NEET MDS Lessons
Physiology
Heart is a hollow muscular organ , that is located in the middle mediastinum between the two bony structures of the sternum and the vertebral column ( a very important location for applying Cardiopulmonary Resuscitation - CPR- ) .
It has a shape of clenched fist , which weighs about 300 grams ( with mild variation between male and female ).
Heart has an apex that is anteriorly , inferiorly , and leftward oriented , and a base , that is posteriorly , superiorly and rightward oriented .
In addition to its apex and base the heart has anterior , posterior and left surfaces.
The wall of the heart is composed of three layers :
1. Endocardium : The innermost layer , which lines the heart chambers and is in direct contact with the blood . It is composed of endothelial cells that are similar to those , that line the blood vessels , and of connective tissue too.
Endocardium has a smooth surface that prevents blood clotting, as it ensures laminar blood flow .
Clinical Physiology
Endocarditis is the inflammation of the endocardium , which is resistant to antibiotic treatment and difficult to cure.Endocarditis usually involves heart valves and chordae tendineae too.
2. Myocardium : The middle layer of the cardiac wall . It is the thickest among the three layers , and is composed of two types of cardiac muscles :
a. contractile muscle cells (form about 98-99% of the cardiac muscle ) .
b- non-contractile muscle cells ( form about 1-2 % of the cardiac muscles and are the cells that form excitatory-conductive system of the heart).
The cardiac muscle cells are similar to the skeletal muscles in that they are striated , but similar to the smooth muscles in being involuntary and connected to each others via gap junctions , that facilitate conduction of electrical potential from one cell to the others. Desmosomes adhere cardiac muscle cells to each others .
3- Epicardium : is the outermost and protective layer of the heart . It is composed of connective tissue , and form the inner layer of the pericardium ( visceral pericardium - see bellow).
Pericardium:
The heart is surrounded by a fluid-fill sac , which is known as pericardium . Pericardium is composed of two layers ( doubled layer membrane ) , between which a fluid-fill pericardial cavity exist .
The outer layer is called fibrous pericardium , while the inner layer is called serous pericardium , which is subdivided into parietal pericardium and visceral pericardium . The visceral pericardium is the previously mentioned outermost layer of heart ( epicardium) .
Pericardial sac plays an important role in protection of heart from external hazards and infections , as it fixes the heart and limits its motion. It also prevents excessive dilation of the heart.
Clinical physiology:
When there is excessive fluid in the pericardial cavity as a result of pericardial effusion , a cardiac tamponade will develop . cardiac tamponade means compression of the heart within the pericardial sac , which will prevent the relaxation of the heart ( heart will not be able to fully expand ) , and thus the circulating blood volume will be decreased (obstructive shock) . This is a life threatening situation which has to be urgently cured by pericardiocentesis .
Chambers of the heart :
Heart has four chambers : two atria and two ventricles . The two right and left atria are separated from the two ventricles by the fibrous skeleton , which involves the right ( tricuspid ) and left ( bicuspid ) valves. Right and left atria are separated from each other by the interatrial septum .
The two ventricles are separated by the interventricular septum.Interventricular septum is muscular in its lower thick part and fibrous in its upper thin part.
The two atria holds the blood returning from the veins and empty it only in a given right moment into the ventricles. Ventricles pump the blood into the arteries .
Heart valves :
There are four valves in the heart : Two atrioventricular valves and two semi-lunar valves:
1. Atrioventricular ( AV ) valves: These valves are found between the atria and ventricles , depending on the number of the leaflets , the right atrioventricular valve is also called tricuspid valve (has three leaflets ) , while the left one is called bicuspid valve (has two leaflets ) . The shape of the bicuspid valve is similar to the mitre of bishop , so it is also called the mitral valve.
The leaflets of the valves are attached to fibrous threads (composed of collagen fibers ) , known as chordae tendineae , which from their side are attached to papillary muscles in the ventricles. These valves prevent backward flow of blood from ventricles during the systole.
2. Semi-lunar valves :
These valves are located on the base of the arteries ( aorta and pulmonary artery ) . They prevent the backward flow of blood from the arteries into ventricles.
The structure of the semilunar valves is quite different from that of the AV valves , as they have crescent-shaped cusps that do not have chorda tendinea , instead these cusps are like pockets which are filled of blood when it returns to the ventricles from the lumen of arteries during the diastole , so they get closed and prevent the backward flow of blood.
Oxygen Uptake in the Lungs is Increased About 70X by Hemoglobin in the Red Cells
- In the lungs oxygen must enter the blood
- A small amount of oxygen dissolves directly in the serum, but 98.5% of the oxygen is carried by hemoglobin
- All of the hemoglobin is found within the red blood cells (RBCs or erythrocytes)
- The hemoglobin content of the blood is about 15 gm/deciliter (deciliter = 100 mL)
- Red cell count is about 5 million per microliter
Each Hemoglobin Can Bind Four O2 Molecules (100% Saturation)
- Hemoglobin is a protein molecule with 4 protein sub-units (2 alphas and 2 betas)
- Each of the 4 sub-units contains a heme group which gives the protein a red color
- Each heme has an iron atom in the center which can bind an oxygen molecule (O2)
- The 4 hemes in a hemoglobin can carry a maximum of 4 oxygen molecules
- When hemoglobin is saturated with oxygen it has a bright red color; as it loses oxygen it becomes bluish (cyanosis)
The Normal Blood Hematocrit is Just Below 50%
- Blood consists of cells suspended in serum
- More than 99% of the cells in the blood are red blood cells designed to carry oxygen
- 25% of all the cells in the body are RBCs
- The volume percentage of cells in the blood is called the hematocrit
- Normal hematocrits are about 40% for women and 45% for men
At Sea Level the Partial Pressure of O2 is High Enough to Give Nearly 100% Saturation of Hemoglobin
- As the partial pressure of oxygen in the alveoli increases the hemoglobin in the red cells passing through the lungs rises until the hemoglobin is 100% saturated with oxygen
- At 100% saturation each hemoglobin carries 4 O2 molecules
- This is equal to 1.33 mL O2 per gram of hemoglobin
- A person with 15 gm Hb/deciliter can carry:
- Max O2 carriage = 1.33 mL O2/gm X 15 gm/deciliter = 20 mL O2/deciliter
- A plot of % saturation vs pO2 gives an S-shaped "hemoglobin dissociation curve"
- At 100% saturation each hemoglobin binds 4 oxygen molecules
At High Altitudes Hemoglobin Saturation May be Well Below 100%
- At the alveolar pO2 of 105 mm Hg at sea level the hemoglobin will be about 97% saturated, but the saturation will fall at high altitudes
- At 12,000 feet altitude alveolar pO2 will be about 60 mm Hg and the hemoglobin will be 90% saturated
- At 29,000 feet (Mt. Everest) alveolar pO2 is about 24 mm Hg and the hemoglobin will be only 42% saturated
- At very high altitudes most climbers must breath pure oxygen from tanks
- During acclimatization to high altitude the hematocrit can rise to about 60%- this increases the amount of oxygen that can be carried
- Hematocrits above 60% are not useful because the blood viscosity will increase to the point where it impairs circulation
Pain, Temperature, and Crude Touch and Pressure
General somatic nociceptors, thermoreceptors, and mechanoreceptors sensitive to crude touch and pressure from the face conduct signals to the brainstem over GSA fibers of cranial nerves V, VII, IX, and X.
The afferent fibers involved are processes of monopolar neurons with cell bodies in the semilunar, geniculate, petrosal, and nodose ganglia, respectively.
The central processes of these neurons enter the spinal tract of V, where they descend through the brainstem for a short distance before terminating in the spinal nucleus of V.
Second-order neurons then cross over the opposite side of the brainstem at various levels to enter the ventral trigeminothalamic tract, where they ascend to the VPM of the thalamus.
Finally, third-order neurons project to the "face" area of the cerebral cortex in areas 3, 1, and 2 .
Discriminating Touch and Pressure
Signals are conducted from general somatic mechanoreceptors over GSA fibers of the trigeminal nerve into the principal sensory nucleus of V, located in the middle pons.
Second-order neurons then conduct the signals to the opposite side of the brainstem, where they ascend in the medial lemniscus to the VPM of the thalamus.
Thalamic neurons then project to the "face" region of areas 3, I, and 2 of the cerebral cortex.
Kinesthesia and Subconscious Proprioception
Proprioceptive input from the face is primarily conducted over GSA fibers of the trigeminal nerve.
The peripheral endings of these neurons are the general somatic mechanoreceptors sensitive to both conscious (kinesthetic) and subconscious proprioceptive input.
Their central processes extend from the mesencephalic nucleus to the principal sensory nucleus of V in the pons
The subconscious component is conducted to the cerebellum, while the conscious component travels to the cerebral cortex.
Certain second-order neurons from the principal sensory nucleus relay proprioceptive information concerning subconscious evaluation and integration into the ipsilateral cerebellum.
Other second-order neurons project to the opposite side of the pons and ascend to the VPM of the thalamus as the dorsal trigeminothalamic tract.
Thalamic projections terminate in the face area of the cerebral cortex.
Normal Chemical Composition of Urine
Urine is an aqueous solution of greater than 95% water, with a minimum of these remaining constituents, in order of decreasing concentration:
Urea 9.3 g/L.
Chloride 1.87 g/L.
Sodium 1.17 g/L.
Potassium 0.750 g/L.
Creatinine 0.670 g/L .
Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites).
Urine is sterile until it reaches the urethra, where epithelial cells lining the urethra are colonized by facultatively anaerobic gram-negative rods and cocci. Urea is essentially a processed form of ammonia that is non-toxic to mammals, unlike ammonia, which can be highly toxic. It is processed from ammonia and carbon dioxide in the liver.
Sensory pathways include only those routes which conduct information to the conscious cortex of the brain. However, we will use the term in its more loosely and commonly applied context to include input from all receptors, whether their signals reach the conscious level or not.
Bleeding Disorders
A deficiency of a clotting factor can lead to uncontrolled bleeding.
The deficiency may arise because
- not enough of the factor is produced or
- a mutant version of the factor fails to perform properly.
Examples:
- von Willebrand disease (the most common)
- hemophilia A for factor 8 deficiency
- hemophilia B for factor 9 deficiency.
- hemophilia C for factor 11 deficiency
In some cases of von Willebrand disease, either a deficient level or a mutant version of the factor eliminates its protective effect on factor 8. The resulting low level of factor 8 mimics hemophilia A.
Basic Properties of Gases
A. Dalton's Law of Partial Pressures
1. partial pressure - the "part" of the total air pressure caused by one component of a gas
Gas Percent Partial Pressure (P)
ALL AIR 100.0% 760 mm Hg
Nitrogen 78.6% 597 mm Hg (0.79 X 760)
Oxygen 20.9% l59 mm Hg (0.21 X 760)
CO2 0.04% 0.3 mm Hg (0.0004 X 760)
2. altitude - air pressure @ 10,000 ft = 563 mm Hg
3. scuba diving - air pressure @ 100 ft = 3000 mm Hg
B. Henry's Law of Gas Diffusion into Liquid
1. Henry's Law - a certain gas will diffuse INTO or OUT OF a liquid down its concentration gradient in proportion to its partial pressure
2. solubility - the ease with which a certain gas will "dissolve" into a liquid (like blood plasma)
HIGHest solubility in plasma Carbon Dioxide
Oxygen
LOWest solubility in plasma Nitrogen
C. Hyperbaric (Above normal pressure) Conditions
1. Creates HIGH gradient for gas entry into the body
2. therapeutic - oxygen forced into blood during: carbon monoxide poisoning, circulatory shock, asphyxiation, gangrene, tetanus, etc.
3. harmful - SCUBA divers may suffer the "bends" when they rise too quickly and Nitrogen gas "comes out of solution" and forms bubbles in the blood