Renal system anatomy and physiology

The workhorses of the urinary system are the kidneys which are the twin, bean-shaped organs in your body that clear harmful substances by filtering your blood. They’re like a water purification plant that helps clean the drinking water for a city. They also regulate blood pH, volume, pressure, osmolality as well as produce hormones.

The kidneys are located between the T12 and L3 vertebrae, and they’re partially protected by ribs 11 and 12--which are the floating ribs. The kidneys are roughly the size of a fist and are retroperitoneal, meaning they sit behind the peritoneal membrane alongside the vertebral column. The right kidney is pushed down by the liver so it sits slightly lower than the left kidney.

In the middle of each kidney there is an indentation that forms the renal hilum. This is the entry and exit point for the ureter, renal artery and renal vein, lymphatics, and nerves going into and coming out of the kidney.

The kidney is surrounded by three layers of tissue. On the outside is the renal fascia which is a thin layer of dense connective tissue that anchors the kidney to its surroundings. The middle layer, or the adipose capsule, is a fatty layer that protects the kidney from trauma. And the deepest layer, called the renal capsule, is a smooth, transparent sheet of dense connective tissue that gives the kidney its distinctive shape.

If you take a cross-section of the kidney, there are two main parts. The inner portion is the renal medulla and the outside rim is the renal cortex. The medulla is made up of 10 to 18 renal pyramids with the base of the pyramids facing the renal cortex and the tips of the pyramids, called renal papilla—or nipples, pointing towards the center of the kidney. The renal papilla project into minor calyces which join together to form major calyces which funnel into the renal pelvis. Urine collects in the renal pelvis and then heads out of the kidney through the ureter.

The renal cortex can be divided into an outer cortical zone and an inner juxtamedullary zone. There are also sections of the cortex called renal columns, which extend down into the medulla separating the renal pyramids from each other. Each renal pyramid and the renal cortex above it is called a renal lobe.

So an adult’s kidneys filter about 150 liters of blood every day. If we assume that there are 5 liters of blood in the body, that means that the entire blood volume gets filtered about 30 times a day, which is more than once every hour. Because of this, the kidneys get about a quarter of the cardiac output which is blood getting pumped out of the left ventricle.

To reach the kidneys, blood flows from the aorta into the left and right renal arteries. As these renal arteries enter the kidney, they divide into segmental arteries and then into interlobar arteries which pass through the renal columns then to arcuate arteries that go over the bases of the renal pyramids and then into cortical radiate arteries which supply the cortex. The cortical radiate arteries continue to divide eventually forming afferent arterioles that split into a tiny bundle of capillaries called the glomerulus.

And the glomerulus is the site where blood filtration starts. Interestingly, once the blood leaves these glomeruli it does not enter into venules. Instead the glomerulus funnels blood into efferent arterioles which divide into capillaries a second time.

These peritubular capillaries then reunite to become the cortical radiate veins, then the arcuate veins, then interlobar veins and finally into the left and right renal veins which connect to the inferior vena cava. The flow of the veins are similar to the arteries but in reverse, the only difference is that there’s a segmental artery but no segmental vein.

Within each kidney, there are about 1 million nephrons, and each nephron is made up of a renal corpuscle and a renal tubule. The renal corpuscle is where blood filtration starts and it includes the glomerulus, which is the tiny bed of capillaries, and the Bowman’s capsule which is made of renal cells that surround the glomerulus.

As blood flows into the glomerulus, water and some solutes in the blood like sodium are able to pass through the endothelial lining of the capillary, move across its basement membrane, through the epithelial lining of the nephron and finally into the Bowman’s space of the nephron itself—at which point it is called filtrate.

The epithelium of the nephron is made of specialized cells called podocytes which wrap around the basement membrane like the tentacles of an octopus. Between these tentacle-like projections are tiny gaps called filtration slits that act like a sieve allowing only small particles like water, glucose and ionic salts to pass through while blocking large proteins and red blood cells.

As the filtrate leaves the Bowman’s capsule it flows into the renal tubule, which is surrounded by the peritubular capillaries. Now, before we dive too far in here, let’s redraw the nephrons that the structure of the renal tubule as a little more accurate.

Alright so, the renal tubule itself can be divided into the proximal convoluted tubule, the nephron loop—also known as the loop of Henle—which made up of the descending limb and the ascending limb, the distal convoluted tubule, and finally the collection ducts which ultimately send the urine to the minor calyces. Here, the filtrate becomes fine tuned based on what the body wants to keep versus what it wants to discard, with water and solutes getting passed back and forth between the filtrate in the lumen of the renal tubule and the blood in the peritubular capillaries.