NEET MDS Lessons
Physiology
The Kidneys
The kidneys are the primary functional organ of the renal system.
They are essential in homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and the regulation of blood pressure (by maintaining salt and water balance).
They serve the body as a natural filter of the blood and remove wastes that are excreted through the urine.
They are also responsible for the reabsorption of water, glucose, and amino acids, and will maintain the balance of these molecules in the body.
In addition, the kidneys produce hormones including calcitriol, erythropoietin, and the enzyme renin, which are involved in renal and hemotological physiological processes.
Anatomical Location
The kidneys are a pair of bean-shaped, brown organs about the size of your fist. They are covered by the renal capsule, which is a tough capsule of fibrous connective tissue.
Right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right.
The right kidneys lie just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen.
Resting on top of each kidney is an adrenal gland (adrenal meaning on top of renal), which are involved in some renal system processes despite being a primarily endocrine organ.
They are considered retroperitoneal, which means that they lie behind the peritoneum, the membrane lining of the abdominal cavity.
The renal artery branches off from the lower part of the aorta and provides the blood supply to the kidneys.
Renal veins take blood away from the kidneys into the inferior vena cava.
The ureters are structures that come out of the kidneys, bringing urine downward into the bladder.
Internal Anatomy of the Kidneys
There are three major regions of the kidney:
1. Renal cortex
2. Renal medulla
3. Renal pelvis
The renal cortex is a space between the medulla and the outer capsule.
The renal medulla contains the majority of the length of nephrons, the main functional component of the kidney that filters fluid from blood.
The renal pelvis connects the kidney with the circulatory and nervous systems from the rest of the body.
Renal Cortex
The kidneys are surrounded by a renal cortex
The cortex provides a space for arterioles and venules from the renal artery and vein, as well as the glomerular capillaries, to perfuse the nephrons of the kidney. Erythropotein, a hormone necessary for the synthesis of new red blood cells, is also produced in the renal cortex.
Renal Medulla
The medulla is the inner region of the parenchyma of the kidney. The medulla consists of multiple pyramidal tissue masses, called the renal pyramids, which are triangle structures that contain a dense network of nephrons.
At one end of each nephron, in the cortex of the kidney, is a cup-shaped structure called the Bowman's capsule. It surrounds a tuft of capillaries called the glomerulus that carries blood from the renal arteries into the nephron, where plasma is filtered through the capsule.
After entering the capsule, the filtered fluid flows along the proximal convoluted tubule to the loop of Henle and then to the distal convoluted tubule and the collecting ducts, which flow into the ureter. Each of the different components of the nephrons are selectively permeable to different molecules, and enable the complex regulation of water and ion concentrations in the body.
Renal Pelvis
The renal pelvis contains the hilium. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters—the urine-bearing tubes that exit the kidney and empty into the urinary bladder. The renal pelvis connects the kidney to the rest of the body.
Supply of Blood and Nerves to the Kidneys
• The renal arteries branch off of the abdominal aorta and supply the kidneys with blood. The arterial supply of the kidneys varies from person to person, and there may be one or more renal arteries to supply each kidney.
• The renal veins are the veins that drain the kidneys and connect them to the inferior vena cava.
• The kidney and the nervous system communicate via the renal plexus. The sympathetic nervous system will trigger vasoconstriction and reduce renal blood flow, while parasympathetic nervous stimulation will trigger vasodilation and increased blood flow.
• Afferent arterioles branch into the glomerular capillaries, while efferent arterioles take blood away from the glomerular capillaries and into the interlobular capillaries that provide oxygen to the kidney.
• renal vein
The veins that drain the kidney and connect the kidney to the inferior vena cava.
• renal artery
These arise off the side of the abdominal aorta, immediately below the superior mesenteric artery, and supply the kidneys with blood.
Chemical Controls of Respiration
A. Chemoreceptors (CO2, O2, H+)
1. central chemoreceptors - located in the medulla
2. peripheral chemoreceptors - large vessels of neck
B. Carbon Dioxide Effects
1. a powerful chemical regulator of breathing by increasing H+ (lowering pH)
a. hypercapnia Carbon Dioxide increases ->
Carbonic Acid increases ->
pH of CSF decreases (higher H+)- >
DEPTH & RATE increase (hyperventilation)
b. hypocapnia - abnormally low Carbon Dioxide levels which can be produced by excessive hyperventilation; breathing into paper bag increases blood Carbon Dioxide levels
C. Oxygen Effects
1. aortic and carotid bodies - oxygen chemoreceptors
2. slight Ox decrease - modulate Carb Diox receptors
3. large Ox decrease - stimulate increase ventilation
4. hypoxic drive - chronic elevation of Carb Diox (due to disease) causes Oxygen levels to have greater effect on regulation of breathing
D. pH Effects (H+ ion)
1. acidosis - acid buildup (H+) in blood, leads to increased RATE and DEPTH (lactic acid)
E. Overview of Chemical Effects
Chemical Breathing Effect
increased Carbon Dioxide (more H+) increase
decreased Carbon Dioxide (less H+) decrease
slight decrease in Oxygen effect CO2 system
large decrease in Oxygen increase ventilation
decreased pH (more H+) increase
increased pH (less H+) decrease
Blood is a liquid tissue. Suspended in the watery plasma are seven types of cells and cell fragments.
- red blood cells (RBCs) or erythrocytes
- platelets or thrombocytes
- five kinds of white blood cells (WBCs) or leukocytes
- Three kinds of granulocytes
- neutrophils
- eosinophils
- basophils
- Two kinds of leukocytes without granules in their cytoplasm
- lymphocytes
- monocytes
- Three kinds of granulocytes
The endocrine system along with the nervous system functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion and interpretation of impulses. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities
The bulk of the pancreas is an exocrine gland secreting pancreatic fluid into the duodenum after a meal. However, scattered through the pancreas are several hundred thousand clusters of cells called islets of Langerhans. The islets are endocrine tissue containing four types of cells. In order of abundance, they are the:
- beta cells, which secrete insulin and amylin;
- alpha cells, which secrete glucagon;
- delta cells, which secrete somatostatin, and
- gamma cells, which secrete a polypeptide of unknown function.
Beta Cells
Beta cells secrete insulin in response to a rising level of blood sugar
Insulin affects many organs. It
- stimulates skeletal muscle fibers to
- take up glucose and convert it into glycogen;
- take up amino acids from the blood and convert them into protein.
- acts on liver cells
- stimulating them to take up glucose from the blood and convert it into glycogen while
- inhibiting production of the enzymes involved in breaking glycogen back down (glycogenolysis) and
- inhibiting gluconeogenesis; that is, the conversion of fats and proteins into glucose.
- acts on fat (adipose) cells to stimulate the uptake of glucose and the synthesis of fat.
- acts on cells in the hypothalamus to reduce appetite.
Diabetes Mellitus
Diabetes mellitus is an endocrine disorder characterized by many signs and symptoms. Primary among these are:
- a failure of the kidney to retain glucose .
- a resulting increase in the volume of urine because of the osmotic effect of this glucose (it reduces the return of water to the blood).
There are three categories of diabetes mellitus:
- Insulin-Dependent Diabetes Mellitus (IDDM) (Type 1) and
- Non Insulin-Dependent Diabetes Mellitus (NIDDM)(Type 2)
- Inherited Forms of Diabetes Mellitus
Insulin-Dependent Diabetes Mellitus (IDDM)
IDDM ( Type 1 diabetes)
- is characterized by little or no circulating insulin;
- most commonly appears in childhood.
- It results from destruction of the beta cells of the islets.
- The destruction results from a cell-mediated autoimmune attack against the beta cells.
- What triggers this attack is still a mystery, although a prior viral infection may be the culprit.
Non Insulin-Dependent Diabetes Mellitus (NIDDM)
Many people develop diabetes mellitus without an accompanying drop in insulin levels In many cases, the problem appears to be a failure to express a sufficient number of glucose transporters in the plasma membrane (and T-system) of their skeletal muscles. Normally when insulin binds to its receptor on the cell surface, it initiates a chain of events that leads to the insertion in the plasma membrane of increased numbers of a transmembrane glucose transporter. This transporter forms a channel that permits the facilitated diffusion of glucose into the cell. Skeletal muscle is the major "sink" for removing excess glucose from the blood (and converting it into glycogen). In NIDDM, the patient's ability to remove glucose from the blood and convert it into glycogen is reduced. This is called insulin resistance. NIDDM (also called Type 2 diabetes mellitus) usually occurs in adults and, particularly often, in overweight people.
Alpha Cells
The alpha cells of the islets secrete glucagon, a polypeptide of 29 amino acids. Glucagon acts principally on the liver where it stimulates the conversion of glycogen into glucose (glycogenolysis) which is deposited in the blood.
Glucagon secretion is
- stimulated by low levels of glucose in the blood;
- inhibited by high levels, and
- inhibited by amylin.
The physiological significance of this is that glucagon functions to maintain a steady level of blood sugar level between meals.
Delta Cells
The delta cells secrete somatostatin. Somatostatin has a variety of functions. Taken together, they work to reduce the rate at which food is absorbed from the contents of the intestine. Somatostatin is also secreted by the hypothalamus and by the intestine.
Gamma Cells
The gamma cells of the islets secrete pancreatic polypeptide. No function has yet been found for this peptide of 36 amino acids.
Production of Hormones
The kidneys produce and interact with several hormones that are involved in the control of systems outside of the urinary system.
Calcitriol. Calcitriol is the active form of vitamin D in the human body. It is produced by the kidneys from precursor molecules produced by UV radiation striking the skin. Calcitriol works together with parathyroid hormone (PTH) to raise the level of calcium ions in the bloodstream. When the level of calcium ions in the blood drops below a threshold level, the parathyroid glands release PTH, which in turn stimulates the kidneys to release calcitriol. Calcitriol promotes the small intestine to absorb calcium from food and deposit it into the bloodstream. It also stimulates the osteoclasts of the skeletal system to break down bone matrix to release calcium ions into the blood.
Erythropoietin. Erythropoietin, also known as EPO, is a hormone that is produced by the kidneys to stimulate the production of red blood cells. The kidneys monitor the condition of the blood that passes through their capillaries, including the oxygen-carrying capacity of the blood. When the blood becomes hypoxic, meaning that it is carrying deficient levels of oxygen, cells lining the capillaries begin producing EPO and release it into the bloodstream. EPO travels through the blood to the red bone marrow, where it stimulates hematopoietic cells to increase their rate of red blood cell production. Red blood cells contain hemoglobin, which greatly increases the blood’s oxygen-carrying capacity and effectively ends the hypoxic conditions.
Renin. Renin is not a hormone itself, but an enzyme that the kidneys produce to start the renin-angiotensin system (RAS). The RAS increases blood volume and blood pressure in response to low blood pressure, blood loss, or dehydration. Renin is released into the blood where it catalyzes angiotensinogen from the liver into angiotensin I. Angiotensin I is further catalyzed by another enzyme into Angiotensin II.
Angiotensin II stimulates several processes, including stimulating the adrenal cortex to produce the hormone aldosterone. Aldosterone then changes the function of the kidneys to increase the reabsorption of water and sodium ions into the blood, increasing blood volume and raising blood pressure. Negative feedback from increased blood pressure finally turns off the RAS to maintain healthy blood pressure levels.
Blood Pressure
Blood moves through the arteries, arterioles, and capillaries because of the force created by the contraction of the ventricles.
Blood pressure in the arteries.
The surge of blood that occurs at each contraction is transmitted through the elastic walls of the entire arterial system where it can be detected as the pulse. Even during the brief interval when the heart is relaxed — called diastole — there is still pressure in the arteries. When the heart contracts — called systole — the pressure increases.
Blood pressure is expressed as two numbers, e.g., 120/80.
Blood pressure in the capillaries
The pressure of arterial blood is largely dissipated when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7.5 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total cross-sectional area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases.
Blood pressure in the veins
When blood leaves the capillaries and enters the venules and veins, little pressure remains to force it along. Blood in the veins below the heart is helped back up to the heart by the muscle pump. This is simply the squeezing effect of contracting muscles on the veins running through them. One-way flow to the heart is achieved by valves within the veins
Exchanges Between Blood and Cells
With rare exceptions, our blood does not come into direct contact with the cells it nourishes. As blood enters the capillaries surrounding a tissue space, a large fraction of it is filtered into the tissue space. It is this interstitial or extracellular fluid (ECF) that brings to cells all of their requirements and takes away their products. The number and distribution of capillaries is such that probably no cell is ever farther away than 50 µm from a capillary.
When blood enters the arteriole end of a capillary, it is still under pressure produced by the contraction of the ventricle. As a result of this pressure, a substantial amount of water and some plasma proteins filter through the walls of the capillaries into the tissue space.
Thus fluid, called interstitial fluid, is simply blood plasma minus most of the proteins. (It has the same composition and is formed in the same way as the nephric filtrate in kidneys.)
Interstitial fluid bathes the cells in the tissue space and substances in it can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid.
Near the venous end of a capillary, the blood pressure is greatly reduced .Here another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and thus a somewhat greater concentration of water. This difference sets up an osmotic pressure. Although the osmotic pressure is small, it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid reenters the capillary here.
Control of the Capillary Beds
An adult human has been estimated to have some 60,000 miles of capillaries with a total surface area of some 800–1000 m2. The total volume of this system is roughly 5 liters, the same as the total volume of blood. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. So a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. During vigorous exercise, for example, capillary beds in the skeletal muscles open at the expense of those in the viscera. The reverse occurs after a heavy meal.
The walls of arterioles are encased in smooth muscle. Constriction of arterioles decreases blood flow into the capillary beds they supply while dilation has the opposite effect. In time of danger or other stress, for example, the arterioles supplying the skeletal muscles will be dilated while the bore of those supplying the digestive organs will decrease. These actions are carried out by
- the autonomic nervous system.
- local controls in the capillary beds