THE RENAL SYSTEM

Introduction

The chemical composition of body fluids is important for the well-being of the cells of the body. The circulatory system is mainly responsible for the physical transport of fluids but not for the composition of those fluids. This function is largely the responsibility of the kidneys.

Although they help with various physiological functions, the kidneys' main roles are the removal of wastes and the maintenance of the body's water balance. The functions of the kidneys can be summarised as follows:

1. Control of the body's water balance. The amount of water in the body must be balanced against the amount of water which we drink and the amount we lose in urine and sweat etc.

2. Regulation of blood pressure via the renin-angiotensin-aldosterone system

3. Regulation of blood electrolyte balance - Na+, Ca2+, K+ etc.

4. Excretion of metabolic wastes such as urea, creatinine and foreign substances such as drugs and the chemicals we ingest with our food

5. Help in the regulation of the body’s acid base balance

6. Regulation of red blood cell production via the hormone erythropoietin

7. Help in the production of vitamin D

As this list indicates, the renal system is very important to the normal functioning of the body.

THE STRUCTURE OF THE RENAL SYSTEM

Urine is produced in the kidneys from water and wastes extracted from the blood.

The rest of the urinary system is concerned with the storage and ducting of the urine to the outside of the body - Figure 01.

Figure 01 - Structure of the renal system

The kidneys are large, bean shaped organs which lie on the dorsal side of the visceral cavity, roughly level with the waistline. Blood is supplied to the kidneys by the renal arteries which branch off the aorta. The kidneys and are drained by the the renal veins into the inferior vena cava. From the kidneys, urine passes to the urinary bladder via the ureters. Urine is passed to the outside environment via the urethra (this is routed differently in males and females).

MACROSTRUCTURE OF THE KIDNEY

The kidneys are protected by a tough fibrous coat called the renal capsule. Under

the capsule, the arrangement of nephrons and capillaries in the kidney produce the appearance of distinct regions when viewed in longitudinal section. The outer cortex region surrounds darker triangular structures called pyramids which collectively form the medulla. The inner part of the kidney, the renal pelvis, collects the urine draining from the nephron tubules and channels it into the ureter - Figure 02.

Figure 02 - Sectioned view of the kidney

MICROSTRUCTURE OF THE KIDNEY

The basic functional unit of the kidney is the nephron. There are over one million nephrons in each human kidney and together they are responsible for the complex water regulation and waste elimination functions of the kidneys. The heads of the nephrons are in the cortical region and the tubular component then descends through the medulla and eventually drains into the renal pelvis - Figure 03.

Figure 03. Arrangement of nephrons in the kidney

The key area of interface between the circulatory system and the tubular part of the kidney is the knot of glomerular capillaries in the Bowman's capsule. Those liquid parts of the blood that are able to cross through the filtration membrane of the capillaries pass into the Bowman's capsule and then into the tubular section of the nephron - Figure 04. The filtration membrane only allows water to pass through it and small molecules that will dissolve in water such as waste (urea, creatinine etc.) glucose, amino acids and ions. Large proteins and blood cells are too large to be filtered and remain in the blood.

Figure 04. The Bowman's capsule and glomerulus

The filtered fluid or filtrate enters the proximal tubule and then into the loop of Henle which is the part of the nephron which dips in and out of the medulla. From

the loop of Henle, the filtrate travels through the distal tubule and then into a common collecting duct which passes through the medulla and into the renal pelvis - Figure 05.

 

Figure 05. Structure of the nephron

PERITUBULAR CAPILLARIES

The nephrons are surrounded by a fine network of capillaries called the peritubular capillaries. These perform an important role in direct secretion, selective reabsorption and the regulation of water (see below).

DIRECT SECRETION

In addition to glomerular filtration, some substances are secreted directly from the adjacent peritubular capillaries into the proximal tubule. These substances include potassium ions and some hormones.

SELECTIVE REABSORPTION

Ultrafiltration is indiscriminate except for size of particle and useful substances are filtered from the blood as well as wastes. This situation is obviously unsatisfactory as the body would soon be depleted of amino acids, glucose and sodium etc. which would need to be replenished from external sources. To resolve this problem, useful substances in the filtrate are reabsorbed back into the peritubular capillaries as the filtrate passes along the tubule, leaving only the wastes which are eliminated in the urine. This process is shown in the animation in Figure 06.

Figure 06. Selective reabsorption of essential nutrients

WATER REGULATION BY THE KIDNEYS

The water content of the body can vary depending on various factors. Hot weather and physical activity such as exercise make us sweat and so lose body fluids. Drinking tends to be at irregular intervals when socially convenient. This means that sometimes the body has too little water and needs to conserve it and sometimes too much water and needs to get rid of it. Most of the control of water conservation takes place in the distal and collecting tubules of the nephrons under control of anti-diuretic hormone, (ADH), sometimes called vasopressin. This hormone is released by the posterior pituitary under control of the hypothalamus in the mid-brain area. The hypothalamus monitors the water content of the blood. If the blood contains too little water (indicating dehydration) then more ADH is released. If the blood contains too much water (indicating over-hydration) then less

ADH is released into the blood stream - Figure 07.

Figure 07. Release of ADH from the posterior pituitary into the bloodstream

ADH released from the pituitary travels in the blood stream to the peritubular capillaries of the nephron. ADH binds to receptors on the distal and collecting tubules of the nephrons which causes water channels to open in the tubule walls. This allows water to diffuse through the tubule walls into the interstitial fluid where it is collected by the peritubular capillaries. The more ADH present, the more water channels are open and the more water is reabsorbed - Figure 08.

Figure 08 Reabsorption of water from the filtrate under the influence of ADH

Over 99% of the filtrate produced each day can be reabsorbed. The amount of water reabsorbed from the filtrate back into the blood depends on the water situation in the body. When the body is dehydrated, most of the filtrate is reabsorbed but note that even in cases of extreme of water shortage, the kidneys will continue to produce around 500 ml of urine each day in order to perform their excretory function.

THE MICTURION REFLEX

Micturition is another word for urination and in most animals it happens automatically. As the bladder fills with urine, stretch receptors in the wall of the bladder send signals to the parasympathetic nerves to relax the band of smooth muscle that forms the internal urethral sphincter. As the muscle relaxes, the urethra opens and urine is voided to the outside environment.

A second sphincter, the external urethral sphincter is skeletal muscle controlled by motor neurons - Figure 09. These neurons are under conscious control and this means we are able to exercise control over when and where we urinate. This control is a learned response that is absent in the new-born infant.

Figure 09. The urinary bladder and urethra

RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM

The long-term control of blood pressure is via the renin-angiotensin-aldosterone (RAA) system. This system is also one of the body's compensatory mechanisms to a fall in blood pressure. The kidneys release renin into the bloodstream and this converts angiotensinogen to angiotensin I which in turn is converted to angiotensin II by angiotensin converting enzyme in the capillaries of the lungs. Under the influence of Angiotensin II, aldosterone levels increase. This increases blood sodium levels by decreasing the amount of salt excreted by the kidneys. Retaining salt instead of excreting it into urine increases the osmolarity of the blood and so the blood volume. As the volume increases, so does the blood pressure. Angiotensin II is also a potent vasoconstrictor which raises blood pressure by increasing vascular resistance - Figure 10.

Figure 10. The Renin, angiotensin, aldosterone response to a fall in blood pressure

ACID BASE BALANCE

The body controls the acidity of the blood very carefully because any deviation from the normal pH of around 7.4 can cause problems - especially with the nervous system. Deviations in pH can occur due to trauma or diseases such as diabetes, pneumonia and acute asthma. The mechanisms that resist and redress pH change are...

1. Minor changes in pH are resisted by plasma proteins acting as buffers in the blood.

2. Adjustment to the rate and depth of breathing. An increase in acidity (decrease in pH) increases the rate and depth of breathing which gets rid of carbon dioxide from the blood and so reduces acidity.

3. The kidneys respond to changes in blood pH by altering the excretion of acidic or basic ions in the urine. If the body becomes more acidic, the kidneys excrete acidic hydrogen ions (H+) and conserve basic bicarbonate ions (HCO3-). If the body becomes more basic, the kidneys excrete basic bicarbonate ions and conserve acidic hydrogen ions.

Together, these three mechanisms maintain tight control over the pH of the body.

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