Circulatory system 1

The Blood

• The surrounding environment of cells and tissues is composed of body fluids, blood and interstitial/tissue fluid.
• Blood connects tissues to each other, maintains internal environment stability, and transports molecules to and from cells.
• Blood carries nutrients absorbed from the intestine, oxygen from the air and waste products from tissues.
• Human blood circulates in a closed system.
• Human blood is composed of blood plasma and blood cells
• The total blood volume in adults is about 5 litres, and at least 50% of that volume is blood plasma.

Blood Plasma

• Blood plasma is composed of 90% water.
• The remaining 10% contains ions and organic molecules.
• Blood plasma consists of cations such as sodium, potassium, and calcium, and anions such as chloride and bicarbonate.
• Blood plasma consists of small organic molecules like glucose, amino acids and lipids, proteins, carbamide/urea and uric acid.
• Albumins in blood regulate the osmotic pressure of blood to transport fatty acids and bile acids.
• Globulins transport carbohydrate side chains involved in transport and immune responses.
• Fibrinogen is a protein involved in blood coagulation after injuries.

Red Blood Cells (Erythrocytes)

• Erythrocytes are biconcave cells and the most abundant cell type in human blood, with 5 million/1 mm3 blood.
• Erythrocytes are produced in the bone marrow through haematopoiesis.
• Erythrocytes' DNA content gradually decreases during maturation for four to five days, and their nuclei disappear.
• Haemoglobin concentration increases in the cytoplasm of red blood cells during maturation.
• Haemoglobin is a complex protein of four subunits with an iron-containing heterocyclic ring called porphyrin and a polypeptide chain called globin.
• Haemoglobin is able to carry respiratory gases, transporting oxygen from the respiratory system to body tissues and carbon dioxide from tissues back to respiratory organs.
• As oxyhaemoglobin, it transiently binds oxygen and delivers it to tissues.
• Haemoglobin's oxygen-binding capacity is mostly influenced by the oxygen pressure.
• The pulmonary alveoli are surrounded by blood capillaries, providing ideal conditions for the formation of oxyhaemoglobin.
• Oxygen is released in blood capillaries found in different body tissues where oxygen pressure is lower.
• Carbon dioxide generated during cells' metabolic processes enters red blood cells through simple diffusion.
• Carbon dioxide and water are converted to carbonic acid (H2CO3) by enzymes in erythrocytes.
• Carbonic acid then dissociates into hydrogen (H+) and hydrogen carbonate (HCO3−) ions.
• Haemoglobin takes up hydrogen ions, increasing hydrogen carbonate ions' concentration in red blood cell cytoplasm.
• Higher concentration gradient between blood plasma and red blood cells facilitates the diffusion of HCO3− outward.
• Chloride ions (Cl−) flow into erythrocytes, maintaining the equilibrium of negative charges across the membrane.
• In the lung, diffused carbon dioxide the pressure of which is lower back into pulmonary alveoli.
• Carbon dioxide is released from carbonic acid generated from hydrogen and hydrogen carbonate ions.
• Reduced HCO3 leads to Cl- diffusion back into blood plasma.
• Lifespan of RBCs is about 120 days, after which they are degraded in the spleen.
• The heme group released from RBCs is converted to a yellow pigment called bilirubin in the liver and is secreted into the small intestine.
• Anaemia includes diseases where red blood cells or functioning haemoglobin molecules are less than normal.
• Differing causes include decreased erythrocyte production from malnutrition.

White Blood Cells (Leukocytes)

• Leukocytes differ in morphology and function.
• Granulocytes have a short lifespan of a few days, are produced in bone marrow, step out capillaries by amoeboid movement, engulf bacteria/foreign substances, degrade engulfed material. They release the content of their cytoplasmic granules, thus killing pathogens.
• Monocytes are synthesized in the bone marrow, enter the blood after a while, leave the circulatory system, step into tissues by amoeboid movement, function as immune cells (macrophages), and engulf/break down foreign substances/pathogens/cellular debris.
• Lymphocytes are produced in the bone marrow, mature in the organs of the immune system, and specifically recognize and inactivate foreign molecules.

Platelets (Thrombocytes)

• Thrombocytes are cell fragments originating in cells synthesized in bone marrow.
• Thrombocytes are responsible for blood coagulation.
• Platelets attach to each other and blood vessel walls to close openings when blood vessels are damaged.
• In serious injuries, chemical reactions occur, turning soluble fibrinogen into insoluble fibrin molecules.
• These fibers with platelets create a network and seal gaps.
• Thrombin, a protein from prothrombin, is required for the fibrinogen → fibrin transition.
• Formation of both fibrin and thrombin requires calcium ions, and Vitamin K helps synthesize prothrombin; several other coagulation factors are also required.

Blood Clotting

• Blood coagulation normally takes 5–10 minutes in humans.
• Haemophilia affects males primarily and is inherited in an X-linked recessive manner and is a condition when this process requires hours

Anatomy and Physiology of the Human Circulatory System

• The circulatory system transports respiratory gases, nutrients, and other molecules (e.g., hormones), defends the body against pathogens, and maintains homeostasis by controlling body temperature and pH.
• The cardiovascular and lymphatic systems work together in the circulatory system.
• The parts of the cardiovascular system are the heart, blood vessels, and blood.
• Vertebrates have a closed cardiovascular system; blood never leaves blood vessels and the heart; lymph flows freely into/out of tissues in the lymphatic system.

The Human Heart

• The heart pumps respiratory gases, nutrients, and other molecules.
• The heart's wall has three layers: the pericardium (outermost thin layer), the myocardium (thick middle layer of heart muscle and coronary arteries), and the endocardium (inner layer of endothelial cells).
• The heart is divided into two sections by a vertical wall, creating two sides.
• Each half has a ventricle and an atrium.
• Carbon dioxide-rich blood flows from all tissues into the right atrium through the veins, blood enriched in oxygen as it arrives in the left atrium.
• Blood is then pumped into the ventricles.
• The wall of the heart is thinner in the atria because blood is only sent against a low pressure.
• The ventricles have a thicker heart muscle layer because they pump into arteries at much higher pressures.
• Cardiac valves are located at the atrium-ventricle and ventricle-artery boundaries to maintain one-way blood flow.
• Heart function is based on the rhythmic contraction/relaxation of muscle (cardiac cycle).
• The atria and ventricles contract alternatively.
• Atrium-ventricle boundary valves close during atrial contraction and relaxing ventricles, blood cannot flow back to the atria, high pressure causes ventricle-artery boundary valves to open, blood flows to body and lung.
• As blood flows into the blood vessels, pressure in the ventricles drops, and values at the ventricle-artery boundaries close leading into backwards blood flowing into the atria, and cycle restarting.
• The first and second heart sounds result from the closing and opening of valves: a "lub-dub" sound.
• The heart cycle occurs in adults in a resting state approximately 75 times every minute.
• The contraction of a ventricle releases around 70 ml blood per minute which means that a ventricle pumps approximately 5 liters blood into the circulatory system.

The Cardiac Cycle

• Systole is the contraction, and diastole is the relaxation of the heart muscle.
• Blood pressure is generally represented as systolic blood pressure (the pressure measured in big arteries) over diastolic blood pressures. Normal blood pressure is considered 120/80 mmHg (millimetres of mercury).

The Impulse-Generating System of the Heart

• Heart muscle cells can generate their own action potential.
• The sinoatrial or sinus node, in the right atrium, causes atrial contractions through the electric impulse it generates.
• This impulse is transmitted to the atrioventricular node with an 80 ms delay before passing to the Tawara's branches (left and right branches) of the bundles of His and the Purkinje fibers.
• Action potential changes during heart action can be measured through electrocardiogram (ECG).

Blood Vessels

• The circulation of blood is maintained in a closed circulatory system by the heart.
• The system is a network of arteries, veins, and capillaries.
• Arteries and arterioles deliver blood from the heart to all body tissues.
• Arteries' walls are three layers consisting of endothelial cells (lumen), smooth muscle (middle layer) and connective tissue (outer layer).
• When the left ventricle contracts, blood flows into the aorta, which branches into smaller arteries and becomes narrower; this difference in blood pressure means there is decreased blood pressure.
• Veins and venules transport waste products and oxygen from tissues/lungs back to heart.
• Compared to arteries, blood pressure in veins/venules is very low.
• Walls contain a smooth muscle layer thinner than arteries allowing them to easily dilate and compress.
• Skeletal muscle contraction facilitates blood flow towards the heart in veins, aided by venous valves that prevent blood from flowing backward.

Exchange in Capillaries

• A difference in pressure exists between the two ends of capillaries.
• Capillary walls are a single layer of endothelial cells functioning as a semi-permeable membrane.
• Blood plasma proteins cannot leave capillaries because of osmotic pressure.
• Water/nutrients leave capillaries because blood pressure is higher on arterial end than osmotic pressure of proteins in blood.
• At venous end, blood pressure is lower than proteins' osmotic pressure in blood, leading to water/waste products entering capillaries.
• Bigger molecules like plasma proteins can be taken into veins by lymph capillaries.