The Urinary System
|Organs and functions of the urinary system: (See Figure 26.1)
1) the urinary bladder - stores urine and contracts to release it to the urethra
2) urethra - carries urine from the urinary bladder
3) ureters carry urine from the kidney to the bladder
4) kidney - maintains homeostasis of the blood. It would be tempting to list the kidney's function as production of urine, and it certainly does this. But the production of urine is a side effect of the kidney's true function, to regulate the homeostasis of: electrolytes and water, acid-base, nutrients, wastes, and toxins, even oxygen and blood pressure.
The detailed structure of #'s 1 through 3 will be discussed later. We will begin with the kidney.
| The Kidney (See Figure 26.3)
The kidney is composed of several years and is covered with a fibrous capsule, the renal capsule. The outer layer of the kidney is the cortex. It contains the major (upper) portion of the nephrons. The middle layer of the kidney is the medulla. It is composed of the triangular shaped pyramids and the renal columns. The pyramids contain the collecting tubules and loops of Henle, the lower portion of the nephrons. These tubules run nearly parallel to one another and give the pyramids a grain which leads to their points or papillae. The renal columns are regions between the pyramids in which blood vessels run to and from the cortex. The papilla of each pyramid projects into a funnel-shaped area known as the calyx. The calyces (plural of calyx) collect the urine released from the papillae and allow it to drain into a large area known as the renal pelvis and then into the ureter.
The blood supply of the kidney is paramount in its function. The two kidneys receive between 15 and 20% of the body's systemic blood flow at rest. The renal artery branches into lobar and then interlobar arteries. These pass through the renal columns toward the cortex. Arcuate arteries branch into the cortex and lead to interlobular arteries which distribute the blood evenly throughout the cortex to the afferent arterioles which serve the nephrons. Blood flow leaving the nephrons returns by veins of the corresponding names.
|The Nephron: (See Figures 26.4 and 26.5) - The nephrons are the
functional units of the kidney, i.e. individually and collectively they perform
the functions of the kidney.
Each nephron is served with blood by the afferent arteriole. This vessel brings blood into a capillary tuft called the glomerulus. Blood leaving the glomerulus flows into the efferent arteriole. Usually an arteriole flows into a venule. But in this case the efferent arteriole flows into more capillaries, the peritubular capillaries, and, in juxtamedullary neurons (see below), the vasa recta. The peritubular capillaries and vasa recta then lead to venules and the venous drainage of the kidney.
A capillary tuft differs from a capillary bed in that it does not perfuse a tissue like a capillary bed does. Instead this capillary tuft is a condensed mass of capillaries which allows substances to escape by filtration. The capillaries of this tuft are surrounded by specialized cells which form the inner (visceral) layer of Bowman's capsule. (See Figures 26.7 and 26.8)These specialized cells are called podocytes (foot cells) because they have processes called pedicels which interdigitate or interlace producing openings called filtration slits. The capillaries are fenestrated in order to allow a large amount of filtration. The outer (parietal) layer of Bowman's capsule consists of epithelial cells with tight junctions and serves to contain the filtrate in the capsular space.
The Bowman's capsule opens into the proximal convoluted tubule which leads to the loop of Henle. The loop of Henle has a descending limb which passes into the medulla, recurves, and becomes the ascending limb which leads back up to the distal convoluted tubule in the cortex. Most human nephrons are termed cortical nephrons because their corpuscles are located in the mid to outer cortex and their loops of Henle are very short and pass only into the outer medulla. But a small portion are called juxtamedullary nephrons and their loops travel deep into the inner medulla. These nephrons are important in concentrating the urine by increasing the amount of water reabsorbed. Distal convoluted tubules lead into collecting tubules and ducts which pass through the medullary pyramids to the papillae. See [Orientation of the Nephron] diagram.
|Step 1 in urine formation, Filtration - Fluid pressure forces water and dissolved substances out of the blood into Bowman's capsule. Filtration averages 125 ml/min for your two kidneys. This amounts to about 180 Liters per day. Since we urinate an average of 1500 ml per day, more than 99% must be returned to the blood. Filtration involves the small molecules: water, electrolytes, urea, glucose, amino acids. It does not involve the blood proteins or cells. The large amount of filtration is the result of the porous glomerular membrane and filtration slits in the visceral layer of Bowman's capsule.|
|Step 2, Reabsorption - The return of substances from the filtrate to the blood and interstitial
fluid. The major substances reabsorbed are water, NaCl, glucose, and amino acids. Some of
the urea, together with other salts are also reabsorbed.
Water is reabsorbed by osmosis. Entering the proximal convoluted tubule the filtrate is very dilute compared to the blood. 65% of water reabsorption occurs from the PCT as a result of this osmotic gradient. As the filtrate enters the descending limb of the loop of Henle, especially in juxtamedullary nephrons with long loops, it is exposed to increasingly hypertonic medulla. This pulls at least another 20% of absorbable water out of the filtrate. Reabsorption in this area is termed obligatory because it must occur due to the osmolarity of the surrounding interstitial fluid. As the filtrate enters the ascending limb the tubule becomes impermeable to water. Otherwise it might actually diffuse back into the tubule as the osmotic gradient reverses. When the filtrate, now nearly urine, passes through the medulla again in the collecting tubule it is once again exposed to the hypertonicity of the deep medulla. This has the potential to pull more water out by osmosis. But reabsorption of water from the collecting tubule is facultative because it is under control of the hormone ADH (See below).
|The Countercurrent Mechanisms to increase water reabsorption:
1) The Countercurrent Multiplier - This mechanism works in the loop of Henle to increase water reabsorbed from the descending limb as a result of salt reabsorbed from the ascending limb. The term countercurrent comes from the fact that fluid is moving in opposite directions in the two limbs of the loop. This magnifies the effect of transport from one limb on transport from the other limb. The same principle is at work in heat exchangers used in industry.
2) The countercurrent exchange of salt in the vasa recta. The vasa recta has descending and ascending limbs too. Blood flowing into the medulla in the descending limb picks up salt from the hypertonic medulla. As the surrounding medullary fluid becomes more and more salty toward the papilla the gradient increases and more and more salt is picked up by the descending vasa recta limb. But as the blood heads back up to the cortex in the ascending limb of the vasa recta, the interstitial fluid becomes less and less salty. This causes the gradient to reverse and salt diffuses back out of the vasa recta into the medulla. This helps to conserve salt and keep the medulla hypertonic.
3) Urea is also reabsorbed, passively, from the nephron and this too helps to keep the surrounding fluid hypertonic, pulling water. This same urea will be filtered later and may in fact be reabsorbed again. Overall though, urea experiences a net loss from the body because more is filtered and released in the urine than is reabsorbed.
|Step 3, Secretion
Secretion is the release by active transport of substances into the filtrate. It is accomplished by the tubular lining cells. The substances released are usually derived from the blood in the peritubular capillaries. Actually secretion has already been going on but it is the third process we consider. It begins in the proximal convoluted tubule and continues in the distal convoluted tubule and the collecting tube. It is done for three purposes:
1) to release any residues from toxins and drugs which haven't bee filtered;
2) to establish electrolyte balance. Since positive ions, namely sodium, are reabsorbed, positive ions must be secreted in exchange. The first choice is potassium, K+. In addition negative ions will be managed. This usually means chloride, Cl-, will either be secreted or will diffuse down its electrochemical gradient. Other anions may be available for release such as sulfate, but certain ions will never be secreted. For example, bicarbonate will always be retained to help manage the buffering capacity of blood.
3) acid - base balance. Usually this means getting rid of acid. The first choice for this is H+. Hydrogen ions are derived from the reaction of carbon dioxide and water, just as they are in the rbc and in stomach lining cells. The reaction yields carbonic acid which dissociates into H+and HCO3-as you've already learned. The bicarbonate produced is retained for the buffer (as mentioned above) and exchanged for chloride, called the chloride shift. Hydrogen ions can be secreted during moderately acidic conditions, but when you have more severed acidity they reach their limit, called the tubular maximum. At that point they neutralize some of the H+ with NH2 and NH3 groups derived from certain amino acids. The result is ammonium ions, NH4+ , which are secreted during these more severely acidic conditions. During extreme acidity they can also secrete phosphoric acid.
Since the hydrogen ions and ammonium ions are also cations, less potassium is secreted during acidic conditions as well. Since conserving potassium may be important for many people, consuming liquids which are acidic as well as contain potassium are important in supplying the needed potassium and encouraging it to be retained by the body. Citrus juice, although containing potassium, does not acidify the blood greatly, but cranberry juice, grape juice, watermelon etc. work well. Cranberry juice also acidifies the urine which can help discourage bacteria and some types of kidney stones. Cranberry juice also reduces the adherence of bacteria onto the walls of the urinary tract thus reducing urinary tract infections.
|The ADH Mechanism for controlling
Reabsorption in the collecting tubule is controlled by a hormone from the posterior pituitary gland known as ADH, anti-diuretic hormone. Actually this hormone is released by nerve fibers coming from the hypothalamus and stored in the pituitary. ADH is then released into the blood on command of the hypothalamus. The hypothalamus responds to high blood osmolarity. Increased osmolarity results from water loss and dehydration from sweating, vomiting and the like, and from simply not taking in enough replacement water. ADH allows water to be reabsorbed from the collecting tubule and not leave the body with the urine. The water is reabsorbed by osmosis due to medullary hypertonicity. Lack of ADH causes the production of a large amount of dilute urine, a condition called diabetes insipidus.
|Juxtaglomerular Apparatus (JGA) and the tubuloglomerular mechanism in
The juxtaglomerular apparatus is a place where the distal convoluted tubule lies close to the glomerulus and to the afferent and efferent arterioles. Within the JGA is a group of cells lining the distal tubule called the macula densa cells. These cells monitor the rate of filtrate flow in the distal tubule, which is directly related to the glomerular filtration rate (GFR) and the glomerular pressure. It also monitors the salt levels. In response to rising salt levels and reduced GFR the macula densa cells do two things:
1) The macula densa causes the juxtaglomerular cells lining the arterioles to secrete renin. Renin acts as an enzyme to cause a substance already in the blood, angiotensinogen, to undergo a structural change to become angiotensin I, which is then converted to angiotensin II by angiotensin converting enzyme. See [Angiotensin Converting Enzyme, ACE]. Angiotensin II acts as a vasoconstrictor, first causing vasoconstriction in the efferent arteriole. Since the efferent arteriole is the outflow from the glomerulus, constricting it rapidly raises glomerular pressure. Angiotensin II also causes the adrenal cortex to release aldosterone. Aldosterone acts on the distal convoluted tubule to increase Na+ reabsorption. More sodium reabsorption means more water reabsorption, and more water reabsorption means and increase in blood pressure.
2) The macula densa also acts directly on the afferent arteriole and cause it to vasodilate. So at the same time the efferent arteriole is constricting, the afferent arteriole is dilating bringing in more blood and the combination dramatically raises glomerular pressure and GFR.
The only mechanism responsive to high blood pressure is the direct myogenic autoregulation of the afferent arteriole. This vessel, like others in the body, responds to high pressure with vasoconstriction. This reduces blood flow into the glomerulus and brings GFR back down to normal levels. This mechanism works only for transitory pressure increases and is not effective against sustained hypertension.
|The heart-renal connection:
Several mechanisms are at work to respond to excessively high or excessively low blood volume. These mechanisms are based on pressoreceptors located in the heart, primarily the right atrium, and in other regions such as the aortic and carotid sinuses. These receptors are constantly monitoring blood volume and pressure by responding to physical stretch.
h When blood volume and pressure rises excessively, stretch acts to inhibit ADH secretion and release ANF (atrial natriuretic factor) which dilates the afferent arteriole and reduces Na+ reabsorption. These actions effectively release fluid into the urine thus reducing the blood volume.
Absence of stretch due to low blood volume acts to release significant amounts of ADH (vasopressin) which acts to generally vasoconstrict arterioles throughout the body. These actions increase blood pressure and blood volume.
|Normal and Abnormal Constituents of Urine: (See Table 26.2)|
|The Ureter, Bladder, and the Micturition Reflex -
(See Figures 26.18, 26.20)
Urine travels to the urinary bladder through the ureter by peristalsis. The ureter has two layers of smooth muscle which work like smooth muscle in the intestine, except they are in reversed position (longitudinal toward the inside, circular toward the outside). The ureter is lined with transitional epithelium to allow for stretch and reduce back pressure on the kidney.
The bladder is also lined with transitional epithelium and has many rugae for expansion. The bladder's detrusor muscle consists of three layers like the stomach's and also serves for compression. At the lower end of the bladder the ureteral openings form a triangle with the urethra which is called the trigone. The trigone has longitudinal folds which funnel the urine toward the urethra. These folds help squeeze the ureteral openings closed when micturition occurs.
The urethra varies from a short tubule in females to a longer tubule in males with several sections (see diagram). Near the bladder the urethra is lined with transitional epithelium and near the external os it is stratified squamous, while in the middle it is pseudostratified columnar epithelium.
When urine pressure stimulates presso-receptors in the bladder wall it triggers a parasympathetic reflex which stimulates mild detrusor contractions and relaxation of the internal urethral sphincter. Pathways to the brain stimulate the sense of a need to urinate. Then, when conditions are appropriate, additional parasympathetic stimuli result in micturition and voluntary stimuli relax the external sphincter.
|Disorders of the Urinary Tract:|
Revised: November 12, 2006
NEXT: The Endocrine System