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.
Urine is a waste byproduct formed from excess water and metabolic waste molecules during the process of renal system filtration. The primary function of the renal system is to regulate blood volume and plasma osmolarity, and waste removal via urine is essentially a convenient way that the body performs many functions using one process. Urine formation occurs during three processes:
Filtration
Reabsorption
Secretion
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.
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.
Secretion
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.
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 thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands.
The Thyroid Gland
The thyroid gland synthesizes and secretes:
- thyroxine (T4) and
- calcitonin
T4 and T3
Thyroxine (T4 ) is a derivative of the amino acid tyrosine with four atoms of iodine. In the liver, one atom of iodine is removed from T4 converting it into triiodothyronine (T3). T3 is the active hormone. It has many effects on the body. Among the most prominent of these are:
- an increase in metabolic rate
- an increase in the rate and strength of the heart beat.
The thyroid cells responsible for the synthesis of T4 take up circulating iodine from the blood. This action, as well as the synthesis of the hormones, is stimulated by the binding of TSH to transmembrane receptors at the cell surface.
Diseases of the thyroid
1. hypothyroid diseases; caused by inadequate production of T3
- cretinism: hypothyroidism in infancy and childhood leads to stunted growth and intelligence. Can be corrected by giving thyroxine if started early enough.
- myxedema: hypothyroidism in adults leads to lowered metabolic rate and vigor. Corrected by giving thyroxine.
- goiter: enlargement of the thyroid gland. Can be caused by:
- inadequate iodine in the diet with resulting low levels of T4 and T3;
- an autoimmune attack against components of the thyroid gland (called Hashimoto's thyroiditis).
2. hyperthyroid diseases; caused by excessive secretion of thyroid hormones
Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.
Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.
Calcitonin
Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca2+) circulating in the blood. These cells monitor the level of circulating Ca2+. A rise in its level stimulates the cells to release calcitonin.
- bone cells respond by removing Ca2+ from the blood and storing it in the bone
- kidney cells respond by increasing the excretion of Ca2+
Both types of cells have surface receptors for calcitonin.
Because it promotes the transfer of Ca2+ to bones, calcitonin has been examined as a possible treatment for osteoporosis
- Sensory:
- Somatic (skin & muscle) Senses:
Postcentral gyrus (parietal lobe). This area senses touch, pressure, pain, hot, cold, & muscle position. The arrangement is upside-down (head below, feet above) and is switched from left to right (sensations from the right side of the body are received on the left side of the cortex). Some areas (face, hands) have many more sensory and motor nerves than others. A drawing of the body parts represented in the postcentral gyrus, scaled to show area, is called a homunculus . - Vision:
Occipital lobe, mostly medial, in calcarine sulcus. Sensations from the left visual field go to the right cortex and vice versa. Like other sensations they are upside down. The visual cortex is very complicated because the eye must take into account shape, color and intensity. - Taste:
Postcentral gyrus, close to lateral sulcus. The taste area is near the area for tongue somatic senses. - Smell:
The olfactory cortex is not as well known as some of the other areas. Nerves for smell go to the olfactory bulb of the frontal cortex, then to other frontal cortex centers- some nerve fibers go directly to these centers, but others come from the thalamus like most other sensory nerves - Hearing:
Temporal lobe, near junction of the central and lateral sulci. Mostly within the lateral sulcus. There is the usual crossover and different tones go to different parts of the cortex. For complex patterns of sounds like speech and music other areas of the cortex become involved.
- Somatic (skin & muscle) Senses:
- Motor:
- Primary Motor ( Muscle Control):
Precentral gyrus (frontal lobe). Arranged like a piano keyboard: stimulation in this area will cause individual muscles to contract. Like the sensory cortex, the arrangement is in the form of an upside-down homunculus. The fibers are crossed- stimulation of the right cortex will cause contraction of a muscle on the left side of the body. - Premotor (Patterns of Muscle Contraction):
Frontal lobe in front of precentral gyrus. This area helps set up learned patterns of muscle contraction (think of walking or running which involve many muscles contracting in just the right order). - Speech-Muscle Control:
Broca's area, frontal lobe, usually in left hemisphere only. This area helps control the patterns of muscle contraction necessary for speech. Disorders in speaking are called aphasias.
- Primary Motor ( Muscle Control):
- Perception:
- Speech- Comprehension:
Wernicke's area, posterior end of temporal lobe, usually left hemisphere only. Thinking about words also involves areas in the frontal lobe. - Speech- Sound/Vision Association:
Angular gyrus, , makes connections between sounds and shapes of words
- Speech- Comprehension:
The large intestine (colon)
The large intestine receives the liquid residue after digestion and absorption are complete. This residue consists mostly of water as well as materials (e.g. cellulose) that were not digested. It nourishes a large population of bacteria (the contents of the small intestine are normally sterile). Most of these bacteria (of which one common species is E. coli) are harmless. And some are actually helpful, for example, by synthesizing vitamin K. Bacteria flourish to such an extent that as much as 50% of the dry weight of the feces may consist of bacterial cells. Reabsorption of water is the chief function of the large intestine. The large amounts of water secreted into the stomach and small intestine by the various digestive glands must be reclaimed to avoid dehydration.
Phases of cardiac cycle :
1. Early diastole ( also called the atrial diastole , or complete heart diastole) : During this phase :
- Atria are relaxed
- Ventricles are relaxed
- Semilunar valves are closed
- Atrioventricular valves are open
During this phase the blood moves passively from the venous system into the ventricles ( about 80 % of blood fills the ventricles during this phase.
2. Atrial systole : During this phase :
- Atria are contracting
- Ventricles are relaxed
- AV valves are open
- Semilunar valves are closed
- Atrial pressure increases.the a wave of atrial pressure appears here.
- P wave of ECG starts here
- intraventricular pressure increases due to the rush of blood then decrease due to continuous relaxation of ventricles.
The remaining 20% of blood is moved to fill the ventricles during this phase , due to atrial contraction.
3. Isovolumetric contraction : During this phase :
- Atria are relaxed
- Ventricles are contracting
- AV valves are closed
- Semilunar valves are closed
- First heart sound
- QRS complex.
The ventricular fibers start to contract during this phase , and the intraventricular pressure increases. This result in closing the AV valves , but the pressure is not yet enough to open the semilunar valves , so the blood volume remain unchanged , and the muscle fibers length also remain unchanged , so we call this phase as isovolumetric contraction ( iso : the same , volu= volume , metric= length).
4. Ejection phase : Blood is ejected from the ventricles into the aorta and pulmonary artery .
During this phase :
- Ventricles are contracting
- Atria are relaxed
- AV valves are closed
- Semilunar valves are open
- First heart sound
- Intraventricular pressure is increased , due to continuous contraction
- increased aortic pressure .
- T wave starts.
5. Isovolumetric relaxation: This phase due to backflow of blood in aorta and pulmonary system after the ventricular contraction is up and the ventricles relax . This backflow closes the semilunar valves .
During this phase :
- Ventricles are relaxed
- Atrial are relaxed
- Semilunar valves are closed .
- AV valves are closed.
- Ventricular pressure fails rapidly
- Atrial pressure increases due to to continuous venous return. the v wave appears here.
- Aortic pressure : initial sharp decrease due to sudden closure of the semilunar valve ( diacrotic notch) , followed by secondary rise in pressure , due to elastic recoil of the aorta ( diacrotic wave) .
- T wave ends in this phase