RENAL PHYSIOLOGY ASSIGNMENT

Name: Ogunlade Dolapo Precious

Matric number: 18/020810

Department: Pharmacy

1. List the functions of Kidney

Answer

A. ROLE IN HOMEOSTASIS: i. Excretion of Waste Products.

ii. Maintenance of Water Balance.

iii. Maintenance of Electrolyte Balance.

iv. Maintenance of Acid–Base Balance.

B. HEMOPOIETIC FUNCTION.

C. ENDOCRINE FUNCTION.

D. REGULATION OF BLOOD PRESSURE.

E. REGULATION OF BLOOD CALCIUM LEVEL.

2. What does the nephron look like and how does it carry out its function?

i. Each nephron in the mammalian kidney is a long tubule, or extremely fine tube, about 30–55 mm
(1.2–2.2 inches) long. At one end this tube is closed, expanded, and folded into a double-walled cuplike
structure. This structure, called the renal corpuscular capsule, or Bowman’s capsule, encloses a cluster
of microscopic blood vessels—capillaries—called the glomerulus.

ii. The nephrons work through a two-step process: the glomerulus filters the blood, and the tubule
returns needed substances to the blood and removes wastes. Each nephron has a glomerulus to filter
the blood and a tubule that returns needed substances to the blood and pulls out additional wastes.

3. Urine is formed from three (3) processes namely, Filtration, Selective Reabsorption and Tubular
Secretion. How does the kidney carry out these processes?

i. Filtration: During filtration, blood enters the afferent arteriole and flows into the glomerulus where
filterable blood components, such as water and nitrogenous waste, will move towards the inside of the
glomerulus, and nonfilterable components, such as cells and serum albumins, will exit via the efferent
arteriole. These filterable components accumulate in the glomerulus to form the glomerular filtrate.

Normally, about 20% of the total blood pumped by the heart each minute will enter the kidneys to
undergo filtration; this is called the filtration fraction. The remaining 80% of the blood flows through the
rest of the body to facilitate tissue perfusion and gas exchange.

ii. Selective reabsorption: The next step is reabsorption, during which molecules and ions will be
reabsorbed into the circulatory system. The fluid passes through the components of the nephron (the
proximal/distal convoluted tubules, loop of Henle, the collecting duct) as water and ions are removed as
the fluid osmolarity (ion concentration) changes. In the collecting duct, secretion will occur before the
fluid leaves the ureter in the form of urine.

iii. During secretion some substances±such as hydrogen ions, creatinine, and drugs—will be removed
from the blood through the peritubular capillary network into the collecting duct. The end product of all
these processes is urine, which is essentially a collection of substances that has not been reabsorbed
during glomerular filtration or tubular reabsorbtion.

Urine is mainly composed of water that has not been reabsorbed, which is the way in which the body
lowers blood volume, by increasing the amount of water that becomes urine instead of becoming
reabsorbed. The other main component of urine is urea, a highly soluble molecule composed of
ammonia and carbon dioxide, and provides a way for nitrogen (found in ammonia) to be removed from
the body. Urine also contains many salts and other waste components. Red blood cells and sugar are not
normally found in urine but may indicate glomerulus injury and diabetes mellitus respectively.

4. How is urine concentrated and diluted by the kidney? Note the roles of Antidiuretic Hormone (ADH)
and Urea cycling.

The loop of Henle is critical to the ability of the kidney to concentrate urine. The high concentration of
salt in the medullary fluid is believed to be achieved in the loop by a process known as countercurrent
exchange multiplication. The principle of this process is analogous to the physical principle applied in the
conduction of hot exhaust gases past cold incoming gas so as to warm it and conserve heat. That
exchange is a passive one, but in the kidney the countercurrent multiplier system uses energy to “pump”
sodium and chloride out of the ascending limb of the loop into the medullary fluid. From there it enters
(by diffusion) the filtrate (isotonic with plasma) that is entering the descending limb from the proximal
tubule, thus raising its concentration a little above that of plasma. As this luminal fluid in turn reaches
the ascending limb, and subsequently the distal tubule, it in turn provides more sodium to be pumped
out into the surrounding fluid or blood, if necessary, and transported (by diffusion) back into the
descending limb; this concentrating process continues until the osmotic pressure of the fluid is sufficient
to balance the resorptive power of the collecting ducts in the medulla, through which all of the final
urine must pass. This resorptive capacity in the ducts is regulated by antidiuretic hormone (ADH), which
is secreted by the hypothalamus and stored in the posterior pituitary gland at the base of the brain. In
the presence of ADH, the medullary collecting ducts become freely permeable to solute and water. As a
consequence, the fluid entering the ducts (en route to the renal pelvis and subsequent elimination)
acquires the concentration of the interstitial fluid of the medulla; i.e., the urine becomes concentrated.
On the other hand, in the absence of ADH, the collecting ducts are impermeable to solute and water,
and, thus, the fluid in the lumen, from which some solute has been removed, remains less concentrated
than plasma; i.e., the urine is dilute.

The secretion of ADH by the hypothalamus and its release from the posterior pituitary is part of a
feedback mechanism responsive to the tonicity of plasma. This interrelation between plasma osmotic
pressure and ADH output is mediated by specific and sensitive receptors at the base of the brain. These
receptors are particularly sensitive to sodium and chloride ions. At normal blood tonicity there is a
steady receptor discharge and a steady secretion of ADH. If the plasma becomes hypertonic (i.e., has a
greater osmotic pressure than normal), either from the ingestion of crystalloids such as common salt, or
from shortage of water, receptor discharge increases, triggering increased ADH output, and more water
leaves the collecting ducts to be absorbed into the blood. If the osmotic pressure of plasma becomes
low, the reverse is the case. Thus water ingestion dilutes body fluids and reduces or stops ADH
secretion; the urine becomes hypotonic, and the extra water is excreted in the urine.

The situation is complex because there are also receptors sensitive to changes in blood volume that
reflexively inhibit ADH output if there is any tendency to excessive blood volume. Exercise increases ADH
output and reduces urinary flow. The same result may follow emotional disturbance, fainting, pain, and
injury, or the use of certain drugs such as morphine or nicotine. Diuresis is an increased flow of urine
produced as the result of increased fluid intake, absence of hormonal activity, or the taking of certain
drugs that reduce sodium and water reabsorption from the tubules. If ADH secretion is inhibited by the
drinking of excess water, or by disease or the presence of a tumour affecting the base of the brain,
water diuresis results; and the rate of urine formation will approach the rate of 16 millilitres per minute
filtered at the glomeruli. In certain disorders of the pituitary in which ADH secretion is diminished or
absent—e.g., diabetes insipidus—there may be a fixed and irreversible output of a large quantity of
dilute urine.

5. Describe the mechanism of renal pH regulation.

The renal regulation of the body’s acid-base balance addresses the metabolic component of the
buffering system. Whereas the respiratory system (together with breathing centers in the brain) controls
the blood levels of carbonic acid by controlling the exhalation of CO2, the renal system controls the
blood levels of bicarbonate. A decrease of blood bicarbonate can result from the inhibition of carbonic
anhydrase by certain diuretics or from excessive bicarbonate loss due to diarrhea. Blood bicarbonate
levels are also typically lower in people who have Addison’s disease (chronic adrenal insufficiency), in
which aldosterone levels are reduced, and in people who have renal damage, such as chronic nephritis.
Finally, low bicarbonate blood levels can result from elevated levels of ketones (common in unmanaged
diabetes mellitus), which bind bicarbonate in the filtrate and prevent its conservation. Bicarbonate ions,
HCO3–, found in the filtrate, are essential to the bicarbonate buffer system, yet the cells of the tubule
are not permeable to bicarbonate ions. The steps involved in supplying bicarbonate ions to the system
are summarized below:

Step 1: Sodium ions are reabsorbed from the filtrate in exchange for H+ by an antiport mechanism in the
apical membranes of cells lining the renal tubule.

Step 2: The cells produce bicarbonate ions that can be shunted to peritubular capillaries.

Step 3: When CO2 is available, the reaction is driven to the formation of carbonic acid, which dissociates
to form a bicarbonate ion and a hydrogen ion.

Step 4: The bicarbonate ion passes into the peritubular capillaries and returns to the blood. The
hydrogen ion is secreted into the filtrate, where it can become part of new water molecules and be
reabsorbed as such, or removed in the urine.

It is also possible that salts in the filtrate, such as sulfates, phosphates, or ammonia, will capture
hydrogen ions. If this occurs, the hydrogen ions will not be available to combine with bicarbonate ions
and produce CO2. In such cases, bicarbonate ions are not conserved from the filtrate to the blood,
which will also contribute to a pH imbalance and acidosis.

The hydrogen ions also compete with potassium to exchange with sodium in the renal tubules. If more
potassium is present than normal, potassium, rather than the hydrogen ions, will be exchanged, and
increased potassium enters the filtrate. When this occurs, fewer hydrogen ions in the filtrate participate
in the conversion of bicarbonate into CO2 and less bicarbonate is conserved. If there is less potassium,
more hydrogen ions enter the filtrate to be exchanged with sodium and more bicarbonate is conserved.

Chloride ions are important in neutralizing positive ion charges in the body. If chloride is lost, the body
uses bicarbonate ions in place of the lost chloride ions. Thus, lost chloride results in an increased
reabsorption of bicarbonate by the renal system.