The Kidneys
– marvels of filtration
The human kidney, weighing between 120 and 160 g, is a complex physico-chemical factory which is essential for purifying the blood (See Diagram). The two kidneys are located on either side of the backbone in the region of the loins. They control the fluid balance of the body and keep constant the composition and volume of the extra-cellular liquid in which all the cells of the body are bathed. In this way the functioning of all the cells of the body is optimised. When there is too much liquid or too much of dissolved substances, the kidneys ensure that excesses are eliminated. If there is a shortage of water, the kidneys reduce the excretion of water without affecting the essential elimination of metabolic end-products. All these processes are carried out in such a way that the kidneys control the water and salt content of the blood and excrete the waste products of protein metabolism (urea) and purine metabolism (uric acid), as well as any toxins.
For this purpose, blood flows through the kidneys at an astounding rate – 1.2 litres of blood each minute, which is four times their own weight. The specific perfusion of an organ is a term indicating the amount of blood flowing through it per minute in proportion to its weight. This amounts to (1,200 cm3 blood/min)/(280 g kidney weight) = 4.3 cm3/(g x min). This is appreciably higher than for the other major organs like the brain, liver, or heart muscle. Every day 1,700 litres of blood flow through the kidneys. This is more than twenty times the entire body weight and 340 times the total amount of blood in the body, which is about 5 litres. The quantity of blood passing through the kidneys in one hour is 15 times the entire volume of the blood in the circulatory system.
There are about 2.5 million renal glomeruli (Latin singular glomerulus = the diminutive of glomus = cluster; plural: glomeruli) which, together with a similar quantity of renal tubules (having a total length of nearly 100 km), contribute to a cleverly designed physical filtering process, actually a molecular sieve.
The volume of liquid filtered by the glomeruli per unit of time provides a measure of the excretory capacity of the kidneys. This is called the glomerular filtration rate (GFR) which is about 120 cm3 per minute. This means that approximately 180 litres, which is 60 times as much as the volume of all the blood plasma (the liquid part of the blood, being about three litres), are filtered and purified every day. The total volume of extra-cellular fluid in the body (including blood) is about 14 litres, so even this amount passes 13 times per day through the kidney filter. The renal glomeruli thus excrete 180 litres of ultra-filtrate (primary urine) per day. This high volume is required to properly excrete metabolic products through the capillary walls. But if this enormous quantity of urine were passed directly, the water loss would be horrendous. This volume of urine would fill 18 buckets a day, and we would never be able to leave the toilet. What’s more, we would have to drink a similar quantity of water each day.
However, the all-wise Creator set up an ingenious principle whereby more than 99 per cent of the water and the major portion of other vital substances are returned to the body. As the filtrate travels further along the renal tubules (See diagram), water, glucose, and sodium chloride are recovered and returned to the blood. The recovery ratio is about 100:1, so that only between 1 and 1.6 litres of urine are eliminated, depending on the quantity of ingested liquid and on other water losses (e. g. sweat).
In cross-section (See diagram), the kidney displays an outer granular cortex and an inner, radially striped medullary layer. The nephron (Greek nephron = kidney) is the smallest functional renal unit. It comprises the glomerulus, together with the blood-transporting arterioles, the renal tubule, and the papillary duct (See left part of the diagram). Granular in appearance, the renal glomeruli are an extremely well-designed apparatus. The vas afferens1 (diameter between 20 and 50 μm), conveying the incoming blood, branches into a bundle of very fine capillaries with a diameter of only 7 μm = 0.007 mm. The tiny vessels in this clump (about 30 loops; see the right hand part of the diagram) subsequently combine to form the vas efferens which carries the blood away. This is a marvellously constructed system.
The tuft of blood vessels is enclosed in a doublewalled container known as Bowman’s capsule, whose diameter is only about 0.17 mm. The place where the blood enters and leaves, is called the vascular pole. The urinary pole, where a single conduit emerges, is located at the other end of the capsule. The walls of the looped capillaries act like a filter with extremely fine pores. These allow water and other small molecules to be squeezed out through them, but blood corpuscles and large protein molecules cannot pass through.
The result of this ultra-filtration is that 180 litres of “primary” urine is pressed from 1,700 litres of blood during a 24 hour period, comprising a loss of about 10 per cent by volume. But most of this “loss” is recovered by means of the retro-absorption taking place in the renal tubules, tiny tubes which extend from the urinary poles of the capsules. Water, glucose, and other substances are recovered from the primary urine and returned to the blood. A renal tubule starts with a convoluted segment which joins directly to a straight segment, followed by the thin loop of Henle. This leads into another convoluted segment, the distal tubule (see the left diagram). This discharges urine into a minor collecting duct, which becomes a larger collecting tube (the papillary duct). Bundles of these end in bulges of the renal medulla (renal papillae). These in turn are incorporated in cuplike extensions (renal calyces) of the renal pelvis (See diagram), where they discharge their urine. This is subsequently carried to the bladder by the ureter.
Continual contemplation of God’s Word acts in our lives like purifying kidneys, as Jesus told his disciples: “Now ye are clean through the word which I have spoken unto you.” (John 15:3). The redemptive power of the blood of Jesus is the foundation of purification; in the Word of God the church is told that “Christ loved the church and gave himself up to her to make her holy, cleansing her by washing with water through the Word” (Eph 5:25-26).
We cannot live without kidneys. If someone’s sins have not been forgiven, and all the accumulated dross is not eliminated, then he is spiritually dead. Nobody can have everlasting life without spiritual kidneys, as Jesus said: “Let the (spiritually) dead bury their (physically) dead” (Matt 8:22).
