Comprehensive Study Notes on Blood Physiology, Cardiovascular Systems, and Excretory Mechanisms

Department of Physics: Blood, its Structure and Function

The following notes pertain to the Department of Physics at Nnamdi Azikiwe Universi, Faculty of Physical Sciences, P.M.B. 5025, Awka, Nigeria. The blood is described as a fluid consisting of two primary components: the fluid portion, known as the plasma, and the cellular elements. These cellular elements include the leulocytes (white blood cells), erythrocytes (red blood cells), and thromborges (platelets). Blood is circulated throughout the body to carry oxygen and nutrients to tissues, while simultaneously collecting and disposing of metabolic waste materials.

The Physiology of Red and White Blood Cells

Red blood cells (RBC), also termed erythrocytes, are characterized as flat, circular, and biconcave discs. They are produced in the anthro cytes bone marrow and are notable for having no nucleus. They contain a respiratory pigment called haemoglos (haemoglobin), which reacts with oxygen to form oxyhaemoglobin. The primary function of RBCs is to transport oxygen in the form of oxyhaemoglobin. These cells have an average life span of approximately $120\,\text{days}$.

White blood cells (WBC), or leulocytes, are irregular in shape and possess a nucleus. While they are larger than red blood cells, they are fewer in number. They are produced in the bone marrow, lymph nodes, and the spleen. Their primary role is defense; they are phagocytic in function, meaning they defend the body against infection by engulfing pathogens.

Blood Platelets and the General Functions of Blood

Blood platelets, also known as thrombocytes, may be square, star-like, or irregular in shape. Like other blood cells, thrombocytes are produced in the red bone marrow. Their essential function is aiding in the clotting of blood, which prevents excessive bleeding and the entry of bacteria into the body through open wounds.

The overall functions of blood are multifaceted. First, defense is a critical role; WBC cells combat invading bacteria or viruses through phagocytosis. Furthermore, some WBCs produce antibodies to destroy or inactivate invading pathogens, while others produce antitoxins to neutralize toxins produced by bacteria or viruses. Second, blood assist in the regulation of body temperature by ensuring the even distribution of heat. Third, it serves a transport function for $O_2$, $H_2O$, $CO_2$, food materials, hormones, and other products of metabolism. Fourth, the lymphatic system, involving pale watery fluid called lymph, uses valves to maintain a one-way flow for stability. Finally, the formation of clots by platelets seals wounds to prevent bleeding and pathogen entry.

Anatomy of the Circulatory System and Cardiac Structure

In vertebrates, the circulatory system is composed of organs including the heart, blood vessels, and blood. It is defined as a closed system, meaning the blood never leaves the network of blood vessels. This is distinct from many invertebrates, such as arthropods, which have an open circulatory system where a heart pumps hemolymph into a body cavity rather than through vessels. Simple organisms like sponges and comb jellies (Ctenophora) or diploblasts from the phylum Coelenterata (e.g., Coidana) lack a circulatory system entirely. In vertebrates, the system is divided into systemic circulat and plumonery arculats.

The heart is a muscular organ located in the middle compartment of the chest between the lungs, an area called the mediastinum. In humans, the heart is divided into four chambers: the upper left and right abía (atria) and the lower right and left ventricles. The right atrium and ventricle are collectively known as the right heart, with the left chambers known as the left heart.

Cardiac Rhythm, the Pacemaker, and Pulmonary Circulation

The heart's natural rhythm generator is the pacemaker, located in the sinoatrial (SA) node. This node produces electrical impulses that control the rate of contraction of the cardiac muscle, effectively regulating the heart's sinus rhythm. This electrical current travels through an atrioventricular node and along the conducting system of the heart. A healthy adult heart typically beats at a resting rate of approximately $72\,\text{beats per minute}$. While exercise causes a temporary increase in heart rate, it is beneficial for long-term health as it eventually lowers the resting rate.

In the human circulatory system, deoxygenated blood enters the heart via the right atrium from the superior and inferior vanae cavae and passes into the right ventricle. From there, it is pumped into the plumonony circulat to the lungs, where it receives oxygen and releases carbondioxide. Oxygenated blood then returns to the left atrium, passes to the left ventricle, and is pumped through the aorta into the systemic circulat. It travels through arteries, arterioles, and capillaries, where nutrients and substances are exchanged with cells. After losing oxygen and gaining carbondioxide, the blood returns to the heart via venules and veins.

Cardiovascular Health and Clinical Considerations

Blood flows in a single direction through the heart thanks to heart valves, which prevent backflow. The organ is encased in a protective sac called the pericardium, which contains a small amount of fluid. The heart wall itself consists of three layers: the epicardium, myocardium, and endocardium. Cardiovascular diseases are a major global health concern, accounting for $30\%$ of all human deaths as of $2008$. Risk factors for coronary artery disease include smoking, being overweight, lack of exercise, high cholesterol, HBP (high blood pressure), and diabetes. Symptoms may include chest pain or shortness of breath. Diagnosis involves taking a medical history, listening to heart sounds with a stethoscope, and using tools such as an ECG (electrocardiogram) or an echocardiogram (ultrasound).

Principles of the Excretory System

Excretion is defined as the removal of waste products or by-products of metabolism that accumulate due to chemical activities within body cells. This process is essential for maintaining water balance, removing poisonous or unwanted substances, controlling body temperature, and maintaining salt balance (homeostasis). Egestion, by contrast, is the elimination of undigested food in animals through the anus.

Across different species, the excretory organs vary significantly. Unicellular organisms like Amoeba and Paramecium utilize the contractile vacuole. Flatworms such as Planaria, flukes, and tapeworms use flame cells. Annelids, including earthworms, utilize nephridia. Insects use Malpighian tubules. Mammals, birds, amphibians, and reptiles use the skin, kidneys, and liver. In plants, the process is handled via stomata and lenticells.

Mechanisms of Excretion in Plants

Plants do not have a specialized excretory system because they are stationary and manufacture their own food, resulting in minimal metabolic waste. Primary waste products including water, carbon dioxide ($CO_2$), and oxygen ($O_2$) diffuse out through intercellular spaces. In leaves and young stems, gases escape through stomata, while in old stems, they escape through lenticells. In roots, gases pass through the general root surface.

Excess water in plants is removed as water vapour through transpiration. In specific plants like tomato and potato, excess water is released in liquid form as droplets at leaf margins, a process known as guttation. Other waste products may be converted into harmless insoluble forms like calcium oxalate and calcium carbonate, or stored in cell vacuoles and walls as glycosides, tannins, alkaloids, or plant oils. These products are often removed when leaves fall.

Mammalian Excretion and the Human Urinary System

The mammalian excretory organs include a pair of kidneys, the lungs, and the liver. In females, the urinary opening is separate from and dorsal to the vaginal opening. In males, the urethra serves as a common opening for both sperm and urine, known as the urinogenital opening (penis), while the female equivalent is the rudimentary clitoris.

The human urinary system (renal system) consists of the kidneys, ureters, bladder, and urethra. Its purpose is to eliminate waste, regulate blood volume and pressure, control electrolyte/metabolite levels, and regulate blood pH. The kidneys possess an extensive blood supply via the renal arteries and exit via the renal veins. The kidneys are bean-shaped, brownish organs held in place by fatty tissue. Each is covered by a tough fibrous membrane called the capsule. An endocrine gland, the adrenal gland, sits above each kidney. The depression on the concave edge is the hilum, where the renal artery enters and the renal vein and ureter exit.

Internal Kidney Structure and the Role of the Nephron

A cross-section of the kidney reveals the cortex, medulla, and pelvis. Each kidney is composed of functional units called nephrons. A nephron consists of a malpighian body (a thin-walled cup called Bowman's capsule fitted with a knot of capillaries called the glomerulus). This continues into a convoluted and ciliated tube that includes a U-shaped bend known as the loop of Henle. The tube is closely associated with networks of blood capillaries.

Urine formation involves three main steps. First, ultrafiltration occurs in the Bowman's capsule, where water, salts, sugar, and urea filter out of the blood (while blood cells and large proteins stay in the stream). This fluid is called the glomerular filtrate. Second, selective reabsorption occurs as the filtrate passes through the proximal convoluted tubule and the loop of Henle; water, sugar, amino acids, and some salts are reabsorbed into the capillaries. Third, the fluid becomes more concentrated in the distal portion of the tubule, forming urine, which consists of water, urea, and small amounts of uric acid, ammonium compounds, mineral salts, and creatinine.

Osmoregulation, pH Control, and the Role of Other Organs

The kidney plays a vital role in osmoregulation, maintaining the osmotic concentration of blood plasma to prevent cells from swelling (if plasma is too dilute) or shrinking (if plasma is too concentrated). It also maintains the acid-base balance (pH) by excreting hydrogen ($H^+$) ions and producing ammonia ($NH_3$) at the distal portion to react with acid, forming mineral salts that are excreted. Normal urine is slightly acidic.

Other organs contribute to excretion as well. The lungs excrete carbon dioxide ($CO_2$) and water vapour formed during respiration. The liver prepares materials for the kidney. Through deamination, the liver breaks down excess amino acids into a keto-acid (stored as glycogen) and an amino group, which is converted to ammonia and then to urea. The liver also performs detoxification, converting toxins from food preservatives, drugs, and bacteria into harmless forms. The skin helps maintain fluid balance and excrets excess mineral salts and water through sweat pores.

Detailed Functions of the Liver

The liver's functions are critical for survival and include:

1. Carbohydrate metabolism: Regulating blood sugar by storing excess glucose as glycogen under the influence of insulin.
2. Bile formation: Breaking down red blood cell pigments and producing bile to emulsify fats.
3. Deamination of proteins: Removing the amino group ($NH_2$) from excess amino acids to produce urea.
4. Storage of irons: Retaining iron from decomposed red blood cells.
5. Manufacture of plasma proteins: Producing proteins like fibrinogen for blood clotting.
6. Maintenance of body heat: distributing heat produced by high metabolic rates evenly through blood circulation.
7. Detoxification: Changing poisonous substances from the intestines or drugs into harmless substances.
8. Storage of vitamins: Storing fat-soluble vitamins such as vit AVHD.

Questions & Discussion

Note: More urine is produced on a cold day than on a hot dry day because on a cold day, the body undergoes an increased metabolic rate to produce more heat and maintain body temperature, while less water is lost through sweat. Additionally, some individuals, such as diabetes patients or women suffering from vesico-vaginal frere (fistula), may pass urine frequently if the sphincter muscle at the neck of the bladder is destroyed or compromised.