Module Three 13.4 How do exchanges in the kidney help us to maintain the internal environment in mam

Kidneys: function and structure

Humans have two kidneys. They are bean-shaped organs, approximately 11.5 cm long, situated in the abdominal cavity, just below the ribcage, one on either side of the spine. They are embedded in fatty tissue for protection.

The kidneys are essentially blood filters. Blood is brought to the kidneys to be filtered by the renal artery. After filtering, the blood is taken away again by the renal vein.

The kidneys have four functions:

1. Regulation of blood water levels
2. Reabsorption of useful substances into the blood
3. Adjustment of the levels of salts and ions in the blood
4. Excretion of urea and other metabolic wastes

Excretion of urea

Urea is a waste product from the breakdown of proteins. If more protein is eaten than is needed the body will have an excess of amino acids, which cannot be stored by the body. The excess amino acids are broken down by the liver, with urea as a by-product. Urea is toxic, and must be excreted from the body, so it is passed into the blood to be filtered out by the kidneys. It is then passed to the bladder for storage as urine before leaving the body.

A longitudinal section through a kidney shows three distinct areas:

1. an outer region called the cortex
2. a middle region called the medulla, and
3. an inner region leading to the ureter, called the pelvis. (If you're wondering why it has the same name as the bones of the hip, it's because they both look a bit like a wash-hand basin - in Latin, pelvis.)

Kidneys: how they work

The solid-looking kidney is in fact made up of a mass of minute tubes - some 750,000 in each kidney - called nephrons. Each nephron starts in the cortex and runs down into the pelvis. It is in the nephrons that the filtering takes place.

Each nephron consists of a hollow cup-shaped structure called the Bowman's capsule (sometimes called the renal capsule), and a long narrow tubule which ends in the pelvis. This tubule has three distinct parts:

1. the first convoluted tubule
2. the loop of Henle, and
3. the second convoluted tubule

Filtering is a three-stage process:

1. Ultrafiltration

Blood from the renal artery is supplied to each Bowman's capsule by arterioles and capillaries - which eventually form a ball of tiny capillaries inside the capsule called a glomerulus. Blood in the glomerulus is at a relatively high pressure and this forces many of the substances dissolved in the blood through the capillary walls (which are only one cell thick, remember) into the capsule.

Ultrafiltration removes both waste products and essential nutrients from the blood -in fact, it filters out everything except red blood cells and blood plasma protein molecules which are too big to pass through the capillary walls. Obviously the body needs these nutrients, so they must be recaptured. And that's exactly what happens in stage 2..

2. Reabsorption
Blood now leaves the Bowman's capsule in another arteriole and then flows through many tiny capillaries that wind themselves round and round the tubules, which now contain the substances which have been filtered out in the glomerulus - the glomerular filtrate. The essential nutrients are then REABSORBED via active transport through the capillary walls into the blood.

All the glucose, amino acids, hormones, vitamins, and some salts are reabsorbed into the capillary blood as the filtrate passes through the first convoluted tubule. With these solutes now removed the glomerular filtrate becomes dilute, and water is reabsorbed into the blood by osmosis.

In the loop of Henle the ion content is adjusted to maintain the blood pH of between 7.3 - 7.4. As the fluid passes on through the second convoluted tubule some more adjustments are made and further absorption of water and salt may take place, depending on the current needs of the body.

3. Release of wastes
All the urea, together with any excess water, salt and ions, continue through the tubule into the pelvis, where they are collected and pass as urine via the ureter into the bladder, from where they are excreted.

The table summarises the changes in blood content as it passes through the nephron:

Before ultrafiltration the blood contains:	The glomerular filtrate contains:	Reabsorbed from tubule?
Blood cells	-	-
Plasma proteins	-	-
Glucose	Glucose	Yes
Amino acids	Amino acids	Yes
Hormones	Hormones	Yes
Vitamins	Vitamins	Yes
Salts	Salts	Yes
Water	Water	Yes
Urea	Urea	No

Human Anatomy Online - The Urinary System

abpi - The Kidneys and Body Balance

Treating kidney disease

People who suffer from kidney failure may be treated either by using a dialysis machine or by having a healthy kidney transplanted.

Dialysis Machine

• a person's blood flows between partially permeable membranes.

· waste urea passes out from the blood into the dialysis fluid.

It is important that useful substances in the blood, such as glucose and mineral ions, are not lost. Treatment by dialysis restores the concentrations of dissolved substances in the blood to normal levels and has to be carried out at regular intervals.

THE ADVANTAGES AND DISADVANTAGES OF PERITONEAL DIALYSIS

ADVANTAGES	DISADVANTAGES
• Patient's involvement in self-care	• Four exchanges per day
• Control over schedule	• Permanent external catheter
• Less diet & fluid restriction	• Change of body image
• More steady physical condition as it provides slow, continuous therapy	• Potential weight gain
• Most similar to original kidneys. Can be done in the night as in automated peritoneal dialysis	• Some risks of infection
• Provide less severe cardiovascular instabilities in patients with underlying heart disease	• If on automated peritoneal dialysis, one will be tied onto a machine in the night
	• Storage space is needed for supplies

Kidney transplantation

This enables a diseased kidney to be replaced by a healthy one from a donor. However, the donor's kidney may be rejected by the immune system unless precautions are taken. To prevent rejection:

• a donor kidney with a 'tissue-type' similar to that of the recipient is used;
• the bone marrow of the recipient is treated with radiation to stop white cell production;
• the recipient is kept in sterile conditions for some time after the transplant operation;
• the recipient is treated with drugs that suppress the immune response.

During a transplant operation, transfusion of blood may be required. This blood must be matched with the blood group of the recipient to prevent agglutination.

THE ADVANTAGES AND DISADVANTAGES OF KIDNEY TRANSPLANTATION

ADVANTAGES	DISADVANTAGES
• Absence of need for frequent dialysis treatment	• Need for frequent physician visits
• Better quality of life	• Pain, discomfort of surgery
• Better health	• Risk of transplant rejection
• Reduced medical cost after first year	• Prone to infections
• No diet and fluid intake restriction	• Some risks of infection
• Provide less severe cardiovascular instabilities in patients with underlying heart disease	• On lifelong medications

Kidney Transplants

The Treatment of Kidney Failure

In the ABO system of blood grouping there are:

• antigens on the surface of red blood cells;
• antibodies in the plasma.