If you were to cut a kidney in half, you would see the following parts:
- Renal capsule - a thin, outer membrane that helps protect the kidney
- Cortex - a lightly colored outer region
- Medulla - a darker, reddish-brown, inner region
- Renal pelvis - a flat, funnel-shaped cavity that collects the urine into the ureters
If you look closely at the cortex and medulla, you can see many tiny, tubular structures that stretch across both regions perpendicular to the surface of the kidney. In each kidney, there are one million of these structures, called nephrons. The nephron is the basic unit of the kidney. It's a long, thin tube that is closed at one end, has two twisted regions interspaced with a long hairpin loop, ends in a long straight portion and is surrounded by capillaries.
The parts of the nephron are as follows:
- Bowman's capsule - This closed end at the beginning of the nephron is located in the cortex.
- Proximal convoluted tubule or proximal tubule - The first twisted region after the Bowman's capsule; it's in the cortex.
- Loop of Henle - A long, hairpin loop after the proximal tubule, it extends from the cortex down into the medulla and back.
- Distal convoluted tubule or distal tubule - This second twisted portion of the nephron after the loop of Henle is located in the cortex.
- Collecting duct - This long straight portion after the distal tubule that is the open end of the nephron extends from the cortex down through the medulla.
Each part of the nephron has different types of cells with different properties -- this is important in understanding how the kidney regulates the composition of the blood.
The nephron has a unique blood supply compared to other organs:
- Afferent arteriole - connects the renal artery with the glomerular capillaries
- Glomerular capillaries - coiled capillaries that are inside the Bowman's capsule
- Efferent arteriole - connects the glomerular capillaries with the peritubular capillaries
- Peritubular capillaries - located after the glomerular capillaries and surrounding the proximal tubule, loop of Henle, and distal tubule
- Interlobular veins - drain the peritubular capillaries into the renal vein
The kidney is the only organ of the body in which two capillary beds, in series, connect arteries with veins. This arrangement is important for maintaining a constant blood flow through and around the nephron despite fluctuations in systemic blood pressure.
Regulating the composition of the blood involves the following:
- Keeping the concentrations of various ions and other important substances constant
- Keeping the volume of water in your body constant
- Removing wastes from your body
- Keeping the acid/base concentration of your blood constant
The kidney does this by a combination of three processes:
- It filters 20 percent of the plasma and non-cell elements from the blood into the inside of the nephron (the lumen).
- It reabsorbs the components that the body needs from the lumen back into the blood.
- It secretes some unwanted components from the blood into the lumen of the nephron.
Anything (fluid, ions, small molecules) that has not been reabsorbed from the lumen gets swept away to form the urine, which ultimately leaves the body. Through these processes, the blood is maintained with the proper composition, and excess or unwanted substances are removed from the blood into the urine.
Next, we'll look at how the kidneys regulate blood composition by three main processes: