(3.3) Some details of how the kidney works in conjunction with the urinary and blood systems

Be able to identify on drawings, diagrams and images, the ureters, bladder and urethra.

The urinary system (simplified diagram on the right)

The 'dirty' blood' i.e. 'used' blood that need's processing enters the kidneys from the renal artery - it is also supplying the kidneys with oxygen.

The purified or 'cleaned' blood exits the kidney from the renal vein, but it is also depleted in oxygen - used up by the kidney cells respiring to power all the filtration and transport processes.

Apart from the processed blood, the waste fluid from the kidney filtration process, urine, containing mainly urea, passes into the ureter tube and stored in the bladder prior to excretion.

 

Nephrons and the kidney ultrafiltration system

How does the kidney filter out the 'unwanted stuff' and reabsorb useful substances still in the blood?

It's extremely complicated, and I hope my 'simplified' diagram plus text explain how it all works!

The out structure of the kidney, the cortex and medulla, are not shown on the above diagram.

The renal cortex, also called the kidney cortex, is a part of your kidney, which is itself part of the urinary tract.

The renal cortex (kidney cortex) is where the filtering units of the kidney begin.

The renal cortex surrounds the inside of the organ, which is called the medulla and the renal cortex is where the nephrons, the blood-filtering units of the kidney begin.

 

What does the renal cortex do?

As part of your urinary tract, the renal cortex is involved in many kidney functioning and not just in getting rid of body wastes. Kidneys also control:

The balance of electrolytes and water,  your body’s blood pressure and amount of extracellular fluid (outside the cells),  red blood cell production,  the balance between acids and bases.,  Vitamin D production,  the balance between calcium and phosphate (a very busy organ!).

The part of the kidney that does the actual separation is called a nephron - the filtration unit and there are millions of them!.

Nephrons are the functional units in the kidney, each consisting of a glomerulus, Bowman's capsule and its associated very long thin tubule (kidney tubule), through which the glomerulus filtrate passes before emerging as urine.

Overall the nephron is like a long thin tubular membrane surrounded by tiny arteries of blood - into which reabsorbed materials will pass. - one grey 'squiggly' artery is shown on the diagram.

A glomerulus is a tiny ball-shaped structure composed of capillary blood vessels actively involved in the filtration of the blood to form urine. The glomerulus is one of the key structures that make up the nephron, the functional unit of the kidney.

Bowman's capsule is a cup-like sack at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac.

The parts of the nephron after the renal artery are as follows (not shown in detail on the above simplified diagram):

tiny arteries of the glomerulus membrane sac ==> Bowman's capsule (filter) ==> reabsorption kidney tubule sections (proximal convoluted loop ==> loop of Henle ==> distal convoluted loop)

In the nephron loop sections the selective reabsorption take place.

Adaptation note: Animals inhabiting areas which suffer from drought conditions have an extended Henle loop (part of a nephron) to reabsorb more water.
 

How does the nephron complex function?

(i) The liquid part of the blood from the renal artery, contains glucose, ions (e.g. sodium Na⁺ from salt - sodium chloride), water, urea and other substances, is forced and filtered under high pressure through the glomerulus into the tissue of the Bowman's capsule - this is where the main filtration process occurs.

Larger molecules like proteins or even larger blood cells cannot pass through the membranes of the Bowman's capsule into the extended length of the nephron - the residue remains in the bloodstream - the tiny arteries of the glomerulus - this is the ultrafiltration process.

The smaller molecules/ions like water, sodium chloride, urea and sugars pass through the membrane filters of the Bowman's capsule from the blood into the nephron of the kidney.

(ii) As the filtered liquid flows through the glomerulus of the nephron useful substances are selectively reabsorbed.

All the glucose is selectively reabsorbed back into the bloodstream against the concentration gradient - so energy is needed for 'active transport', since spontaneous diffusion will not take place in the desired direction for reabsorption.

Any residual ions required are also reabsorbed back into the bloodstream e.g. sufficient salt that is needed, any excess is passed on and eventually excreted in urine.

Water is reabsorbed, but the amount dictated by the level of the ADH hormone (feedback system described in a later section on this page).

Adaptation note: Animals living in very dry regions, prone to drought, have longer loops of Henle to reabsorb more water.

(iii) The residual waste substances e.g. excess water, excess ions and urea, which are not reabsorbed, flow out of the nephrons as urine, through collecting ducts which all merge into the ureter and down into the storage bladder.  The urine is eventually excreted from the bladder through the urethra.

(iv) Active transport moves substances against the concentration gradient - opposing the normal diffusion direction.

Active transport requires a net energy input from respiration and involves protein carrier molecules to transport material through a membrane against the concentration gradient.

This happens in conveying glucose through the membranes of the kidney tubules.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points on how the kidney works

What Is the Kidney and What Does It Do?

The kidneys are two bean-shaped organs located in the lower back. They are part of the urinary (excretory) system and play a vital role in homeostasis by:

• Filtering the blood

• Removing waste products (like urea)

• Regulating water and ion balance

• Controlling blood pressure and pH

Each kidney contains about 1 million nephrons, which are the functional units that carry out filtration and reabsorption.

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How the Kidney Works: Step-by-Step

1. Ultrafiltration (in the Bowman's Capsule)

• Blood enters the glomerulus (a ball of capillaries) under high pressure.

• Small molecules like water, glucose, urea, and ions are forced out into the Bowman’s capsule.

• Large molecules (proteins, blood cells) stay in the blood.

This process is called ultrafiltration.

2. Selective Reabsorption (in the tubules)

• As the filtrate passes through the proximal tubule, loop of Henle, and distal tubule, useful substances are reabsorbed:

  • All glucose (by active transport)

  • Some water (by osmosis)

  • Some ions (by diffusion or active transport)

This ensures the body keeps what it needs.

3. Water Reabsorption (in the collecting duct)

• Controlled by the hormone ADH (Anti-Diuretic Hormone).

• If the body is dehydrated, more water is reabsorbed, producing concentrated urine.

• If the body has excess water, less is reabsorbed, producing dilute urine.

4. Excretion

• The remaining fluid (urine) contains urea, excess water, and ions.

• It flows through the ureter to the bladder, then out via the urethra.

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Why the Kidney Is Important in Human Biology

• Maintains homeostasis by regulating water and salt balance.

• Removes toxic waste (urea) from protein metabolism.

• Prevents dehydration or overhydration.

• Regulates blood pressure and pH.

• Demonstrates how the endocrine system (via ADH) and excretory system work together.

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Exam Tips

• Use key terms: nephron, ultrafiltration, selective reabsorption, ADH, urea, homeostasis.

• Be able to label a diagram of the nephron.

• Understand how the kidney responds to different hydration levels.

• Compare urine composition in different scenarios (e.g. after exercise versus after drinking lots of water).

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Keywords, phrases and learning objectives for this part on how the kidney works

When given a diagram, or other information, be able to describe and explain details of how the kidney works in conjunction with the urinary and blood systems including the functions of the renal vein, nephrons, filtration process, renal cortex, Bowman's capsule, glomerulus membranes and selective absorption or reabsorption of water and glucose.

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