Together, the glomerulus and Bowman's capsule make up the renal corpuscle of the nephron. These parts of the nephron filter blood and are vital to our body's survival. In this lesson, you'll explore the major corpuscular structures and their functions.
Cardiac Output to the Kidneys
About 25% of your cardiac output goes to your kidneys.
Without a doubt, I'm sure you know that your kidneys are critical for life. In fact, they are so important that every minute about 25% of your heart's output, known as cardiac output, goes to your kidneys. This is because your kidneys are essential in the filtration of blood and the maintenance of water balance throughout your entire body. In this lesson, we'll get into the very first part of your blood's transit through the nephrons, which are the functional units of the kidneys.
As blood is pumped out of the heart, it will pass through the aorta before reaching the kidney. Once this blood enters the kidneys, it will eventually enter the first part of each nephron. This part is a network of capillaries responsible for the filtration of blood that is called a glomerulus.
The Bowman's Capsule
Each glomerulus is surrounded by a single layer of epithelium called the Bowman's capsule. If you can imagine inserting your fist into a large water balloon, your fist would be the glomerulus, and the balloon would be the Bowman's capsule enveloping your fist. The space between the sides of the balloon would be called the Bowman's space.
The Renal Corpuscle
Together, the glomerulus and its surrounding Bowman's capsule are called a renal corpuscle. This structure is located in the renal cortex. You should also be aware that the nephron is composed of two main parts: the renal tubule and the renal corpuscle. The renal corpuscle, in essence, is the first structure involved in the nephron's formation of urine, while the renal tubule takes over thereafter. In addition, the renal corpuscle's role is often referred to as the ultrafiltration of blood.
The renal corpuscle is the first structure involved in the formation of urine.
The Afferent and Efferent Arteriole
At this point, you're probably wondering how the process of ultrafiltration occurs. In essence, blood enters the glomerulus through a small artery supplying the glomerulus with unfiltered blood called the afferent arteriole. Another vessel, a small artery carrying filtered blood away from the glomerulus, is called the efferent arteriole. These two arterioles play a key role in helping the glomeruli's capillaries perform the process of ultrafiltration.
Here's what I mean. If you were to go outside and connect a hose to a faucet and turn the faucet on, water would flow freely out the open end of the hose. If you were to take the open end and close off half of the opening with your finger without adjusting the faucet's output, the pressure inside the hose would increase since the water is being held back from coming out freely and you haven't decreased the amount of water coming into the hose from the faucet.
Likewise, thanks to a pressure difference between the afferent and efferent arterioles, your blood can be filtered by the glomerulus. Normally, the efferent arteriole's lumen, which is the central space where blood flows, is a smaller diameter than the afferent arteriole's lumen. This means that the 'open' end of the hose, the efferent arteriole, is smaller than the faucet's end, the afferent arteriole. This difference creates a sort of back-up of blood in the glomerulus itself, and this back-up of blood means the pressure rises within the glomerulus as it did in our hose.
The increased pressure in the glomerulus forces certain ions and molecules to leak out of the blood through small holes within capillary walls, through the thin Bowman's capsule, and into the Bowman's space. While there are some exceptions, in general you should be aware that under normal conditions, the holes in the capillary walls are not big enough and the pressure within the glomerulus is not large enough to allow anything and everything to leak through. For example, tiny ions like calcium, potassium, and sodium will go through. Little molecules like water, urea, amino acids (which combine to form proteins), and glucose (a small carbohydrate) will also go through.
Small ions and molecules are able to pass through the capillary walls.
However, cells and larger protein structures like albumin and hemoglobin shall not pass! Only small, and preferably positively charged, ions and molecules can pass through. In addition, you should understand that this process of pressurized filtration of blood by the glomerulus per unit of time is known as the glomerular filtration rate, or 'GFR' for short.
With that in mind, let's review the important points of our lesson. The glomerulus is a network of renal capillaries responsible for the filtration of blood. The glomerulus is supplied with blood by a small artery carrying unfiltered blood called the afferent arteriole. The efferent arteriole is a small artery that carries filtered blood away from the glomerulus and back into general circulation. Each glomerulus is surrounded by a single layer of epithelium called the Bowman's capsule, and the glomerulus and its surrounding Bowman's capsule is called a renal corpuscle.
Following this lesson, you should be able to:
- Describe the anatomical structure and function of the renal corpuscle, which includes the glomerulus and Bowman's capsule
- Explain how blood flows through the afferent and efferent arterioles
- Summarize how ultrafiltrate flows through the kidneys