys)
Water - regulated to maintain proper balance (via kidneys, skin)
Salts - like sodium and potassium ions (via kidneys, sweat)

👀 Excretory Organs

Several organs help with excretion:

Kidneys - filter blood to remove urea, excess water and salts
Lungs - remove carbon dioxide
Skin - removes water, salts and small amounts of urea through sweat
Liver - produces urea from amino acids and breaks down toxins

Kidney Structure and Function

The kidneys are bean-shaped organs located at the back of the abdominal cavity, one on each side of the spine. Each kidney is about the size of your fist and plays a crucial role in filtering blood and producing urine.

External Structure of the Kidney

Looking at a kidney from the outside, you'll notice:

Renal artery - brings blood containing waste products to the kidney
Renal vein - carries filtered blood away from the kidney
Ureter - tube that carries urine from the kidney to the bladder
Renal capsule - tough outer layer that protects the kidney

Internal Structure of the Kidney

If we cut a kidney in half, we would see three main regions:

🔳 Cortex

The outer region of the kidney where blood filtration begins. Contains the Bowman's capsules and convoluted tubules of nephrons.

🔴 Medulla

The inner region containing pyramid-shaped structures. The loops of Henle and collecting ducts run through this region.

🔵 Pelvis

The central collecting area where urine gathers before flowing into the ureter and on to the bladder.

The Nephron: The Functional Unit

Each kidney contains about one million tiny filtering units called nephrons. These microscopic structures are where the actual work of the kidney happens. Let's explore the parts of a nephron and how they function:

🔎 Nephron Structure

Each nephron consists of:

Bowman's capsule - cup-shaped structure that surrounds the glomerulus
Glomerulus - a ball of capillaries where blood filtration occurs
Proximal convoluted tubule - first coiled section of the tubule
Loop of Henle - U-shaped tube extending into the medulla
Distal convoluted tubule - second coiled section
Collecting duct - where urine from multiple nephrons collects

🛠 Blood Supply

The nephron has a specialized blood supply:

Afferent arteriole - brings blood to the glomerulus
Efferent arteriole - carries blood away from the glomerulus
Peritubular capillaries - surround the tubules to reabsorb useful substances
Vasa recta - long capillaries that run alongside the loop of Henle

How the Kidney Works

The kidneys filter about 180 litres of fluid from the blood each day, but we only produce about 1-2 litres of urine. This means the kidneys are very efficient at recovering useful substances. The formation of urine involves three main processes:

1. Ultrafiltration

This is the first step in urine formation and occurs in the glomerulus:

Blood enters the glomerulus through the afferent arteriole at high pressure
The efferent arteriole is narrower, creating high pressure in the glomerulus
This pressure forces small molecules out of the blood and into Bowman's capsule
The filtrate contains water, glucose, amino acids, salts and waste products like urea
Blood cells and proteins are too large to pass through and remain in the blood

2. Selective Reabsorption

As the filtrate passes through the tubules, useful substances are reabsorbed back into the blood:

Proximal convoluted tubule: Reabsorbs glucose, amino acids, some salts and most of the water
Loop of Henle: Creates a concentration gradient in the medulla for water reabsorption
Distal convoluted tubule: Fine-tunes salt balance under hormonal control
Collecting duct: Reabsorbs more water when the body needs it (controlled by ADH)

3. Secretion

Some substances move from the blood into the tubules to be excreted:

Hydrogen ions - to control blood pH
Potassium ions - to maintain electrolyte balance
Drugs and toxins - to remove them from the body

Case Study Focus: Diabetes and the Kidney

Diabetes mellitus can severely impact kidney function. In uncontrolled diabetes, high blood glucose levels can overwhelm the kidney's ability to reabsorb glucose. This leads to glucose appearing in the urine (glycosuria). Over time, high blood glucose damages the tiny blood vessels in the kidneys, leading to diabetic nephropathy - a leading cause of kidney failure. This is why regular kidney function tests are important for people with diabetes.

Osmoregulation: Controlling Water Balance

The kidneys play a crucial role in maintaining the water balance of the body through a process called osmoregulation. This is controlled by hormones:

💦 ADH Mechanism

Antidiuretic Hormone (ADH) controls water reabsorption:

When the body is dehydrated, osmoreceptors in the brain detect concentrated blood
The pituitary gland releases more ADH
ADH makes the collecting ducts more permeable to water
More water is reabsorbed, producing concentrated urine
When well-hydrated, less ADH is released, producing dilute urine

⚠ Kidney Disorders

Common kidney problems include:

Kidney stones - mineral deposits that can block urine flow
Urinary tract infections - bacterial infections in any part of the urinary system
Chronic kidney disease - gradual loss of kidney function
Kidney failure - when kidneys can no longer filter waste effectively

Dialysis and Kidney Transplants

When kidneys fail, patients may need dialysis to filter their blood artificially. This can be done using a machine (haemodialysis) or using the lining of the abdomen (peritoneal dialysis). A more permanent solution is a kidney transplant. Interestingly, we can live normally with just one functioning kidney, which is why living kidney donation is possible. The UK performs around 3,000 kidney transplants annually, but thousands more remain on waiting lists.

Keeping Your Kidneys Healthy

You can help keep your kidneys functioning well by:

Staying well-hydrated
Maintaining a healthy blood pressure
Eating a balanced diet low in salt
Exercising regularly
Avoiding excessive alcohol and certain medications
Managing conditions like diabetes that can damage kidneys

🧠 Test Your Knowledge!