The kidneys are located in the back wall of the abdomen, high, just below the diaphragm, and behind the peritoneum, retroperitoneal (retro = behind).
Each kidney is shaped like a big bean, with the ureter jutting out of its hilus. This is also the entry and exit point for the blood vessels and the nerves.
In the cross-section (see diagram), three areas are visible in the kidney:
In this diagram, you can also see the location and shape of one nephron and its tubule; this is the functional unit of the kidney.
The next 5 pages in this chapter are dedicated to the different structures (and functions) of the nephron.
The medulla is shaped like a pyramid with the base pointing towards the cortex and the apex (= papillae) pointing towards the calices and the pelvis.
The calices and the pelvis collect the urine flowing from the tubules and empty them into the ureter and ultimately, after passage through the ureters, into the bladder.
Because the kidneys filter so much blood all the time (about 1 litre/minute!), the kidneys have a very rich blood supply. In fact, the kidneys obtain the highest portion of the cardiac stroke volume of any organ in the body: 20%! (20% of 5 litres/min = 1 litre/min).
2.
The arterial blood comes directly from the abdominal aorta through the renal arteries (see diagram in the previous page).
3.
Inside the kidneys, the renal artery branches into 5-6 large segmental arteries that pass between the medullar pyramids to the cortex. There they perfuse the nephrons (see next page).
All this blood also has to go back to the heart. Therefore, there is also a large supply of veins that pretty well mirrors the arterial system. Ultimately, the renal veins drain back into the abdominal vena cava.
Not shown in the diagram, but not to be forgotten, are the lymph vessels and the nerves going to the kidneys.
The nerves are mostly fibres from the autonomic nervous system and of the sympathetic branches. They are mostly involved in the regulation of the blood flow and determine how much blood actually flows to the nephron.
