The kidneys

The kidneys are bean-shaped organs approximately 10 cm in length. They are situated on the posterior wall of the abdominal cavity and lie on either side of the spine at the level between the twelfth thoracic and the third lumbar vertebrae. Their medial surface is concave and has an indentation or notch called the hilum. They are covered and held in place by fibrous connective tissue and protected by adipose tissue (fat), which insulates and protects the kidneys.

Blood is brought to the kidneys via the renal artery and taken away via the renal vein. These vessels enter the kidney at the hilum, together with the nerves and lymphatic vessels. One ureter leaves each kidney at the hilum and leads into the bladder.

The kidneys are composed of three distinct layers:

Capsule

1 | a tough fibrous outer layer or capsule that encases and protects the kidney

^^ the cortex, which lies in the middle, and is dark red in colour

^^ the medulla, which is the inner layer, and is reddish brown in colour.

Each kidney is made up of over a million functional units called nephrons, where the kidney processes of filtration and selective reabsorption occur (Figure 2.24).

Blood entering the kidney via the renal artery passes into fine capillary networks, which surround the tubules of the nephrons. The first phase of kidney function is the filtration of substances from the capillary blood into the kidney. These substances pass from the blood in the glomerulus into the Bowman's capsule of the kidney: they include water, mineral salts, glucose, toxins, uric acid and urea. These pass through the tubules to be eliminated but some substances are reabsorbed if the body needs them.

The substances to be eliminated from the body form urine and include the waste products of protein metabolism, i.e. urea, uric acid, ammonia; toxins; certain mineral salts and some water. These substances pass along the tubules into the ureters and bladder to be eliminated.

Hilum Renal artery

Renal vein

Pelvis

Ureter

Capsule

Renal vein

Pelvis

Ureter

Cortex

Medulla

Figure 2.23 Section through the kidney.

Cortex

Medulla

Figure 2.23 Section through the kidney.

Bowman's capsule Efferent arteriole Glomerulus Renal artery

Renal vein

Kidney tubules

Loop of Henle

The nephron.

Kidney tubules

Figure 2.24

Loop of Henle

The nephron.

Other substances, needed by the body, are reabsorbed into the bloodstream. These substances pass from the kidney tubules back into the capillary blood: this is known as selective reabsorption. Most of the water, all of the glucose, some sodium ions and vitamin C are reabsorbed into the blood. Hormones regulate the reabsorption of water and sodium depending on body requirements.

Through these functions of filtration and reabsorption, the kidneys adjust the balance of water, sodium ions and other substances leaving the body, with those entering the body. In this way the kidneys regulate the composition and volume of blood and maintain a stable internal environment for the tissues, known as homeostasis.

Functions of the kidney: ® formation of urine

® elimination of toxic waste substances that are harmful to the body ® regulation of water balance in the body ® regulation of sodium level and other electrolytes ® maintenance of normal pH level of the blood ® the kidneys also influence blood pressure.

These functions of the kidneys maintain homeostasis and are vital for sustaining life: kidney failure will result in death. Kidney dialysis is a treatment for maintaining life if the kidneys fail to function. The patient is connected to a machine that circulates their blood through special tubing immersed in dialysing fluid containing prescribed substances necessary for treatment. Any unwanted or harmful substances are removed from the blood and its constituents are balanced. The procedure must be carried out at regular intervals to sustain life. A kidney transplant will offer the chance of near normal life to those fortunate enough to receive one. Unfortunately, there are not enough kidneys available to meet the demand.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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