The Kidneys

Di Prospero, Santoro, Massoni

12.3: the human excretory system

excretion

2.

Kidney tubles

3.

Urine formation

4.

Filtration

1.

The kidneys

6.

The bladder

7.

Kidney diaysis

8.

Kidney transplants

5.

Reabsorption

Index

• Cortex: the outer portion of the kidney between the renal capsule and the renal medulla;
• Medulla: the inner part of the kidney, divided into a number of sections known as the renal pyramids;
• the pelvis.

The kidneys

The kidneys are located behind the intestines, at the back of the abdomen. They can be divided into three main parts:

• The proximal tubule;
• The loop of Henle;
• The distal tubule;
• The collecting tubule;
• The collecting duct.

A kidney tubule is the largest part ofthe nephron, the functional unit of the kidney. It has the function of modifying, through different processes, the excess fluid until it becomes urine ready to be secreted. This organ has an overall length of 30-40 mm. The kidney tubule can be divided into three portions:

Kidney tubules

As blood passes through the kidneys, it is filtered by removing most of the urea, excess water and salts. During this process, useful substances like glucose are taken back into the body. The final liquid produced is a solution of urea and salts in water called urine, that flows out of the kidneys along the ureters and into the bladder, where it is stored before being released. The kidneys adjust theamount of urine they produce according to the needs of the body.

3. Urine formation

The part of the kidney responisble of filtrating the blood; there are thousands in the cortex of each one of them. Every kidney is shaped like a cup and with a glomerulus (tangle of blood capillaries) in its middle. Process of filtrationThe blood vessel brings blood from the other parts of the body to the kidneys through their cortex. There, the renal capsules squeelze the smaller molecules inside the kidney through small holes

Renal capsule

4. Filtration

The fluid now present in the renal capsule is a solution of glucose, salts and urea dissolved in water. Part of these substances need to be kept in the blood and not be expelled. Once these substances are reabsorbed, they are taken back into the blood passing in capillaries wrapped around each kidney. These organs are extremely efficient at reabsorbing water and putting it back in circulation. The remaining fluid, now mostly water and urea, is now fully urine and directed towards thecollecting duct.

5. Reabsorption

After reabsorption, the urine flows into the ureters, which take it to the bladder. This is an organ responsible for storing urine, which has stretchy walls for holding large quantities of liquid.Leading out of the bladder is the urethra, which has a sphincter muscle at the top, usually tightly closed so that the urine doesn't flow out of the body. This muscle can be conciously controlled by adult animals.

6. The bladder

+ onwards

The person’s blood flows through the machine and back into their body. Inside the machine, the blood is separated from a fluid by a partially permeable membrane. The fluid contains water, glucose, salts and other substances that should be present in the blood. As the patient’s blood passes through the tubes, the substances in the fluid diffuse out through the partially permeable membrane. For example due to the fact that there isn’t urea in the dialysis fluid, the urea in patient’s blood diffuses out into the fluid.

7. Kidney dialysis

The person receiving the transplant is the recipient, and the person from whose body the organ was taken is the donor. Organs for transplants must be removed quickly from a body and kept cold. Sometimes, however, the donor may be alive. The biggest problem isn’t the transplant operation, but the possibility of rejection, the process in which the recipient’s immune system could recognise the donor organ as being “foreign”, and attacks it. Drugs called “immunosuppressants” ,which stop the white blood cells working efficiently, are given to the recipient to decrease the chances of rejection. The chances of rejection are reduced if the donor is a close relative of the recipient. If there is not a relative who can donate an organ, then may be opened a world-wide search, looking for a potential donor with similar antigens to the recipient.

8. Kidney transplants

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