Each Kidney Cleans Your Blood Through About a Million Microscopic Tubes

Every living cell produces waste as it works. Breaking down proteins releases nitrogen, which the liver turns into urea, and respiration releases carbon dioxide. If these wastes built up, they would poison the cells. Excretion is the removal of such wastes made inside the body.

This part covers the excretory organs, the urinary system, the structure of the kidney, its blood supply and the nephron. Part 2 then follows how urine is actually formed.

The lungs, kidneys, skin and liver are the main excretory organs, each removing different wastes.

- Lungs — carbon dioxide and some water vapour, in exhaled air
- Kidneys — urea, excess water and excess salts, as urine
- Skin — water, salts and a small amount of urea, as sweat
- Liver — makes urea from excess amino acids, and breaks down old haemoglobin into bile pigments, which leave the body with the faeces

The kidneys are the chief excretory organs, because they remove most of the nitrogen waste and also control how much water and salt the body keeps.

An everyday example. On a hot afternoon in May, a cricket match leaves your shirt soaked and salty, while you pass less urine than usual. The skin and kidneys are sharing the work of removing water and salt.

The misconception. Passing faeces is not excretion. Undigested food never entered the body's cells, so removing it is egestion; only the bile pigments in faeces are true excretory waste.

The urinary system is made of two kidneys, two ureters, a urinary bladder and a urethra; each kidney has an outer cortex, an inner medulla and a funnel-shaped pelvis.

The parts in order:

- Kidneys — form urine
- Ureters — carry urine from each kidney to the bladder
- Urinary bladder — a muscular bag that stores urine
- Urethra — carries urine out of the body, controlled by a ring of muscle, the sphincter

External structure. The kidneys are bean-shaped, dark red organs at the back of the abdomen, one on each side of the backbone; the right kidney sits slightly lower because the liver lies above it. The inner, curved side has a notch, the hilum, where the ureter and blood vessels join.

Internal structure of a kidney cut lengthwise:

- Cortex — the outer, dark, granular region
- Medulla — the inner region, divided into cone-shaped renal pyramids
- Pelvis — the funnel-shaped space that collects urine and leads into the ureter

An everyday example. A doctor checking for kidney stones often orders an ultrasound of the kidneys, ureters and bladder, following the same order urine takes.

The link. The cortex and medulla look different because different parts of the nephrons lie in each, as the next two sections show.

Blood enters each kidney through the renal artery and leaves through the renal vein, passing on the way through a glomerulus and then a network of capillaries around the tubule.

The path of blood:

$$\text{aorta}\to \text{renal artery}\to \text{afferent arteriole}\to \text{glomerulus}\to \text{efferent arteriole}$$
$$\to \text{capillaries around the tubule}\to \text{renal vein}\to \text{inferior vena cava}$$

- Renal artery — brings oxygenated blood, carrying urea
- Afferent arteriole — carries blood into the glomerulus
- Glomerulus — a knot of capillaries where filtration happens
- Efferent arteriole — carries blood out of the glomerulus
- Renal vein — returns blood with far less urea

Why pressure is high. The efferent arteriole is narrower than the afferent arteriole, so blood is squeezed into the glomerulus faster than it can leave. That high pressure drives filtration.

An everyday example. A garden hose with your thumb over the end sprays harder, because narrowing the outlet raises the pressure behind it.

The boundary case. Blood passes through two capillary networks one after the other — the glomerulus and then the capillaries around the tubule — unlike most organs, which have only one.

A nephron is the kidney's working unit: a cup-shaped capsule around a glomerulus, followed by a long coiled and looped tubule that ends in a collecting duct.

Regions and functions:

- Malpighian capsule — the Bowman's capsule, a double-walled cup, together with the glomerulus inside it; lies in the cortex and filters the blood
- Proximal convoluted tubule — coiled tube in the cortex; reabsorbs glucose, amino acids and most of the water and salts
- Loop of Henle — U-shaped loop dipping into the medulla; helps reabsorb water and concentrate urine
- Distal convoluted tubule — second coiled tube in the cortex; further reabsorption and secretion of some wastes
- Collecting duct — receives fluid from many nephrons, reabsorbs more water and carries urine to the pelvis

Each kidney contains about a million nephrons, which gives an enormous filtering surface.

An everyday example. A tea strainer holds back the leaves and lets the liquid through, just as the capsule holds back blood cells and proteins.

The link. Capsules and coiled tubules lie in the cortex, and loops and collecting ducts in the medulla — which explains why the two regions look different.