The Kidney

Generalities.

    We have two kidneys. They are located in the lumbar region of the abdominal cavity, apposed to the back. Usually, the left kidney is slightly higher than the right kidney. The main functions of the kidneys are to maintain the ionic balance of extracellular fluid and filter the blood to eliminate toxic waste.

    It is by virtue of balancing the retention and excretion of water and electrolytes that the kidneys help to maintain an ideal composition of blood and extracellular fluids. For example, renal function must adjust to the changes of our water and salts consumption, and to the various salt loss caused by perspiration (sweat production), vomiting or diarrhea. The kidneys also have the function of removing various metabolic wastes such as urea and creatinine, as well as many foreign substances such as drugs, food additives, pesticides or other substances that have no nutritional utilities. Each day, to accomplish proper filtering, the kidneys require a minimum of 500 ml of water to form urine. So if a person does not drink at least 500ml of water daily, it will lose blood volume to death.

    The blood enters the kidney via the renal artery. Approximately 20-25% of cardiac output passes through the kidneys. Then, at the level of the glomerulus (localized in the renal cortex), the plasma is extracted and passes in a long U­shaped tubules (located in the medulla and cortex) to be filtered. The urine that is produced will be more or less concentrated in order to maintain plasma homeostasis. Once formed, the urine is collected in the middle of the kidney, and flow along the ureter to the bladder, where it is temporarily stored. The adult bladder can hold 250 to 400ml of urine before the pressure inside the bladder stimulates an urge to urinate.

    The bladder is a bag that can stretch to accommodate a certain volume of urine and contract to drain that urine. Contraction of the bladder produces the pressure required to expel urine into the urethra to the meatus. In women, the urethra is short and the urinary meatus is located in the vulva between the clitoris and the vagina. In men, the urethra is longer and the urinary meatus is located at the tip of the penis. For anatomical details, please refer to pages on the reproductive system of man and woman.

    In the figures below you will discover the anatomical details of the kidney. You will find the renal artery and vein, the cortex, medulla, the hilum, pyramids and renal columns. The figure on the right shows a cast of the arterial vasculature in red and the renal excretion in yellow. To achieve this, a red polymer was injected into the renal artery and a yellow polymer was injected into the urethra. In this case, the veins have not been injected.

The nephron.

    The nephron is the functional unit of the kidney. In the kidney, the nephron and vascular system are entangled, and this is what allows the filtration of the blood.

    Each kidney contains about one million nephrons. The upper part of the nephron, composed mainly of the proximal tubule, distal tubule and the glomerulus. They are located in the renal cortex, while the lower part, the nephron loops (loops of Henle), are located in the medulla of the kidney. Collecting ducts run across the kidney, from the cortex to the medulla and empty into the renal pelvis.

    Intimately linked to this system of tubules, there is a complex vascular system. The blood enters the kidney via a renal artery which divides in a lot of afferent arterioles, one for each nephron. Unlike arterioles in the rest of our body which divide into capillaries to deliver oxygen, the afferent arterioles of the kidney rather give birth to a ball-shaped network of fenestrated capillaries called the glomerulus. The glomerulus is encapsulated in a structure called the Bowman's capsule. It is at that level that part of the blood, the serum or a primitive urine passes into Bowman's capsule and travel the nephron's tubular system. When exiting the glomerulus, the capillary network reunite in an arteriole, an efferent arteriole. And, another particular characteristic of the renal vascular network is that the efferent arteriole forms another network of capillaries which are intimately linked to tubules of the nephron. At this level, they are called the peritubular capillaries. This peritubular network has two functions, it feeds the kidney tissue proper and participate in the exchange, mostly recapture, of substances between the urine in the tubular system and the blood. Finally, this network of peritubular capillaries merge into a venule, and in a renal vein which exit the kidney to return the blood to the heart.

    As we just said, it is in the glomerulus that the blood is filtered, the filtrate accumulates in the Bowman's capsule. This filtration, or ultrafiltration, is performed because there is more blood pressure in the afferent arteriole than the efferent arteriole, hence more pressure in the glomerulus than in the Bowman's capsule. In addition, the capillaries in the glomeruli are fenestrated, meaning they have perforated walls, which allows the plasma to pass from the capillaries into the Bowman's capsule. The rate at which blood is filtered is called the glomerular filtration rate (GFR), and this rate is usually tightly controlled.

    Several mechanisms are involved in this control. There are self-regulatory mechanisms ensuring that even if the systemic arterial pressure (the blood pressure) changes, the kidney will succeed, within certain limits, to maintain a constant glomerular filtration. This is mainly done by adjusting the caliber of the afferent arterioles, using vasoconstriction or vasodilation. This could be achieved by two distinct mechanisms: a myogenic mechanism and tubuloglomerular feedback mechanism. The myogenic mechanism is actually a simple vascular reflex that when there is more tension on the arterial wall, there is an increased tonus, vascular contraction. Conversely, blood vessels will dilate when the blood pressure decrease. allowing more blood to go through and less stress on the arterial wall. The tubuloglomerular feedback mechanism, on the other hand, involve a special portion of the distal tubule. This is the portion that passes between the afferent and efferent arterioles. This area, called the macula densa, contains specialized secretory cells. These cells are intimately associated with other cells, the juxtaglomerular cells (the granular cells), which are located in the terminal portion of the afferent arteriole to provoke its contraction and therefore reduce the glomerular filtration rate. Together, these structures form the juxtaglomerular apparatus which detects the concentration of sodium chloride in the distal tubule and secrete paracrine substances (hormones locally active: possibly endothelin for vasoconstriction and bradykinin for vasodilation) to act on the afferent arteriole to decrease or increase the glomerular filtration rate (GFR).

    Another mechanism also participates in the control of glomerular filtration rate. This mechanism involves the sympathetic nervous system which can be excited or inhibited in response to the stimulation of baroreceptors (blood pressure sensors) located in the carotid sinus and the aortic arch. Activation of the sympathetic nervous system, due to bleeding for example, will cause vasoconstriction of the afferent arteriole to decrease the GRF, and the production of urine. This will help to retain body fluid. Conversely, when blood volume increases, like after drinking a lot of water, the activity of the sympathetic nervous system will decrease, relaxing the afferent arteriole allowing more blood to filter and produce more urine. This will reduce the excess in blood volume.

    Once the filtrate (the primitive urine) leaves the glomerulus, it travels along the proximal tubule, the loop of Henle, distal tubule and collecting duct. Throughout this trip very complex processes will concentrate this filtrate in urine per see. These processes involved diffusion, active transport and reabsorption of various substances. All of which controlled by membrane transport pumps, certain hormones and gradients in osmolarity. It would be too complex, and out the scope of this web site, to explain these mechanisms in details. I provide you with a table and some figures showing how the composition of urine can change along the tubule system, and how various hormonal systems are involved in the reabsorption or excretion of salts and water.

Urination.

    Urination is the action of evacuating the urine. The urine which is produced by the kidneys must first be stored in the bladder. Then, when the bladder is full enough, it must empty. If we did not have the ability to store the urine produced by the kidneys, it would constantly drip out our body.

    The figures on the left show the location of the organs responsible for urinary excretion. As you can see, there is a close relationship between the urinary tract and reproductive system: in men, urine leaves the tip of the penis, whereas in women, it is expelled at the vulva between the clitoris and the vagina. The bladder is located within the abdominal cavity, as close as possible to the exit, for the urethra to be the shortest possible and thus offer the least resistance to flow.

    Before being evacuated, the urine, produced by the kidneys, must first be stored in the bladder. It is the ureter that connect the kidneys to the bladder. At this level, the urine does not flow only by gravity, but also with the help of peristaltic contractions of ureteral smooth muscles. Then, the ureter enters, obliquely, deep into the bladder. When the bladder fills, the weight of the urine press against a portion of the ureter preventing the backflow of urine to the kidney.

    The bladder walls contains smooth muscles, which stretches and contracts. These muscle are called the detrusor muscles. When the bladder fills with urine, the muscle fibers stretch and that stimulates stretch sensitive receptors. This stimulation causes a reflex, passing through the spinal cord, that activates the fibers of the parasympathetic nervous system innervating the muscle of the bladder. This activation causes the contraction of the bladder's smooth muscles. When the bladder contracts, the pressure increases and this produces the urge to urinate. The higher the pressure in the bladder, the more the urge to urinate becomes difficult to control.

    As safeguard mechanism, there are sphincters that prevent involuntarily urination. A sphincter is a ring of striated muscle which surrounds a conduit and, when contracted, prevents the flow of liquid through that conduit. With regards to the urinary system, there are two sphincters: an internal urethral sphincter that is part of the bladder itself and not under voluntary control, and an external urethral sphincter that is under voluntary control. When there is no contraction of the bladder, the internal sphincter closes the opening of the bladder but, upon bladder contraction and increased pressure, it open. Meanwhile, the external sphincter, under voluntary control, remains constricted (closed) unless we voluntary relax it. At the time of voluntary voiding, in addition to relaxing this external sphincter, the stimulation of our parasympathetic nervous system increases bladder contraction and urine is expelled with some pressure. In young children, the micturition reflex is rather accidental and the brain does not have control, or have little control, over this reflex. It is only with training that the control of the external sphincter is reinforced.

    We voluntarily control that reflex, up to a certain limit, when the bladder is really full and the pressure becomes almost unbearable, this reflex becomes uncontrollable. At this point, we need to urgently find a toilet, otherwise the accident is imminent. It is a matter of health.