Chapter 10: Homeostasis

____________________ is a steroid hormone that is secreted by the adrenal cortex in response to decreased blood pressure. Decreased blood pressure stimulates the release of renin from juxtaglomerular cells in the kidney. Renin then cleaves angiotensinogen, a liver protein, to form angiotensin I. This peptide is then metabolized by angiotensin-converting enzyme in the lungs to form angiotensin I. This peptide is then metabolized by angiotensisn-converting enzyme in the lungs to from angiotensin II, which promotes release of aldosterine from the adrenal cortex. Aldosterone works by altering the ability of the distal convoluted tubule and collecting duct to reabsorb sodium. Remember that water does not move on its own, but rather travels down its osmolarity gradient. Thus, if we absorb more sodium, water will flow with it. This reabsorption of isotonic fluid has the net effect of increasing blood volume and therefore blood pressure. Aldosterone will also increase potassium and hydrogen ion excretion.

Aldosterone

_______________________ is a peptide hormone synthesized by the hypothalamus and released by the posterior pituitary in response to high blood osmolarity. It directly altering the permeability of the collecting duct, allowing more water to be reabsorbed by making the cell junctions of the duct leaky. Increased concentration in the interstitium (hypertonic to the filtrate) will then cause the reabsorption of water from the tubule. Alcohol and caffeine both inhibit ADH release and lead to the frequent excretion of dilute urine. In addition to the kidneys, the cardiovascular system also regulates blood pressure, specifically by vasoconstricting or vasodilating in order to maintain blood pressure. Constriction of the afferent arteriole will lead to a lower pressure of blood reaching the glomeruli, which are adjacent to the juxtaglomerular cells. Therefore, this vasoconstriction will secondarily lead to renin release, which will help raise blood pressure also.

Antidiuretic hormone (ADH, also known as vasopressin)

Visible in kidney figure is structure of nephron. Around the glomerulus is a cuplike structure known as _____________. Bowman's capsule leads to a ling tubule with many distinct areas; in order, these are the proximal convuluted tubule, descending and ascending limbs of the Loop of Henle, distal convoluted tubule, and collecting duct. The kidney's ability to excrete waste is intricately tied to the specific placement of these structures and their physiology.

Bowman's capsule

Filtration continued: Most circumstances, fluid will flow from the glomerulus into ________________. However, various pathologies can cause derangements of this flow. Consider what might happen if the ureter was obstructed by a kidney stone. An obstruction would result in a buildup of urine behind the stone. Eventually, enough fluid will build up and cause distention of the renal pelvis and nephrons. What will happen to filtration in this case? The hydrostatic pressure in Bowman's space would increase to the point that filtration could no longer occur because there would be excessive pressure opposing movement of fluid into the nephron. The filtrate is similar in composition to blood but does not contain cells or proteins due to the filter's ability to select based on size. In other words, molecules or cells larger than glomerular pores will remain in the blood. As described earlier, blood remaining in the glomerus then travels into the efferent arterioles , which empty into the vasa recta. The filtrate is isotonic to blood so that neither the capsule not capillaries swell. Our kidneys filter about 180 liters per day, which is approximately 36 times our blood volume. This means the entire volume of a person's blood is filtered about every 40 minutes.

Bowman's space

Reabsorption: Some compounds that are filtered or secreted may be taken back up for use via _____________. Certain substances are almost reabsorbed , such as glucose, amino acids, and vitamins. In addition, hormones such as antidiuretic hormone (ADH or vasopressin) and aldosterone can alter the quantity of water reabsorbed within the kidney in order to maintain blood pressure.

Reabsorption

By both weight and size, the _______________ is the largest organ in our bodies. It makes up about 16 percent of total body weight, on average. Skin is one of the major components of nonspecific immune defense, protecting us from exposure to the elements and invasion by pathogens.

skin (integument)

Thermoregulation: We have already mentioned that the skin protects us from the elements and microbes. It also has other functions, including ultraviolet protection (via melanin) and transduction of sensory information from the outside world. Another function of the skin is thermoregulation. Thermoregulation is achieved by sweating, piloerection, vasodilation, and vasoconstriciton. Sweating is an excellent cooling mechanism that is controlled by the ______________ nervous system. When body temperature rises above the set point determined by the hypothalamus, thermoregulation processes must occur in order to rid the body of heat. Postganglionic sypathetic neurons that utilize acetylcholine innervate sweat glands and promote the secretion of water with certain ions onto the skin. Heat is then absorbed from the body as the water molecules undergo a phase change to evaporate. The production of sweat itself is not the main mechanism of cooling; it is the evaporation of water from the skin which absorbs body heat. At the same time, arteriolar vasodilation occurs to maximize heat loss. This brings a large quantity of blood to the skin, which accelerates, the evaporation of sweat by maximizing heat energy available for liquid-gas phase change. In cold conditions, arrector pili muscles contract, causing the hairs of the skin to stand up on end (piloerection). This helps to trap a layer of heated air near the skin. The arterioles that feed the capillaries of the skin constrict, limiting the quantity of blood reaching the skin. Skeletal muscle may also begin to contract rapidly, causing shivering. Shivering requires a sizable amount of ATP, however, a significant portion of the energy from ATP is converted into thermal energy. In addition to those mechanisms, humans possess a layer of fat just below the skin. This fat helps to insulate the body. In addition to this fat, which is called white fat, brown fat may be present, especially in infants. Brown fat has much less efficient electron transport chain, which means that more heat energy is released as fuel is burned. The skin also helps to maintain osmolarity of the body. This is because the skin is relatively impermeable to water. This prevents not only the entrance of water through the skin, but also loss of water from the tissues. This becomes very important in cases such as burns or large losses of skin as dehydration of the tissues becomes a real threat to survival.

autonomic

Acid-Base Balance: The __________________ is the major regulator of blood pH. (Refer to buffer equation). In CH 6 we talked about how the respiratory system can contribute to acid-base balance by increasing or decreasing respiratory rate. If the blood pH is too low, then increasing the respiratory rate blows off more CO2 and favors conversion of H+ and HCO3- to water and carbon dioxide, increasing the pH. If the blood pH is too high, then decreasing the respiratory rate causes opposite effects. The respiratory system can react to derangements of pH quickly. What can the excretory system do to contribute? The kidneys are able to selectively increase or decrease the secretion of hydrogen ions and bicarbonate. When blood pH is too low, the kidneys excrete more hydrogen ions and increase reabsorption of hydrogen ions. This is slower than the respiratory response, but is a highly effective way for the body to maintain acid-base balance.

bicarbonate buffer system

The final concentration of urine will depend largely on the permeability of the ________________, which is responsive to both aldosterone and antidiuretic hormone (ADH or vasopressin). As permeability of the collecting duct increases, so too does water reabsorption, resulting in further concentration of the urine. The reabsorbed water enters the intersitium and makes its way to the vasa recta, where it reeneters the bloodstream to once again become part of the plasma. The collecting duct almost always reabsorbs water, but the amount is variable. When the body is very well hydrated, the collecting duct will be fairly impermeable to salt and water. When in conservation mode, ADH and aldosterone will each act to increase reabsorption of water in the collecting duct, allowing for greater water retention and more concentrated urine output. Ultimately, anything not reabsorbed from the tubule by the end of the collecting duct will be excreted; the collecting duct is the point of no return. After that there are nor further opportunities for reabsorption. As the filtrate leaves the tubule, it collects in the renal pelvis. The fluid, which carries mostly urea, uric acid, and excess ions (sodium, potassium, magnesium, and calcium), flows through the ureter to the bladder where it is stored until voiding.

collecting duct

Each kidney is subdivided into what two structures?

cortex and medulla.

The ________ is the kindey's outermost layer, while the ____________ of the kidney sits within the cortex.

cortex, medulla

The _____________ also consist of multiple layers. The upper layer is right below the epidermis and is the papillary layer, which consist of loose connective tissue. Below the papillary layer is the denser reticular layer. Sweat glands, blood vessels, and hair follicles originate in the dermis. Most sensory receptors are also located in the dermis. Merkel cells (discs), for example, are sensory receptors present at the epidermal-dermal junction. These cells are connected to sensory neurons and are responsible for deep pressure and texture sensation within the skin. Other sensory organs in the skin include free nerve endings, which respond to pain; Meissner's corpuscles, which respond to light touch; Ruffini endings, which respond to stretch; and pacinian corpuscles, which respond to deep pressure and vibration

dermis

Filtrate from the proximal convoluted tubule then enters the _________________________, which dives deep into the medulla before turning around to become the _______________________. The descending limb is permeable only to water and the medulla has an ever-increasing osmolarity as the descending limb travels deeper into it. Think for a moment how this would affect the flow of water. As the descending limb traverses deeper into the medulla, the increasing interstitial concentration favors the outflow of water from the descending limn, which is reabsorbed into the vasa recta. The kidney is capable of altering the osmolarity of the interstitium. This creates a gradient that, coupled with selective permeability of the nephron, allows maximal reabsorption and conservation of water. In the normal physiological state, the osmolarity in the cortex is approximately the same as that of the blood and remains at that level. Deeper in the medulla, the osmolarity in the interstitum can range from isotonic with blood (when trying to excrete water) to four times as concentrated (when trying to conserve water). If the concentration is the same in the tubule and in the interstitium, there is no driving force (gradient), and the water will be lost in the urine. If the interstitium is more concentrated, then water will move out of the tubule, into the interstitium, and eventually back into the blood. Together, the vasa recta and nephron create a countercurrent multiplier system. This means the flow if filtrate through the loop of Henle is in the opposite direction from the flow of blood through the vasa recta. If the two flowed in the same direction, they would quickly reach equilibrium and the kidney would be unable to reabsorb as much water. By making the two flow in opposite directions, the filtrate is constantly being exposed to hypertonic blood, which allows maximal reabsorption of water. As the descending limb transitions to become the ascending limb of the loop of Henle, a change in permeability occurs, The ascending limb is only permeable to salts and is impermeable to water. So while the descending limb maximizes water reabsorption by taking advantage of increasing medullary osmolarity, the ascending limb maximizes salt reabsorption by takin advantage of decreasing medullary osmolarity. At the transition from inner to outer medulla, the loop of Henle becomes thicker in what is termed the diluting segment. This is not because the lumen within the tube has enlarged, but because the cells lining the tube are larger. These cells contain large amounts of mitochondria, which allows the reabsorption of sodium and chloride by active transport. Indeed, because so much salt is reabsorbed while water is stuck in the nephron, the filtrate actually becomes hypotonic compared to the interstitium. While we tend to focus on the concentrating abilities of the nephron, this segment is noteworthy because it is the only portion of the nephron that can produce urine that is more dilute than the blood. This is important during periods of overhydration and provides a mechanism for eliminating excess water. At the beginning of the loop of Henle, the filtrate is isotonic to the interstitium. Thus, from the beginning of the loop of Henle to the end, there is a slight degree of dilution. Far more important, however, is the fact that the volume of the filtrate has been significantly reduced, demonstrating a net reabsorption of a large volume of water.

descending limb of the loop of Henle, ascending limb of the loop of Henle.

The bladder has a muscular lining known as the _____________. Parasympathetic activity causes the detrusor muscle to contract. However, in order to leave body, urine must pass through two sphincters-the internal and external urethral sphincters. The internal urethral sphincter, consisting of smooth muscle, is contracted in its normal state. Because the internal sphincter is made of smooth muscle, it is under involuntary control. The external urethral sphincter consist of skeletal muscle and is under voluntary control. When bladder is full, stretch receptors convey to nervous system that bladder requires emptying. This causes parasympathetic neurons to fire, and the detrusor muscle contracts. This contraction also causes internal sphincter relaxation. This reflex is known as micturition reflex. The next step is up to the individual. The person can choose to relax the external sphincter to urinate, or can maintain the tone of the external sphincter to prevent urination. This can cause a few moments of discomfort, but the reflex usually dissipates in a few minutes. However, if the bladder is not emptied, then the process will begin anew shortly thereafter. Urination itself is facilitated by the contraction of the abdominal musculature, which increases pressure within the abdominal cavity, resulting in compression of the bladder and increased urine flow rate.

detrusor muscle

Next, filtrate enters the ___________________. This responds to aldosterone, which promotes sodium reabsorption. Because sodium ions are osmotically active particles, water will follow the sodium, concentrating the urine and decreasing its volume. The DCT is also a site of waste product secretion, like the PCT.

distal convoluted tubule (DCT)

The ____________ is also subdivided into layers called strata. From the deepest layer outward, these are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale contains stem cells and is responsible for proliferation of keratinocytes, the predominant cells of the skin, that produce keratin. In the stratum spinosum, these cells become connected to each other; this layer is also the site of Langerhans cells. In the stratum lucidum is only present in thick, hairless skin, such as skin of soles of feet and palms, and is nearly transparent. Finally, the stratum corneum contains up to several dozen layers of flattened keratinocytes, forming a barrier that prevents invasion y pathogens and that helps to prevent loss of fluids and salt. Hair projects above the skin, and there are openings for sweat and sebaceous glands. Other cells of the epidermis: In the epidermis, the main cells are keratinocytes, Keratin, mentioned above, is resistant to damage and provides protection against injury, water, and pathogens. Calluses form from the excessive keratin deposition in areas of repeated strain due to friction; they provide protection to avoid future damage. Fingernails and hair are also formed fro keratin and are produced by specialized cells in the skin. Melanocytes are a cell type derived from neural crest cells and found in the stratum basale. These cells produce melanin, a pigment that serves to protect the skin from DNA damage caused by UV radiation. Once produced, the pigment is transferred to the keratinocytes. All humans actually have comparable numbers of melanocytes;skin color is caused by varying level of activity of melanocytes. More active melanocytes result in darker skin tones. Upon exposure to UV radiation, melanocytes increase activity, resulting in darker skin color. Langerhans cells are actually special macrophages that reside within the stratum spinosum. These cells are capable of presenting antigens to T-cells in order to activate the immune system.

epidermis

The _____________ system consist of the kidneys, ureters, bladder, and urethra.

excretory system

The _______________ serves many functions, including regulation of blood pressure, blood, osmolarity, acid-base balance, and removal of nitrogenous waste. The kidney plays an essential role in these functions

excretory system

Filtration: The nephron's first function is _____________. In the kidneys, approximately 20 percent of blood that passes through the glomerulus is filtered as fluid into Bowman's space. The collected fluid is known as filtrate. The movement of fluid into Bowman's space is governed by Starling forces, which account for the pressure differentials in both hydrostatic and oncotic pressures between the blood and Bowman's space. The hydrostatic pressure in the glomerulus is significantly higher that in Bowman's space, which causes fluid to move into the nephron. On the other hand, the osmolarity of blood is higher than that of Bowman's space , resulting in pressure opposing the movement of fluid into the nephron. However, the hydrostatic pressure is much larger than the oncotic pressure, so the net flow is still from blood into the nephron.

filtration

Each kidney also has a renal __________, which is a deep slit in the center of its medial surface. The widest part of the ureter, the renal pelvis, spans almost the entire width of the renal hilum. The renal artery, renal vein, and ureter enter and exit through the renal hilum.

hilum

The ____________ is a layer of connective tissue that connects the skin to the rest of the body. This layer contains fat and fibrous tissue.

hypodermis

Structure: The skin has several layers. Starting from the deepest layer and working outward, these layers are the __________________________________. Skin is derived from the ectoderm.

hypodermis (subcutaneous layer), dermis, and epidermis.

Nephron Function: The kidney uses mechanisms such as filtration, secretion, and reabsorption to produce urine and to regulate blood volume and osmolarity. However, the function of the nephron isn't quite that simple. In fact, renal physiology is often considered one of the most difficult topics covered in medical school. Kidney has 2 main goals: Keep what body needs and lose what it doesn't, and concentrate urine to conserve water. The kidneys allow the human body to reabsorb certain materials for reuse, while also selectively eliminating waste. For example, glucose and amino acids are not usually present in urine because kidney is able to reabsorb these substances for later use. On the contrary, waste products like hydrogen, and potassium ions, ammonia, and urea remain in filtrate and are excreted. Finally, water is reabsorbed in large quantities in order to maintain blood pressure and adequate hydration. Below we will discuss the occurrences of each segment of the nephron. Note Bowman's capsule, proximal convoluted tubule, and distal convoluted tubule are primarily focused on identity of the particles in the urine. In contrast, loop of Henle and collecting duct are primarily focused on volume and concentration of urine.

kidney

The ________ are two bean-shaped structures located behind the digestive organs at the level of the bottom rib. The functional unit of the kidney is the nephron; each kidney has approximately 1 million nephrons. All of the nephrons eventually empty into the renal pelvis, which narrows to form the ureter. Urine travels through the ureter to the bladder. From the bladder, urine is transported through the urethra to exit the body.

kidneys

Osmoregulation: The _____________ of the blood must be tightly controlled to ensure correct oncotic pressures within the vasculature. A note on terminology: Osmotic pressure is the "sucking" pressure that draws water into the vasculature caused by all dissolved particles. Oncotic pressure, is the osmotic pressure that is attributable to dissolved proteins specifically. Blood osmolarity is usually maintained at approximately 290 milliosmoles (mOsm) per liter. The kidneys control osmolarity by modulating the reabsorption of water and by filtering and secreting dissolved particles. When blood osmolarity is low, excess water will be excreted, while solutes will be reabsorbed in higher concentrations. In contrast, when blood osmolarity is high, water reabsorption increases and solute excretion increases.

osmolarity

The kidney has one of the few portal systems in the body. A _____________ consist of two capillary beds in series through which blood must travel before returning to the heart. The renal artery branches out, passes through the medulla, and enters the cortex as afferent arterioles. The highly convoluted capillary tufts derived from these afferent arterioles are known as glomeruli. After blood passes through a glomerulus, the efferent arterioles then form a second capillary bed. These capillaries surround the loop of Henle and are known as vasa recta.

portal system

The filter first enters the _______________________. In this region, amino acids, glucose, water-soluble vitamins, and majority of salts are reabsorbed along with water. Almost 70 percent of filtered sodium will be reabsorbed here, but the filtrate remains isotonic to the intersitium, as other solutes and a large volume of water are also reabsorbed. Solutes that enter the interstitium- the connective tissue surrounding the nephron-are picked up by the vasa recta to be returned to the blood stream for reuse within the body. The PCT is also the site of secretion for a number of waste products, including hydrogen ions, potassium ions, ammonia, and urea.

proximal convoluted tubule (PCT)

Secretion: In addition to filtering blood, nephrons are able to ____________ salts, acids, bases, and urea directly into the tubule by either active or passive transport. The quantity and identity of the substances secreted into the nephrons are directly related to the needs of the body at that time. For example, a diet heavy in meat results in the intake of large amount of protein, which contains a significant amount of nitrogen. Ammonia (NH3) is a byproduct of the metabolism of nitrogen containing compounds and, as a base, can disturb the pH of blood and cells. The liver converts the ammonia to urea, a neutral compound, which travels to the kidney and is secreted into the nephron for excretion in the urine. The kidneys are capable of eliminating ions or other substances when present in relative excess in the blood, such as potassium cations, hydrogen ions, or metabolites of medications. Secretion is also a mechanism for excreting wastes that are simply too large to pass through glomerular pores.

secrete

Functions of the Excretory System: The kidneys use osmolarity gradients and selective permeability to filter, secrete, and reabsorb materials in the process of making _____________. However, these processes have larger implications for the human body as a whole. The selective elimination of water and solutes allows the kidneys, in conjugation with the endocrine, cardiovascular, and respiratory systems, to control blood pressure, blood osmolarity, and acid-base balance.

urine

Osmoregulation: The kidney filters blood to form __________. The composition and quantity of urine is determined by present state of body. For example, if blood volume is low and blood osmolarity is high, then it is most beneficial to the body to maximally retain water. This results in low-volume, highly concentrated urine. Likewise, a patient receiving large amounts of intravenous fluids is likely to produce a larger volume of less concentrated urine. Thus, the primary job of the kidneys is to regulate blood volume and osmolarity. In order to do this, kidney function may be divided into three different processes: filtration, secretion, and reabsorption.

urine